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Full-scale pressure measurements on a Sparkman and Stephens

24-foot sailing yacht

Citation for published version: Viola, IM & Flay, RGJ 2010, 'Full-scale pressure measurements on a Sparkman and Stephens 24-foot sailing yacht', Journal of Wind Engineering and Industrial Aerodynamics, vol. 98, no. 12, pp. 800-807. https://doi.org/10.1016/j.jweia.2010.07.004

Digital Object Identifier (DOI): 10.1016/j.jweia.2010.07.004

Link: Link to publication record in Edinburgh Research Explorer

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Published In: Journal of Wind Engineering and Industrial Aerodynamics

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Viola, I.M., Flay, G.J.F. Full-scale pressure measurements on a Sparkman and Stephens 24-foot sailing yacht J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807. doi:10.1016/j.jweia.2010.07.004

Abstract The aerodynamics of a Sparkman and Stephens 24-foot sailing yacht was investigated. Full-scale pressure measurements were performed on the mainsail and the genoa in upwind condition. Pressure taps were adopted to measure the pressures on three horizontal sections on the windward and leeward sides of the two sails. Several trims and apparent wind angles were tested. The present paper shows the pressure distributions on the sails and correlates the measured pressures with the flow pattern. In particular, leading-edge laminar separation bubble, turbulent reattachment and turbulent separation are discussed. Pressure measurements are also adopted to draw some trim guidelines.

1. Introduction

Sail aerodynamics has been widely investigated in the last half century and it is common practice to characterize it by aerodynamic force components, whilst pressure on sails is rarely measured.

Forces are commonly measured at model-scale in wind tunnels. Boundary-layer wind tunnels are designed to model the atmospheric boundary-layer, and special devices allow the vertical wind profile to be twisted to model the twisted flow resulting from the sum of the atmospheric boundary layer and the boat speed in full-scale. The yacht model is placed onto a balance, which allows the aerodynamic forces to be measured. The sails are flexible to allow several sail trims to be investigated. Various sail shapes, yacht heels, apparent wind angles, etc, are tested to investigate the relationship between different geometries and the aerodynamic forces.

One of the main limitations of wind tunnel tests is the inability to reach full-scale Reynolds number. In fact, most of the wind tunnels used for yacht testing do not allow wind speeds larger than 10 m/s and model-scales larger than 1/10th. The wind speed in full-scale is between 3 and 15 m/s and hence, the model-scale wind-speed should be between 30 and 150 m/s in order to match full-scale Reynolds numbers. This exceeds the maximum speeds achievable. Moreover, sails are thin membranes, which would not support the aerodynamic loads of a 30 m/s air speed.

Pressures on sails have rarely been measured either in model-scale or in full-scale. In model-scale, pressure measurements are complicated because sails are flexible in order to be trimmed, the weight of the pressure transducers affects the sail shape, and the dimension of the transducer affects the flow field. When solid and thick sails are adopted to support the transducer weight, the sails can only be trimmed with difficulty.

In full-scale, pressure measurements are complicated because of the unsteady wind environment and the large number of parameters involved. The atmospheric boundary layer is fully turbulent and does not usually provide a steady wind environment. The sails are flexible and the shapes change with the wind speed. The boat heels and pitches continuously due to the waves and the wind.

The differential pressure across a sail is 3 orders of magnitude smaller than the absolute value of the pressure. In fact, atmospheric pressure is between 95,000 Pa and 105,000 Pa, whilst the differential pressure across sails is of the order of magnitude of the dynamic pressure, which is roughly 15 Pa in a 5 m/s wind. Hence, the required accuracy of the pressure instrumentation is of the order of magnitude of one Pascal. To increase the accuracy, the differential pressure across the sail instead of the absolute pressure is measured.

If the pressure distributions on the windward and leeward sides of the sail need to be measured individually (instead of only the differential values between the two faces), a reference pressure is necessary. In particular, to relate the pressures with the test conditions, the undisturbed far field conditions should be chosen as the reference static and dynamic pressure. Ideally, a differential pressure transducer should be connected through pressure tubes to the far field and to the sail. Because this is impossible, a stable reference pressure, possibly representative of the far field pressure, should be found somewhere onboard the vessel.

