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Loading and stability of a late 16th century Portuguese Indiaman
Journal of Archaeological Science 39 (2012) 2835e2844
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Loading and stability of a late 16th century Portuguese Indiaman T.A. Santos a, N. Fonseca a, F. Castro b, *, T. Vacas a a b
Centre of Marine Technology and Engineering, Technical University of Lisbon, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Nautical Archaeology Program, Department of Anthropology, Texas A&M University, 105 Anthropology Building, College Station, TX 77843-4352, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2010 Received in revised form 13 April 2012 Accepted 17 April 2012
The stability characteristics of 16th century ships are not known with certainty, but safety issues related to floatability, stability and overloading were a cause of concern at the time. The aim of the paper is to advance knowledge in this field by developing a set of loading conditions for a typical Portuguese ship of this epoch, for both the voyage from Lisbon to India and the return voyage. This allows testing the reconstruction of the presumable Nossa Senhora dos Mártires as well as to use this reconstruction to bring a better understanding of safety and loading issues on the Portuguese East India route. Given the uncertainties about the loading conditions, several hypotheses are tested, varying the amount of ballast, the degree of overloading and the distribution of weights on board, and allowing the development of a range of plausible loading arrangements. The stability of the ship is then assessed using modern tools to develop the limit KG curve for compliance with a modern stability criterion applicable to large sailing vessels. The case study ship is a plausible reconstruction based on the analysis of nautical archaeological remains, contemporary documents and the use of modern naval architecture methods. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Shipbuilding history Nautical archaeology Portuguese Indiamen 16th century seafaring
1. Introduction The ships of the Portuguese discoveries and maritime expansion are still not well known, which is related to the lack of shipwrecks excavated by nautical archaeologists. Furthermore, during the 15th and 16th centuries there were no standard procedures for technical drawing and design documentation. A new line of research is emerging which will bring new insight into the technical characteristics of the historic ships. It consists of combining the scientific fields of naval history and nautical archaeology, with the naval architecture, including advanced computational and experimental methods (Brandt and Hochkirch, 1995; Castro and Fonseca, 2006). This methodology is being applied to reconstruct and investigate an early 17th century Portuguese Indiaman, one of the largest ships of its time, able to carry large amounts of cargo along the longest sea route of that time. The reconstruction being carried out is partially based on the analysis of remains excavated during the 1990s at the mouth of the river Tagus, near Lisbon (Afonso, 1998; Castro, 2000, 2005a,b). Based on the excavation data and on 16th and early 17th century Portuguese shipbuilding treatises, Castro (2003 and 2007)
* Corresponding author. Tel.: þ1 979 8456220; fax: þ1 979 8454070. E-mail address: [email protected] (F. Castro). 0305-4403/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2012.04.031
proposed a reconstruction of the hull. Fonseca et al. (2005) and Santos et al. (2006, 2007) describe in detail the procedure to obtain the hull bodylines from the analysis of the archaeological remains and the methods in the contemporary shipbuilding treatises and present a preliminary analysis of the ship’s floatability and stability. This paper introduces new methodologies to analyze the floatability and stability of this ship. The main goals are to test the reconstruction of the presumable Nossa Senhora dos Mártires as well as to use this reconstruction to bring a better understanding of safety and loading issues on the Portuguese East India route. Historic documents were analyzed which allow a characterization of the ship’s loading process step by step (Castro et al., 2010). The loading process is simulated numerically and the floatability and stability analyzed. This analysis led to new conclusions, including, for example, the amount of ballast that the ship would need to carry in the outbound voyage and in the return voyage. This is relevant since until now the amount of ballast in this type of ships was a subject of debate. Additional historic documents regarding the operation of the ships and the sail plan in different weather conditions were also analyzed. This analysis led to the definition of realistic sailing conditions applied in the stability analysis. Furthermore, this paper presents a comprehensive study of the ship’s loading conditions, an assessment of the compliance with the US Coast Guard criterion
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(Code of Federal Regulations, 1983) and the derivation of limit KG curves for various combinations of sails.
information because some of its timbers had construction marks inscribed.
2. A Portuguese Indiaman: the Nossa Senhora dos Mártires
3. Reconstruction of the ship’s lines plan, structure and rigging
The East Indiaman Nossa Senhora dos Mártires was probably built in the Ribeira das Naus shipyard, Lisbon, during the first years of the 17th century. The ship departed from Lisbon to India, on its maiden voyage, on the 21st of March 1605 under the command of Captain Manuel Barreto Rolim. It arrived at Goa on the 28th of September after a journey without incidents, for a short stay, and left for Cochim, to load the hold with peppercorns and other spices. After a short four months’ sojourn, on January 16th 1606, the loaded ship left Cochim to Lisbon, together with another ship, the nau Salvação. The voyage went without problems and the ship stopped in the Azores Islands in June, before arriving near Lisbon in the middle of a severe Southwest storm on September the 13th. The ship dropped anchors outside the mouth of the Tagus River, but two days later the mooring cables broke and the captain decided to attempt entering the river. On the morning of September 15, Nossa Senhora dos Mártires touched the rocky bottom at the entrance of the Tagus Mouth in heavy following or stern quartering seas and, possibly, against a strong tidal current. The ship’s hull was broken against the rocks in a matter of hours. On September 19 about 200 bodies had already washed ashore, together with an immense black tide of peppercorns that stretched over several miles. As soon as possible, the royal officials rescued all the valuable goods they could recover. As the years passed, the wreck was forgotten. In 1992, a group of divers found the remains of a ship wrecked approximately at the same location, which was later excavated by archaeologists between 1996 and 2000 (Afonso, 1998). The hull remains were preserved in a layer of peppercorns and the excavation uncovered a number of artifacts consistent with the period of the voyage of Nossa Senhora dos Mártires. For example, one of the astrolabes has an inscribed date of 1605, the year of the ship’s maiden voyage. All the evidence indicates that these were the remains of Nossa Senhora dos Mártires. Fig. 1 (left) presents part of the remains ship structure, namely a portion of the bottom, including part of the keel, an apron, 11 frames and some of the planking. In spite of the importance of these ships for the history of the European discoveries and maritime expansion, this is the first Portuguese East Indiaman of this period to be archaeologically explored. Alves et al. (1998) and Castro (2005a) analyzed these remains and concluded that they belong to the ship bottom forward of midship, as shown in Fig. 1 (right). This part of the hull and structure provided important clues for the understanding of the methods and techniques employed by shipbuilders at that period. The preserved hull remains yielded an impressive amount of
3.1. Portuguese documents on shipbuilding A few contemporary documents, namely shipbuilding treatises and lists of materials to build ships, were fundamental in the reconstruction of the ships of the India Route. The first important text is Fernando Oliveira’s Livro da fábrica das naus (Book on the Fabrication of Ships), dated to around 1580, describes a 600 tonéis Indiaman (the tonel being a barrel with approximately 1.5 m in height and 1 m in maximum diameter) but is incomplete. The number of tonéis was a measure of the volume of the ship’s hold. The second coeval text considered is an anonymous list of the timbers necessary to build a three-decked, 600 tonéis Indiaman for the India route included in a codex named Livro Náutico (Nautical Book), in Lisbon’s National Library, and dating to circa 1590 (Domingues (2004)). The third text is a manuscript titled Livro primeiro da arquitectura naval (First Book of Naval Architecture), by João Baptista Lavanha, written sometime around 1600, unfortunately also incomplete (Lavanha, 1996). It describes a ship of 500 tonéis in great detail although only up to the first deck. The fourth text is Manoel Fernandez’ Livro de traças de carpintaria (Book of Shipwright Drawings) dated to 1616, and comprising a large number of lists of dimensions and drawings illustrating a number of ship types of this period, including a four-decked Indiaman. 3.2. Hull lines plan and rigging plan A reconstruction of the hull lines drawing was initially proposed by Castro (2005a). It was based on the analysis of the archaeological remains, namely the timber’s scantlings and the carpenters’ marks. From the texts above, the recipe that adjusted better to the archaeological data was the one presented in Oliveira (1580) for a 18 rumos of keel (27.72 m) Indiaman. After concluding that the preserved part of the hull was most likely part of the bottom forward of midships, the remaining hull was reconstructed from the list of proportions and algorithms of Oliveira’s treatise. The missing information, namely regarding some details of the castles, decks, hatches, bulwarks, and fittings, were obtained from other contemporary documents. Details of the method can be found in Castro (2005)a,b and Santos et al. (2006). Fig. 2 presents the resulting lines plan and Table 1 the main particulars of the ship. The reconstruction of the vessel’s internal space was based on Oliveira’s treatise for an 18 rumos of keel Indiaman (Oliveira, 1580), resulting in a three-decked ship, with stern and forecastles (Fig. 3),
Fig. 1. Remains of the presumed Nossa Senhora dos Mártires, (left photo by Francisco Alves, CNANS) and relation with the reconstructed hull (drawing by Kevin Gnadinger).
