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Experimental validation of spray deflectors for high speed craft
Ocean Engineering 191 (2019) 106482
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Experimental validation of spray deflectors for high speed craft Bogdan Molchanov a,b,c , Svante Lundmark b,c , Mirjam Fürth c ,∗, Mathew Green c a
Aalto University, Espoo, Finland KTH Royal Institute of Technology, Stockholm, Sweden c Stevens Institute of Technology, Hoboken, NJ, USA b
ARTICLE Keywords: High speed craft Spray Resistance Towing tank Planing craft
INFO
ABSTRACT A planing craft is a type of marine vehicle that is supported predominantly by hydrodynamic forces at high speeds. It has widespread applications in recreational and military use as well as for search and rescue operations. Although spray rails are a mature technology, there is limited experimental data illustrating their hydrodynamic effects. Furthermore, continuous efforts are being made to improve the efficiency, safety and performance of planning craft. In this paper, the efficiency of the spray resistance reduction technologies – spray rails and spray deflectors – are compared in model scale through towing tank testing in calm water and irregular waves. This study examines the influences of the technologies on the total resistance and running position of the hull in calm water and on impact accelerations experienced at the center of gravity and bow in irregular waves. The experimental results are discussed in connection with established semi-empirical methods for predicting the performance of planing craft and improvements for further testing of the spray deflection methods are proposed.
1. Introduction High Speed Craft (HSC) have many military and civilian uses, they are used for defensive purposes, for search and rescue, and for recreation among other things. These vessels allow for speedy operation in archipelagos and coastal waters where islands and other obstacles are plentiful and the water is shallow. With the recent increases in the power to weight ratio of outboard engines and the use of composite materials for hull structures, the use of HSC has become even more widespread (De Marco et al., 2017). With a larger and broader user base comes an increased need for technical improvements aimed specifically at HSC. The last 30 years have seen an increase in research focusing on HSC but many flow phenomena are still not fully understood and relatively few publications exist that address this issue. The hull of an HSC acts as a lifting surface to lift the craft out of its displacement mode. This causes a significant reduction in the wetted surface area, and thus, the resistance. This reduction allows the craft to reach high speeds without a linear increase in power consumption, unlike a displacement hull. As the speed increases, the hull moves from displacement mode through transition mode to reach planing mode. The resistance of an HSC can be decomposed into induced drag, appendage resistance, frictional resistance, aerodynamic resistance and spray resistance (Larsson and Hoyte, 2010). Payne (1982) concluded that wave making resistance tends to zero in the planing speed region while spray resistance becomes significant. Savitsky et al. (2007) show
that spray resistance can constitute over 15% of the total resistance at planing speeds. The flow dynamics around HSC can be predicted using theoretical or experimental modeling. Theoretical modeling can be grouped into numerical methods such as potential flow-based methods, viscous flow based methods and semi-empirical techniques (Yousefi et al., 2013). The method by Savitsky (1964) is the most widely used semiempirical method for evaluation of HSC and has over the past decades been improved through further model testing at the Davidson Laboratory (Savitsky and Brown, 1976; Savitsky et al., 2007). However, the flow around HSC is difficult to simulate, especially when taking dynamic instabilities into consideration. Therefore, the main method of evaluating the performance of HSC is often towing tank testing (Yousefi et al., 2013). Because of the high importance of the spray resistance, as mentioned earlier, specialized methods and designs exist for predicting and reducing spray. Such designs include spray rails and spray deflectors which both serve to reduce the spray area. Accurately evaluating the efficiency of such designs is therefore of high importance for future improvements to HSC design. A method of predicting the spray area, its contribution to total drag, and design guidelines for locating spray rails was published by Savitsky et al. (2007) and Begovic and Bertorello (2012). Some of the key definitions and areas used by Savitsky (1964) are shown in Fig. 1.
∗ Corresponding author. E-mail address: [email protected] (M. Fürth).
https://doi.org/10.1016/j.oceaneng.2019.106482 Received 30 April 2019; Received in revised form 26 July 2019; Accepted 20 September 2019 Available online 3 October 2019 0029-8018/© 2019 Elsevier Ltd. All rights reserved.
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Nomenclature Acronyms 𝐵𝐻 𝐶𝐺 𝐻𝑆𝐶 𝐼𝑇 𝑇 𝐶 𝐿𝐶𝐺 𝑆𝐷1 𝑆𝐷2 𝑆𝐿𝑅
Bare Hull Center of Gravity High Speed Craft International Towing Tank Conference Longitudinal Center of Gravity [m] Spray Deflector Configuration 1 Spray Deflector Configuration 2 𝑉𝑠 Speed Length Ratio [ √ ] 2
𝑆𝑅
Spray Rails
𝐿𝑤𝐿
Fig. 1. Underwater view of the planing hull: spray area is bounded by the dry area in the forepart and by the pressure area in the aft.
Greek symbols 𝛽 𝛥 ∇ 𝜏
Deadrise angle [deg] Ship displacement [N] Displaced volume [m3 ] Trim angle [deg]
used by De Marco et al. (2017) and Taunton et al. (2010) the deflectors run at an angle to the center line, are straight, and have no curvature in contrast to the steps used by Sverchkov (2010). The spray deflectors were shown to reduce the total resistance by about 30% using a Reynolds Averaged Navier–Stokes (RANS) based model with a Volume of Fluid (VOF) method (Olin et al., 2016). However, no comparison with experimental results was done since experiments focusing on the effects of spray are scarce. Wielgosz et al. (2018) tried to validate the results experimentally, however, their results were inconclusive due to lack of repeatability, mainly due to the small wetted surface and the model being fixed in heave and pitch. Numerical modeling of HSC is notoriously difficult and Lotfi et al. (2015) achieved a 5%–31% error in the trim angle compared to Taunton et al. (2010), for a model that was free to heave and pitch. The objective of this study is to improve the knowledge regarding the effects of spray deflection technologies on the resistance and accelerations of HSC. This is done by determining the reduction of the spray resistance due to the redirection of the flow, and by evaluating the efficiency of spray rails and deflectors on a hull specifically designed to generate high spray forces. Furthermore, the accelerations of the hull in waves are measured for three hull configurations; bare hull, spray rails, and deflectors designs. The results from the experimental investigation, that took place in the Davidson Laboratory towing tank, are compared to semi-empirical estimations by Savitsky et al. (2007).
Symbols 𝑎
1 10
𝑎1 3
𝐹 𝑛∇ 𝑔 𝐻 𝐻𝑠 𝐿𝑜𝑎 𝐿𝑤𝑙 𝑅𝑇 𝑇𝑝
Highest one tenth average acc. [g] Highest one third average acc. [g] Volumetric Froude number Gravitational constant [m∕s2 ] Heave measured at CG [m] Significant wave height [m] Model length overall [m] Model wetted length [m] Measured total resistance [N] Significant wave period [s]
The spray area located between the stagnation line and the spray edge is formed by flow reflected forward from the stagnation line. Since the spray is in contact with the hull, frictional resistance is present in the spray area. Therefore, the spray causes an effective increase in the wetted surface area. The resistance can be lowered by forcing detachment of the flow and thus reducing the wetted surface. Clement (1964) showed that total resistance of an HSC can be reduced by detaching the flow in the spray area, showing a reduction of the total resistance of up to 18% using spray rails. The same principle has shown to be true for stepped hulls, where the step detaches the flow and thus reduces the spray area and the associated resistance (Savitsky and Morabito, 2010; Faltinsen, 2006). There is a limited amount of experimental evaluation of spray rails. Early investigations include Clement (1964), with recent experimental investigations of spray rails having been conducted in South Korea (Lee et al., 2010; Seo et al., 2016). Spray deflectors are a novel method of redirecting the spray and thus reducing the spray area. The spray deflectors are essentially a step located in the spray area that reflects the oncoming spray sheet aftward and downward. Equally, there are few experimental studies on the effect of stepped hulls on the performance of HSC, in fact there is only one series of systematic evaluations of stepped hulls, by Taunton et al. (2010). They studied the influence of transverse steps on the performance characteristics of HSC, and showed a significant reduction in resistance, for both single and double steps. Cucinotta et al. (2017) also showed that a transverse step reduced the drag resistance, however, this was only evident at higher Froude numbers. A unique insight to the flow phenomena at the step was gained by De Marco et al. (2017) who conducted experiments on a stepped hull with a transparent bottom. Unlike the transverse steps
2. Experimental setup 2.1. Spray deflection technologies The focus of the present study is to compare performance of two existing spray deflection technologies: spray rails and spray deflectors. The working principles of the technologies are shown in Fig. 2. For bare hull configuration (BH), top of Fig. 2, 100% of the spray area is present, so BH serves as a baseline for the comparison of spray rails and deflectors. The spray rails configuration (SR) is shown in the middle of Fig. 2. Similar to the experiment set up by Clement (1964), the spray rails in this study are short and located in the expected spray area. The rails are longitudinal stripes running parallel to the keel line and have a sharp edge that facilitates flow separation from the hull surface. The design guidelines by Savitsky et al. (2007) were used for the geometry and placement of the spray rails. To this date, the spray rails technology has been widely used in the industry, but its efficiency could be improved by harvesting kinetic energy from the otherwise wasted transversal flow component of the spray. The deflectors configuration is shown at the bottom of Fig. 2. Unlike spray rails, the deflectors cannot be retrofitted. The spray deflector is a step designed for a specific speed range — it is most effective when placed in the spray area, close and parallel to the expected stagnation 2
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Fig. 3. Side renderings of the four hull configurations. From top to bottom: BH, SR, SD1, SD2.
