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Application of composite materials to yacht rigging
Application of composite materials to yacht rigging
12
R. Pemberton, J. Graham-Jones University of Plymouth, Plymouth, UK
12.1 Introduction Whilst sailing vessels have developed from the stone-age dug-out canoes to the m odern America's Cup hydrofoiling catamarans, the fundamental requirement of a yacht’s rigging has not changed. It must work in combination with the mast and hull to provide a platform for the sails to generate as much thrust as possible. Ideally, this should be achieved with minimal aerodynamic drag penalty. Whilst this covers the performance requirements of rigging, there are additional safety requirements. The rigging must also be capable of supporting the mast and sails in the strongest of foreseeable wind conditions. In this regard, rigging becomes a critical component on a yacht, for without it the vast majority of yachts masts would fall down, causing damage to the yacht, with the associated risk of broken rigging fouling the propeller, rendering the yacht and crew vulnerable to the elements. Historically, as rig design has developed, there has always been a drive to exploit the best materials on offer. Rigging has the somewhat contradictory requirements of high strength with as small a cross section as possible, to minimise the associated aerodynamic resistance. This chapter will briefly outline the development of rigs, and the range of materials that have been used for rigging, to give a background to where composite materials sit in their application to rigging. The requirements of the mast designer are also discussed, along with the implications for the hull and sail designer and what might be expected for future rigging materials.
12.2 Rig development Whilst the fundamental requirements of a rig may not have changed, the means of harnessing the winds’ power has significantly advanced. The exact start of seafaring and sailing is difficult to place and a matter of debate amongst archaeologists. There is evidence that the as early as c.5000 BC, boats were being built with masts, with a possible mast foot found in a bitumen-preserved section of a reed boat (Carter and Crawford, 2010). It is certain that by the Bronze Age (c.3100 BC), the depiction of sails on the Naqada vases leads archaeologists to believe that sailing and rigs were quite developed (Edwards et al., 1970). Marine Applications of Advanced Fibre-Reinforced Composites. http://dx.doi.org/10.1016/B978-1-78242-250-1.00012-0 Copyright © 2016 Elsevier Ltd. All rights reserved.
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These early rigs were square sails, predominantly in the case of the Nile, to sail with the wind and allow vessels to return against the current. In terms of rigging, simple stays were used to support the mast in the fore and aft direction, with some lateral support offered by the stays fixed to the outboard parts of the hull. With time came the desire to sail at angles closer to the wind’s direction, and so square sails were rotated to align closer to the crafts centreline as opposed to perpendicular to it. The extra lateral support required for this came about through the increase in beam of the hulls. Early rig designs developed, from square sails with horizontal spars called yards typically seen in Viking ships which had one square sail supported by a head yard (wooden pole) and were attached to control ropes at the sail’s foot (loose foot). In HMS Victory, several square sails were in positions above each other on the masts. The foot (base) of one sail became the head (top) yard of the next going down. Rig designs developed, from square sails to lug sails (a quadrilateral sail bent upon a yard that crosses the mast obliquely.) where the yards at the top were hauled to the top of the mast and yards angled skyward A lug sail is a quadrilateral shape (Figure 12.1) and very easy to raise and control. Lug sails are still popular today for their simple constructions and traditional look. Looe, in Cornwall, runs a bi-annual Lugger Race linking back to the traditional fishing boats that used Lug sails. A variation of this is the Chinese lugsail more commonly called Junk rig, or sampan rig. The main difference is the use of horizontal battens to control the sail. As ships became larger, greater sail areas are required to overcome hull and Peak Yard
He
Mast
ad
Halyard
Leech
Luf f
Throat Centre of Effort
Clew Foot
Tack
Mast
Figure 12.1 Lug sail rig.
Boom Downhaul
Application of composite materials to yacht rigging281
wave making drag. Lug rigs were limited as the yards could not exceed the maximum length of a tree. Gaff rig sails are still limited to a maximum tree length. However, attaching the yard (now called gaff) to the mast via a loop of rope at the throat and raising the gaff to similar angles resulted in a greater sail area. Additionally, the boom was also attached via the tack to the mast further increasing sail area. Gaff rig sails (Leather, 2001) similar to Lug sails are still used by modern boat manufacturers, e.g. the Cornish Crabber (www.cornishcrabbers.co.uk). With improving materials and adhesives longer masts became possible which lead to the Bermudan rig becoming the most popular for modern boats. Considering the sail developments from Square, Lug, to Gaff (a spar rising aft from a mast to support the head of a quadrilateral fore-and-aft sail) and Bermudan (triangular sail) rigs, the method of rigging the mast has not evolved at a similar pace. The rigging was still taken to the outboard side of the hull, and so the overall height of the mast was governed by the beam of the boat and the size and type of rigging used. The end of the nineteenth century saw a significant change in rig design, in that the lateral support offered by rigging to a mast was increased by using spreaders, bars which increase the angle between the rigging and the mast, before the rigging runs down to the deck. Photographs of the Royal Yacht, ‘Britannia’, sailing in the 1890s show a set of spreaders on the mast, and the development of taller rigs coincided with the development of radio and radio masts, hence these rigs were often referred to as Marconi rigs. Figure 12.2 shows the typical staying arrangement for a multi-spreader modern racing yacht rig. The longitudinal support of the rig is provided by the backstays (running
Jumpers
ksta y
bac
ing
ast b
V4
D4
V3
D3
V2
D2
V1
D1
Spreader 3
Spreader 2
Ch ec
ks tay
Run n
Top m
Spreader 4
stay
e For
ack s
tay
V5
Spreader 1
Figure 12.2 A typical modern racing yacht rig (stern at left and bow at right).
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aft) and the forestay (running forwards, also known as the headstay). The tensions in all of the backstays are adjustable whilst sailing, and often in the forestay as well. The lateral support for the rig is provided by the shrouds and the spreaders. Shrouds are normally rope, wire, or rods in tension attached symmetrically on both port and starboard sides of the vessel. Typically, they attach to the mast in several locations: at the mast head (Cap Shrouds) or mid-way (Lower Shrouds), as well as either side of the mast. A typical yacht could have six different shrouds to provide lateral support the mast. Shrouds ideally attach high up the mast, thus structures projecting from the mast, i.e. Spreaders, increase the angle of the shroud at the deck fixings (chainplates) providing more mast support. The shrouds are often made up of discontinuous elements, named from the bottom up numerically and by whether they are vertical (V) or diagonal (D). Hence the lowest vertical and diagonal shroud would be called V1 and D1 respectively. The specification and sizing of these rigging elements will be discussed later, the following section covers the choices of materials which were, and are currently, available for use as rigging.
12.3 Rigging materials In addition to the sail plans developing, so too were the materials being used for rigging. One of the difficulties in establishing the history of rigging is that rigging and sails are rarely preserved archaeologically. The information on what was used by our ancestors must be gleaned from the remaining wooden elements, or from contemporary historical accounts, such as the Norse sagas, or artistic depictions. Before the advent of international trade routes, the materials used for rigging were whatever was locally available, and as such the range is wide. The techniques for producing the rigging ropes may have been similar throughout time and globally, but the materials may have been leather, seaweed, papyrus, willow or even animal hairs. The pre-dominant material for much of the history of rigging has been hemp. From its first uses for rope production in Asia in 4000 BCE, hemp rope spread throughout the world as the seafarers sailed and traded with countries further afield (McCaskill, 2009). It was only in the 19th century when abacá (a member of the banana family) became the preferred material for rope production, that its use decreased. Many would not have known of this change, as abacá became known as Manila Hemp, due to it being a native plant of the Philippines. Further details of fibre rope technology are given by McKenna et al. (2004). As the industrial revolution brought about a decrease in the cost of producing iron and steel, so these materials began to replace the traditional organic materials. Developments in the mining industry in Germany in the 1830s saw wrought iron rope replacing hemp for use in underground cable carriages (Wilhelm Albert, 2014). These inventions were adopted by seafarers, with a report from the International Exhibition of 1862 stating that the use of cast iron rigging was common place for the previous 10 years, and 1863 saw the launch of the British ship Seaforth, which is documented as being the first to have both steel spars and rigging (Anderson and Anderson, 2012).