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
D1 D2 D3
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Fig. 2. Lines drawing of Nossa Senhora dos Mártires.
and capacity for 600 tonéis. A digital 3D model was developed to test several hypothesis for the internal division and appropriation of the ship’s space (Wells, 2008). Santos et al. (2007) demonstrated that this reconstructed model has a real capacity near 600 tonéis, enough to accommodate the ballast, cargo and provisions necessary for the six months voyage to India. The reconstruction of the masts and yards, including their dimensions, material and arrangement was proposed by Castro (2005b). This work was based on coeval documents such as the Livro Náutico (c. 1590) of unknown author, Fernandez (1616) and Sousa (1630). According to these sources, which are fairly consistent, the lengths and diameters of the ship’s masts and yards could be obtained from the ship’s keel length and are most lengths and diameters of the masts and spars were simple fractions of the main mast’s. The dimensions of the sails were determined from the lengths of the masts and yards, and are fairly consistent with the written sources. Fig. 8 shows the reconstruction of the masts and yards and corresponding sail plan. The total sail area is 1789 m2. 4. Reconstruction of loading conditions typical of the India route An accurate estimate of the ship’s weight distribution is a difficult task, but an important one, since the correct assessment of the ship’s floatability, stability and safety, depends on the correct estimate of the displacement and position of the centre of gravity. The methodology adopted consists on decomposing the ship’s total weight in different components and then carrying out estimates for each one, based on archaeological and documental data. The Table 1 Main particulars of the reconstructed Nossa Senhora dos Mártires. Length overall of the hull (m) Length between perpendiculars (m) Length at the waterline (m) Beam(m) Draft (m) Depth to main deck (m) Depth to gun deck (m) Displacement (t) Sailplane area (m2)
50.4 38.0 38.0 13.2 5.0 8.2 5.9 1331.0 1789.0
following components have been considered: hull, masts and yards, sails, rigging (shrouds, etc.), anchors and ship’s boats, artillery, ballast, cargo, crew, soldiers, passengers and supplies. A detailed model of the complete ship and cargo has been reconstructed. A comprehensive description of the weight estimation for several of the components was given by Santos et al. (2007) and only the final results are presented here. Sections 4.1 and 4.2 complement these values with comprehensive estimations of the amount ballast and cargo carried by the ship.
4.1. Ballast The amount of ballast taken onboard any large ship of this period is unknown nowadays and subject to debate. Ballast was taken to consist of broken limestone (1.55 t/m3) and was carried directly above the keel, inside the lower hold. Some texts, such as Brito (1735) and Sebastião Themudo (Lavanha, 1996, 237e239) mention heights of ballast in the hold between 6 palmos (1.5 m) and 9 palmos (2.25 m), but this would depend of course on the type and size of the ship. Blot (1994) hypothesized that a 18th century ship with 64 guns carried 270 t of ballast. All things equal and taking into account the dimensions of that ship and the dimensions of the Nossa Senhora dos Mártires, its ballast should amount to 154 t. On the other hand, taking into account the ballast found in the Molasses Reef and Highborn Cay wrecks (Oertling, 1989a, 1989b), the amount of ballast could be higher, at around 200 t. In Santos et al. (2007), a value of 175 t was justified. A scientific approach is applied here to confirm these estimates. It is known that these ships were loaded in the middle of Tagus river (Lisbon) using barges. The same presumably happened in India. Calculations have shown that the ship is not stable upright in light condition unless about 130 t of ballast are at the bottom of the lower hold. This value represents then a lower limit for the ballast. The loading proceeded using barges to carry the cargo, water, wine and provisions to the ship. As reported in Malheiro do Vale (1988), water was one of the first items to be loaded onboard, using for this purpose tonéis. These were loaded using, at first, a stern hatch on the ship’s side called sisbordo (Fig. 3). This hatch leads to the aft part of the first deck, but it would be used to load both the lower hold and the first deck. On the other hand, this hatch can only be used as
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1,79 m
Poop deck 1,79 m
Quarterdeck
bulwark
1,79 m
8,32 m
1,79 m
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1,79 m
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1,79 m
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Hold Fig. 3. Reconstruction of the general arrangement of Nossa Senhora dos Mártires.
long as its lower edge remains above the waterline, otherwise the ship may start flooding through the open hatch. According with the details given in the Livro Náutico (c. 1590), this hatch was located roughly between 4.0 m and 5.75 m forward of the sternpost. The lower corner of the stern hatch (in this case, due to the substantial sheer, the forward-most extreme) would be located 4.7 m above the baseline. When the lower hold and first deck were full, the loading process would continue using also barges but lifting up and then down the various items through the ship’s hatches on the main deck. The stern hatch was closed meanwhile. Taking in consideration this loading process, two criteria were used to determine the amount of ballast carried onboard a 600 tonéis Indiaman: Positive metacentric height of at least 0.2 m when starting the loading; Positive freeboard at stern hatch of at least 0.2 m after loading the lower hold and first deck; These criteria guarantee an acceptable minimum safety level in terms of stability and floatability throughout the loading process. Fig. 4 shows the metacentric height (vertical distance between the centre of gravity and the transverse metacentre, this distance being a measure of the ship’s stability), the freeboard at the stern hatch, the height of ballast in the hold, the free-height left in the lower hold and the trim of the ship. All quantities are presented as a function of the amount of ballast in the hold (horizontal axis). The upper graph corresponds to the ballast condition (ship without any cargo), while in lower graph represents ship with the hold and the lower deck fully loaded. The amount and characteristics of the cargo loaded in Lisbon are described in Section 4.2. According with these calculations, the ballast at departure from Lisbon would be between 150 t, required for a metacentric height of 0.19 m when the ship was in the light condition (upper graph in Fig. 4), and 250 t required to have at least 0.18 m of freeboard at the stern hatch when ship has the lower hold and the first deck fully loaded (lower Fig. 4). The free-height in the hold for 175 t of ballast would then be about 2.55 m and for 250 t it would still be of 2.3 m. For an amount of ballast over 350 t, the free-height comes under 2 m and this appears to be too small for cargo stowage or indeed for standing on top of the ballast. With 175 t of ballast, the height of ballast is over 6 palmos and for 250 t it is over 7 palmos. These values appear reasonable taking into account the limited information provided in contemporary text mentioned above. On the other hand, these limit values provide sufficient metacentric height at the beginning of loading, as well as a minimum freeboard at the stern hatch when loading the hold and lower deck is completed. Both these parameters are essential for safe loading of the ship.