Fig. 4. Assembled hull bottom configurations. From left to right: BH, SR, SD1. Fig. 2. Top: Bare hull (BH) with 100% of the spray area present. Middle: Spray rails (SR) only affect the vertical flow component, thus reducing wetted area and generating lift. Bottom: Spray deflectors (SD) affect horizontal and vertical flow components, thus reducing wetted area, generating extra lift and thrust.
of the hull to resemble Savitsky (1964), Savitsky et al. (2007) semiempirical model. The bow shape is curved to ensure fair hydrodynamic characteristics in waves and at pre-planing speeds where the bow is in contact with water. Due to the selected prismatic hull form, the results of this study are presumably partially valid for warped hull forms, which are a common design choice.
Table 1 Main dimensions of hull. Parameter
Value
Deadrise: 𝛽 [deg] Beam: B [m] Length overall: 𝐿𝑜𝑎 [m] Length of waterline: 𝐿𝑤𝑙 [m] Displacement: 𝛥 [kg] Displaced volume: ∇ [m3 ] 𝐿𝑤𝑙 ∕𝐵 [–] LCG from stern [m]
20 0.36 1.80 1.68 22.68 0.021 4.67 0.55
2.3. Design and material The model was milled out of marine grade closed cell foam. The exterior of the hull was coated with several layers of epoxy and spray paint to improve structural integrity and reduce surface roughness. A 12.7 mm deep pocket was milled out of the bottom of the prismatic hull section to house PVC plate inserts. The inserts were used to change between the bare hull and the deflectors hull configurations. For the BH, the insert was a flat plate occupying the entire pocket volume. Conversion from BH to SR configuration was done by attaching three equally spaced short spray rails to the bare hull on both sides of the hull. The inserts for the spray deflectors (SD1 and SD2) occupy the pocket area forward of the expected stagnation lines. Since the selected speed range is broad and spray deflectors are designed only for a specific design speed, two deflectors configurations SD1 and SD2 were built with design speeds 𝐹 𝑛∇ = 4.3 and 5.87 respectively, while only one SR configuration was needed as the SR were manufactured long enough for all speeds and stagnation lines. The spray area and stagnation line were predicted using (Savitsky et al., 2007). Side renderings of the hull configurations are shown in Fig. 3 with the bottom view of the three hull configurations shown in Fig. 4.
line. The spray deflectors work by reflecting the oncoming spray sheet aftward and downward. Therefore, when located close to the stagnation line, deflectors may remove an even larger portion of the spray area compared to spray rails. In addition, deflectors generate both lift and thrust from the kinetic energy contained in the spray. 2.2. Model parameters A modular hull design was selected in order to accommodate all configurations in a single model. This allows for all of the equipment to be attached to the main model, limiting re-calibration of sensors when configurations are changed. Additionally, the modular design helps to achieve the same model scale, geometrical and mass properties for all configurations. The hull dimensions are shown in Table 1. The main particulars of the hull are based on Variant 3 of the United States Coast Guard’s 47 ft Motor Lifeboat (Soletic, 2010). The main body is prismatic without chine-flats in order for the wetted part
2.4. Instrumentation and measurements The experiments were performed in the Davidson Laboratory high speed towing tank. The tank is 95 m long, 5 m wide and has variable 3
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Fig. 5. Test matrix for the four hull configurations. The BH, SR, SD1 and SD2 configurations are compared at speeds shown in magenta boxes. The interim testing speeds for SD1 and SD2 are enclosed in dashed boxes. The low BH speeds are shown separately in black boxes. Speeds are shown in 𝐹 𝑛∇ .
depth of around 1.9 m. The towing carriage can achieve a constant test speed of about 18 m/s. During the calm water tests, the heave at LCG, trim and total resistance of the model were measured. For the irregular waves tests the number of pitch and heave encounters was recorded in accordance with ITTC recommendations (ITTC, 2017b) and the vertical accelerations at the bow and LCG were measured. The instrumentation and sensor details and exact locations are shown in Appendix A.
Fig. 6. Calm water drag results for the four hull configurations. BH blue open circles, SR red asterisks, SD1 green diamonds & SD2 purple squares.
2.5. Calm water tests The purpose of calm water tests was to determine the effects of spray deflection technologies on the total resistance. Therefore, the displacement and running position of the different hull configurations should ideally match at the same towing speeds. Thus, drag generated in the pressure and dry area should be the same for all configurations tested at the same speed. The difference in total resistance is a direct result of differences in the spray area drag between the hull configurations. For each test, the total resistance, dynamic trim and heave at the LCG position were measured. To ensure repeatability of the results, each test speed was repeated at least two times for each hull configuration. Each run was filmed from the aft and starboard with overwater and underwater pictures taken during the steady planing state. A total of 5 speeds were selected for comparison of the three hull types. The BH and SR configurations were tested at speeds boxed in magenta in Fig. 5. SD1 and SD2 were compared to BH and SR configurations at their design speeds and at two adjacent speeds. The configurations SD1 and SD2 were also towed at interim speed intervals (𝐹 𝑛∇ =3.91, 4.70, 5.48 & 6.26) to check for any irregularities in the running position and measured drag associated with the stagnation line crossing the deflectors. Finally, the BH configuration was validated using the semi-empirical method presented by Savitsky et al. (2007). Therefore, lower speeds shown separately on the left of Fig. 5 were tested for the BH configuration to allow for comparison with the predicted performance.
Fig. 7. Percentage drag reduction for SR, SD1, and SD2 compared to BH as a function of 𝐹 𝑛∇ .
operating conditions for offshore planing vessels. The hull configurations were towed in head seas generated using a 2-parameter Pierson– Moskowitz (P&M) spectrum. The spectrum had a significant wave height 𝐻𝑠 of 96 mm and a period 𝑇𝑝 of 1.557 s. In accordance with ITTC recommendations (ITTC, 2017b), the models were towed at the same speed until an average of 100 heave and pitch encounters was reached. The average highest third 𝑎1∕3 and highest tenth 𝑎1∕10 accelerations were calculated from the top 1/3rd and 1/10th measured accelerations.
2.6. Irregular wave tests The irregular wave tests were designed to assess the influence of spray deflection technologies on the magnitude of impact accelerations. The hull configurations were loaded to the same displacement and LCG position. During the tests, accelerations at the bow and near the LCG were measured, see A for exact locations. The BH, SR and SD1 configurations were tested in waves at 𝐹 𝑛∇ =1.47 and 4.30. The lower speed corresponds to Speed to Length Ratio (SLR) 2 and is suited for large motion amplitudes. The higher speed is the design speed for the SD1 deflectors and results in high impact accelerations (Savitsky and Brown, 1976). SD2 was not tested in waves as its design speed is not suitable for operation in waves. For the wave spectrum, sea conditions associated with a Beaufort scale force of 2 were selected, because they are frequently encountered
3. Results and discussion 3.1. Results of calm water tests In this section, the towing speeds, total resistance, and heave are shown in non-dimensional units 𝐹 𝑛∇ , 𝑅𝑇 ∕𝛥 and 𝐻∕∇1∕3 respectively. The colors in the figures are consistent for each configuration, where blue stands for BH, red for SR, green for SD1, and purple for SD2. The calm water tests show a significant reduction in total resistance for both spray rails and deflectors technologies, as shown in Fig. 6. Fig. 7 shows comparison of SR, SD1 and SD2 against BH configuration. 4
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Fig. 8. Photos showing the spray along the hull of the model in different conditions. The top picture is the BH configuration, the middle picture is the SR configuration, and the bottom picture is the SD1 configuration. Arrows indicate the location of the commencement of spray for each hull configuration.
Fig. 9. Calm water dynamic trim results for the four hull configurations.
The bars represent percentage decrease in total resistance for each configuration when compared to BH drag. The deflectors have shown consistent drop in total resistance with increasing speed. SD1 and SD2 reduced drag by 14.5% and 20% respectively at design speeds. At overlap speed 𝐹 𝑛∇ 5.09, the SD2 removed about 2% more drag than SD1. This slight variation in resistance may have been caused by differences in deflectors location or by small variations in dynamic trim angle. At higher speeds, the total resistance of SR has decreased by up to 9%. The spray rails were submerged at 𝐹 𝑛∇ 3.52 causing a slight increase in resistance. All results are summarized in Table 2. Fig. 8 shows running position and spray pattern for BH, SR and SD1 configurations at 𝐹 𝑛∇ 4.3. The arrows in Fig. 8 show the differences in the spray patterns. For BH, there is a thin sheet of spray separating from the chine forward of the LCG (heave post). For the SR configuration, the spray separates approximately at the LCG and additional spray sheets are created at the spray rails. Finally, for SD1, the spray separates aft of the LCG. The spray area is dry and the main spray is more turbulent than in the other two cases. Despite encouraging results, the reduction in the total resistance for the deflectors configurations cannot be attributed to the deflectors interacting with the spray area alone. As seen in Figs. 9 and 10, the running trim of SD1 and SD2 configurations was higher than the trim of the BH and SR by 0.8–1.2◦ . An attempt to adjust the LCG of BH to match the trims of SD1 and SD2 resulted in porpoising. The higher trim means the pressure and dry area are not consistent across all hull configurations and a direct comparison is not feasible. Therefore, SD1 and SD2 have reduced the total resistance of the hull by both deflecting the oncoming spray and by causing the hull to attain a more favorable trim angle. For the SR configuration, on the other hand, the difference in trim is negligible, so the total resistance can be directly compared to the BH configuration. The major source of trim discrepancies is the location of the spray deflectors, which were designed for the stagnation line predicted using Savitsky method (Savitsky et al., 2007). From Judge et al. (2017) we know that small changes in the hull shape can have a large difference in the trim of a model. As discussed in the next section, the semi-empirical method is insufficient to accurately predict BH’s running position. The BH and SR were tested first and achieved almost the same running position, with the exception of heave shown in Fig. 11, which left no room for adjustments to deflectors’ design when significant offset in trim was observed. In contrast to dynamic trim, which was changing at the same rate for the three configurations (Fig. 9), the heave of SD1 shown in Fig. 11 is almost the same as heave of SR for lower 𝐹 𝑛∇ but it deviates and approaches BH at 𝐹 𝑛∇ 5.09.