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Steel rope development has not stood still and is currently a mature product, due to its ease of manufacture and fitting, in addition to its relatively low cost, and hence remains the most common rigging choice for smaller yachts and dinghies. SAE316 grade austenitic nickel–chromium stainless steel (Cr16Ni10Mn2Mo2Si1), known as the marine grade stainless steel, is used in rigging and has increased resistance to chloride corrosion due to the 2–3% molybdenum compared to type SAE304 stainless steel (Cr18Ni8Mn2Si1). The wire sizes range from 3 to 16 mm. The most basic circular cross-section strand wire construction (wire rope classification is 1 × 7) has six strands hexagonal close packed around a single core strand. The next size up is overwrapped by a further 12 strands to produce 1 × 19 strand wire. These individual wires may be further grouped to produce other common arrangements used in rigging, notably 7 × 7 (49 wires), 7 × 19 (133 wires) and 6 × 36 (216 wires). Increasing the number of wires produces greater flexibility for the same size. Thus for running rigging (used for raising, lowering and controlling sails), a 7 × 19 would be used, as this rigging needs flexibility. Standing rigging supporting the mast needs high strength and stiffness thus 1x19 is used. On larger yachts, however, the desire for a rigging option with less stretch than wire, lead to further developments with steel. Rod Rigging first appeared on yachts in the late 1960s although its original development was done approximately 50 years previously, but not with yachts in mind. The requirements of war can often drive innovation, and during the First World War, the early biplanes had rigging wire between their sets of wings to produce a light, stiff, structure. Increasing the stiffness of this structure would have meant an increase in the diameter of the rigging wire used, which would have come at a cost of extra weight and aerodynamic drag. The Scottish firm of Macomber & Whyte made rigging from steel rods as an alternative to wire, as the elongation of the rods was typically 35% less under the same load as the equivalent wire (Spurr, 1982). The slow progression from aircraft rigging into yacht rigging was due to a number of early failures, predominantly fatigue failures of the end fittings of the rods, where they met either the mast or the chainplates. Chainplates are metal plates with many holes used to fasten a stay or shroud via a turnbuckle to a sail boat hull. A turnbuckle is used to allow articulation of the wire to the fixed chainplate. It wasn’t until the 1960s when this problem was solved by the development of cold forming heads onto the rods. The heads were swellings with a smooth radius around their edge, which could fit into specially designed fittings on the mast or hull. By 1970 all of the US America’s Cup 12MR yachts had chosen rod rigging in preference to wire. Rod rigging then found acceptance in the wider market outside of high end racing yachts, and until the last 10–15 years, was the default choice for the rigging of large yachts. These improvement in low drag and lower weight, are ignored by most coastal and offshore smaller yacht users who still use wire rigging as it cheaper and has better damage tolerance as when individual wires fail the fracture does not run completely through the whole wire rope. Damage can become visible to inspection. This damage tolerance is a key advantage in composite rigging. The marine environment is a notoriously corrosive one, and so the rods are made from a Nitronic 50 (Ni50: Fe56Cr22Ni12.5Mn5Mo2.25Si1C0.06) high quality stainless steel.
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The obvious downside of steel is its relatively high specific gravity of 7.5 in comparison with 1.5 of many composite constructions. Rigging is normally changed due to concerns over undetectable corrosion fatigue crack bringing the mast down. Stainless rod or wire rigging could pass a visual inspection but fail without warning. Non-destructive testing of metal rigging is expensive and unreliable. On smaller yachts, as a general guide standing rigging should be replaced every 12 years, 15 years maximum in salt water, or 15–20 years in fresh water. The replacement cycle depends on its use: if you are racing or sailing offshore, then earlier replacements are required due to the great loads normally on a 3 or 5 year cycle, or as recommended by classification society. Where boats do so little actual sailing, or remove the mast over winter, or are over-specified for the received loads, or are in a boat yard undergoing restoration over many years, then longer replacement cycle times are possible. Wire rigging normally fails where the wire enters the swaged fittings. Although less common failure at chainplates, tangs turnbuckles and toggles are possible. The potential weight savings, and more importantly, reductions in centre of gravity, lead rigging manufacturers to experiment with composite materials.
12.4 The composite age As sailcloth, hull laminates and running rigging took advantage of the favourable material properties composites offer for a given weight, naturally the progression to experimenting with these materials in standing rigging would follow. This main materials used are aramid, polybenzobisoxazole (PBO) and carbon, which will be described below.
12.4.1 Aramid (trade names: DuPont Kevlar® or Teijin Twaron®) The use of aramid fibres in sailing began in the late 1970s with America’s Cup syndicates making their sails from cloth containing it (Stansell, 1983). Aramid fibres have high tensile strength to weight ratio whilst also exhibiting low stretch which meant it was natural they should be used in rigging. Aramid fibres will degrade in sunlight and high UV environment. Thus the main load carrying aramid fibres are kept parallel encased within a polyethylene braided cover for protection. Further, aramid fibres are strongly hygroscopic relative to PBO or carbon, which provides an easy route for moisture ingress and associated inhomogeneity in properties. Whilst the material properties of aramids are not as good as other materials (e.g. PBO below), the lower cost of aramids, combined with its resilience to both fatigue and abrasion resistance means they still have a place in the market (Marples, 2015). The use of aramids tends to be more for checkstays and running backstays, which can often see damage due to contact with the sails, or sail battens. In this application, aramids resistance to impact makes it a suitable material choice. In compression, aramids are not ideal and kink due to the linearly arranged fibres.
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12.4.2 Polybenzobisoxazole/PBO (trade name: Toyobo Zylon®) Although produced in resin form in the late 1950s (USA Pat., 1955), PBO fibres were not seen as a suitable material for use in yacht rigging until the mid-1990s. Since aramid fibres had already been used as a rigging element, it was a natural progression to use PBO, because of its improved properties. PBO has a tensile modulus of 270 GPa and a tensile strength of 5.8 GPa (compared to 113 GPa and 2.8 GPa respectively for Kevlar). Typically, continuous PBO fibre tows are wound around metal thimble end fittings, before the fibres are covered with a braid and protective sheath under tension. The protective sheath is required due to the degradation of material properties with exposure to UV light (Marples, 2015). The use of PBO as a rigging material began in the early 2000s, and although it can have a greater weight saving over some carbon fibre rigging systems, the vulnerability to UV light does mean it is important to be conservative when assuming a lifespan within rigging. This has not stopped many boats sailing many tens of thousands of miles with PBO rigging, but it is expected, that with time it will lose market share to carbon fibre rigging. There is currently an outstanding lawsuit with the manufactures of PBO regarding the degradation of properties, in an instance where PBO was used within bulletproof vests which failed with fatal consequences (United States ex rel. Westrick v. Second Chance Body Armor, Inc. Second Chance Body Armor Inc., et al., 2013).
12.4.3 Carbon fibre From its first uses in racing yacht rudders, the high Young’s modulus and tensile strength of carbon fibre along with its corrosion resistance made it an attractive material to use in many high-performance, marine applications. The use of carbon fibre within rigging was a natural progression, particularly as many modern mast tubes were being made from it. There are a variety of methods of manufacturing carbon fibre rigging, which are often specific to particular manufacturers. The main types of construction are pultruded rods, rigid carbon rigging or carbon fibre/thermoplastic rigging (Dawson, 2015). These methods will be outlined in this section:
12.4.3.1 Pultruded rods One of the early methods of using carbon was developed by the company that now is Composite Rigging and the product is called EC6. Small diameter (approx.: 1 mm) pultruded carbon rods are grouped together. The ends of the rods are fixed in a resin, and moulded into a conical shape, which then fit into a titanium socket. The rods are then covered with a braid to protect them and keep them grouped together. The separate carbon elements provide a natural protection against crack propagation, and so have good fatigue and impact resistance properties. For a given weight of carbon, there will be a slight increase in diameter of the overall rigging rod compared to rigid carbon, as there is a packing factor for the pultruded rods fitting into a circle, plus there is the braided cover.
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12.4.3.2 Rigid carbon Carbo-Link (Carbo-Link, n.d.), Southern Spars (Southern Spars, n.d) and Hall Spars and Rigging (Hall Spars and Rigging, n.d.) are companies that produce solid carbon rigging. Whilst there are differences in the details of the production methods, the overall processes are similar. Epoxy prepreg tows of fibres are wound around end fittings, the fibres are consolidated under tension using shrink-wrap material and then the stay is cured. Each company has their own style of end fittings and their curing processes differ Hall Spars prepreg carbon is cured in a traditional oven, whereas Carbo-Link take advantage of the conductive nature of carbon fibre, and generate the heat to cure the resin by applying an electric current to the stay, in what is known as the Ohmic curing process. As the final product is a rigid object, both Hall Spars and Carbo-Link have been able to offer section shapes other than round with the aerodynamic gains which will be discussed later.