Concluding, and bearing in mind that Portuguese ships are known to have offloaded ballast in Goa quite substantially, to the point of silting up the mooring area, it is assumed that in the return voyage a 600 tonéis Indiaman would be carrying 175 t of ballast, that is towards the lower range. When leaving from Lisbon, it would be carrying around 250 t. This implies that 75 t would be dumped in the river in Goa or Cochim, in order to maximize the ship’s cargo capacity. 4.2. Cargo Evaluating the weight of cargo carried onboard an India Route Indiaman is a difficult task. First, it is important to keep in mind that the cargo was different when leaving Lisbon and when leaving India. Ships went to India relatively light and returned severely overloaded, as reported in Brito (1735), frequently causing shipwrecks. When leaving from Lisbon, ships typically took copper, tin, and lead ingots, silver and gold coins, and a small number of other merchandises, such as ivory, as it was recently observed in the remains of an outbound Portuguese Indiaman found in Namibia and tentatively dated to 1533 (Werz, 2009). The type of cargos taken onboard also varied along the 16th century and depended quite often on what was available in Lisbon in each particular year. In general, it is clear that the amount of cargo in the hold is much less when leaving Lisbon than when the ship is leaving from India on the return trip. In this study it is assumed that no more than 100 t of cargo are taken in the hold when departing from Lisbon, as this is close to the average cargo (111 t) found in detailed documents from the early 16th century, which have been published by Godinho (1987). Unfortunately, we are less well informed for the beginning of the next century, but the same author points out that silver coins became even more important as cargo, so we can assume that when leaving Lisbon the Nossa Senhora dos Mártires was probably taking a cargo no larger than 100 t. Regarding the return voyage the cargo aboard a Indiaman consisted mainly of peppercorns, weighting between 3000 and 5000 quintais (1 quintal ¼ 58.75 Kg), as indicated by Castro (2005a). Costa (1997) indicates some cargo weights for ships returning from India and 4500 quintais (265 t) is a usual cargo weight for ships of this size. Fig. 5 shows the average cargo when arriving in Lisbon per ship in different years, using data extracted from Godinho (1987). Of course, this cargo depends on the average size of the ships and whether the cargo available in India was enough to load completely the various ships departing for Portugal. However, the average size of the ships appears to have grown faster between 1498 and 1520 than during the remaining decades of the 16th century (Costa, 1997). On the other hand, great efforts were generally made in India to fill the ships and if enough
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
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cargo was not available, then the ships would stay in India, so the average cargoes in Fig. 5 are probably not far from the ship’s maximum capacities and probably relate, on average, to ships not far from our 600 tonéis Indiaman. Nevertheless, large variations in cargo amounts exist, with values between 4000 and 6000 quintais being fairly common. In this study a standard cargo of 4500 quintais in the return voyage is assumed. From the numerical model of Nossa Senhora dos Mártires, the available volume in the lower hold above the ballast can be found to be 606 m3 and, taking into account that the pepper density is approximately 0.5 kg/dm3, the volume required for the 4500 quintais is 530 m3. This indicates that the estimated cargo of pepper could be carried entirely in the lower hold, which is consistent to the loading scheme described by Falcão (1607). The centre of gravity of the pepper can be estimated to be located 19.6 m forward of the aft perpendicular and 2.83 m above the baseline. 4.3. Loading conditions Fig. 6 shows a comparison between the loading conditions when departing from Lisbon and when departing from India for the return trip. The main differences are the smaller amount of ballast in the return voyage, which is more than compensated by the large amount of cargo carried onboard. When departing from Lisbon the lower hold is largely empty, therefore it was decided to stow there 86 t of water and wine in barrels. This also helps with ballasting the ship since the amount of cargo is very light in this condition. It is useful pointing out that the hull represents a large part of the ship’s displacement and the large amount of ballast that was taken onboard. Finally, it is possible to conclude that provisions (water, wine and hardtack) represent a heavier load than the cargo itself or, indeed, the hull. Interesting conclusions are obtained when the percentages of the displacement which consist of cargo, passengers and consumables (deadweight) are compared with several modern ship types. In fact this late 16th century ship has a deadweight fraction similar to a modern general cargo ship. However, in the modern ships more than 80% of the deadweight is actually cargo deadweight (which generates revenue) while in the Nossa Senhora dos Mártires only about 40% of the deadweight is actually cargo deadweight. The remaining 60% of the deadweight consist of
Sails Departure from Lisbon
Rigging
Return from India
Passengers Boat Light Artillery Crew Anchors Soldiers Heavy Artillery Masts and yards Other supplies Biscuit Ballast Water/Wine Cargo Hull 0,0
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Fig. 6. Comparison of loading conditions.
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ballast and supplies for the six months voyage. If one takes in consideration that these ships were designed to carry a mix of cargo and passengers, they don’t appear as inefficient when compared with modern day vessels, except for the large amount of ballast required for stability. 5. Assessment of the loading process We have mentioned that these ships were loaded afloat and starting with the large water casks containing enough provisions for six months (Malheiro do Vale, 1988). Supplies, weapons, and other necessary goods were loaded with boats and barges, carrying them from the royal warehouses, the Armazens da Ribeira das Naus. The amounts were registered by warehouse clerks before embarking in the contracted boats and then were discharged onboard the ship, where a parallel registry was carried out by the ship’s main officers. Passengers and soldiers with their belongings were the last to be taken onboard, in the last days before departure. At Goa or Cochim the loading process was carried in similar ways, but the hold was mainly filled with peppercorns. Smaller portions of cargo, such as pots with spices, bales of silk or cotton, lather bags with jewels, porcelains and pieces of furniture, were loaded in the various decks and castles. According with many authors, such as D’Intino (1998), who have studied documents from the 16th and early 17th centuries, overloading of the ships was a common problem, mainly regarding the spaces in the upper decks and superstructures, where passengers and crew tended to carry far more personal cargo than was officially allowed. Documents from this time show that the negative effects on the ship’s stability of this cargo located so high in the ship were already known. The upper graph on Fig. 7 shows the stability and floatation of the ship during the various loading stages at Lisbon. The horizontal axis, from 1 to 4, represents the various loading conditions during the process. After being ballasted with 250 t, the ship starts loading, with 1.1 m freeboard at the stern hatch and 0.75 m of metacentric height. It is assumed that at this stage the ship is already fully equipped with sails, rigging, anchors, cables, ship’s boats, and artillery. Trim in this condition is 0.75 aft. Cargo and water/wine are then loaded in the hold after which the lower deck is also loaded with hardtack and more water/wine. When the lower deck is fully loaded the ship’s metacentric height has risen to 1.35 m and the freeboard at the stern hatch is only 0.25 m. The stern hatch is then closed, made watertight or at least weathertight, and the loading is continued using the hatches in the main deck. When fully loaded the ship’s metacentric height is 1.2 m and the stern hatch is about 0.1 m below the waterline. The lower graph of Fig. 7 shows the stability and floatation of the ship during the various loading stages in India. Again, the ship starts from the ballast condition and already equipped with all its outfitting. For the 175 t of ballast considered, the ship’s metacentric height is 0.35 m and the freeboard at the stern hatch is 1.25 m. When the hold and lower deck are fully loaded, respectively with cargo and with water/wine, the metacentric height is 1.3 m and the freeboard at the stern hatch is 0.13 m. Loading then continues and when the ship has its second deck loaded and the crew and passengers are taken onboard, the stern hatch is already 0.35 m below the waterline. If the overloading of the stern and forward castles is taken in consideration, the stern hatch may be as much as 0.65 m below the waterline. There are texts such as Brito (1735) which suggest that the stern hatch was not supposed to be under water when leaving from India as this caused the ship to take in even more water than was already common in the wooden ship’s of this epoch.
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
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Loading Condition Fig. 7. Ship’s stability during various stages of loading in Lisbon (upper graph) and in Goa (lower graph).
6. Development of limit KG curve according with modern intact stability criterion Santos et al. (2007) have studied the floatability and stability of the presumed Nossa Senhora dos Mártires, mainly for the return loading condition. It was found that the ship could comply with a demanding modern stability criterion such as that of the US Coast Guard (Code of Federal Regulations, 1983). The main reason for this compliance, which may seem surprising, is the small length to breadth ratio of this ship or, in other words, the large breadth of the ship. On the other hand, the sail area is not excessive for a ship of this size, which also helps in complying with the criterion. It is worth pointing out that calculations were carried out under the assumption that the ship was watertight to the main deck, that is assuming that the stern side hatch and gun ports were watertight. Texts from the time indicate that these openings were secured before leaving from India and that its opening was rather difficult when, for example, deploying the ship’s boat was necessary during the return voyage, see Brito (1735).
In this section a more comprehensive study of the ship’s loading conditions is presented, including the loading conditions for the voyage to India, with its characteristically lighter cargo. Compliance of these conditions with the US Coast Guard criterion is assessed using the limit KG curves for various combinations of sails (KG represents the height of the centre of gravity, G, in relation to the keel, K). The US Coast Guard criterion was developed specifically for large sailing ships and it requires that the three numerals are larger than minimum values and also that the angle of extinction of stability is larger than 90 . The numerals are related to: a) protection against water on deck, b) resistance against down-flooding the interior of the ship, c) ability to resist a knockdown leading to capsize. Fig. 8 shows the sail areas now proposed and those given in the so-called Harvard Codex (Ataíde, 1588-1633). It may be seen that the match between both sources is quite acceptable, but the main interest of the later reference is that it gives the areas of the sail’s bonnets. Thus it allows the development of a reduced sail area condition used in bad weather. When the weather worsened the
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Fig. 8. Reconstructed sail plan (Castro, 2009) and sail areas from the Harvard Codex and from Santos et al. (2007).
topsails, spritsail and main and foresail bonnets were taken off (Malhão Pereira, 2001). Table 2 shows the effects of these reductions in the sail area. The sail area decreases at first from 1882 m2 to only 1490 m2 by removing the bonnets and the spritsail. If this was not sufficient in bad weather, the tops would also be taken off and this leads to a sail area of only 837.5 m2. The calculation of the lead in percentage of the length of the waterline includes also the effects of the exposed lateral areas of the hull, forecastle and aft castle. With the full sail plane deployed, the centre of effort is about 11% forward of the centre of lateral resistance. As the sails are removed, the lead decreases and for the last condition the centre of effort is actually aft of the centre of lateral resistance. In this last condition both the area and the height of the vertical centre of effort are significantly reduced. Fig. 9 shows the limit KG curve for compliance with the US Coast Guard large sailing ship intact stability criterion. One curve is
shown for each of the three sail plane areas just described. It may be seen that for lower displacements of up to 1300 t significant differences exist between the limit KG for different sail plane areas. In particular, removing the topsails increases the maximum allowable KG by as much as 0.75 m for the lower displacements. Removing the bonnets and spritsail produces a small increase of 0.2 m in the limit KG. This increase is almost uniform over the entire range of displacements. Fig. 9 also shows six loading conditions derived as described above. These conditions are:
Table 2 Characteristics of different sail planes.