Fig. 10. Dynamic trim deviation of SR, SD1, & SD2 from the BH trim. The SR achieved similar trim values (0.1◦ deviation at most) while SD1 and SD2 resulted in consistently higher trim values.
Fig. 11. Calm water heave results for three hull configurations.
5
Ocean Engineering 191 (2019) 106482
B. Molchanov et al. Table 2 Experimental measurements of drag, trim and heave in calm water tests for all hull configurations. 𝐹 𝑛∇
3.52 3.91 4.30 4.70 5.09 5.48 5.87 6.26 6.65
𝑅𝑇 ∕𝛥
𝐻∕𝛥1∕3
𝜏
BH
SR
SD 1
SD 2
BH
SR
SD 1
SD 2
BH
SR
SD 1
SD 2
0.19 – 0.24 – 0.29 – 0.37 – 0.46
0.19 – 0.22 – 0.27 – 0.33 – 0.42
0.17 0.18 0.20 0.22 0.25 – – – –
– – – – 0.24 0.27 0.29 0.32 0.34
4.28 – 3.37 – 2.65 – 2.02 – 1.53
4.30 – 3.32 – 2.55 – 2.01 – 1.52
5.14 4.67 4.22 3.90 3.65 – – – –
– – – – 3.59 3.34 3.16 2.95 2.75
0.124 – 0.142 – 0.153 – 0.160 – 0.165
0.133 – 0.152 – 0.163 – 0.170 – 0.175
0.131 0.142 0.149 0.154 0.155 – – – –
– – – – – – – – –
3.2. Calm water experiments in comparison to semi-empirical model The BH experimental results were compared with Savitsky method (Savitsky et al., 2007). The theoretical total resistance, estimated using model parameters from Table 1, is plotted as a black continuous line, and the experimental results are plotted as blue markers in Fig. 12. It is seen that at 𝐹 𝑛∇ 4.3 the theoretical drag starts to significantly deviate from the experimental results. The Savitsky method (Savitsky et al., 2007) uses semi-empirical relationships to first determine the dynamic trim and wetted area of the hull, and then calculates the total resistance as a sum of spray, wetted, and dry area resistance. Additional total resistance calculations were made using the Savitsky method (Savitsky et al., 2007), but this time parameters from Table 1 were supplemented with experimentally measured trim and wetted area. The results are plotted as orange markers in Fig. 12. The underprediction error of theoretical (black curve) and semitheoretical (orange markers) methods is plotted in Fig. 13 in corresponding colors. Here, the experimental results are taken as a baseline or 100%, the calculated resistance is a share of the 100% (X<100%), and the plotted error is a relative difference between the baseline and the calculated resistance (100%-X%). As seen in Fig. 13, the theoretical method agrees well with the experimental results for a typical range of HSC speeds between 1.47< 𝐹 𝑛∇ <3.52 where the relative error is 3%–5%. At 𝐹 𝑛∇ 1.47, the hull is in the semi-planing regime, resulting in a violation of the semiempirical model; the underprediction in the semi-planing regime has been noted by other researchers (Blount and Fox, 1976). Thus, an error of higher magnitude was anticipated. However, at 𝐹 𝑛∇ >4.30 the theoretical error increases beyond 10%. At these speeds, the dynamic trim starts to approach and exceeds 2◦ , which is the limit of the semi-empirical model. Therefore, the semi-theoretical calculations significantly improve the agreement with experimental values at higher speeds. The prediction error at 𝐹 𝑛∇ 4.30, is comparable in magnitude to drag reduction achieved by SR and SD1 at the same speed. This implies that at higher speeds, application of the semi-empirical model for reverse-calculation of resistance components from the measured total resistance yields inaccurate results and can only be used for indicative purposes.
Fig. 12. Total resistance as a function of 𝐹 𝑛∇ . Determined theoretically using the Savitsky Method (Savitsky et al., 2007) (black line), semi-theoretically using the running position from experiments (orange squares), and experimentally obtained resistance (blue open circles).
extreme value while the average should be less affected by the number of encounters. Statistically, the probability of exceeding 𝑎1∕3 and 𝑎1∕10 is 12% and 4%, respectively (Savitsky and Brown, 1976). This means that significant differences in 𝑎1∕3 and 𝑎1∕10 accelerations experienced by each hull configuration are most likely due to differences in the hull bottom geometry as both SR and SD1 are designed to increase lift compared to BH, as noted from the calm water run heave results (see Fig. 11). On the other hand, the formula developed for impact accelerations in waves by Savitsky and Brown (1976) included the calm water trim (𝜏∕4). This means that the difference in trim observed between the SD1 configuration and the other configurations during the calm water tests could also account for the difference in accelerations during the wave tests. 4. Conclusions
3.3. Irregular wave test results In this paper, the performance of spray deflection technologies – spray rails and spray deflectors – was compared through towing tank testing in calm water and irregular waves. The results presented in this paper are most accurate for the selected hull form and proportions but may be partially valid for varying proportions of prismatic and warped hull forms. The calm water tests have shown a significant reduction in drag for both technologies. Spray rails resulted in drag reductions of up to 9% and only affected the heave of the model. The deflectors configurations yielded even higher reductions in drag ranging from 10% to 25%, but the dynamic trim of the hull differed significantly from the other two configurations.
For irregular waves tests, the vertical accelerations of the BH, SR and SD1 configurations is shown in Figs. 14–17 and Tables 3 and 4. Accelerations at the bow are fairly constant for all configurations (see Figs. 14 and 16), except for the extreme value at the higher tested speed where both SR and SD1 experience higher acceleration than the BH (see Fig. 16). Accelerations at the LCG were greater than BH for both the SR and SD1 configurations, as shown in Figs. 15 and 17. The highest third is most representative of the experienced accelerations while the highest extreme represents the worst-case scenario. The extreme value is more sensitive to the number of encounters: a larger number of encounters may mean a higher change of a larger 6
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B. Molchanov et al. Table 3 Acceleration comparisons for wave runs, 𝐹 𝑛∇ = 1.47. LCG comparison
Bow comparison
Bare Hull
Spray Rails
Deflectors 1
Bare Hull
Spray Rails
Deflectors 1
0.37
+0.08
+0.15
1.48
+0.01
−0.18
[g]
0.56
+0.15
+0.34
2.55
+0.03
+0.09
Extreme [g]
1.09
+0.4
+1.49
4.92
+0.1
+0.11
𝑎 1 [g] 3
𝑎
1 10
Table 4 Acceleration comparisons for wave runs, 𝐹 𝑛∇ = 4.3. LCG comparison
Bow comparison
Bare Hull
Spray Rails
Deflectors 1
Bare Hull
Spray Rails
Deflectors 1
1.01
+0.08
+0.09
2.04
+0.01
−0.28
[g]
1.6
+0.2
+0.33
3.64
−0.03
−0.36
Extreme [g]
3.3
+0.79
+2.98
6.88
+0.87
+0.95
𝑎 1 [g] 3
𝑎
1 10
Fig. 15. Accelerations measured at LCG, 𝐹 𝑛∇ = 1.47. Fig. 13. Comparison of errors in resistance estimation using predicted running position (black) and measured running position (orange) with the experimentally measured resistance.
Fig. 16. Accelerations measured at Bow, 𝐹 𝑛∇ = 4.3.
Fig. 14. Accelerations measured at Bow, 𝐹 𝑛∇ = 1.47.
individual effects cannot be accurately isolated unless the same running trim angle is achieved for all configurations. The present study shows that the semi-empirical model (Savitsky et al., 2007) is insufficient for accurate prediction of running position even for prismatic hulls. Additional CFD analysis or directly measured experimental data should be used for the design of the spray deflectors.
For the tested deflectors configurations, the significant reduction in total resistance is viewed as a combined effect of deflectors redirecting the spray sheet and causing a change in dynamic trim of the hull. This provides insight into how deflectors affect overall performance, but 7
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B. Molchanov et al. Table A.1 Sensor, ballast and plate weights and locations. Nr
Unit Name
XCG [mm]
YCG [mm]
Mass [kg]
1 2 3 4 5 6 7 8 9 10a 10b 11a 11b 11c 11d
Foam Hull Inclinometer Pitch Pivot Box Drag Balance Attachment Bar Heave Post CG Accelerometer Bow Accelerometer Aft Ballast CG Ballast CG Unload Spring Bare Hull Plate Spray Rails SLR 5.9 Plate SLR 8 Plate
787.4 317.5 548.6 548.6 548.6 548.6 444.5 1318.3 76.2 548.6 548.6 650.2 825.5 1104.9 977.9
114.3 152.4 158.8 222.3 304.8 685.8 151.1 151.1 177.8 431.8 – 63.5 63.5 71.1 73.7
9.59 0.35 1.19 2.69 0.71 2.69 0.13 0.13 Varies Varies Varies 3.27 0.09 1.22 1.35
Fig. 17. Accelerations measured at LCG, 𝐹 𝑛∇ = 4.3.