12.4.3.3 Carbon fibre/thermoplastic rigging Navtec Rigging Solutions (n.d.) were at the forefront of developing stainless steel rod rigging and have been active in developing their own carbon rigging. Rather than using a thermoset resin, they use a thermoplastic matrix. In a similar manner to Composite Rigging, they have bundles of small diameter carbon pultrusions, but the carbon is pre-impregnated with a specially developed thermoplastic. The bundle of rods is then loaded and goes through a proprietary process to fuse them into a solid bar. All of the production methods have the benefits that come with using carbon as a rigging material instead of steel. The most significant of these is that for the same stiffness there is a significant reduction in weight, but also there are the added benefits of greater fatigue and corrosion resistance (although it is important to guard against galvanic corrosion of adjacent light alloys). Carbon rigging also does not experience creep in the same way that steel rigging does. In terms of size, both Carbo-Link (n.d.) and Composite Rigging Ltd and Co (n.d.) have made rigging at both extremes of the yachting rigging world. The largest diameter rigging the companies have produced are 60 mm for Carbo-Link and 90 mm for Composite Rigging (Composite Rigging Ltd and Co, n.d.)) (for the main mast on an 85 m ketch). Even at 90 mm, it was still possible to coil the EC six cable (Southern Spars, n.d.) into a 3 m diameter for shipping, whereas at the large diameters such as 60 mm, Carbo-Link would prepare their cable at their factory, and then conduct the final cure on site. A benefit of their production method is that, provided they have electricity and two attachment points to tension the cable, they can cure their product anywhere. At the other end of the spectrum both companies produce rigging less than 2 mm in diameter for use in dinghy rigging market. The construction methods appear to be relatively scalable, although some of the practical issues of going to larger sizes involve the size of stock material for machining end fittings, particularly as titanium is the preferred material for end fittings. Another benefit of using composite rigging is that it allows large yachts the possibility of having continuous rigging. The rigging layout (shown in Figure 12.2) is made up of discontinuous elements, and whilst on smaller yachts (often single spreader rig
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boats) it has been possible with Nitronic rods to have a number of continuous rods for the rigging, this is not practical with larger yachts. With carbon rigging, it has been possible to incorporate the diagonals into the main vertical component and thereby reduce the sizeable end fittings which come with discontinuous rigging. There is often an added expense of doing this, as jigs particular to the boat in question need to be built. This is on top of the already greater expense of using carbon rigging over steel. Producing the raw carbon product is expensive, and whilst some elements can be automated, manufacture of carbon items tends to be labour-intensive, so a certain level of cost is unavoidable. In future, the extra manufacturing costs might be more reasonable if the rigging replacement cycle was extended to 2–3 times that of stainless steel.
12.5 Inspection Due to Nitronic rods being the industry standard, Carbo-Link recommend a similar maintenance and inspection schedule for their solid carbon rigging. Currently for Nitronic rigging a four-yearly service is recommended, where the rods may be (reheaded, i.e. threads checked) and non-destructive testing will occur. A full replacement is recommended every 8 years. In a similar manner Carbo-Link conduct an annual inspection with the rig in situ in the boat, and a 4 yearly service as per Nitronic. However there is no requirement to replace the rigging every 8 years. Provided the surface covering is intact then the rigging should have the same life span as the mast itself (Carbo-Link, 2015). Some PBO manufacturers claim to exceed the lifespan of conventional rigging in lab tests by a factor of three or more. Life expectancy depends on the continued integrity of the protective cover that is placed around the fibres during manufacture. This protects the load-bearing fibres from chafe, UV degradation and humidity. It is too early to see if these predictions on life expectancy are true. However, it would move the onus from the fibre and carbon rigging manufacturer to the yacht owner for maintenance. Regular inspections are required, critically for abrasion or other damage to the protective jacket and carbon termination covers that could expose the core fibres, or problems with termination points. Ideally at regattas, or on long passages, this should ideally be done daily. Soft (rope) attachment of terminals enables connections to be easily visually inspected. This gives more confidence in rope than visual inspection of stainless steel components where possible defects are hidden within a swage fitting that often does not reveal the microscopic cracks that can lead to potentially devastating crevice corrosion (Holmes, 2013).
12.6 Classification society guidelines The high material properties which composite materials offer can only be exploited if the materials are handled in the correct way in the production process. Det Norske Veritas – Germanischer Lloyd (DNV-GL) stipulate requirements for the handling of both carbon and other fibres (GL, 2012). Whilst product development may occur in
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some racing classes who are not concerned with having a certified product, from a commercial point of view, the rigging manufacturers need to have the large superyacht market to make their businesses successful, and these yachts do require certified products. The DNV-GL guidelines provide structural calculations for a rig designer to show compliance based on the materials chosen and the relevant loads, along with handling guidelines specific to the materials used. There are humidity requirements for manufacture of carbon fibre composite rigging, due to the problem during the curing process of vapour bubbles forming voids. There are light exposure time limits for PBO fibre (no exposure to direct sunlight and cumulative 48 h for visible light).
12.7 Rig and rigging design considerations As will be described in Chapter 13 of this book, the combination of a mast, its rigging and sails is a complex aero-elastic problem. The aerodynamic loads (lift & drag) generated are dependent on the geometric shape. This in turn, must be matched to the performance characteristics of the hull, to achieve a boat which performs well, something which is of great importance whether the yacht is a racing yacht or simply for pleasure. As such, it becomes an area where the hull designer, mast designer and sail designer must work together to ensure this complementary product. The process of fitting a rig to the yacht will begin with the hull designer. The hull designer will set the general parameters of the yacht, such as length, beam, displacement and the expected weight of ballast, which will determine the righting moment of the hull form. For the given size of yacht, the hull designer will have an expected sail area for the yacht for it to perform to given criteria. The hull designer will also specify the mast height and sail area required for the boat. In general, a tall mast is favourable, for two reasons: (i) for the same given area, the induced drag due to developing the drive force from the sails will be less, and (ii) the wind speed increases with height, due to the mast being within the earth’s boundary layer (Claughton et al., 1998). At this point, the hull designer will begin working with the mast and sail designer to specify the overall package. A more detailed description of mast design can be found in both Claughton et al. (1998) and Sheahan (1990), but in essence, the rig is expected to withstand a load typically derived from the righting moment of the yacht at 30° heel, as this is usually close to the maximum righting moment for most yachts. The presence of the stays mean that the mast tube is operating in compression, and so specifying the properties becomes one of resisting buckling. The righting moment of a yacht is due in part to the location of the centre of buoyancy (dependent on hull design, and in multihulls the distance between the hulls) and in part due to the location of the centre of gravity. The centre of gravity is significantly affected by the weight of the mast and rigging, due to it having such a significant lever arm. Righting moment is directly linked to yacht performance, in that it is a measure of the yacht’s ability to utilise the winds power to generate the thrust to drive it forward. More righting moment means a yacht can absorb more power, but to withstand that both the sails and the mast require extra structure.