The hypothesis with 10% supplies represents the estimated arrival conditions at Lisbon and India. The cargo in the return voyage consists of 265 t of peppercorns. The overload represents the cargo carried on board by the passengers and crew in the return voyage, mainly in the main deck and superstructures. This is of course very difficult to evaluate as much of this cargo was in fact not legal and accordingly no values exist in the literature.
Full sail plane Reduced sail plane Storm condition
Sail plane area (m2)
Long. centre of effort (m)
Lead (% of LWL)
Vert. centre of effort (m)
1882 1490 837.5
23.0 20.3 19.0
10.8% 3.6% 4.3%
19.3 21.1 16.2
Return from India with 100% supplies and 96 t overload. Return from India with 10% supplies and 96 t overload. Return from India with 100% supplies and standard loading. Return from India with 10% supplies and standard loading. Voyage to India with 100% supplies. Voyage to India with 10% supplies.
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
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6,0
Return 100% overloaded
5,5 Return 10% overloaded
Return 100%
KG (m)
5,0
To India 10%
To India 100%
Return 10%
4,5
4,0 Full sail plan Without tops and bonnets Return 100% Return 100% overloaded To India 100%
3,5
Without bonnets Watertight to 2nd deck Return 10% Return 10% overloaded To India 10%
3,0 600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
Displacement (t) Fig. 9. Limit KG curves as function of the ship displacement for different sail plan configurations, and estimated KG for different loading conditions (symbols).
Furthermore, the amounts of cargo actually allowed per crew member by the Portuguese crown even varied considerably in time. In the voyage to India a standard cargo of 100 t in the hold was considered. Finally, the literature suggests that supplies could indeed also be in excess and had to be assessed when arriving in Lisbon. Comparing these loading conditions with the limit KG curves, shows that when all sails are deployed the ship only complies with the criterion when departing to India, departing from India not overloaded, or when arriving in Lisbon, again not overloaded. Even for these conditions, the margins are only 0.2 m, 0.25 m and there is practically no margin in the third case. For the other three conditions considered, the ship does not comply when all sails are deployed. When the spritsail, bonnets and tops are removed, however, the ship complies with the criterion in all conditions. It is also worth mentioning that the ship’s centre of gravity descends as the supplies are consumed, because these are taken in the upper decks and not in the hold. This is particularly true for the return conditions. In the voyage to India it was assumed that much of the free space in the hold was filled with barrels of water, which is consumed during the voyage. The centre of gravity in this case remains practically at the same vertical position as supplies are consumed both above and below the initial position of centre of gravity. Finally, we have noted the large difference in the ship’s displacement between the departure and arrival conditions. This is caused by the very large amount of supplies carried onboard to feed crewmembers, soldiers and passengers during the six or more months that took each leg of the voyage. Fig. 9 also shows the limit KG curve when the ship’s hull is only taken as watertight up to the second deck and not to the main deck. In this case the ship does not comply with the Coast Guard criterion in any condition. The adverse effects of the much reduced freeboard are clearly visible, especially when the displacement is larger than 1100 t. 7. Conclusions The paper starts by presenting a reconstruction of an early 17th century Portuguese Indiaman, considered representative of late 16th century Portuguese shipbuilding, found at the mouth of Tagus River, in Portugal, including its lines drawing, internal divisions,
and rigging. Using this information together with historic documents, the ship’s loading process was reconstructed step by step. This was found to be a difficult task, involving the formulation of numerous hypotheses. Based on floatability and stability considerations during the ship loading phases, the amount of ballast used in a 600 t Indiaman was estimated as 250 t at departure from Lisbon and 175 t on the return voyage. plausible estimates were obtained for the amount of cargo transported, namely around 100 t when departing from Lisbon and 265 t at departure from India. Finally, the stability of the ship was evaluated against the US Coast Guard criterion, showing that the ship marginally complies with the criterion in the fully loaded condition at the departure from India. However, if overloaded at the departure from India or in light (10% of supplies) arrival conditions, it would not comply with such a demanding criterion. Furthermore, compliance with the stability criterion is based on the hypothesis that the ship is watertight to the main deck, as contemporary texts indicate that every effort was made to make the hatches fully watertight. Without this hypothesis, compliance in any loading condition is not possible. As an overall conclusion, one can state that the plausibility of the proposed ship reconstruction is cross checked in different ways, including information retrieved from contemporary documents, and all evidence points to a consistent reconstruction.
References Afonso, S.L., 1998. Nossa Senhora dos Mártires, The Last voyage. Verbo/Expo’98, Lisbon. Alves, F., Castro, C., Rodrigues, P., Garcia, C., Aleluia, M., 1998. Archeology of a shipwreck. In: dos Martires, Nossa Senhora (Ed.), The Last Voyage. Catalogue of the Portuguese Pavilion at EXPO’98, Verbo, Lisbon. Ataíde, A., 1588-1633. Codices of D. António de Ataíde, vols. 3. Ms. 4794f of the Houghton Library of Harvard University (microfilm at Lisbon’s Biblioteca Central de Marinha). Blot, J.Y., 1994. Interpretação do Naufrágio do Navio S. Pedro de Alcântara na Costa de Peniche (1786). In: Proceedings of 6as Jornadas Técnicas de Engenharia Naval, pp. 3e1 a 3e49, Viana do Castelo, Portugal. Brandt, H., Hochkirch, K., 1995. The sailing properties of the Hanse cog in comparison with other cargo sailships. Ship Technology Research 42 (1), 3e20. Brito, B.G., 1735. História Trágico-Marítima em que se escrevem chronologicamente os Naufragios que tiverão as Naos de Portugal, depois que se poz em exercicio a Navegação da Índia. Publicações Europa-América (1984), Lisbon. Castro, F., Fonseca, N., 2006. Sailing the Pepper Wreck: a proposed methodology to understand an early 17th-century Portuguese Indiaman. International Journal of Nautical Archaeology 35.1, 97e103.
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Castro, F., Fonseca, N., Wells, A., 2010. Outfitting the Pepper Wreck. Historical Archaeology 44 (2), 14e34. Castro, F., 2000. Os Destroços de uma Nau da Índia na Foz do Tejo, Lisboa, Portugal. In: Actas do 3 Congresso de Arqueologia Peninsular, vol. 8, “Terrenos da Arqueologia da Península Ibérica”, Porto, ADECAP, pp. 475e495. Castro, F., 2003. The Pepper Wreck, and early 17th-century Portuguese Indiaman at the mouth of the Tagus River, Portugal. The International Journal of Nautical Archeology 32 (1), 6e23. Castro, F., 2005a. The Pepper Wreck. Texas A&M University Press, College Station, Texas, USA. Castro, F., 2005b. Rigging the Pepper Wreck: Part I e Masts and Yards. The International Journal of Nautical Archeology 34 (1), 112e124. Castro, F., 2007. Rising and Narrowing: 16th-Century Geometric algorithms used to design the bottom of ships in Portugal. The International Journal of Nautical Archaeology 36.1, 148e154. Castro, F., 2009. Rigging the pepper wreck. Part II e Sails. International Journal of Nautical Archaeology 38.1, 105e115. Code of Federal Regulations, 1983. Subdivision and Stability Regulations: Final Rules, vol. 46. Code of Federal Regulations. Ch. 1. Costa, L.F., 1997. Naus e Galeões na Ribeira de Lisboa e A Construção Naval no século XVI para a Rota do Cabo. Patrimonia Historica, Cascais. D’Intino, R., 1998. A Gente do Mar na Carreira da Índia. In: Catalogue of the Portuguese Pavilion at EXPO’98. Verbo, Lisbon. Domingues, F.C., 2004. Os Navios do Mar Oceano e Teoria e Empiria na Arquitectura Naval Portuguesa dos Séculos XVI e XVII. Centro de História da Universidade de Lisboa, Lisbon. Falcão, L.F., 1607. Livro em que se contem toda a fazenda e real patrimonio dos reinos de Portugal, India, e ilhas adjacentes e outras particularidades, ordenado por Luiz de Figueiredo Falcao, secretario de el rei Filippe II. Imprensa Nacional Casa da Moeda (1859), Lisbon. Fernandez, M., 1616. Livro de traças de carpintaria com todos os modelos e medidas para se fazerem toda a navegaçao, assi de alto bordo como de remo traçado. Academia de Marinha (1989), Lisbon. Fonseca, N., Santos, T.A., Castro, F., 2005. Study of the intact stability of a Portuguese Nau from the early XVII century. In: Guedes Soares, C., Garbatov, Y., Fonseca, N. (Eds.), 2005. Maritime Transportation and Exploitation of Ocean and Coastal
Resources, vol. 1. Taylor & Francis Group, London, United Kingdom, pp. 893e900. Godinho, V.M., 1987. Os Descobrimentos e a Economia Mundial, 2a edição. Editorial Presença, Lisbon. Lavanha, J.B., 1996. Livro Primeiro da Architectura Naval. Academia de Marinha, Lisbon, Portugal. Livro Nautico, c. 1590. Livro Nautico ou Meio pratico da Construcao de Navios e Galés Antigas, Cod. 2257, Biblioteca Nacional de Lisboa. In Os Navios do Mar Oceano e Teoria e Empiria na Arquitectura Naval Portuguesa dos Séculos XVI e XVII, Domingues, F.C. Centro de História da Universidade de Lisboa, Lisboa, Portugal. Malhão Pereira, J.M. 2001. Norte dos Pilotos Guia dos Curiozos de Manuel dos Sanctos Rapozo, Um Livro de Marinharia do século XVIII: Estudo Crítico, Dissertação de Mestrado, Faculdade de Ciências Sociais e Humanas, Departamento de História, Lisboa. Malheiro do Vale, A.J., 1988. Nau de Pedra. Edição da Revista da Armada. Lisboa. Oertling, T.J., 1989a. The Highborn Cay Wreck: the 1986 field season. International Journal of Nautical Archaeology 18 (3), 244e253. Oertling, T.J., 1989b. The Molasses Reef wreck hull analysis: final report. International Journal of Nautical Archaeology 18 (3), 229e243. Oliveira, F., 1580. O Liuro da fabrica das nãos. Academia de Marinha e Museu Marítimo de Macau (1995), Macau, PRC. Santos, T.A., Fonseca, N., Castro, F., 2006. Stability characteristics of an early XVII century Portuguese Nau. In: Proceedings of the 9th International Conference on the Stability of Ships and Ocean Vehicles (STAB’2006), September, 25e29, Rio de Janeiro, Brazil, vol. 1, pp. 69e80. Santos, T.A., Fonseca, N., Castro, F., October 2007. Naval Architecture Applied to the Reconstruction of an Early XVII Century Portuguese Nau. Marine Technology 44 (4), 254e267. Sousa, G. c., 1630. Conta das medidas de uma nau da Índia, in Coriosidades de Gonçallo de Sousa, c. 1630, manuscript 3074, Reservados. Biblioteca Geral da Universidade de Coimbra. Wells, A., 2008, Virtual Reconstruction of a 17th-Century Portuguese Nau, Master Thesis, Texas A&M University. Werz, B., 2009. The Oranjemund shipwreck, Namibia. The excavation of sub-Saharan Africa’s oldest discovered wreck. Journal of Namibian Studies (6), 81e106.