The wave tests have shown that the current deflectors design does not significantly affect the vertical accelerations compared to spray rails at the bow. At the LCG, SD1 has a significantly higher acceleration compared to SR, especially the highest extreme value. In general, for the deflectors, the data shows lowered average accelerations at the bow, increased average acceleration at the LCG and increased extreme accelerations across all conditions.
Fig. A.1. Positions of sensors and weights in model.
5. Further work Drag and pitch
Additional experiments are required with improved deflectors placement and geometry for more direct comparison of spray rails and deflectors configurations in waves. The spray deflectors geometry could be improved if their thickness would taper from a maximum at the chine to almost zero at the keel line. Correctly located and tapered deflectors may eliminate the problem of the stagnation line crossing the deflectors near the chine and being too far from the deflectors at the keel. Since the thickness of the tapered deflectors is practically zero at the keel, it will also eliminate problems associated with the tip of the deflectors re-entering the water. Finally, more insight is required to understand the spray phenomenon. The current spray force prediction method is based on limited semi-empirical data presented in Savitsky et al. (2007). A method for direct measurement of the spray drag for a range of speeds, running positions, and model sizes would be invaluable for further development of the spray deflection technologies.
To measure the drag, a Linear Variable Differential Transformer equipped drag balance is used, rated for 222.4 N (50 lbf) meaning that it can measure forces up to 222 N, with an absolute error of ±0.06 N. The drag balance is then mounted on a pitch pivot box which measures the pitch angle through a rotational potentiometer. This measurement is used for the wave runs to accurately measure the number of waves the hull encounters and is calibrated statically to within ±0.0161 deg. The drag balance is number 4 in Table A.1 and Fig. A.1 while the pitch pivot box is number 3. The pitch pivot box with the drag balance is mounted to the heave post and to the hull and thus becomes the place where the thrust from the monorail carriage is transferred to the hull.
Heave
Acknowledgments This research would not have been possible without the support of the faculty, staff and students of Davidson Laboratory at Stevens Institute of Technology, especially Professor Raju Datla and Uihoon Chung who have been most helpful. Further, we would like to thank Jonas Danielsson at Petestep AB, Sweden, for many helpful discussions regarding stepped hulls.
The heave is measured from a potentiometer mounted by the heave post calibrated to within ± 0.2 mm. The heave post and sensor is number 6 in Table A.1 and in Fig. A.1.
Accelerometers
Appendix A. Sensors and measurements
For the waves, a set of accelerometers are used to measure the acceleration at certain points along the hull. The sensors are 30g Schaevitz closed loop torque-balance transducers which can be mounted anywhere on the hull. In this application, one is put as close to the LCG position as possible and another is mounted a distance forwards closer to the bow as shown in Fig. A.1. The accelerometers are calibrated statically to within ±0.03g. The accelerometers are numbers 7 and 8 in Table A.1 and in Fig. A.1.
For calm water the interesting measurements are drag, pitch and heave. For wave runs the focus is on accelerations at the CG, accelerations at the bow, number of wave encounters, and number of wave peaks and troughs. Data was collected over 120 ft for each run. An unload spring was used at the CG to help control model weight; the spring does affect the dynamic pitching motions in waves but does not affect calm water runs. 8
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B. Molchanov et al. Table B.1 Total combined uncertainty, 𝑢′𝑐 ,in percent for each hull and speed in 𝐹 𝑛𝑉 with reference to resistance. 𝐹 𝑛∇
3.52
BH SR SD1 SD2
0.42 0.16 0.18
3.91
4.30
0.18
0.62 0.13 0.15
4.70
0.39
5.09 0.82 1.43 0.69 0.77
5.48
5.87
6.26
0.33 1.87 0.42
0.48
Clement, E.P., 1964. Reduction of Planing Boat Resistance by Deflection of the Whisker Spray. Technical Report DTMB-1929, David Taylor Model Basin, pp. 1–23. Cucinotta, F., Guglielmino, E., Sfravara, F., 2017. An experimental comparison between different artificial air cavity designs for a planing hull. Ocean Eng. 140, 233–243. De Marco, A., Mancini, S., Miranda, S., Scognamiglio, R., Vitiello, L., 2017. Experimental and numerical hydrodynamic analysis of a stepped planing hull. Appl. Ocean Res. 64, 135–154. Faltinsen, O.M., 2006. Hydrodynamics of High-Speed Marine Vehicles. Cambridge University Press, Cambridge. ITTC, 2017a. 28th International Towing Tank Conference, General Guide for Uncertainty Analysis in Resistance Tests. In: Recommended Procedures and Guidelines, vol. 7.5-02-02-02. ITTC, 2017b. 28th International Towing Tank Conference, High Speed Marine Vehicles, Seakeeping Tests. In: Recommended Procedures and Guidelines, vol. 7.5-02-05-04. Judge, C., Beaver, B., Zseleczky, J., 2017. The 30th American Towing Tank Conference. In: An Evaluation of Planing Boat Trim Measurements, ATTC, West Bethesda, Maryland. Larsson, L., Hoyte, R.C., 2010. The Principles of Naval Architecture Series: Ship Resistance and Flow, first ed. The Society of Naval Architects and Marine Engineers. Lee, J.-G., Jung, K.-H., Suh, S.-B., Ho-Hwan, C., Lee, I., 2010. A study on the hull form design of semi-planing round-bilge craft. J. Ocean Eng. Technol. 24, 59–65. Lotfi, P., Ashrafizaadeh, M., Esfahan, R.K., 2015. Numerical investigation of a stepped planing hull in calm water. Ocean Eng. 94, 103–110. Olin, L., Altimira, M., Danielsson, J., Rosen, A., 2016. Numerical modelling of spray sheet deflection on planing hulls. Proc. Inst. Mech. Eng. A 231 (4), 811–817. Payne, P.R., 1982. The spray volume shed by an uncambered planing hull in steady planing. Ocean Eng. 9 (4), 373–384. Savitsky, D., 1964. Hydrodynamic design of planing hulls. Mar. Technol. 1 (1), 71–95. Savitsky, D., Brown, P.W., 1976. Procedures for hydrodynamic evaluation of planing hulls in smooth and rough water. Mar. Technol. 13 (4), 381–400. Savitsky, D., DeLorme, M.F., Datla, R., 2007. Inclusion of whisker spray drag in performance prediction method for high-speed planing hulls. Mar. Technol. 44 (1), 35–56. Savitsky, D., Morabito, M., 2010. Surface wave contours associated with the forebody wake of stepped planing hulls. Mar. Technol. 47, 1–16. Seo, J., Choi, H.-K., Jeong, U.-C., 2016. Model tests on resistance and seakeeping performance of wave-piercing high-speed vessel with spray rails. Int. J. Nav. Archit. Ocean Eng. 8 (5), 442–455. Soletic, L., 2010. Seakeeping of a systematic series of planing hulls. In: The Second Chesapeake Power Boat Symposium. Sverchkov, A., International conference on ship drag reduction (SMOOTH-ships). In: Application of Air Cavities on High-Speed Ships in Russia, 2010, Istanbul, Turkey. Taunton, D.J., Hudson, D.A., Shenoi, R., 2010. Characteristics of a series of high speed hard chine planing hulls-part 1: performance in calm water. Int. J. Small Craft Technol. 152, 55–75. Wielgosz, C., Fürth, M., Datla, R., Chung, U., Rosen, A., Danielsson, J., Experimental validation of numerical drag prediction of novel spray deflector design. In: Proceedings 13th International Marine Design Conference, 2018, pp. 491–497. Yousefi, R., Shafaghat, R., Shakeri, M., 2013. Hydrodynamic analysis techniques for high-speed planing hulls. Appl. Ocean Res. 42, 105–113.
6.65 0.07
0.53
0.96
Inclinometer To measure the static and running trim, a gravity referenced inclinometer is used. Made by Schaevitz, it is their LSO series which is a fully self-contained, fluid damped, flexure suspension, servo inclinometer calibrated to within ± 0.0005 deg. The inclinometer is number 2 in Table A.1 and Fig. A.1. Sensor positions The appendages, such as sensors and plates, and their positions are listed in Table A.1 and shown in Fig. A.1. In Table A.1, XCG is the distance measured from the stern to the center point of each appendage and YCG is measured from the keel. Appendix B. Error and repeatability The measurements shown are the averages of the measurements for the repeated experiments. The inherent uncertainty from sensors and fluctuations in the environment can be combined to predict the total uncertainty of the measurements. In this paper the ITTC’s guidelines for calculating the uncertainty of measured data points was used (ITTC, 2017a). The total combined uncertainty, 𝑢′𝑐 , for each speed and hull in percent is shown in Table B.1.
References Begovic, E., Bertorello, C., 2012. Resistance assessment of warped hullform. Ocean Eng. 56, 28–42. Blount, D.L., Fox, D.L., 1976. Small-craft power prediction. Mar. Technol. 13, 14–45.