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As shown in Figure 12.1, the modern yacht rig is a complex arrangement of mast tube, with spreaders and rigging, the combination of which transfers the loads from the sails into the hull, driving it forward. The mast designer looks to optimise this combination for low weight and low aerodynamic drag whilst also being both structurally stiff and strong enough to allow the sails to perform to their optimum. A key defining factor in the design of a rig is the number of spreaders. This, in combination with their length, sets the angle the highest shroud sees with the tip of the mast, thereby controlling the compression the mast tube will experience. The length of the spreaders is often governed by the available shroud base, which is the width between the attachment points, or chainplates, on the hull. The greater the distance between port and starboard chainplates together with a greater distance between forestay and backstay, increase the rigidity and lessens the load in the rigging. More spreaders mean smaller panels, or sections, of mast between supports. This leads to less material being required in the mast tube, which is beneficial from a weight perspective, but also aerodynamically, as the tube is smaller and creates less drag. The downside of this is the associated aerodynamic drag associated with an extra set of spreaders. Another method of reducing the size of the mast tube is to have the widest shroud base possible. This will mean the shroud angle at the top of the mast is greater and the compression in the mast is reduced. This does however conflict with some of the sail designer’s requirements. The performance of the boat, when sailing close to the wind, is directly related to how close to the centreline the sails can be pulled in, without stalling the sails. This can be limited by the width of the shroud base, so the sail designer would prefer a narrower shroud base. The sail and mast designer will discuss what the limitations are, and assess the effects on upwind performance caused by shroud base. This is not the same for all boats either, it is less of a concern for yachts that spend a large amount of time racing downwind, such as the Open 60 class round the world racing yachts, than in boats that race on upwind downwind courses. In the Open 60 class, deck spreaders have been used to extend the shroud base outside of the limits of the hull, so as to be able to reduce weight in the overall rig package. With multihull sailing vessels, far greater loads need to be supported when compared to mono-hulls. Normally a mono-hull has its greatest righting force at around 60°, due to a static mass. In a strong gust, a mono-hull will heel away up to 90° from the wind reducing the effective sail area, spilling wind and reducing the loads on rigging. In strong gusts catamarans and trimarans normally heel up to 10° and 20° respectively, due to the wider distance between the centre of buoyancy of leeward hull and the centre of gravity. On catamarans the stability force (GZ) reaches a maximum at around 7° of tilt, when the windward hull is completely lifted out of the water. In trimarans, the maximum GZ is when the leeward float is being submerged, or when the main hull is lifted out of the water typically at around 15° of heel. Thus for a given gust a multihull will present a greater effective sail area to the wind, and associated load. These greater loads are partially reduced by the wider shroud base, typically 50% wider or more than a similar mono-hull. However, the greater form stability in multihulls means a higher mast can be carried. In addition to these requirements, the shape of the sails can be varied by the shape of the mast and how it deforms under both sail and sailor-induced load. The mast
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d esigner and sail designer will be working together to get a bend distribution to the mast that gives the optimum sail shape across the height of the sail, and can vary to handle different wind conditions and strengths. The sail designers need to know the overall stiffness of the rig to be able to calculate what shape to make the sail. In the case of the headsail on the yacht, the shape of the leading edge of the sail can be controlled by the tension in the forestay. A tighter forestay means a flatter sail, but as the forestay is a catenary, there are limits to how straight it can be. These are related to the forestay size and material, but also the structural strength of the hull. The forestay is often tensioned and sag removed by tensioning the backstay, but if the hull is not sufficiently stiff, this will just impart bending on the hull and not reduce sag in the forestay. This is a good example of the sail, mast and hull designer having to work together, to match their individual components to get the optimum performance. In practice, sail designers will often assume typical forestay sag figures, as given in Table 12.1. With all this being an interlinked and iterative process, there may be a few iterations required between hull, mast and sail designer, to establish the exact righting moment to design to, due to the influence of rig weight on righting moment. The level of integration and analysis required to achieve this is dependent on the level of competition/ value associated with the project. Although there is an ever increasing use of composite rigging, because of its long history of use, the industry benchmark material is still Nitronic 50 rod rigging, and often designers will look to achieve an equivalent stiffness to a rod rigged rig, but with the benefit of increased righting moment due to the lighter composite rigging. Designing a new boat with this in mind is easier than replacing a rig in an existing rod rigged boat. Many of the components on the yacht may be structurally designed for the existing righting moment. In these instances, it may be possible to reduce the overall weight of the yacht whilst maintaining the existing righting moment, which will often lead to a performance gain. In terms of maximising performance, it is of great importance to reduce the aerodynamic drag, or windage of all the rigging elements. Table 12.2 shows the variation in drag of different possible rigging cross sections. The reduction in windage which was made in moving from wire to rod was a welcome improvement for sailors, and as can be seen, the potential to reduce windage by using an elliptical cross section is influencing some carbon rigging manufacturers to work in this direction. Making exact comparisons of windage can be difficult, as it is not due to the rigging wires alone, but also the end fittings and spreaders. When considering the rigging Table 12.1 Values
for typical forestay sag for types of yacht
Type of yacht
Typical forestay sag %
Grand Prix Racer Racer/Cruiser Cruiser/Racer Bluewater Cruiser
0.75 1.00 1.25 1.50
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Table 12.2 Comparison of aerodynamic drag of different rigging types, from data published in (Marchaj, 2000) Rigging type
Drag relative to A
Plain smooth wire 0.500″ Ø
1.0
Braided wire 0.515″ Ø
1.19
Elliptical strut (t/c 2.8) chord 1.0″
0.28
Two tangent wires 0.500″ and 0.375″ Ø
0.79
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alone Humphreys Yacht Design (2015) compared the weight and windage data for Ni50 Rod and EC6 rigging for a 54 foot racing yacht. For the complete rig, there was a 4% increase in frontal area and 2% increase in lateral area, whilst the overall reduction in total rig weight was 11%, with the actual rigging itself being 55% lighter in EC6 than Ni50 Rod. As composite rigging allows the possibility to match end fittings into spreaders rather more than rod, it may well be possible to gain back some of this area increase, to achieve the same, if not better values than Nitronic 50. Whilst performance gains may appear to be of more interest to the racing yacht fraternity, the design demands placed on recreational yachts and superyachts mean there are significant benefits for them with composite rigging. In order to access many ports and anchorages, cruising yachts and superyachts have restricted keel depths, thus limiting their righting moment. This coupled with extra weight in the mast for mechanisms to furl the sails, mean that gains in righting moment are often very welcome. Humphreys Yacht Design have seen up to a 65% reduction in standing rigging weight, translating to a 20% overall reduction in terms of total rig and rigging weight in using carbon rigging in a cruising yacht. Whilst the benefits to performance in increasing righting moment have already been highlighted, there are additional performance gains in having reduced weight in the rig. This will reduce both the roll and pitch inertias of the vessel, which results in reduced motions of the yacht when sailing through waves.
12.8 Future trends As with the move from Manila hemp ropes to steel wire ropes, the increased use of composite materials for yacht rigging is a continuation of the development of rigging being driven by the most suitable material available at the time. As with steel ropes,
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or rod rigging, the use of carbon fibres for rigging took a number of decades after both their invention and application by other industries. With this in mind, what are the materials currently available that we can expect to see being used in the rigging industry in the future? The obvious candidate for a replacement to carbon would be graphene, as this currently is the highest strength to weight ratio of any material currently tested (Lee et al., 2013). Much is made in the popular scientific media of graphene being a material that could be used in multiple applications. Cost-effective industrial scale production methods are currently being developed, and these would need to be more mature, before it is realistic to consider graphene in rigging applications. Never-theless, there is continued development in producing longer fibre lengths (NPG Asia Materials, 2014). As with previous discoveries of new materials, such as Kevlar or PBO (USA Pat., 1955), there is always a time delay between discovery and producing quantities in a form that can be used for engineering applications. Another factor that will affect the potential use of graphene is that it has a number of attractive material properties (it is conductive and inherently two-dimensional so possible to produce in films), demand from other industries may mean the price is prohibitive for use in rigging. The current technology of using carbon is still relatively young and except for some special applications, in general it is intermediate modulus carbon that is used in carbon rigging, so there is still scope available to develop within carbon fibre making it unlikely that we will see it replaced for the foreseeable future.
12.9 Summary The increased use of composite rigging in modern yacht rigs is an example of what sailors have done throughout history. They choose the highest performance material available at the time, to have a lightweight, strong and stiff method of supporting their masts. Carbon appears to have become the material of choice, and it is likely that this will continue and become more the case if higher modulus fibres are applied, and further weight and windage reductions are possible. Whilst steel is still the industry benchmark, primarily the reduced weight of carbon is seeing it become an ever popular choice for standing rigging, despite the additional cost. There is sufficient scope to see further development within carbon, before any other material will replace it.
Acknowledgements The authors would like to thank Luke McCallum of Carbo-Link AG; Hasso Hoffmeister of DNV-GL; Dr Helen Farr of University of Southampton; Tom Humphreys of Humphreys Yacht Design; Jeremy Elliott of North Sails; and Scott Vogel of Composite Rigging Ltd & Co.