Contents lists available at SciVerse ScienceDirect
Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas
Loading and stability of a late 16th century Portuguese Indiaman T.A. Santos a, N. Fonseca a, F. Castro b, *, T. Vacas a a b
Centre of Marine Technology and Engineering, Technical University of Lisbon, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Nautical Archaeology Program, Department of Anthropology, Texas A&M University, 105 Anthropology Building, College Station, TX 77843-4352, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2010 Received in revised form 13 April 2012 Accepted 17 April 2012
The stability characteristics of 16th century ships are not known with certainty, but safety issues related to floatability, stability and overloading were a cause of concern at the time. The aim of the paper is to advance knowledge in this field by developing a set of loading conditions for a typical Portuguese ship of this epoch, for both the voyage from Lisbon to India and the return voyage. This allows testing the reconstruction of the presumable Nossa Senhora dos Mártires as well as to use this reconstruction to bring a better understanding of safety and loading issues on the Portuguese East India route. Given the uncertainties about the loading conditions, several hypotheses are tested, varying the amount of ballast, the degree of overloading and the distribution of weights on board, and allowing the development of a range of plausible loading arrangements. The stability of the ship is then assessed using modern tools to develop the limit KG curve for compliance with a modern stability criterion applicable to large sailing vessels. The case study ship is a plausible reconstruction based on the analysis of nautical archaeological remains, contemporary documents and the use of modern naval architecture methods. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Shipbuilding history Nautical archaeology Portuguese Indiamen 16th century seafaring
1. Introduction The ships of the Portuguese discoveries and maritime expansion are still not well known, which is related to the lack of shipwrecks excavated by nautical archaeologists. Furthermore, during the 15th and 16th centuries there were no standard procedures for technical drawing and design documentation. A new line of research is emerging which will bring new insight into the technical characteristics of the historic ships. It consists of combining the scientific fields of naval history and nautical archaeology, with the naval architecture, including advanced computational and experimental methods (Brandt and Hochkirch, 1995; Castro and Fonseca, 2006). This methodology is being applied to reconstruct and investigate an early 17th century Portuguese Indiaman, one of the largest ships of its time, able to carry large amounts of cargo along the longest sea route of that time. The reconstruction being carried out is partially based on the analysis of remains excavated during the 1990s at the mouth of the river Tagus, near Lisbon (Afonso, 1998; Castro, 2000, 2005a,b). Based on the excavation data and on 16th and early 17th century Portuguese shipbuilding treatises, Castro (2003 and 2007)
* Corresponding author. Tel.: þ1 979 8456220; fax: þ1 979 8454070. E-mail address: [email protected] (F. Castro). 0305-4403/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2012.04.031
proposed a reconstruction of the hull. Fonseca et al. (2005) and Santos et al. (2006, 2007) describe in detail the procedure to obtain the hull bodylines from the analysis of the archaeological remains and the methods in the contemporary shipbuilding treatises and present a preliminary analysis of the ship’s floatability and stability. This paper introduces new methodologies to analyze the floatability and stability of this ship. The main goals are to test the reconstruction of the presumable Nossa Senhora dos Mártires as well as to use this reconstruction to bring a better understanding of safety and loading issues on the Portuguese East India route. Historic documents were analyzed which allow a characterization of the ship’s loading process step by step (Castro et al., 2010). The loading process is simulated numerically and the floatability and stability analyzed. This analysis led to new conclusions, including, for example, the amount of ballast that the ship would need to carry in the outbound voyage and in the return voyage. This is relevant since until now the amount of ballast in this type of ships was a subject of debate. Additional historic documents regarding the operation of the ships and the sail plan in different weather conditions were also analyzed. This analysis led to the definition of realistic sailing conditions applied in the stability analysis. Furthermore, this paper presents a comprehensive study of the ship’s loading conditions, an assessment of the compliance with the US Coast Guard criterion
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(Code of Federal Regulations, 1983) and the derivation of limit KG curves for various combinations of sails.
information because some of its timbers had construction marks inscribed.