9
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Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Experimental validation of spray deflectors for high speed craft Bogdan Molchanov a,b,c , Svante Lundmark b,c , Mirjam Fürth c ,∗, Mathew Green c a
Aalto University, Espoo, Finland KTH Royal Institute of Technology, Stockholm, Sweden c Stevens Institute of Technology, Hoboken, NJ, USA b
ARTICLE Keywords: High speed craft Spray Resistance Towing tank Planing craft
INFO
ABSTRACT A planing craft is a type of marine vehicle that is supported predominantly by hydrodynamic forces at high speeds. It has widespread applications in recreational and military use as well as for search and rescue operations. Although spray rails are a mature technology, there is limited experimental data illustrating their hydrodynamic effects. Furthermore, continuous efforts are being made to improve the efficiency, safety and performance of planning craft. In this paper, the efficiency of the spray resistance reduction technologies – spray rails and spray deflectors – are compared in model scale through towing tank testing in calm water and irregular waves. This study examines the influences of the technologies on the total resistance and running position of the hull in calm water and on impact accelerations experienced at the center of gravity and bow in irregular waves. The experimental results are discussed in connection with established semi-empirical methods for predicting the performance of planing craft and improvements for further testing of the spray deflection methods are proposed.
1. Introduction High Speed Craft (HSC) have many military and civilian uses, they are used for defensive purposes, for search and rescue, and for recreation among other things. These vessels allow for speedy operation in archipelagos and coastal waters where islands and other obstacles are plentiful and the water is shallow. With the recent increases in the power to weight ratio of outboard engines and the use of composite materials for hull structures, the use of HSC has become even more widespread (De Marco et al., 2017). With a larger and broader user base comes an increased need for technical improvements aimed specifically at HSC. The last 30 years have seen an increase in research focusing on HSC but many flow phenomena are still not fully understood and relatively few publications exist that address this issue. The hull of an HSC acts as a lifting surface to lift the craft out of its displacement mode. This causes a significant reduction in the wetted surface area, and thus, the resistance. This reduction allows the craft to reach high speeds without a linear increase in power consumption, unlike a displacement hull. As the speed increases, the hull moves from displacement mode through transition mode to reach planing mode. The resistance of an HSC can be decomposed into induced drag, appendage resistance, frictional resistance, aerodynamic resistance and spray resistance (Larsson and Hoyte, 2010). Payne (1982) concluded that wave making resistance tends to zero in the planing speed region while spray resistance becomes significant. Savitsky et al. (2007) show
that spray resistance can constitute over 15% of the total resistance at planing speeds. The flow dynamics around HSC can be predicted using theoretical or experimental modeling. Theoretical modeling can be grouped into numerical methods such as potential flow-based methods, viscous flow based methods and semi-empirical techniques (Yousefi et al., 2013). The method by Savitsky (1964) is the most widely used semiempirical method for evaluation of HSC and has over the past decades been improved through further model testing at the Davidson Laboratory (Savitsky and Brown, 1976; Savitsky et al., 2007). However, the flow around HSC is difficult to simulate, especially when taking dynamic instabilities into consideration. Therefore, the main method of evaluating the performance of HSC is often towing tank testing (Yousefi et al., 2013). Because of the high importance of the spray resistance, as mentioned earlier, specialized methods and designs exist for predicting and reducing spray. Such designs include spray rails and spray deflectors which both serve to reduce the spray area. Accurately evaluating the efficiency of such designs is therefore of high importance for future improvements to HSC design. A method of predicting the spray area, its contribution to total drag, and design guidelines for locating spray rails was published by Savitsky et al. (2007) and Begovic and Bertorello (2012). Some of the key definitions and areas used by Savitsky (1964) are shown in Fig. 1.
∗ Corresponding author. E-mail address: [email protected] (M. Fürth).
https://doi.org/10.1016/j.oceaneng.2019.106482 Received 30 April 2019; Received in revised form 26 July 2019; Accepted 20 September 2019 Available online 3 October 2019 0029-8018/© 2019 Elsevier Ltd. All rights reserved.
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Nomenclature Acronyms 𝐵𝐻 𝐶𝐺 𝐻𝑆𝐶 𝐼𝑇 𝑇 𝐶 𝐿𝐶𝐺 𝑆𝐷1 𝑆𝐷2 𝑆𝐿𝑅
Bare Hull Center of Gravity High Speed Craft International Towing Tank Conference Longitudinal Center of Gravity [m] Spray Deflector Configuration 1 Spray Deflector Configuration 2 𝑉𝑠 Speed Length Ratio [ √ ] 2
𝑆𝑅
Spray Rails
𝐿𝑤𝐿
Fig. 1. Underwater view of the planing hull: spray area is bounded by the dry area in the forepart and by the pressure area in the aft.
Greek symbols 𝛽 𝛥 ∇ 𝜏
Deadrise angle [deg] Ship displacement [N] Displaced volume [m3 ] Trim angle [deg]
used by De Marco et al. (2017) and Taunton et al. (2010) the deflectors run at an angle to the center line, are straight, and have no curvature in contrast to the steps used by Sverchkov (2010). The spray deflectors were shown to reduce the total resistance by about 30% using a Reynolds Averaged Navier–Stokes (RANS) based model with a Volume of Fluid (VOF) method (Olin et al., 2016). However, no comparison with experimental results was done since experiments focusing on the effects of spray are scarce. Wielgosz et al. (2018) tried to validate the results experimentally, however, their results were inconclusive due to lack of repeatability, mainly due to the small wetted surface and the model being fixed in heave and pitch. Numerical modeling of HSC is notoriously difficult and Lotfi et al. (2015) achieved a 5%–31% error in the trim angle compared to Taunton et al. (2010), for a model that was free to heave and pitch. The objective of this study is to improve the knowledge regarding the effects of spray deflection technologies on the resistance and accelerations of HSC. This is done by determining the reduction of the spray resistance due to the redirection of the flow, and by evaluating the efficiency of spray rails and deflectors on a hull specifically designed to generate high spray forces. Furthermore, the accelerations of the hull in waves are measured for three hull configurations; bare hull, spray rails, and deflectors designs. The results from the experimental investigation, that took place in the Davidson Laboratory towing tank, are compared to semi-empirical estimations by Savitsky et al. (2007).
Symbols 𝑎
1 10
𝑎1 3
𝐹 𝑛∇ 𝑔 𝐻 𝐻𝑠 𝐿𝑜𝑎 𝐿𝑤𝑙 𝑅𝑇 𝑇𝑝
Highest one tenth average acc. [g] Highest one third average acc. [g] Volumetric Froude number Gravitational constant [m∕s2 ] Heave measured at CG [m] Significant wave height [m] Model length overall [m] Model wetted length [m] Measured total resistance [N] Significant wave period [s]
The spray area located between the stagnation line and the spray edge is formed by flow reflected forward from the stagnation line. Since the spray is in contact with the hull, frictional resistance is present in the spray area. Therefore, the spray causes an effective increase in the wetted surface area. The resistance can be lowered by forcing detachment of the flow and thus reducing the wetted surface. Clement (1964) showed that total resistance of an HSC can be reduced by detaching the flow in the spray area, showing a reduction of the total resistance of up to 18% using spray rails. The same principle has shown to be true for stepped hulls, where the step detaches the flow and thus reduces the spray area and the associated resistance (Savitsky and Morabito, 2010; Faltinsen, 2006). There is a limited amount of experimental evaluation of spray rails. Early investigations include Clement (1964), with recent experimental investigations of spray rails having been conducted in South Korea (Lee et al., 2010; Seo et al., 2016). Spray deflectors are a novel method of redirecting the spray and thus reducing the spray area. The spray deflectors are essentially a step located in the spray area that reflects the oncoming spray sheet aftward and downward. Equally, there are few experimental studies on the effect of stepped hulls on the performance of HSC, in fact there is only one series of systematic evaluations of stepped hulls, by Taunton et al. (2010). They studied the influence of transverse steps on the performance characteristics of HSC, and showed a significant reduction in resistance, for both single and double steps. Cucinotta et al. (2017) also showed that a transverse step reduced the drag resistance, however, this was only evident at higher Froude numbers. A unique insight to the flow phenomena at the step was gained by De Marco et al. (2017) who conducted experiments on a stepped hull with a transparent bottom. Unlike the transverse steps
2. Experimental setup 2.1. Spray deflection technologies The focus of the present study is to compare performance of two existing spray deflection technologies: spray rails and spray deflectors. The working principles of the technologies are shown in Fig. 2. For bare hull configuration (BH), top of Fig. 2, 100% of the spray area is present, so BH serves as a baseline for the comparison of spray rails and deflectors. The spray rails configuration (SR) is shown in the middle of Fig. 2. Similar to the experiment set up by Clement (1964), the spray rails in this study are short and located in the expected spray area. The rails are longitudinal stripes running parallel to the keel line and have a sharp edge that facilitates flow separation from the hull surface. The design guidelines by Savitsky et al. (2007) were used for the geometry and placement of the spray rails. To this date, the spray rails technology has been widely used in the industry, but its efficiency could be improved by harvesting kinetic energy from the otherwise wasted transversal flow component of the spray. The deflectors configuration is shown at the bottom of Fig. 2. Unlike spray rails, the deflectors cannot be retrofitted. The spray deflector is a step designed for a specific speed range — it is most effective when placed in the spray area, close and parallel to the expected stagnation 2
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Fig. 3. Side renderings of the four hull configurations. From top to bottom: BH, SR, SD1, SD2.