With regard to the static far-field pressure, the following two remarks should to be considered. An incoming atmospheric disturbance can change the atmospheric pressure by several Pascal per minute. Hence, the far field pressure can change significantly during the acquisition period. Moreover, the vertical pressure gradient due to hydrostatic effects is of the order of 10 Pa/m. Hence, on a 10 m high mast, the static pressure at the top of the sail is roughly 100 Pa larger than the pressure at the bottom of the sail height.

With regard to the dynamic far-field pressure, the following two remarks should to be considered. The differential pressure across the sail varies linearly with the dynamic pressure and thus with the square of the wind speed. Hence, when a gust increases the wind speed by 1m/s, the dynamic pressure increases by roughly 5 Pa. Moreover, the dynamic pressure is higher at the highest sail sections than at the lowest sail sections, due to the atmospheric boundary layer.

The difficulties above were overcome in the present paper, which discuss the pressure distributions on the sails of a 24-foot sailing yacht. Differential pressures were measured, with the reference pressure being the pressure inside the yacht cabin, which was supposed representative of the far field static pressure. The reference dynamic pressure was measured with Pitot static probes located at several locations onboard. The pressure on three horizontal sail sections of the mainsail and genoa were measured for different sail trims.

The present paper shows the pressure distribution on a typical modern sail-plan measured with a state-of-the-art measuring system. Pressure distributions on sails have been rarely published, particularly in full scale. As far as known by the present authors, the only published full-scale measured pressure distribution was presented by Warner & Ober in 1925. The innovative contributions in the present paper are the larger number of pressure transducers used, which allow a high-resolution map of the sail pressure distributions; the high resolution of the transducers, which allows small pressure differences due to the sail trim to be measured; the high sampling frequency, which allows mean values taking into account the high frequency fluctuations to be measured accurately.

The design of high-performance sails is performed with numerical methods, which compute aerodynamic forces. Several candidate sails are processed, and a Velocity Prediction Program (VPP) uses the aerodynamic forces to compute the boat speeds achieved with each candidate sail, Pressure distributions and the aerodynamic forces are computed numerically and are not measured. Indeed, complicated phenomena are involved, such as the laminar to turbulent transition, leading edge separation and reattachment, and the trailing edge separation, which are all modelled with difficulty. Therefore, experimental validation is essential to check on the numerical modeling accuracy. Wind tunnel measurements are often used to validate the numerical computations; but wind tunnel modelling does not take into account the lowest frequency components of the atmospheric boundary-layer turbulence and the yacht dynamics. Moreover, the model-scale Reynolds number that can be achieved in wind-tunnel testing is one order of magnitude lower than the full-scale Reynolds number. Therefore, full-scale pressure measurements, such as the present measurements, have to be used to validate the numerical methods.

2. State of the Art

Until the beginning of the last century, the aerodynamics of sailing yachts was described by empirical formulae. For instance, Skene (1938) quoted the “Martin’s Formula” p=0.004.U2 giving the mean pressure load “p” (in pounds per square foot) on the sails due to the wind speed “U” (in miles per hour).

During the first half of the last century, yacht builders and designers such as the Herreshoff family, and Sparkman & Stephens, turned wind tunnels and towing tanks into essential tools for yacht design. Unfortunately, a collection of their research is unknown to the authors.

Between 1915 and 1921, Warner & Ober performed several experiments at the Massachusetts Institute of Technology (MIT) to find the relationship between the performance of the sails of a yacht compared to the wings of an airplane and in 1923 they performed the first full-scale pressure measurements on the S-class yacht Papoose (Warner & Ober 1925). They measured the pressure with manometers connected to three sections of the mainsail and one section of the jib, and investigated the difference between a mainsail with and without battens, and the effect of one sail on the pressure distribution of the other sail.

In the early 1960s, Marchaj (1964) measured with manometers the pressure on eight sections of a model-scale mainsail in the University of Southampton Wind Tunnel.

In the 1970s, Gentry (1971) was interested in the slot effect due to jib/mainsail interaction and investigated the pressure distributions with an Analogue Field Plotter on a 2D model. He was involved in the design of the masts for the successive America’s Cup defences Courageous, Freedom and Liberty in the 1974 and 1977, 1980, 1983 America’s Cup defences respectively (Gentry 1988). The pressure distribution on the mast/mainsail was investigated with the CFD code of the Boeing Commercial Airplane Company.