2. A Portuguese Indiaman: the Nossa Senhora dos Mártires
3. Reconstruction of the ship’s lines plan, structure and rigging
The East Indiaman Nossa Senhora dos Mártires was probably built in the Ribeira das Naus shipyard, Lisbon, during the first years of the 17th century. The ship departed from Lisbon to India, on its maiden voyage, on the 21st of March 1605 under the command of Captain Manuel Barreto Rolim. It arrived at Goa on the 28th of September after a journey without incidents, for a short stay, and left for Cochim, to load the hold with peppercorns and other spices. After a short four months’ sojourn, on January 16th 1606, the loaded ship left Cochim to Lisbon, together with another ship, the nau Salvação. The voyage went without problems and the ship stopped in the Azores Islands in June, before arriving near Lisbon in the middle of a severe Southwest storm on September the 13th. The ship dropped anchors outside the mouth of the Tagus River, but two days later the mooring cables broke and the captain decided to attempt entering the river. On the morning of September 15, Nossa Senhora dos Mártires touched the rocky bottom at the entrance of the Tagus Mouth in heavy following or stern quartering seas and, possibly, against a strong tidal current. The ship’s hull was broken against the rocks in a matter of hours. On September 19 about 200 bodies had already washed ashore, together with an immense black tide of peppercorns that stretched over several miles. As soon as possible, the royal officials rescued all the valuable goods they could recover. As the years passed, the wreck was forgotten. In 1992, a group of divers found the remains of a ship wrecked approximately at the same location, which was later excavated by archaeologists between 1996 and 2000 (Afonso, 1998). The hull remains were preserved in a layer of peppercorns and the excavation uncovered a number of artifacts consistent with the period of the voyage of Nossa Senhora dos Mártires. For example, one of the astrolabes has an inscribed date of 1605, the year of the ship’s maiden voyage. All the evidence indicates that these were the remains of Nossa Senhora dos Mártires. Fig. 1 (left) presents part of the remains ship structure, namely a portion of the bottom, including part of the keel, an apron, 11 frames and some of the planking. In spite of the importance of these ships for the history of the European discoveries and maritime expansion, this is the first Portuguese East Indiaman of this period to be archaeologically explored. Alves et al. (1998) and Castro (2005a) analyzed these remains and concluded that they belong to the ship bottom forward of midship, as shown in Fig. 1 (right). This part of the hull and structure provided important clues for the understanding of the methods and techniques employed by shipbuilders at that period. The preserved hull remains yielded an impressive amount of
3.1. Portuguese documents on shipbuilding A few contemporary documents, namely shipbuilding treatises and lists of materials to build ships, were fundamental in the reconstruction of the ships of the India Route. The first important text is Fernando Oliveira’s Livro da fábrica das naus (Book on the Fabrication of Ships), dated to around 1580, describes a 600 tonéis Indiaman (the tonel being a barrel with approximately 1.5 m in height and 1 m in maximum diameter) but is incomplete. The number of tonéis was a measure of the volume of the ship’s hold. The second coeval text considered is an anonymous list of the timbers necessary to build a three-decked, 600 tonéis Indiaman for the India route included in a codex named Livro Náutico (Nautical Book), in Lisbon’s National Library, and dating to circa 1590 (Domingues (2004)). The third text is a manuscript titled Livro primeiro da arquitectura naval (First Book of Naval Architecture), by João Baptista Lavanha, written sometime around 1600, unfortunately also incomplete (Lavanha, 1996). It describes a ship of 500 tonéis in great detail although only up to the first deck. The fourth text is Manoel Fernandez’ Livro de traças de carpintaria (Book of Shipwright Drawings) dated to 1616, and comprising a large number of lists of dimensions and drawings illustrating a number of ship types of this period, including a four-decked Indiaman. 3.2. Hull lines plan and rigging plan A reconstruction of the hull lines drawing was initially proposed by Castro (2005a). It was based on the analysis of the archaeological remains, namely the timber’s scantlings and the carpenters’ marks. From the texts above, the recipe that adjusted better to the archaeological data was the one presented in Oliveira (1580) for a 18 rumos of keel (27.72 m) Indiaman. After concluding that the preserved part of the hull was most likely part of the bottom forward of midships, the remaining hull was reconstructed from the list of proportions and algorithms of Oliveira’s treatise. The missing information, namely regarding some details of the castles, decks, hatches, bulwarks, and fittings, were obtained from other contemporary documents. Details of the method can be found in Castro (2005)a,b and Santos et al. (2006). Fig. 2 presents the resulting lines plan and Table 1 the main particulars of the ship. The reconstruction of the vessel’s internal space was based on Oliveira’s treatise for an 18 rumos of keel Indiaman (Oliveira, 1580), resulting in a three-decked ship, with stern and forecastles (Fig. 3),
Fig. 1. Remains of the presumed Nossa Senhora dos Mártires, (left photo by Francisco Alves, CNANS) and relation with the reconstructed hull (drawing by Kevin Gnadinger).
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
D1 D2 D3
0
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D1 D2
1
6
7
8
9
10
11
12
13
14
15
16
17
D3
18
19
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IV III II
I
I
II
III IV
Fig. 2. Lines drawing of Nossa Senhora dos Mártires.
and capacity for 600 tonéis. A digital 3D model was developed to test several hypothesis for the internal division and appropriation of the ship’s space (Wells, 2008). Santos et al. (2007) demonstrated that this reconstructed model has a real capacity near 600 tonéis, enough to accommodate the ballast, cargo and provisions necessary for the six months voyage to India. The reconstruction of the masts and yards, including their dimensions, material and arrangement was proposed by Castro (2005b). This work was based on coeval documents such as the Livro Náutico (c. 1590) of unknown author, Fernandez (1616) and Sousa (1630). According to these sources, which are fairly consistent, the lengths and diameters of the ship’s masts and yards could be obtained from the ship’s keel length and are most lengths and diameters of the masts and spars were simple fractions of the main mast’s. The dimensions of the sails were determined from the lengths of the masts and yards, and are fairly consistent with the written sources. Fig. 8 shows the reconstruction of the masts and yards and corresponding sail plan. The total sail area is 1789 m2. 4. Reconstruction of loading conditions typical of the India route An accurate estimate of the ship’s weight distribution is a difficult task, but an important one, since the correct assessment of the ship’s floatability, stability and safety, depends on the correct estimate of the displacement and position of the centre of gravity. The methodology adopted consists on decomposing the ship’s total weight in different components and then carrying out estimates for each one, based on archaeological and documental data. The Table 1 Main particulars of the reconstructed Nossa Senhora dos Mártires. Length overall of the hull (m) Length between perpendiculars (m) Length at the waterline (m) Beam(m) Draft (m) Depth to main deck (m) Depth to gun deck (m) Displacement (t) Sailplane area (m2)
50.4 38.0 38.0 13.2 5.0 8.2 5.9 1331.0 1789.0
following components have been considered: hull, masts and yards, sails, rigging (shrouds, etc.), anchors and ship’s boats, artillery, ballast, cargo, crew, soldiers, passengers and supplies. A detailed model of the complete ship and cargo has been reconstructed. A comprehensive description of the weight estimation for several of the components was given by Santos et al. (2007) and only the final results are presented here. Sections 4.1 and 4.2 complement these values with comprehensive estimations of the amount ballast and cargo carried by the ship.
4.1. Ballast The amount of ballast taken onboard any large ship of this period is unknown nowadays and subject to debate. Ballast was taken to consist of broken limestone (1.55 t/m3) and was carried directly above the keel, inside the lower hold. Some texts, such as Brito (1735) and Sebastião Themudo (Lavanha, 1996, 237e239) mention heights of ballast in the hold between 6 palmos (1.5 m) and 9 palmos (2.25 m), but this would depend of course on the type and size of the ship. Blot (1994) hypothesized that a 18th century ship with 64 guns carried 270 t of ballast. All things equal and taking into account the dimensions of that ship and the dimensions of the Nossa Senhora dos Mártires, its ballast should amount to 154 t. On the other hand, taking into account the ballast found in the Molasses Reef and Highborn Cay wrecks (Oertling, 1989a, 1989b), the amount of ballast could be higher, at around 200 t. In Santos et al. (2007), a value of 175 t was justified. A scientific approach is applied here to confirm these estimates. It is known that these ships were loaded in the middle of Tagus river (Lisbon) using barges. The same presumably happened in India. Calculations have shown that the ship is not stable upright in light condition unless about 130 t of ballast are at the bottom of the lower hold. This value represents then a lower limit for the ballast. The loading proceeded using barges to carry the cargo, water, wine and provisions to the ship. As reported in Malheiro do Vale (1988), water was one of the first items to be loaded onboard, using for this purpose tonéis. These were loaded using, at first, a stern hatch on the ship’s side called sisbordo (Fig. 3). This hatch leads to the aft part of the first deck, but it would be used to load both the lower hold and the first deck. On the other hand, this hatch can only be used as
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1,79 m
Poop deck 1,79 m
Quarterdeck
bulwark
1,79 m
8,32 m
1,79 m
Main deck
1,79 m
2nd deck
1,79 m
3rd deck
1,79 m 1,92 m
Sisbordo
3,59 m
Hold Fig. 3. Reconstruction of the general arrangement of Nossa Senhora dos Mártires.
long as its lower edge remains above the waterline, otherwise the ship may start flooding through the open hatch. According with the details given in the Livro Náutico (c. 1590), this hatch was located roughly between 4.0 m and 5.75 m forward of the sternpost. The lower corner of the stern hatch (in this case, due to the substantial sheer, the forward-most extreme) would be located 4.7 m above the baseline. When the lower hold and first deck were full, the loading process would continue using also barges but lifting up and then down the various items through the ship’s hatches on the main deck. The stern hatch was closed meanwhile. Taking in consideration this loading process, two criteria were used to determine the amount of ballast carried onboard a 600 tonéis Indiaman: Positive metacentric height of at least 0.2 m when starting the loading; Positive freeboard at stern hatch of at least 0.2 m after loading the lower hold and first deck; These criteria guarantee an acceptable minimum safety level in terms of stability and floatability throughout the loading process. Fig. 4 shows the metacentric height (vertical distance between the centre of gravity and the transverse metacentre, this distance being a measure of the ship’s stability), the freeboard at the stern hatch, the height of ballast in the hold, the free-height left in the lower hold and the trim of the ship. All quantities are presented as a function of the amount of ballast in the hold (horizontal axis). The upper graph corresponds to the ballast condition (ship without any cargo), while in lower graph represents ship with the hold and the lower deck fully loaded. The amount and characteristics of the cargo loaded in Lisbon are described in Section 4.2. According with these calculations, the ballast at departure from Lisbon would be between 150 t, required for a metacentric height of 0.19 m when the ship was in the light condition (upper graph in Fig. 4), and 250 t required to have at least 0.18 m of freeboard at the stern hatch when ship has the lower hold and the first deck fully loaded (lower Fig. 4). The free-height in the hold for 175 t of ballast would then be about 2.55 m and for 250 t it would still be of 2.3 m. For an amount of ballast over 350 t, the free-height comes under 2 m and this appears to be too small for cargo stowage or indeed for standing on top of the ballast. With 175 t of ballast, the height of ballast is over 6 palmos and for 250 t it is over 7 palmos. These values appear reasonable taking into account the limited information provided in contemporary text mentioned above. On the other hand, these limit values provide sufficient metacentric height at the beginning of loading, as well as a minimum freeboard at the stern hatch when loading the hold and lower deck is completed. Both these parameters are essential for safe loading of the ship.