Fig. 4. Assembled hull bottom configurations. From left to right: BH, SR, SD1. Fig. 2. Top: Bare hull (BH) with 100% of the spray area present. Middle: Spray rails (SR) only affect the vertical flow component, thus reducing wetted area and generating lift. Bottom: Spray deflectors (SD) affect horizontal and vertical flow components, thus reducing wetted area, generating extra lift and thrust.
of the hull to resemble Savitsky (1964), Savitsky et al. (2007) semiempirical model. The bow shape is curved to ensure fair hydrodynamic characteristics in waves and at pre-planing speeds where the bow is in contact with water. Due to the selected prismatic hull form, the results of this study are presumably partially valid for warped hull forms, which are a common design choice.
Table 1 Main dimensions of hull. Parameter
Value
Deadrise: 𝛽 [deg] Beam: B [m] Length overall: 𝐿𝑜𝑎 [m] Length of waterline: 𝐿𝑤𝑙 [m] Displacement: 𝛥 [kg] Displaced volume: ∇ [m3 ] 𝐿𝑤𝑙 ∕𝐵 [–] LCG from stern [m]
20 0.36 1.80 1.68 22.68 0.021 4.67 0.55
2.3. Design and material The model was milled out of marine grade closed cell foam. The exterior of the hull was coated with several layers of epoxy and spray paint to improve structural integrity and reduce surface roughness. A 12.7 mm deep pocket was milled out of the bottom of the prismatic hull section to house PVC plate inserts. The inserts were used to change between the bare hull and the deflectors hull configurations. For the BH, the insert was a flat plate occupying the entire pocket volume. Conversion from BH to SR configuration was done by attaching three equally spaced short spray rails to the bare hull on both sides of the hull. The inserts for the spray deflectors (SD1 and SD2) occupy the pocket area forward of the expected stagnation lines. Since the selected speed range is broad and spray deflectors are designed only for a specific design speed, two deflectors configurations SD1 and SD2 were built with design speeds 𝐹 𝑛∇ = 4.3 and 5.87 respectively, while only one SR configuration was needed as the SR were manufactured long enough for all speeds and stagnation lines. The spray area and stagnation line were predicted using (Savitsky et al., 2007). Side renderings of the hull configurations are shown in Fig. 3 with the bottom view of the three hull configurations shown in Fig. 4.
line. The spray deflectors work by reflecting the oncoming spray sheet aftward and downward. Therefore, when located close to the stagnation line, deflectors may remove an even larger portion of the spray area compared to spray rails. In addition, deflectors generate both lift and thrust from the kinetic energy contained in the spray. 2.2. Model parameters A modular hull design was selected in order to accommodate all configurations in a single model. This allows for all of the equipment to be attached to the main model, limiting re-calibration of sensors when configurations are changed. Additionally, the modular design helps to achieve the same model scale, geometrical and mass properties for all configurations. The hull dimensions are shown in Table 1. The main particulars of the hull are based on Variant 3 of the United States Coast Guard’s 47 ft Motor Lifeboat (Soletic, 2010). The main body is prismatic without chine-flats in order for the wetted part
2.4. Instrumentation and measurements The experiments were performed in the Davidson Laboratory high speed towing tank. The tank is 95 m long, 5 m wide and has variable 3
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Fig. 5. Test matrix for the four hull configurations. The BH, SR, SD1 and SD2 configurations are compared at speeds shown in magenta boxes. The interim testing speeds for SD1 and SD2 are enclosed in dashed boxes. The low BH speeds are shown separately in black boxes. Speeds are shown in 𝐹 𝑛∇ .
depth of around 1.9 m. The towing carriage can achieve a constant test speed of about 18 m/s. During the calm water tests, the heave at LCG, trim and total resistance of the model were measured. For the irregular waves tests the number of pitch and heave encounters was recorded in accordance with ITTC recommendations (ITTC, 2017b) and the vertical accelerations at the bow and LCG were measured. The instrumentation and sensor details and exact locations are shown in Appendix A.
Fig. 6. Calm water drag results for the four hull configurations. BH blue open circles, SR red asterisks, SD1 green diamonds & SD2 purple squares.
2.5. Calm water tests The purpose of calm water tests was to determine the effects of spray deflection technologies on the total resistance. Therefore, the displacement and running position of the different hull configurations should ideally match at the same towing speeds. Thus, drag generated in the pressure and dry area should be the same for all configurations tested at the same speed. The difference in total resistance is a direct result of differences in the spray area drag between the hull configurations. For each test, the total resistance, dynamic trim and heave at the LCG position were measured. To ensure repeatability of the results, each test speed was repeated at least two times for each hull configuration. Each run was filmed from the aft and starboard with overwater and underwater pictures taken during the steady planing state. A total of 5 speeds were selected for comparison of the three hull types. The BH and SR configurations were tested at speeds boxed in magenta in Fig. 5. SD1 and SD2 were compared to BH and SR configurations at their design speeds and at two adjacent speeds. The configurations SD1 and SD2 were also towed at interim speed intervals (𝐹 𝑛∇ =3.91, 4.70, 5.48 & 6.26) to check for any irregularities in the running position and measured drag associated with the stagnation line crossing the deflectors. Finally, the BH configuration was validated using the semi-empirical method presented by Savitsky et al. (2007). Therefore, lower speeds shown separately on the left of Fig. 5 were tested for the BH configuration to allow for comparison with the predicted performance.
Fig. 7. Percentage drag reduction for SR, SD1, and SD2 compared to BH as a function of 𝐹 𝑛∇ .
operating conditions for offshore planing vessels. The hull configurations were towed in head seas generated using a 2-parameter Pierson– Moskowitz (P&M) spectrum. The spectrum had a significant wave height 𝐻𝑠 of 96 mm and a period 𝑇𝑝 of 1.557 s. In accordance with ITTC recommendations (ITTC, 2017b), the models were towed at the same speed until an average of 100 heave and pitch encounters was reached. The average highest third 𝑎1∕3 and highest tenth 𝑎1∕10 accelerations were calculated from the top 1/3rd and 1/10th measured accelerations.
2.6. Irregular wave tests The irregular wave tests were designed to assess the influence of spray deflection technologies on the magnitude of impact accelerations. The hull configurations were loaded to the same displacement and LCG position. During the tests, accelerations at the bow and near the LCG were measured, see A for exact locations. The BH, SR and SD1 configurations were tested in waves at 𝐹 𝑛∇ =1.47 and 4.30. The lower speed corresponds to Speed to Length Ratio (SLR) 2 and is suited for large motion amplitudes. The higher speed is the design speed for the SD1 deflectors and results in high impact accelerations (Savitsky and Brown, 1976). SD2 was not tested in waves as its design speed is not suitable for operation in waves. For the wave spectrum, sea conditions associated with a Beaufort scale force of 2 were selected, because they are frequently encountered
3. Results and discussion 3.1. Results of calm water tests In this section, the towing speeds, total resistance, and heave are shown in non-dimensional units 𝐹 𝑛∇ , 𝑅𝑇 ∕𝛥 and 𝐻∕∇1∕3 respectively. The colors in the figures are consistent for each configuration, where blue stands for BH, red for SR, green for SD1, and purple for SD2. The calm water tests show a significant reduction in total resistance for both spray rails and deflectors technologies, as shown in Fig. 6. Fig. 7 shows comparison of SR, SD1 and SD2 against BH configuration. 4
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Fig. 8. Photos showing the spray along the hull of the model in different conditions. The top picture is the BH configuration, the middle picture is the SR configuration, and the bottom picture is the SD1 configuration. Arrows indicate the location of the commencement of spray for each hull configuration.
Fig. 9. Calm water dynamic trim results for the four hull configurations.
The bars represent percentage decrease in total resistance for each configuration when compared to BH drag. The deflectors have shown consistent drop in total resistance with increasing speed. SD1 and SD2 reduced drag by 14.5% and 20% respectively at design speeds. At overlap speed 𝐹 𝑛∇ 5.09, the SD2 removed about 2% more drag than SD1. This slight variation in resistance may have been caused by differences in deflectors location or by small variations in dynamic trim angle. At higher speeds, the total resistance of SR has decreased by up to 9%. The spray rails were submerged at 𝐹 𝑛∇ 3.52 causing a slight increase in resistance. All results are summarized in Table 2. Fig. 8 shows running position and spray pattern for BH, SR and SD1 configurations at 𝐹 𝑛∇ 4.3. The arrows in Fig. 8 show the differences in the spray patterns. For BH, there is a thin sheet of spray separating from the chine forward of the LCG (heave post). For the SR configuration, the spray separates approximately at the LCG and additional spray sheets are created at the spray rails. Finally, for SD1, the spray separates aft of the LCG. The spray area is dry and the main spray is more turbulent than in the other two cases. Despite encouraging results, the reduction in the total resistance for the deflectors configurations cannot be attributed to the deflectors interacting with the spray area alone. As seen in Figs. 9 and 10, the running trim of SD1 and SD2 configurations was higher than the trim of the BH and SR by 0.8–1.2◦ . An attempt to adjust the LCG of BH to match the trims of SD1 and SD2 resulted in porpoising. The higher trim means the pressure and dry area are not consistent across all hull configurations and a direct comparison is not feasible. Therefore, SD1 and SD2 have reduced the total resistance of the hull by both deflecting the oncoming spray and by causing the hull to attain a more favorable trim angle. For the SR configuration, on the other hand, the difference in trim is negligible, so the total resistance can be directly compared to the BH configuration. The major source of trim discrepancies is the location of the spray deflectors, which were designed for the stagnation line predicted using Savitsky method (Savitsky et al., 2007). From Judge et al. (2017) we know that small changes in the hull shape can have a large difference in the trim of a model. As discussed in the next section, the semi-empirical method is insufficient to accurately predict BH’s running position. The BH and SR were tested first and achieved almost the same running position, with the exception of heave shown in Fig. 11, which left no room for adjustments to deflectors’ design when significant offset in trim was observed. In contrast to dynamic trim, which was changing at the same rate for the three configurations (Fig. 9), the heave of SD1 shown in Fig. 11 is almost the same as heave of SR for lower 𝐹 𝑛∇ but it deviates and approaches BH at 𝐹 𝑛∇ 5.09.