In the 1980s, Wilkinson (1984, 1989, 1990) studied mast/sail interaction on a 2D model- scale section. After these few authors, pressure measurements on sails have rarely been published.

Over the past 3 years, full-scale measurements have made a resurgence and new lightweight small devices have allowed better measurements to be taken. The Yacht Research Unit (YRU) at the University of Auckland is developing a wireless pressure system for full-scale experiments (Flay & Millar 2006), and similar systems were also developed by an Italian research team (Puddu et al. 2006) and by an American team collaborating with the America’s Cup challenger BMW Oracle Racing (Graves et al. 2008). At the current state of the art, these systems have provided pressure measurements at only a few points and have not yet been able to provide a complete pressure map on sails.

In the past year at the YRU wind tunnel, double-surface rigid sails with pressure tubes inside have been tested and the pressure distributions on both the windward and leeward sides of a fully 3D symmetrical spinnaker were measured (Richards & Lasher 2008). The pressures were measured with 8 pressure taps at each of 7 horizontal sections on a 1/25th model-scale International America’s Cup Class (IACC) spinnaker sailing at an apparent wind angle (AWA) of 120°.

Finally, Viola & Flay (2010a and 2010b) measured a complete pressure map on the sail of a 1/15th model-scale AC33-class yacht sailing with mainsail and asymmetric spinnaker. Several trims of three sails, sailing at three AWAs and three heel angles were measured.

3. Full-Scale Pressure Measurement Experimental Setup

3.1 Description of Yacht The yacht Aurelie was kindly provided for the experiment by Madame Emanuela Molinari Viola. The yacht is a Sparkman & Stephens 24-foot cruising yacht with a Bermuda rig, which was designed in the late 1960s by Sparkman & Stephens. She was built in New Zealand in 1978 in fibreglass and polyester resin, with an alloy mast and boom. Table 1 summarizes the main specifications of the Sparkman & Stephens 24’.

3.2 Sails A mainsail with low roach (almost triangular) and a genoa with overlap were used. The fully-batten mainsail was brand new, whilst the genoa was older but in reasonable condition and without battens. Mean values of the sail section shapes were measured from photos taken during the experiments. The main dimensions of the sails are summarized in Table 2.

3.3 Pressure System Surface pressures were measured on the sails with a system developed by the YRU. In particular, Nick Velychko developed the concept, the electrical circuit, the PCB layout, the enclosure design, and the LabVIEW control software, while David Le Pelley developed the LabVIEW data collection and manipulation software. The system has 512 channels, and each of them can acquire up to 3,900 samples per channel per second. The transducers have a pressure range of ±450 Pa and a resolution of 9.25 mV/Pa. All the transducers were pneumatically connected to a reference static pressure. The system is made of 4 boxes, which contain 64 transducers each. To simplify the measurement setup, only 42 transducers were used and hence, only one box was necessary. The box was connected to a laptop, which ran the acquisition software. A standard marine battery supplied 12 V DC to the pressure system. The pressure measurements were initially acquired at 1,000Hz for 5 minutes. Subsequent analysis of those data showed that high frequency signals were

damped by long tubes and hence, the sampling frequency was reduced to 100Hz to reduce the amount of stored data. It was also found that a recording duration of 120 seconds contained several periods of the lowest frequency fluctuations and hence, the subsequent data acquisitions were performed for 120 seconds.

This pressure system has also been used for the measurement of the complete map of the sail pressures on a 1/15th scale model of a AC33-class yacht sailing with mainsail and asymmetric spinnaker (Viola & Flay 2010a and 2010b).

The reference pressure was measured in a locker inside the yacht cabin, which was connected with a 10 m length tube to the box. The locker helped avoid that the static pressure measurement being affected by cabin ventilation. Each transducer was connected to a pressure tap through a tube, which was stuck onto the sails with adhesive tape. The pressure taps were truncated very flat cones with a base diameter of 20 mm and a height of 5 mm. The pressure tap on the top of the cone was connected to a stainless steel tube lying flat against the sail, to which the PVC pressure tube was connected. The pressure taps and the tubes were expected to be sufficiently small not to affect the sail boundary layer. In fact, the diameter of the pressure tube was of the same order of magnitude as the roughness of the sail, due to the cloth panel seams. However, to minimize the interference with the sail boundary layer, the tubes were stuck to the sail along the main flow direction in the region near the pressure taps. Figure 1 shows a schematic drawing of the acquisition system and a photograph of the pressure tap.