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References Anderson, R., Anderson, R.C., 2012. A Short History of the Sailing Ship. Courier Corporation. Carbo-Link AG, Undermülistrasse 25, CH-8320 Fehraltorf, Switzerland, http://www.carbo-link. com. Carbo-Link AG, 2015. Personal communication with Luke McCallum. Carter, R., Crawford, H., 2010. Maritime Interactions in the Arabian Neolithic: The Evidence from H3, As-Sabiyah, an Ubaid-related site in Kuwait. American School of Prehistoric Research Monograph Series, vol. 8. Brill Academic Publishers, (October), http://www. brill.com/maritime-interactions-arabian-neolithic. Claughton, A.R., Wellicome, J.F., Shenoi, R.A., 1998. Sailing Yacht Design: Theory. Addison Wesley Longman, Chicago, USA. Composite Rigging Ltd & Co. 342 Compass Cir, North Kingstown, Rhode Island, USA. Dawson, D., 2015. Yachtbuilding composites: rigged for success. CompositesWorld 1 (3), 60–65. http://www.compositesworld.com/articles/yachtbuilding-composites-rigged-for-success. Edwards, I.E.S., Gadd, C.J., Hammond, N.G.L., 1970. The Cambridge Ancient History. Cambridge University Press, Cambridge. GL, 2012. Rules for Classification and Construction 1 Ship Technology, 4 Rigging Technology 2 Guidelines for Design and Construction of Large Modern Yacht Rigs. Germanischer Lloyd Aktiengesellschaft, Hamburg. Hall Spars & Rigging, 33 Broadcommon Road, Bristol, RI 02809 USA, http://www.hallspars. com. Holmes, R., 2013. Sailing techniques – rigs and rigging. Yachts and Yachting, 27 March. www. yachtsandyachting.co.uk/sailing-techniques/rigs-and-rigging. Humphreys Yacht Design, 2015. Personal communication with Tom Humphreys. Leather, J., 2001. The Gaff Rig Handbook: History, Design, Techniques, Developments. WoodenBoat Books, USA. Lee, G.-H., Cooper, R.C., Joo An, S., Lee, S., Zande, A.v.d., Petrone, N., Hammerberg, A.G., Lee, C., Crawford, B., Oliver, W., Kysar, J.W., Hone, J., 2013. High-strength chemical- vapor–deposited graphene and grain boundaries. Science 340 (6136), 1073–1076. http:// engineering.columbia.edu/even-defects-graphene-strongest-material-world. Marchaj, C.A., 2000. Aero Hydrodynamics of Sailing 3rd ed. Tiller Publishing, Easton, USA (November 2000). Marples, J., 2015. High fiber. Professional Boatbuilder, April. McCaskill, J.R., 2009. Conserving waterlogged rope: a review of traditional methods and experimental research with polyethylene glycol (MA Thesis). Texas A&M University, Texas. http://nautarch.tamu.edu/Theses/pdf-files/McCaskill-MA2009.pdf. McKenna, H.A., Hearle, J.W.S., O’Hear, N., 2004. Handbook of Fibre Rope Technology. Elsevier Technology & Engineering. Woodhead Publishing, ISBN: 9781855736061. Navtec Rigging Solutions, 351 New Whitfield St Guilford, CT 06437. www.navtec.net. NPG Asia Materials, 2014. 6, e113; http://dx.doi.org/10.1038/am.2014.48, published online 18 July. Sheahan, M., 1990. Sailing Rigs and Spars: Installation, Maintenance, Tuning, Haynes Publishing Group, G T Foulis & Co Ltd, UK. Southern Spars, 15 Jomac Place, Rosebank Road, Rosebank, Auckland 1026, New Zealand, www.southernspars.com. Spurr, D., 1982. Is Rod Rigging Here to Stay? Cruising World, May.
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Stansell, J., 1983. The America’s Cup Grand Prix Yachting. New Scientist, June. United States ex rel. Westrick v. Second Chance Body Armor, Inc., et al., No. 1:04-cv-00280, mem. op. (D.D.C., 30 December 2013). USA Pat. No. 2,904,537, 1955. “Wilhelm Albert”. Encyclopaedia Britannica. Retrieved 9 April 2014.
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R. Pemberton, J. Graham-Jones University of Plymouth, Plymouth, UK
12.1 Introduction Whilst sailing vessels have developed from the stone-age dug-out canoes to the m odern America's Cup hydrofoiling catamarans, the fundamental requirement of a yacht’s rigging has not changed. It must work in combination with the mast and hull to provide a platform for the sails to generate as much thrust as possible. Ideally, this should be achieved with minimal aerodynamic drag penalty. Whilst this covers the performance requirements of rigging, there are additional safety requirements. The rigging must also be capable of supporting the mast and sails in the strongest of foreseeable wind conditions. In this regard, rigging becomes a critical component on a yacht, for without it the vast majority of yachts masts would fall down, causing damage to the yacht, with the associated risk of broken rigging fouling the propeller, rendering the yacht and crew vulnerable to the elements. Historically, as rig design has developed, there has always been a drive to exploit the best materials on offer. Rigging has the somewhat contradictory requirements of high strength with as small a cross section as possible, to minimise the associated aerodynamic resistance. This chapter will briefly outline the development of rigs, and the range of materials that have been used for rigging, to give a background to where composite materials sit in their application to rigging. The requirements of the mast designer are also discussed, along with the implications for the hull and sail designer and what might be expected for future rigging materials.
12.2 Rig development Whilst the fundamental requirements of a rig may not have changed, the means of harnessing the winds’ power has significantly advanced. The exact start of seafaring and sailing is difficult to place and a matter of debate amongst archaeologists. There is evidence that the as early as c.5000 BC, boats were being built with masts, with a possible mast foot found in a bitumen-preserved section of a reed boat (Carter and Crawford, 2010). It is certain that by the Bronze Age (c.3100 BC), the depiction of sails on the Naqada vases leads archaeologists to believe that sailing and rigs were quite developed (Edwards et al., 1970). Marine Applications of Advanced Fibre-Reinforced Composites. http://dx.doi.org/10.1016/B978-1-78242-250-1.00012-0 Copyright © 2016 Elsevier Ltd. All rights reserved.
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These early rigs were square sails, predominantly in the case of the Nile, to sail with the wind and allow vessels to return against the current. In terms of rigging, simple stays were used to support the mast in the fore and aft direction, with some lateral support offered by the stays fixed to the outboard parts of the hull. With time came the desire to sail at angles closer to the wind’s direction, and so square sails were rotated to align closer to the crafts centreline as opposed to perpendicular to it. The extra lateral support required for this came about through the increase in beam of the hulls. Early rig designs developed, from square sails with horizontal spars called yards typically seen in Viking ships which had one square sail supported by a head yard (wooden pole) and were attached to control ropes at the sail’s foot (loose foot). In HMS Victory, several square sails were in positions above each other on the masts. The foot (base) of one sail became the head (top) yard of the next going down. Rig designs developed, from square sails to lug sails (a quadrilateral sail bent upon a yard that crosses the mast obliquely.) where the yards at the top were hauled to the top of the mast and yards angled skyward A lug sail is a quadrilateral shape (Figure 12.1) and very easy to raise and control. Lug sails are still popular today for their simple constructions and traditional look. Looe, in Cornwall, runs a bi-annual Lugger Race linking back to the traditional fishing boats that used Lug sails. A variation of this is the Chinese lugsail more commonly called Junk rig, or sampan rig. The main difference is the use of horizontal battens to control the sail. As ships became larger, greater sail areas are required to overcome hull and Peak Yard
He
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Figure 12.1 Lug sail rig.
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wave making drag. Lug rigs were limited as the yards could not exceed the maximum length of a tree. Gaff rig sails are still limited to a maximum tree length. However, attaching the yard (now called gaff) to the mast via a loop of rope at the throat and raising the gaff to similar angles resulted in a greater sail area. Additionally, the boom was also attached via the tack to the mast further increasing sail area. Gaff rig sails (Leather, 2001) similar to Lug sails are still used by modern boat manufacturers, e.g. the Cornish Crabber (www.cornishcrabbers.co.uk). With improving materials and adhesives longer masts became possible which lead to the Bermudan rig becoming the most popular for modern boats. Considering the sail developments from Square, Lug, to Gaff (a spar rising aft from a mast to support the head of a quadrilateral fore-and-aft sail) and Bermudan (triangular sail) rigs, the method of rigging the mast has not evolved at a similar pace. The rigging was still taken to the outboard side of the hull, and so the overall height of the mast was governed by the beam of the boat and the size and type of rigging used. The end of the nineteenth century saw a significant change in rig design, in that the lateral support offered by rigging to a mast was increased by using spreaders, bars which increase the angle between the rigging and the mast, before the rigging runs down to the deck. Photographs of the Royal Yacht, ‘Britannia’, sailing in the 1890s show a set of spreaders on the mast, and the development of taller rigs coincided with the development of radio and radio masts, hence these rigs were often referred to as Marconi rigs. Figure 12.2 shows the typical staying arrangement for a multi-spreader modern racing yacht rig. The longitudinal support of the rig is provided by the backstays (running
Jumpers
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ast b
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Figure 12.2 A typical modern racing yacht rig (stern at left and bow at right).
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aft) and the forestay (running forwards, also known as the headstay). The tensions in all of the backstays are adjustable whilst sailing, and often in the forestay as well. The lateral support for the rig is provided by the shrouds and the spreaders. Shrouds are normally rope, wire, or rods in tension attached symmetrically on both port and starboard sides of the vessel. Typically, they attach to the mast in several locations: at the mast head (Cap Shrouds) or mid-way (Lower Shrouds), as well as either side of the mast. A typical yacht could have six different shrouds to provide lateral support the mast. Shrouds ideally attach high up the mast, thus structures projecting from the mast, i.e. Spreaders, increase the angle of the shroud at the deck fixings (chainplates) providing more mast support. The shrouds are often made up of discontinuous elements, named from the bottom up numerically and by whether they are vertical (V) or diagonal (D). Hence the lowest vertical and diagonal shroud would be called V1 and D1 respectively. The specification and sizing of these rigging elements will be discussed later, the following section covers the choices of materials which were, and are currently, available for use as rigging.