Concluding, and bearing in mind that Portuguese ships are known to have offloaded ballast in Goa quite substantially, to the point of silting up the mooring area, it is assumed that in the return voyage a 600 tonéis Indiaman would be carrying 175 t of ballast, that is towards the lower range. When leaving from Lisbon, it would be carrying around 250 t. This implies that 75 t would be dumped in the river in Goa or Cochim, in order to maximize the ship’s cargo capacity. 4.2. Cargo Evaluating the weight of cargo carried onboard an India Route Indiaman is a difficult task. First, it is important to keep in mind that the cargo was different when leaving Lisbon and when leaving India. Ships went to India relatively light and returned severely overloaded, as reported in Brito (1735), frequently causing shipwrecks. When leaving from Lisbon, ships typically took copper, tin, and lead ingots, silver and gold coins, and a small number of other merchandises, such as ivory, as it was recently observed in the remains of an outbound Portuguese Indiaman found in Namibia and tentatively dated to 1533 (Werz, 2009). The type of cargos taken onboard also varied along the 16th century and depended quite often on what was available in Lisbon in each particular year. In general, it is clear that the amount of cargo in the hold is much less when leaving Lisbon than when the ship is leaving from India on the return trip. In this study it is assumed that no more than 100 t of cargo are taken in the hold when departing from Lisbon, as this is close to the average cargo (111 t) found in detailed documents from the early 16th century, which have been published by Godinho (1987). Unfortunately, we are less well informed for the beginning of the next century, but the same author points out that silver coins became even more important as cargo, so we can assume that when leaving Lisbon the Nossa Senhora dos Mártires was probably taking a cargo no larger than 100 t. Regarding the return voyage the cargo aboard a Indiaman consisted mainly of peppercorns, weighting between 3000 and 5000 quintais (1 quintal ¼ 58.75 Kg), as indicated by Castro (2005a). Costa (1997) indicates some cargo weights for ships returning from India and 4500 quintais (265 t) is a usual cargo weight for ships of this size. Fig. 5 shows the average cargo when arriving in Lisbon per ship in different years, using data extracted from Godinho (1987). Of course, this cargo depends on the average size of the ships and whether the cargo available in India was enough to load completely the various ships departing for Portugal. However, the average size of the ships appears to have grown faster between 1498 and 1520 than during the remaining decades of the 16th century (Costa, 1997). On the other hand, great efforts were generally made in India to fill the ships and if enough
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
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5,0 GM(m) Freeboard(m) Freeheight in hold(m) Height of ballast(m) Trim(º)
4,0
m/º
3,0
2,0
1,0
0,0 50
100
150
200
250
300
350
400
450
500
-1,0 Ballast(t)
5,0 Freeboard (m) GM(m) 4,0
Trim(º)
m/º
3,0
2,0
1,0
0,0 50
100
150
200
250
300
350
400
450
500
-1,0
Ballast(t) Fig. 4. Floatability and stability after ballasting the lower hold (top graph) and after ballasting the lower hold and loading both the lower hold and the first deck (lower graph).
14000
12000
8000
6000
4000
2000
Year Fig. 5. Average cargo per ship at departure from India.
1598
1596
1595
1594
1592
1589
1587
1558
1548
1531
1526
1519
1517
1514
0 1513
Cargo (quintais)
10000
2840
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cargo was not available, then the ships would stay in India, so the average cargoes in Fig. 5 are probably not far from the ship’s maximum capacities and probably relate, on average, to ships not far from our 600 tonéis Indiaman. Nevertheless, large variations in cargo amounts exist, with values between 4000 and 6000 quintais being fairly common. In this study a standard cargo of 4500 quintais in the return voyage is assumed. From the numerical model of Nossa Senhora dos Mártires, the available volume in the lower hold above the ballast can be found to be 606 m3 and, taking into account that the pepper density is approximately 0.5 kg/dm3, the volume required for the 4500 quintais is 530 m3. This indicates that the estimated cargo of pepper could be carried entirely in the lower hold, which is consistent to the loading scheme described by Falcão (1607). The centre of gravity of the pepper can be estimated to be located 19.6 m forward of the aft perpendicular and 2.83 m above the baseline. 4.3. Loading conditions Fig. 6 shows a comparison between the loading conditions when departing from Lisbon and when departing from India for the return trip. The main differences are the smaller amount of ballast in the return voyage, which is more than compensated by the large amount of cargo carried onboard. When departing from Lisbon the lower hold is largely empty, therefore it was decided to stow there 86 t of water and wine in barrels. This also helps with ballasting the ship since the amount of cargo is very light in this condition. It is useful pointing out that the hull represents a large part of the ship’s displacement and the large amount of ballast that was taken onboard. Finally, it is possible to conclude that provisions (water, wine and hardtack) represent a heavier load than the cargo itself or, indeed, the hull. Interesting conclusions are obtained when the percentages of the displacement which consist of cargo, passengers and consumables (deadweight) are compared with several modern ship types. In fact this late 16th century ship has a deadweight fraction similar to a modern general cargo ship. However, in the modern ships more than 80% of the deadweight is actually cargo deadweight (which generates revenue) while in the Nossa Senhora dos Mártires only about 40% of the deadweight is actually cargo deadweight. The remaining 60% of the deadweight consist of
Sails Departure from Lisbon
Rigging
Return from India
Passengers Boat Light Artillery Crew Anchors Soldiers Heavy Artillery Masts and yards Other supplies Biscuit Ballast Water/Wine Cargo Hull 0,0
50,0
100,0
150,0
200,0
250,0
300,0
Weight (t)
Fig. 6. Comparison of loading conditions.
350,0
400,0
450,0
ballast and supplies for the six months voyage. If one takes in consideration that these ships were designed to carry a mix of cargo and passengers, they don’t appear as inefficient when compared with modern day vessels, except for the large amount of ballast required for stability. 5. Assessment of the loading process We have mentioned that these ships were loaded afloat and starting with the large water casks containing enough provisions for six months (Malheiro do Vale, 1988). Supplies, weapons, and other necessary goods were loaded with boats and barges, carrying them from the royal warehouses, the Armazens da Ribeira das Naus. The amounts were registered by warehouse clerks before embarking in the contracted boats and then were discharged onboard the ship, where a parallel registry was carried out by the ship’s main officers. Passengers and soldiers with their belongings were the last to be taken onboard, in the last days before departure. At Goa or Cochim the loading process was carried in similar ways, but the hold was mainly filled with peppercorns. Smaller portions of cargo, such as pots with spices, bales of silk or cotton, lather bags with jewels, porcelains and pieces of furniture, were loaded in the various decks and castles. According with many authors, such as D’Intino (1998), who have studied documents from the 16th and early 17th centuries, overloading of the ships was a common problem, mainly regarding the spaces in the upper decks and superstructures, where passengers and crew tended to carry far more personal cargo than was officially allowed. Documents from this time show that the negative effects on the ship’s stability of this cargo located so high in the ship were already known. The upper graph on Fig. 7 shows the stability and floatation of the ship during the various loading stages at Lisbon. The horizontal axis, from 1 to 4, represents the various loading conditions during the process. After being ballasted with 250 t, the ship starts loading, with 1.1 m freeboard at the stern hatch and 0.75 m of metacentric height. It is assumed that at this stage the ship is already fully equipped with sails, rigging, anchors, cables, ship’s boats, and artillery. Trim in this condition is 0.75 aft. Cargo and water/wine are then loaded in the hold after which the lower deck is also loaded with hardtack and more water/wine. When the lower deck is fully loaded the ship’s metacentric height has risen to 1.35 m and the freeboard at the stern hatch is only 0.25 m. The stern hatch is then closed, made watertight or at least weathertight, and the loading is continued using the hatches in the main deck. When fully loaded the ship’s metacentric height is 1.2 m and the stern hatch is about 0.1 m below the waterline. The lower graph of Fig. 7 shows the stability and floatation of the ship during the various loading stages in India. Again, the ship starts from the ballast condition and already equipped with all its outfitting. For the 175 t of ballast considered, the ship’s metacentric height is 0.35 m and the freeboard at the stern hatch is 1.25 m. When the hold and lower deck are fully loaded, respectively with cargo and with water/wine, the metacentric height is 1.3 m and the freeboard at the stern hatch is 0.13 m. Loading then continues and when the ship has its second deck loaded and the crew and passengers are taken onboard, the stern hatch is already 0.35 m below the waterline. If the overloading of the stern and forward castles is taken in consideration, the stern hatch may be as much as 0.65 m below the waterline. There are texts such as Brito (1735) which suggest that the stern hatch was not supposed to be under water when leaving from India as this caused the ship to take in even more water than was already common in the wooden ship’s of this epoch.