Fig. 10. Dynamic trim deviation of SR, SD1, & SD2 from the BH trim. The SR achieved similar trim values (0.1◦ deviation at most) while SD1 and SD2 resulted in consistently higher trim values.
Fig. 11. Calm water heave results for three hull configurations.
5
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B. Molchanov et al. Table 2 Experimental measurements of drag, trim and heave in calm water tests for all hull configurations. 𝐹 𝑛∇
3.52 3.91 4.30 4.70 5.09 5.48 5.87 6.26 6.65
𝑅𝑇 ∕𝛥
𝐻∕𝛥1∕3
𝜏
BH
SR
SD 1
SD 2
BH
SR
SD 1
SD 2
BH
SR
SD 1
SD 2
0.19 – 0.24 – 0.29 – 0.37 – 0.46
0.19 – 0.22 – 0.27 – 0.33 – 0.42
0.17 0.18 0.20 0.22 0.25 – – – –
– – – – 0.24 0.27 0.29 0.32 0.34
4.28 – 3.37 – 2.65 – 2.02 – 1.53
4.30 – 3.32 – 2.55 – 2.01 – 1.52
5.14 4.67 4.22 3.90 3.65 – – – –
– – – – 3.59 3.34 3.16 2.95 2.75
0.124 – 0.142 – 0.153 – 0.160 – 0.165
0.133 – 0.152 – 0.163 – 0.170 – 0.175
0.131 0.142 0.149 0.154 0.155 – – – –
– – – – – – – – –
3.2. Calm water experiments in comparison to semi-empirical model The BH experimental results were compared with Savitsky method (Savitsky et al., 2007). The theoretical total resistance, estimated using model parameters from Table 1, is plotted as a black continuous line, and the experimental results are plotted as blue markers in Fig. 12. It is seen that at 𝐹 𝑛∇ 4.3 the theoretical drag starts to significantly deviate from the experimental results. The Savitsky method (Savitsky et al., 2007) uses semi-empirical relationships to first determine the dynamic trim and wetted area of the hull, and then calculates the total resistance as a sum of spray, wetted, and dry area resistance. Additional total resistance calculations were made using the Savitsky method (Savitsky et al., 2007), but this time parameters from Table 1 were supplemented with experimentally measured trim and wetted area. The results are plotted as orange markers in Fig. 12. The underprediction error of theoretical (black curve) and semitheoretical (orange markers) methods is plotted in Fig. 13 in corresponding colors. Here, the experimental results are taken as a baseline or 100%, the calculated resistance is a share of the 100% (X<100%), and the plotted error is a relative difference between the baseline and the calculated resistance (100%-X%). As seen in Fig. 13, the theoretical method agrees well with the experimental results for a typical range of HSC speeds between 1.47< 𝐹 𝑛∇ <3.52 where the relative error is 3%–5%. At 𝐹 𝑛∇ 1.47, the hull is in the semi-planing regime, resulting in a violation of the semiempirical model; the underprediction in the semi-planing regime has been noted by other researchers (Blount and Fox, 1976). Thus, an error of higher magnitude was anticipated. However, at 𝐹 𝑛∇ >4.30 the theoretical error increases beyond 10%. At these speeds, the dynamic trim starts to approach and exceeds 2◦ , which is the limit of the semi-empirical model. Therefore, the semi-theoretical calculations significantly improve the agreement with experimental values at higher speeds. The prediction error at 𝐹 𝑛∇ 4.30, is comparable in magnitude to drag reduction achieved by SR and SD1 at the same speed. This implies that at higher speeds, application of the semi-empirical model for reverse-calculation of resistance components from the measured total resistance yields inaccurate results and can only be used for indicative purposes.
Fig. 12. Total resistance as a function of 𝐹 𝑛∇ . Determined theoretically using the Savitsky Method (Savitsky et al., 2007) (black line), semi-theoretically using the running position from experiments (orange squares), and experimentally obtained resistance (blue open circles).
extreme value while the average should be less affected by the number of encounters. Statistically, the probability of exceeding 𝑎1∕3 and 𝑎1∕10 is 12% and 4%, respectively (Savitsky and Brown, 1976). This means that significant differences in 𝑎1∕3 and 𝑎1∕10 accelerations experienced by each hull configuration are most likely due to differences in the hull bottom geometry as both SR and SD1 are designed to increase lift compared to BH, as noted from the calm water run heave results (see Fig. 11). On the other hand, the formula developed for impact accelerations in waves by Savitsky and Brown (1976) included the calm water trim (𝜏∕4). This means that the difference in trim observed between the SD1 configuration and the other configurations during the calm water tests could also account for the difference in accelerations during the wave tests. 4. Conclusions
3.3. Irregular wave test results In this paper, the performance of spray deflection technologies – spray rails and spray deflectors – was compared through towing tank testing in calm water and irregular waves. The results presented in this paper are most accurate for the selected hull form and proportions but may be partially valid for varying proportions of prismatic and warped hull forms. The calm water tests have shown a significant reduction in drag for both technologies. Spray rails resulted in drag reductions of up to 9% and only affected the heave of the model. The deflectors configurations yielded even higher reductions in drag ranging from 10% to 25%, but the dynamic trim of the hull differed significantly from the other two configurations.
For irregular waves tests, the vertical accelerations of the BH, SR and SD1 configurations is shown in Figs. 14–17 and Tables 3 and 4. Accelerations at the bow are fairly constant for all configurations (see Figs. 14 and 16), except for the extreme value at the higher tested speed where both SR and SD1 experience higher acceleration than the BH (see Fig. 16). Accelerations at the LCG were greater than BH for both the SR and SD1 configurations, as shown in Figs. 15 and 17. The highest third is most representative of the experienced accelerations while the highest extreme represents the worst-case scenario. The extreme value is more sensitive to the number of encounters: a larger number of encounters may mean a higher change of a larger 6
Ocean Engineering 191 (2019) 106482
B. Molchanov et al. Table 3 Acceleration comparisons for wave runs, 𝐹 𝑛∇ = 1.47. LCG comparison
Bow comparison
Bare Hull
Spray Rails
Deflectors 1
Bare Hull
Spray Rails
Deflectors 1
0.37
+0.08
+0.15
1.48
+0.01
−0.18
[g]
0.56
+0.15
+0.34
2.55
+0.03
+0.09
Extreme [g]
1.09
+0.4
+1.49
4.92
+0.1
+0.11
𝑎 1 [g] 3
𝑎
1 10
Table 4 Acceleration comparisons for wave runs, 𝐹 𝑛∇ = 4.3. LCG comparison
Bow comparison
Bare Hull
Spray Rails
Deflectors 1
Bare Hull
Spray Rails
Deflectors 1
1.01
+0.08
+0.09
2.04
+0.01
−0.28
[g]
1.6
+0.2
+0.33
3.64
−0.03
−0.36
Extreme [g]
3.3
+0.79
+2.98
6.88
+0.87
+0.95
𝑎 1 [g] 3
𝑎
1 10
Fig. 15. Accelerations measured at LCG, 𝐹 𝑛∇ = 1.47. Fig. 13. Comparison of errors in resistance estimation using predicted running position (black) and measured running position (orange) with the experimentally measured resistance.
Fig. 16. Accelerations measured at Bow, 𝐹 𝑛∇ = 4.3.
Fig. 14. Accelerations measured at Bow, 𝐹 𝑛∇ = 1.47.
individual effects cannot be accurately isolated unless the same running trim angle is achieved for all configurations. The present study shows that the semi-empirical model (Savitsky et al., 2007) is insufficient for accurate prediction of running position even for prismatic hulls. Additional CFD analysis or directly measured experimental data should be used for the design of the spray deflectors.
For the tested deflectors configurations, the significant reduction in total resistance is viewed as a combined effect of deflectors redirecting the spray sheet and causing a change in dynamic trim of the hull. This provides insight into how deflectors affect overall performance, but 7
Ocean Engineering 191 (2019) 106482
B. Molchanov et al. Table A.1 Sensor, ballast and plate weights and locations. Nr
Unit Name
XCG [mm]
YCG [mm]
Mass [kg]
1 2 3 4 5 6 7 8 9 10a 10b 11a 11b 11c 11d
Foam Hull Inclinometer Pitch Pivot Box Drag Balance Attachment Bar Heave Post CG Accelerometer Bow Accelerometer Aft Ballast CG Ballast CG Unload Spring Bare Hull Plate Spray Rails SLR 5.9 Plate SLR 8 Plate
787.4 317.5 548.6 548.6 548.6 548.6 444.5 1318.3 76.2 548.6 548.6 650.2 825.5 1104.9 977.9
114.3 152.4 158.8 222.3 304.8 685.8 151.1 151.1 177.8 431.8 – 63.5 63.5 71.1 73.7
9.59 0.35 1.19 2.69 0.71 2.69 0.13 0.13 Varies Varies Varies 3.27 0.09 1.22 1.35
Fig. 17. Accelerations measured at LCG, 𝐹 𝑛∇ = 4.3.