12.3 Rigging materials In addition to the sail plans developing, so too were the materials being used for rigging. One of the difficulties in establishing the history of rigging is that rigging and sails are rarely preserved archaeologically. The information on what was used by our ancestors must be gleaned from the remaining wooden elements, or from contemporary historical accounts, such as the Norse sagas, or artistic depictions. Before the advent of international trade routes, the materials used for rigging were whatever was locally available, and as such the range is wide. The techniques for producing the rigging ropes may have been similar throughout time and globally, but the materials may have been leather, seaweed, papyrus, willow or even animal hairs. The pre-dominant material for much of the history of rigging has been hemp. From its first uses for rope production in Asia in 4000 BCE, hemp rope spread throughout the world as the seafarers sailed and traded with countries further afield (McCaskill, 2009). It was only in the 19th century when abacá (a member of the banana family) became the preferred material for rope production, that its use decreased. Many would not have known of this change, as abacá became known as Manila Hemp, due to it being a native plant of the Philippines. Further details of fibre rope technology are given by McKenna et al. (2004). As the industrial revolution brought about a decrease in the cost of producing iron and steel, so these materials began to replace the traditional organic materials. Developments in the mining industry in Germany in the 1830s saw wrought iron rope replacing hemp for use in underground cable carriages (Wilhelm Albert, 2014). These inventions were adopted by seafarers, with a report from the International Exhibition of 1862 stating that the use of cast iron rigging was common place for the previous 10 years, and 1863 saw the launch of the British ship Seaforth, which is documented as being the first to have both steel spars and rigging (Anderson and Anderson, 2012).
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Steel rope development has not stood still and is currently a mature product, due to its ease of manufacture and fitting, in addition to its relatively low cost, and hence remains the most common rigging choice for smaller yachts and dinghies. SAE316 grade austenitic nickel–chromium stainless steel (Cr16Ni10Mn2Mo2Si1), known as the marine grade stainless steel, is used in rigging and has increased resistance to chloride corrosion due to the 2–3% molybdenum compared to type SAE304 stainless steel (Cr18Ni8Mn2Si1). The wire sizes range from 3 to 16 mm. The most basic circular cross-section strand wire construction (wire rope classification is 1 × 7) has six strands hexagonal close packed around a single core strand. The next size up is overwrapped by a further 12 strands to produce 1 × 19 strand wire. These individual wires may be further grouped to produce other common arrangements used in rigging, notably 7 × 7 (49 wires), 7 × 19 (133 wires) and 6 × 36 (216 wires). Increasing the number of wires produces greater flexibility for the same size. Thus for running rigging (used for raising, lowering and controlling sails), a 7 × 19 would be used, as this rigging needs flexibility. Standing rigging supporting the mast needs high strength and stiffness thus 1x19 is used. On larger yachts, however, the desire for a rigging option with less stretch than wire, lead to further developments with steel. Rod Rigging first appeared on yachts in the late 1960s although its original development was done approximately 50 years previously, but not with yachts in mind. The requirements of war can often drive innovation, and during the First World War, the early biplanes had rigging wire between their sets of wings to produce a light, stiff, structure. Increasing the stiffness of this structure would have meant an increase in the diameter of the rigging wire used, which would have come at a cost of extra weight and aerodynamic drag. The Scottish firm of Macomber & Whyte made rigging from steel rods as an alternative to wire, as the elongation of the rods was typically 35% less under the same load as the equivalent wire (Spurr, 1982). The slow progression from aircraft rigging into yacht rigging was due to a number of early failures, predominantly fatigue failures of the end fittings of the rods, where they met either the mast or the chainplates. Chainplates are metal plates with many holes used to fasten a stay or shroud via a turnbuckle to a sail boat hull. A turnbuckle is used to allow articulation of the wire to the fixed chainplate. It wasn’t until the 1960s when this problem was solved by the development of cold forming heads onto the rods. The heads were swellings with a smooth radius around their edge, which could fit into specially designed fittings on the mast or hull. By 1970 all of the US America’s Cup 12MR yachts had chosen rod rigging in preference to wire. Rod rigging then found acceptance in the wider market outside of high end racing yachts, and until the last 10–15 years, was the default choice for the rigging of large yachts. These improvement in low drag and lower weight, are ignored by most coastal and offshore smaller yacht users who still use wire rigging as it cheaper and has better damage tolerance as when individual wires fail the fracture does not run completely through the whole wire rope. Damage can become visible to inspection. This damage tolerance is a key advantage in composite rigging. The marine environment is a notoriously corrosive one, and so the rods are made from a Nitronic 50 (Ni50: Fe56Cr22Ni12.5Mn5Mo2.25Si1C0.06) high quality stainless steel.
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The obvious downside of steel is its relatively high specific gravity of 7.5 in comparison with 1.5 of many composite constructions. Rigging is normally changed due to concerns over undetectable corrosion fatigue crack bringing the mast down. Stainless rod or wire rigging could pass a visual inspection but fail without warning. Non-destructive testing of metal rigging is expensive and unreliable. On smaller yachts, as a general guide standing rigging should be replaced every 12 years, 15 years maximum in salt water, or 15–20 years in fresh water. The replacement cycle depends on its use: if you are racing or sailing offshore, then earlier replacements are required due to the great loads normally on a 3 or 5 year cycle, or as recommended by classification society. Where boats do so little actual sailing, or remove the mast over winter, or are over-specified for the received loads, or are in a boat yard undergoing restoration over many years, then longer replacement cycle times are possible. Wire rigging normally fails where the wire enters the swaged fittings. Although less common failure at chainplates, tangs turnbuckles and toggles are possible. The potential weight savings, and more importantly, reductions in centre of gravity, lead rigging manufacturers to experiment with composite materials.
12.4 The composite age As sailcloth, hull laminates and running rigging took advantage of the favourable material properties composites offer for a given weight, naturally the progression to experimenting with these materials in standing rigging would follow. This main materials used are aramid, polybenzobisoxazole (PBO) and carbon, which will be described below.
12.4.1 Aramid (trade names: DuPont Kevlar® or Teijin Twaron®) The use of aramid fibres in sailing began in the late 1970s with America’s Cup syndicates making their sails from cloth containing it (Stansell, 1983). Aramid fibres have high tensile strength to weight ratio whilst also exhibiting low stretch which meant it was natural they should be used in rigging. Aramid fibres will degrade in sunlight and high UV environment. Thus the main load carrying aramid fibres are kept parallel encased within a polyethylene braided cover for protection. Further, aramid fibres are strongly hygroscopic relative to PBO or carbon, which provides an easy route for moisture ingress and associated inhomogeneity in properties. Whilst the material properties of aramids are not as good as other materials (e.g. PBO below), the lower cost of aramids, combined with its resilience to both fatigue and abrasion resistance means they still have a place in the market (Marples, 2015). The use of aramids tends to be more for checkstays and running backstays, which can often see damage due to contact with the sails, or sail battens. In this application, aramids resistance to impact makes it a suitable material choice. In compression, aramids are not ideal and kink due to the linearly arranged fibres.
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12.4.2 Polybenzobisoxazole/PBO (trade name: Toyobo Zylon®) Although produced in resin form in the late 1950s (USA Pat., 1955), PBO fibres were not seen as a suitable material for use in yacht rigging until the mid-1990s. Since aramid fibres had already been used as a rigging element, it was a natural progression to use PBO, because of its improved properties. PBO has a tensile modulus of 270 GPa and a tensile strength of 5.8 GPa (compared to 113 GPa and 2.8 GPa respectively for Kevlar). Typically, continuous PBO fibre tows are wound around metal thimble end fittings, before the fibres are covered with a braid and protective sheath under tension. The protective sheath is required due to the degradation of material properties with exposure to UV light (Marples, 2015). The use of PBO as a rigging material began in the early 2000s, and although it can have a greater weight saving over some carbon fibre rigging systems, the vulnerability to UV light does mean it is important to be conservative when assuming a lifespan within rigging. This has not stopped many boats sailing many tens of thousands of miles with PBO rigging, but it is expected, that with time it will lose market share to carbon fibre rigging. There is currently an outstanding lawsuit with the manufactures of PBO regarding the degradation of properties, in an instance where PBO was used within bulletproof vests which failed with fatal consequences (United States ex rel. Westrick v. Second Chance Body Armor, Inc. Second Chance Body Armor Inc., et al., 2013).
12.4.3 Carbon fibre From its first uses in racing yacht rudders, the high Young’s modulus and tensile strength of carbon fibre along with its corrosion resistance made it an attractive material to use in many high-performance, marine applications. The use of carbon fibre within rigging was a natural progression, particularly as many modern mast tubes were being made from it. There are a variety of methods of manufacturing carbon fibre rigging, which are often specific to particular manufacturers. The main types of construction are pultruded rods, rigid carbon rigging or carbon fibre/thermoplastic rigging (Dawson, 2015). These methods will be outlined in this section:
12.4.3.1 Pultruded rods One of the early methods of using carbon was developed by the company that now is Composite Rigging and the product is called EC6. Small diameter (approx.: 1 mm) pultruded carbon rods are grouped together. The ends of the rods are fixed in a resin, and moulded into a conical shape, which then fit into a titanium socket. The rods are then covered with a braid to protect them and keep them grouped together. The separate carbon elements provide a natural protection against crack propagation, and so have good fatigue and impact resistance properties. For a given weight of carbon, there will be a slight increase in diameter of the overall rigging rod compared to rigid carbon, as there is a packing factor for the pultruded rods fitting into a circle, plus there is the braided cover.