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
Displacement (t)
Displacement (t) Freeboard at sisbordo (m)
2841
GM (m) Trim(º)
1800
1,60
1600
1,40
1400
1,20
1200
1,00
1000
0,80
800
0,60
600
0,40
400
0,20
200
0,00
0
-0,20 Ballast
Hold loaded
First deck loaded
Fully loaded
Loading Condition Displacement (t) Freeboard at sisbordo (m)
GM (m) Trim(º)
1800
1,40
1600
1,20 1,00
1400
Displacement (t)
0,80 1200
0,60
1000
0,40
800
0,20 0,00
600
-0,20 400 -0,40 200
-0,60
0
-0,80 Ballast
Hold loaded
First deck loaded
Second Deck loaded
Crew+Pass
Overloaded
Loading Condition Fig. 7. Ship’s stability during various stages of loading in Lisbon (upper graph) and in Goa (lower graph).
6. Development of limit KG curve according with modern intact stability criterion Santos et al. (2007) have studied the floatability and stability of the presumed Nossa Senhora dos Mártires, mainly for the return loading condition. It was found that the ship could comply with a demanding modern stability criterion such as that of the US Coast Guard (Code of Federal Regulations, 1983). The main reason for this compliance, which may seem surprising, is the small length to breadth ratio of this ship or, in other words, the large breadth of the ship. On the other hand, the sail area is not excessive for a ship of this size, which also helps in complying with the criterion. It is worth pointing out that calculations were carried out under the assumption that the ship was watertight to the main deck, that is assuming that the stern side hatch and gun ports were watertight. Texts from the time indicate that these openings were secured before leaving from India and that its opening was rather difficult when, for example, deploying the ship’s boat was necessary during the return voyage, see Brito (1735).
In this section a more comprehensive study of the ship’s loading conditions is presented, including the loading conditions for the voyage to India, with its characteristically lighter cargo. Compliance of these conditions with the US Coast Guard criterion is assessed using the limit KG curves for various combinations of sails (KG represents the height of the centre of gravity, G, in relation to the keel, K). The US Coast Guard criterion was developed specifically for large sailing ships and it requires that the three numerals are larger than minimum values and also that the angle of extinction of stability is larger than 90 . The numerals are related to: a) protection against water on deck, b) resistance against down-flooding the interior of the ship, c) ability to resist a knockdown leading to capsize. Fig. 8 shows the sail areas now proposed and those given in the so-called Harvard Codex (Ataíde, 1588-1633). It may be seen that the match between both sources is quite acceptable, but the main interest of the later reference is that it gives the areas of the sail’s bonnets. Thus it allows the development of a reduced sail area condition used in bad weather. When the weather worsened the
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Fig. 8. Reconstructed sail plan (Castro, 2009) and sail areas from the Harvard Codex and from Santos et al. (2007).
topsails, spritsail and main and foresail bonnets were taken off (Malhão Pereira, 2001). Table 2 shows the effects of these reductions in the sail area. The sail area decreases at first from 1882 m2 to only 1490 m2 by removing the bonnets and the spritsail. If this was not sufficient in bad weather, the tops would also be taken off and this leads to a sail area of only 837.5 m2. The calculation of the lead in percentage of the length of the waterline includes also the effects of the exposed lateral areas of the hull, forecastle and aft castle. With the full sail plane deployed, the centre of effort is about 11% forward of the centre of lateral resistance. As the sails are removed, the lead decreases and for the last condition the centre of effort is actually aft of the centre of lateral resistance. In this last condition both the area and the height of the vertical centre of effort are significantly reduced. Fig. 9 shows the limit KG curve for compliance with the US Coast Guard large sailing ship intact stability criterion. One curve is
shown for each of the three sail plane areas just described. It may be seen that for lower displacements of up to 1300 t significant differences exist between the limit KG for different sail plane areas. In particular, removing the topsails increases the maximum allowable KG by as much as 0.75 m for the lower displacements. Removing the bonnets and spritsail produces a small increase of 0.2 m in the limit KG. This increase is almost uniform over the entire range of displacements. Fig. 9 also shows six loading conditions derived as described above. These conditions are:
Table 2 Characteristics of different sail planes.
The hypothesis with 10% supplies represents the estimated arrival conditions at Lisbon and India. The cargo in the return voyage consists of 265 t of peppercorns. The overload represents the cargo carried on board by the passengers and crew in the return voyage, mainly in the main deck and superstructures. This is of course very difficult to evaluate as much of this cargo was in fact not legal and accordingly no values exist in the literature.
Full sail plane Reduced sail plane Storm condition
Sail plane area (m2)
Long. centre of effort (m)
Lead (% of LWL)
Vert. centre of effort (m)
1882 1490 837.5
23.0 20.3 19.0
10.8% 3.6% 4.3%
19.3 21.1 16.2
Return from India with 100% supplies and 96 t overload. Return from India with 10% supplies and 96 t overload. Return from India with 100% supplies and standard loading. Return from India with 10% supplies and standard loading. Voyage to India with 100% supplies. Voyage to India with 10% supplies.
T.A. Santos et al. / Journal of Archaeological Science 39 (2012) 2835e2844
2843
6,0
Return 100% overloaded
5,5 Return 10% overloaded
Return 100%
KG (m)
5,0
To India 10%
To India 100%
Return 10%
4,5
4,0 Full sail plan Without tops and bonnets Return 100% Return 100% overloaded To India 100%
3,5
Without bonnets Watertight to 2nd deck Return 10% Return 10% overloaded To India 10%
3,0 600
700
800
900
1000
1100
1200
1300
1400
1500
1600
1700
Displacement (t) Fig. 9. Limit KG curves as function of the ship displacement for different sail plan configurations, and estimated KG for different loading conditions (symbols).
Furthermore, the amounts of cargo actually allowed per crew member by the Portuguese crown even varied considerably in time. In the voyage to India a standard cargo of 100 t in the hold was considered. Finally, the literature suggests that supplies could indeed also be in excess and had to be assessed when arriving in Lisbon. Comparing these loading conditions with the limit KG curves, shows that when all sails are deployed the ship only complies with the criterion when departing to India, departing from India not overloaded, or when arriving in Lisbon, again not overloaded. Even for these conditions, the margins are only 0.2 m, 0.25 m and there is practically no margin in the third case. For the other three conditions considered, the ship does not comply when all sails are deployed. When the spritsail, bonnets and tops are removed, however, the ship complies with the criterion in all conditions. It is also worth mentioning that the ship’s centre of gravity descends as the supplies are consumed, because these are taken in the upper decks and not in the hold. This is particularly true for the return conditions. In the voyage to India it was assumed that much of the free space in the hold was filled with barrels of water, which is consumed during the voyage. The centre of gravity in this case remains practically at the same vertical position as supplies are consumed both above and below the initial position of centre of gravity. Finally, we have noted the large difference in the ship’s displacement between the departure and arrival conditions. This is caused by the very large amount of supplies carried onboard to feed crewmembers, soldiers and passengers during the six or more months that took each leg of the voyage. Fig. 9 also shows the limit KG curve when the ship’s hull is only taken as watertight up to the second deck and not to the main deck. In this case the ship does not comply with the Coast Guard criterion in any condition. The adverse effects of the much reduced freeboard are clearly visible, especially when the displacement is larger than 1100 t. 7. Conclusions The paper starts by presenting a reconstruction of an early 17th century Portuguese Indiaman, considered representative of late 16th century Portuguese shipbuilding, found at the mouth of Tagus River, in Portugal, including its lines drawing, internal divisions,
and rigging. Using this information together with historic documents, the ship’s loading process was reconstructed step by step. This was found to be a difficult task, involving the formulation of numerous hypotheses. Based on floatability and stability considerations during the ship loading phases, the amount of ballast used in a 600 t Indiaman was estimated as 250 t at departure from Lisbon and 175 t on the return voyage. plausible estimates were obtained for the amount of cargo transported, namely around 100 t when departing from Lisbon and 265 t at departure from India. Finally, the stability of the ship was evaluated against the US Coast Guard criterion, showing that the ship marginally complies with the criterion in the fully loaded condition at the departure from India. However, if overloaded at the departure from India or in light (10% of supplies) arrival conditions, it would not comply with such a demanding criterion. Furthermore, compliance with the stability criterion is based on the hypothesis that the ship is watertight to the main deck, as contemporary texts indicate that every effort was made to make the hatches fully watertight. Without this hypothesis, compliance in any loading condition is not possible. As an overall conclusion, one can state that the plausibility of the proposed ship reconstruction is cross checked in different ways, including information retrieved from contemporary documents, and all evidence points to a consistent reconstruction.
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