The wave tests have shown that the current deflectors design does not significantly affect the vertical accelerations compared to spray rails at the bow. At the LCG, SD1 has a significantly higher acceleration compared to SR, especially the highest extreme value. In general, for the deflectors, the data shows lowered average accelerations at the bow, increased average acceleration at the LCG and increased extreme accelerations across all conditions.
Fig. A.1. Positions of sensors and weights in model.
5. Further work Drag and pitch
Additional experiments are required with improved deflectors placement and geometry for more direct comparison of spray rails and deflectors configurations in waves. The spray deflectors geometry could be improved if their thickness would taper from a maximum at the chine to almost zero at the keel line. Correctly located and tapered deflectors may eliminate the problem of the stagnation line crossing the deflectors near the chine and being too far from the deflectors at the keel. Since the thickness of the tapered deflectors is practically zero at the keel, it will also eliminate problems associated with the tip of the deflectors re-entering the water. Finally, more insight is required to understand the spray phenomenon. The current spray force prediction method is based on limited semi-empirical data presented in Savitsky et al. (2007). A method for direct measurement of the spray drag for a range of speeds, running positions, and model sizes would be invaluable for further development of the spray deflection technologies.
To measure the drag, a Linear Variable Differential Transformer equipped drag balance is used, rated for 222.4 N (50 lbf) meaning that it can measure forces up to 222 N, with an absolute error of ±0.06 N. The drag balance is then mounted on a pitch pivot box which measures the pitch angle through a rotational potentiometer. This measurement is used for the wave runs to accurately measure the number of waves the hull encounters and is calibrated statically to within ±0.0161 deg. The drag balance is number 4 in Table A.1 and Fig. A.1 while the pitch pivot box is number 3. The pitch pivot box with the drag balance is mounted to the heave post and to the hull and thus becomes the place where the thrust from the monorail carriage is transferred to the hull.
Heave
Acknowledgments This research would not have been possible without the support of the faculty, staff and students of Davidson Laboratory at Stevens Institute of Technology, especially Professor Raju Datla and Uihoon Chung who have been most helpful. Further, we would like to thank Jonas Danielsson at Petestep AB, Sweden, for many helpful discussions regarding stepped hulls.
The heave is measured from a potentiometer mounted by the heave post calibrated to within ± 0.2 mm. The heave post and sensor is number 6 in Table A.1 and in Fig. A.1.
Accelerometers
Appendix A. Sensors and measurements
For the waves, a set of accelerometers are used to measure the acceleration at certain points along the hull. The sensors are 30g Schaevitz closed loop torque-balance transducers which can be mounted anywhere on the hull. In this application, one is put as close to the LCG position as possible and another is mounted a distance forwards closer to the bow as shown in Fig. A.1. The accelerometers are calibrated statically to within ±0.03g. The accelerometers are numbers 7 and 8 in Table A.1 and in Fig. A.1.
For calm water the interesting measurements are drag, pitch and heave. For wave runs the focus is on accelerations at the CG, accelerations at the bow, number of wave encounters, and number of wave peaks and troughs. Data was collected over 120 ft for each run. An unload spring was used at the CG to help control model weight; the spring does affect the dynamic pitching motions in waves but does not affect calm water runs. 8
Ocean Engineering 191 (2019) 106482
B. Molchanov et al. Table B.1 Total combined uncertainty, 𝑢′𝑐 ,in percent for each hull and speed in 𝐹 𝑛𝑉 with reference to resistance. 𝐹 𝑛∇
3.52
BH SR SD1 SD2
0.42 0.16 0.18
3.91
4.30
0.18
0.62 0.13 0.15
4.70
0.39
5.09 0.82 1.43 0.69 0.77
5.48
5.87
6.26
0.33 1.87 0.42
0.48
Clement, E.P., 1964. Reduction of Planing Boat Resistance by Deflection of the Whisker Spray. Technical Report DTMB-1929, David Taylor Model Basin, pp. 1–23. Cucinotta, F., Guglielmino, E., Sfravara, F., 2017. An experimental comparison between different artificial air cavity designs for a planing hull. Ocean Eng. 140, 233–243. De Marco, A., Mancini, S., Miranda, S., Scognamiglio, R., Vitiello, L., 2017. Experimental and numerical hydrodynamic analysis of a stepped planing hull. Appl. Ocean Res. 64, 135–154. Faltinsen, O.M., 2006. Hydrodynamics of High-Speed Marine Vehicles. Cambridge University Press, Cambridge. ITTC, 2017a. 28th International Towing Tank Conference, General Guide for Uncertainty Analysis in Resistance Tests. In: Recommended Procedures and Guidelines, vol. 7.5-02-02-02. ITTC, 2017b. 28th International Towing Tank Conference, High Speed Marine Vehicles, Seakeeping Tests. In: Recommended Procedures and Guidelines, vol. 7.5-02-05-04. Judge, C., Beaver, B., Zseleczky, J., 2017. The 30th American Towing Tank Conference. In: An Evaluation of Planing Boat Trim Measurements, ATTC, West Bethesda, Maryland. Larsson, L., Hoyte, R.C., 2010. The Principles of Naval Architecture Series: Ship Resistance and Flow, first ed. The Society of Naval Architects and Marine Engineers. Lee, J.-G., Jung, K.-H., Suh, S.-B., Ho-Hwan, C., Lee, I., 2010. A study on the hull form design of semi-planing round-bilge craft. J. Ocean Eng. Technol. 24, 59–65. Lotfi, P., Ashrafizaadeh, M., Esfahan, R.K., 2015. Numerical investigation of a stepped planing hull in calm water. Ocean Eng. 94, 103–110. Olin, L., Altimira, M., Danielsson, J., Rosen, A., 2016. Numerical modelling of spray sheet deflection on planing hulls. Proc. Inst. Mech. Eng. A 231 (4), 811–817. Payne, P.R., 1982. The spray volume shed by an uncambered planing hull in steady planing. Ocean Eng. 9 (4), 373–384. Savitsky, D., 1964. Hydrodynamic design of planing hulls. Mar. Technol. 1 (1), 71–95. Savitsky, D., Brown, P.W., 1976. Procedures for hydrodynamic evaluation of planing hulls in smooth and rough water. Mar. Technol. 13 (4), 381–400. Savitsky, D., DeLorme, M.F., Datla, R., 2007. Inclusion of whisker spray drag in performance prediction method for high-speed planing hulls. Mar. Technol. 44 (1), 35–56. Savitsky, D., Morabito, M., 2010. Surface wave contours associated with the forebody wake of stepped planing hulls. Mar. Technol. 47, 1–16. Seo, J., Choi, H.-K., Jeong, U.-C., 2016. Model tests on resistance and seakeeping performance of wave-piercing high-speed vessel with spray rails. Int. J. Nav. Archit. Ocean Eng. 8 (5), 442–455. Soletic, L., 2010. Seakeeping of a systematic series of planing hulls. In: The Second Chesapeake Power Boat Symposium. Sverchkov, A., International conference on ship drag reduction (SMOOTH-ships). In: Application of Air Cavities on High-Speed Ships in Russia, 2010, Istanbul, Turkey. Taunton, D.J., Hudson, D.A., Shenoi, R., 2010. Characteristics of a series of high speed hard chine planing hulls-part 1: performance in calm water. Int. J. Small Craft Technol. 152, 55–75. Wielgosz, C., Fürth, M., Datla, R., Chung, U., Rosen, A., Danielsson, J., Experimental validation of numerical drag prediction of novel spray deflector design. In: Proceedings 13th International Marine Design Conference, 2018, pp. 491–497. Yousefi, R., Shafaghat, R., Shakeri, M., 2013. Hydrodynamic analysis techniques for high-speed planing hulls. Appl. Ocean Res. 42, 105–113.
6.65 0.07
0.53
0.96
Inclinometer To measure the static and running trim, a gravity referenced inclinometer is used. Made by Schaevitz, it is their LSO series which is a fully self-contained, fluid damped, flexure suspension, servo inclinometer calibrated to within ± 0.0005 deg. The inclinometer is number 2 in Table A.1 and Fig. A.1. Sensor positions The appendages, such as sensors and plates, and their positions are listed in Table A.1 and shown in Fig. A.1. In Table A.1, XCG is the distance measured from the stern to the center point of each appendage and YCG is measured from the keel. Appendix B. Error and repeatability The measurements shown are the averages of the measurements for the repeated experiments. The inherent uncertainty from sensors and fluctuations in the environment can be combined to predict the total uncertainty of the measurements. In this paper the ITTC’s guidelines for calculating the uncertainty of measured data points was used (ITTC, 2017a). The total combined uncertainty, 𝑢′𝑐 , for each speed and hull in percent is shown in Table B.1.
References Begovic, E., Bertorello, C., 2012. Resistance assessment of warped hullform. Ocean Eng. 56, 28–42. Blount, D.L., Fox, D.L., 1976. Small-craft power prediction. Mar. Technol. 13, 14–45.
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