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12.4.3.2 Rigid carbon Carbo-Link (Carbo-Link, n.d.), Southern Spars (Southern Spars, n.d) and Hall Spars and Rigging (Hall Spars and Rigging, n.d.) are companies that produce solid carbon rigging. Whilst there are differences in the details of the production methods, the overall processes are similar. Epoxy prepreg tows of fibres are wound around end fittings, the fibres are consolidated under tension using shrink-wrap material and then the stay is cured. Each company has their own style of end fittings and their curing processes differ Hall Spars prepreg carbon is cured in a traditional oven, whereas Carbo-Link take advantage of the conductive nature of carbon fibre, and generate the heat to cure the resin by applying an electric current to the stay, in what is known as the Ohmic curing process. As the final product is a rigid object, both Hall Spars and Carbo-Link have been able to offer section shapes other than round with the aerodynamic gains which will be discussed later.
12.4.3.3 Carbon fibre/thermoplastic rigging Navtec Rigging Solutions (n.d.) were at the forefront of developing stainless steel rod rigging and have been active in developing their own carbon rigging. Rather than using a thermoset resin, they use a thermoplastic matrix. In a similar manner to Composite Rigging, they have bundles of small diameter carbon pultrusions, but the carbon is pre-impregnated with a specially developed thermoplastic. The bundle of rods is then loaded and goes through a proprietary process to fuse them into a solid bar. All of the production methods have the benefits that come with using carbon as a rigging material instead of steel. The most significant of these is that for the same stiffness there is a significant reduction in weight, but also there are the added benefits of greater fatigue and corrosion resistance (although it is important to guard against galvanic corrosion of adjacent light alloys). Carbon rigging also does not experience creep in the same way that steel rigging does. In terms of size, both Carbo-Link (n.d.) and Composite Rigging Ltd and Co (n.d.) have made rigging at both extremes of the yachting rigging world. The largest diameter rigging the companies have produced are 60 mm for Carbo-Link and 90 mm for Composite Rigging (Composite Rigging Ltd and Co, n.d.)) (for the main mast on an 85 m ketch). Even at 90 mm, it was still possible to coil the EC six cable (Southern Spars, n.d.) into a 3 m diameter for shipping, whereas at the large diameters such as 60 mm, Carbo-Link would prepare their cable at their factory, and then conduct the final cure on site. A benefit of their production method is that, provided they have electricity and two attachment points to tension the cable, they can cure their product anywhere. At the other end of the spectrum both companies produce rigging less than 2 mm in diameter for use in dinghy rigging market. The construction methods appear to be relatively scalable, although some of the practical issues of going to larger sizes involve the size of stock material for machining end fittings, particularly as titanium is the preferred material for end fittings. Another benefit of using composite rigging is that it allows large yachts the possibility of having continuous rigging. The rigging layout (shown in Figure 12.2) is made up of discontinuous elements, and whilst on smaller yachts (often single spreader rig
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boats) it has been possible with Nitronic rods to have a number of continuous rods for the rigging, this is not practical with larger yachts. With carbon rigging, it has been possible to incorporate the diagonals into the main vertical component and thereby reduce the sizeable end fittings which come with discontinuous rigging. There is often an added expense of doing this, as jigs particular to the boat in question need to be built. This is on top of the already greater expense of using carbon rigging over steel. Producing the raw carbon product is expensive, and whilst some elements can be automated, manufacture of carbon items tends to be labour-intensive, so a certain level of cost is unavoidable. In future, the extra manufacturing costs might be more reasonable if the rigging replacement cycle was extended to 2–3 times that of stainless steel.
12.5 Inspection Due to Nitronic rods being the industry standard, Carbo-Link recommend a similar maintenance and inspection schedule for their solid carbon rigging. Currently for Nitronic rigging a four-yearly service is recommended, where the rods may be (reheaded, i.e. threads checked) and non-destructive testing will occur. A full replacement is recommended every 8 years. In a similar manner Carbo-Link conduct an annual inspection with the rig in situ in the boat, and a 4 yearly service as per Nitronic. However there is no requirement to replace the rigging every 8 years. Provided the surface covering is intact then the rigging should have the same life span as the mast itself (Carbo-Link, 2015). Some PBO manufacturers claim to exceed the lifespan of conventional rigging in lab tests by a factor of three or more. Life expectancy depends on the continued integrity of the protective cover that is placed around the fibres during manufacture. This protects the load-bearing fibres from chafe, UV degradation and humidity. It is too early to see if these predictions on life expectancy are true. However, it would move the onus from the fibre and carbon rigging manufacturer to the yacht owner for maintenance. Regular inspections are required, critically for abrasion or other damage to the protective jacket and carbon termination covers that could expose the core fibres, or problems with termination points. Ideally at regattas, or on long passages, this should ideally be done daily. Soft (rope) attachment of terminals enables connections to be easily visually inspected. This gives more confidence in rope than visual inspection of stainless steel components where possible defects are hidden within a swage fitting that often does not reveal the microscopic cracks that can lead to potentially devastating crevice corrosion (Holmes, 2013).
12.6 Classification society guidelines The high material properties which composite materials offer can only be exploited if the materials are handled in the correct way in the production process. Det Norske Veritas – Germanischer Lloyd (DNV-GL) stipulate requirements for the handling of both carbon and other fibres (GL, 2012). Whilst product development may occur in
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some racing classes who are not concerned with having a certified product, from a commercial point of view, the rigging manufacturers need to have the large superyacht market to make their businesses successful, and these yachts do require certified products. The DNV-GL guidelines provide structural calculations for a rig designer to show compliance based on the materials chosen and the relevant loads, along with handling guidelines specific to the materials used. There are humidity requirements for manufacture of carbon fibre composite rigging, due to the problem during the curing process of vapour bubbles forming voids. There are light exposure time limits for PBO fibre (no exposure to direct sunlight and cumulative 48 h for visible light).
12.7 Rig and rigging design considerations As will be described in Chapter 13 of this book, the combination of a mast, its rigging and sails is a complex aero-elastic problem. The aerodynamic loads (lift & drag) generated are dependent on the geometric shape. This in turn, must be matched to the performance characteristics of the hull, to achieve a boat which performs well, something which is of great importance whether the yacht is a racing yacht or simply for pleasure. As such, it becomes an area where the hull designer, mast designer and sail designer must work together to ensure this complementary product. The process of fitting a rig to the yacht will begin with the hull designer. The hull designer will set the general parameters of the yacht, such as length, beam, displacement and the expected weight of ballast, which will determine the righting moment of the hull form. For the given size of yacht, the hull designer will have an expected sail area for the yacht for it to perform to given criteria. The hull designer will also specify the mast height and sail area required for the boat. In general, a tall mast is favourable, for two reasons: (i) for the same given area, the induced drag due to developing the drive force from the sails will be less, and (ii) the wind speed increases with height, due to the mast being within the earth’s boundary layer (Claughton et al., 1998). At this point, the hull designer will begin working with the mast and sail designer to specify the overall package. A more detailed description of mast design can be found in both Claughton et al. (1998) and Sheahan (1990), but in essence, the rig is expected to withstand a load typically derived from the righting moment of the yacht at 30° heel, as this is usually close to the maximum righting moment for most yachts. The presence of the stays mean that the mast tube is operating in compression, and so specifying the properties becomes one of resisting buckling. The righting moment of a yacht is due in part to the location of the centre of buoyancy (dependent on hull design, and in multihulls the distance between the hulls) and in part due to the location of the centre of gravity. The centre of gravity is significantly affected by the weight of the mast and rigging, due to it having such a significant lever arm. Righting moment is directly linked to yacht performance, in that it is a measure of the yacht’s ability to utilise the winds power to generate the thrust to drive it forward. More righting moment means a yacht can absorb more power, but to withstand that both the sails and the mast require extra structure.
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As shown in Figure 12.1, the modern yacht rig is a complex arrangement of mast tube, with spreaders and rigging, the combination of which transfers the loads from the sails into the hull, driving it forward. The mast designer looks to optimise this combination for low weight and low aerodynamic drag whilst also being both structurally stiff and strong enough to allow the sails to perform to their optimum. A key defining factor in the design of a rig is the number of spreaders. This, in combination with their length, sets the angle the highest shroud sees with the tip of the mast, thereby controlling the compression the mast tube will experience. The length of the spreaders is often governed by the available shroud base, which is the width between the attachment points, or chainplates, on the hull. The greater the distance between port and starboard chainplates together with a greater distance between forestay and backstay, increase the rigidity and lessens the load in the rigging. More spreaders mean smaller panels, or sections, of mast between supports. This leads to less material being required in the mast tube, which is beneficial from a weight perspective, but also aerodynamically, as the tube is smaller and creates less drag. The downside of this is the associated aerodynamic drag associated with an extra set of spreaders. Another method of reducing the size of the mast tube is to have the widest shroud base possible. This will mean the shroud angle at the top of the mast is greater and the compression in the mast is reduced. This does however conflict with some of the sail designer’s requirements. The performance of the boat, when sailing close to the wind, is directly related to how close to the centreline the sails can be pulled in, without stalling the sails. This can be limited by the width of the shroud base, so the sail designer would prefer a narrower shroud base. The sail and mast designer will discuss what the limitations are, and assess the effects on upwind performance caused by shroud base. This is not the same for all boats either, it is less of a concern for yachts that spend a large amount of time racing downwind, such as the Open 60 class round the world racing yachts, than in boats that race on upwind downwind courses. In the Open 60 class, deck spreaders have been used to extend the shroud base outside of the limits of the hull, so as to be able to reduce weight in the overall rig package. With multihull sailing vessels, far greater loads need to be supported when compared to mono-hulls. Normally a mono-hull has its greatest righting force at around 60°, due to a static mass. In a strong gust, a mono-hull will heel away up to 90° from the wind reducing the effective sail area, spilling wind and reducing the loads on rigging. In strong gusts catamarans and trimarans normally heel up to 10° and 20° respectively, due to the wider distance between the centre of buoyancy of leeward hull and the centre of gravity. On catamarans the stability force (GZ) reaches a maximum at around 7° of tilt, when the windward hull is completely lifted out of the water. In trimarans, the maximum GZ is when the leeward float is being submerged, or when the main hull is lifted out of the water typically at around 15° of heel. Thus for a given gust a multihull will present a greater effective sail area to the wind, and associated load. These greater loads are partially reduced by the wider shroud base, typically 50% wider or more than a similar mono-hull. However, the greater form stability in multihulls means a higher mast can be carried. In addition to these requirements, the shape of the sails can be varied by the shape of the mast and how it deforms under both sail and sailor-induced load. The mast
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d esigner and sail designer will be working together to get a bend distribution to the mast that gives the optimum sail shape across the height of the sail, and can vary to handle different wind conditions and strengths. The sail designers need to know the overall stiffness of the rig to be able to calculate what shape to make the sail. In the case of the headsail on the yacht, the shape of the leading edge of the sail can be controlled by the tension in the forestay. A tighter forestay means a flatter sail, but as the forestay is a catenary, there are limits to how straight it can be. These are related to the forestay size and material, but also the structural strength of the hull. The forestay is often tensioned and sag removed by tensioning the backstay, but if the hull is not sufficiently stiff, this will just impart bending on the hull and not reduce sag in the forestay. This is a good example of the sail, mast and hull designer having to work together, to match their individual components to get the optimum performance. In practice, sail designers will often assume typical forestay sag figures, as given in Table 12.1. With all this being an interlinked and iterative process, there may be a few iterations required between hull, mast and sail designer, to establish the exact righting moment to design to, due to the influence of rig weight on righting moment. The level of integration and analysis required to achieve this is dependent on the level of competition/ value associated with the project. Although there is an ever increasing use of composite rigging, because of its long history of use, the industry benchmark material is still Nitronic 50 rod rigging, and often designers will look to achieve an equivalent stiffness to a rod rigged rig, but with the benefit of increased righting moment due to the lighter composite rigging. Designing a new boat with this in mind is easier than replacing a rig in an existing rod rigged boat. Many of the components on the yacht may be structurally designed for the existing righting moment. In these instances, it may be possible to reduce the overall weight of the yacht whilst maintaining the existing righting moment, which will often lead to a performance gain. In terms of maximising performance, it is of great importance to reduce the aerodynamic drag, or windage of all the rigging elements. Table 12.2 shows the variation in drag of different possible rigging cross sections. The reduction in windage which was made in moving from wire to rod was a welcome improvement for sailors, and as can be seen, the potential to reduce windage by using an elliptical cross section is influencing some carbon rigging manufacturers to work in this direction. Making exact comparisons of windage can be difficult, as it is not due to the rigging wires alone, but also the end fittings and spreaders. When considering the rigging Table 12.1 Values
for typical forestay sag for types of yacht
Type of yacht
Typical forestay sag %
Grand Prix Racer Racer/Cruiser Cruiser/Racer Bluewater Cruiser
0.75 1.00 1.25 1.50
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Table 12.2 Comparison of aerodynamic drag of different rigging types, from data published in (Marchaj, 2000) Rigging type
Drag relative to A
Plain smooth wire 0.500″ Ø
1.0
Braided wire 0.515″ Ø
1.19
Elliptical strut (t/c 2.8) chord 1.0″
0.28
Two tangent wires 0.500″ and 0.375″ Ø
0.79
Two tangent wires taped 0.500″ and 0.375″ Ø
0.95
A B C
D E
alone Humphreys Yacht Design (2015) compared the weight and windage data for Ni50 Rod and EC6 rigging for a 54 foot racing yacht. For the complete rig, there was a 4% increase in frontal area and 2% increase in lateral area, whilst the overall reduction in total rig weight was 11%, with the actual rigging itself being 55% lighter in EC6 than Ni50 Rod. As composite rigging allows the possibility to match end fittings into spreaders rather more than rod, it may well be possible to gain back some of this area increase, to achieve the same, if not better values than Nitronic 50. Whilst performance gains may appear to be of more interest to the racing yacht fraternity, the design demands placed on recreational yachts and superyachts mean there are significant benefits for them with composite rigging. In order to access many ports and anchorages, cruising yachts and superyachts have restricted keel depths, thus limiting their righting moment. This coupled with extra weight in the mast for mechanisms to furl the sails, mean that gains in righting moment are often very welcome. Humphreys Yacht Design have seen up to a 65% reduction in standing rigging weight, translating to a 20% overall reduction in terms of total rig and rigging weight in using carbon rigging in a cruising yacht. Whilst the benefits to performance in increasing righting moment have already been highlighted, there are additional performance gains in having reduced weight in the rig. This will reduce both the roll and pitch inertias of the vessel, which results in reduced motions of the yacht when sailing through waves.
12.8 Future trends As with the move from Manila hemp ropes to steel wire ropes, the increased use of composite materials for yacht rigging is a continuation of the development of rigging being driven by the most suitable material available at the time. As with steel ropes,
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or rod rigging, the use of carbon fibres for rigging took a number of decades after both their invention and application by other industries. With this in mind, what are the materials currently available that we can expect to see being used in the rigging industry in the future? The obvious candidate for a replacement to carbon would be graphene, as this currently is the highest strength to weight ratio of any material currently tested (Lee et al., 2013). Much is made in the popular scientific media of graphene being a material that could be used in multiple applications. Cost-effective industrial scale production methods are currently being developed, and these would need to be more mature, before it is realistic to consider graphene in rigging applications. Never-theless, there is continued development in producing longer fibre lengths (NPG Asia Materials, 2014). As with previous discoveries of new materials, such as Kevlar or PBO (USA Pat., 1955), there is always a time delay between discovery and producing quantities in a form that can be used for engineering applications. Another factor that will affect the potential use of graphene is that it has a number of attractive material properties (it is conductive and inherently two-dimensional so possible to produce in films), demand from other industries may mean the price is prohibitive for use in rigging. The current technology of using carbon is still relatively young and except for some special applications, in general it is intermediate modulus carbon that is used in carbon rigging, so there is still scope available to develop within carbon fibre making it unlikely that we will see it replaced for the foreseeable future.
12.9 Summary The increased use of composite rigging in modern yacht rigs is an example of what sailors have done throughout history. They choose the highest performance material available at the time, to have a lightweight, strong and stiff method of supporting their masts. Carbon appears to have become the material of choice, and it is likely that this will continue and become more the case if higher modulus fibres are applied, and further weight and windage reductions are possible. Whilst steel is still the industry benchmark, primarily the reduced weight of carbon is seeing it become an ever popular choice for standing rigging, despite the additional cost. There is sufficient scope to see further development within carbon, before any other material will replace it.
Acknowledgements The authors would like to thank Luke McCallum of Carbo-Link AG; Hasso Hoffmeister of DNV-GL; Dr Helen Farr of University of Southampton; Tom Humphreys of Humphreys Yacht Design; Jeremy Elliott of North Sails; and Scott Vogel of Composite Rigging Ltd & Co.
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