Pressure taps were placed on 3 horizontal sections of the two sails. To simplify the experiment, one sail face at a time was measured. Each test condition was repeated on both tacks. Hence, the same sail face was firstly the windward face and then the leeward face. To verify the correspondence of the test condition from one tack to the other, while measuring the pressure on one sail face, a few pressure taps were used to measure the pressure on the other face. Moreover, for each test, 4 pressure taps measured the pressures around the mast, two on the windward side and two on the leeward side respectively.

The measured genoa sections were at 1/4, 1/2 and 3/4 of the luff respectively, whilst the mainsail sections were at 1/4, 1/2 and 800 mm below 3/4 of the luff, respectively. The lowest section of the mainsail was chosen in order to be able to reach the pressure taps from the deck when the main was hoisted. On the top, middle and bottom sections, 6, 9 and 16 pressure taps were used, respectively. Figure 2 shows the sailplan of the yacht Aurelie. The figure shows the two sails with the three measurement sections, and the terminology adopted in the following description. In particular, the leading edge of the sail is also named the luff, while the trailing edge is named the leech; the aft (back) corner of the sails is connected to a rope named a sheet and is used to trim the sail; the vang is a rope used to increase the tension on the mainsail leech, and the backstay is a metal cable used to bend the top of the mast back by pulling on the top.

3.4 Reference Dynamic Pressure Static and dynamic pressures were measured with three Pitot static tubes located on the windward side of the yacht. Two of them were fixed to the shrouds at two different heights. These were oriented approximately 35 degree to windward of the boat’s heading. The third one was fixed on a pole, which held the probe roughly 1.5 m away from the boat stern on the windward side. This latter probe was free to pivot into the wind direction and was found

to be the most reliable of the three of them. The differential pressure between the total probe on the pole and the cabin reference static pressure was taken to be the reference dynamic pressure.

The measurements were performed in September 2009 in the Hauraki Gulf (Auckland, NZ), in roughly 4m/s of breeze. All the tests were performed sailing upwind, with AWAs between 25 and 45 degrees.

4. Results and Discussion

The pressures on the mainsail were measured for different mainsail and genoa trims. For each trim, the boat was sailed on the genoa trim using telltails, which means that the helmsman helmed the boat to keep the optimum course for the genoa trim. In this way, only optimum trims were tested. Hence, the AWA depended on the genoa trim.

The pressures on the genoa were investigated for different genoa trims. The effect of changing the angle of incidence of the genoa was investigated by sailing at a fixed AWA. This investigation allowed the optimum pressure distribution for a fixed AWA to be determined. Then, different AWAs were sailed and the genoa was trimmed according to each AWA.

The following results are shown in terms of pressure coefficients Cp, defined as the difference between the pressure measured on the sail surface p and the reference pressure inside the cabin p∞ , divided by the reference dynamic pressure q measured by the poled Pitot static tube. Note that the vertical axis is reversed in the following figures, showing negative values above and positive values below.

4.1 General Pressure Distribution Trend A general trend of Cp along a sail section can be described. Figure 3 shows a schematic diagram of Cp along a generic section of the genoa and the mainsail. The correlated flow field is also shown. The apparent wind velocity Va is determined by the sum of the boat velocity Vb and the atmospheric true wind Vt.

The windward side of the two sails is a low speed region. Hence, Cp is near 1.0 over the whole section. The leeward side of the genoa shows a suction peak at the leading edge, which is followed by a quick pressure recovery with a local minimum suction at around 10% of the chord. The pressure recovery is related to the laminar separation bubble formed behind the sharp leading edge of the luff, which reattach after the turbulent transition (Abbot & Von Doenhoff 1949; Crompton & Barret 2000). The reattachment location is just in front of the local maximum pressure location (Crompton & Barret 2000). Downstream of the re-attachment, the pressure decreases again due to the section curvature, showing a second suction peak at a location between 20% and 40% of the curve length. After the pressure recovery, the pressure becomes constant due to the trailing edge separation (Thwaites 1969). The Cp variation over the genoa is similar to the Cp measured over the asymmetric spinnaker by the same authors in previous research (Viola & Flay 2010a and 2010b).

The leeward side of the mainsail shows similar trends to the genoa, but the laminar separation occurs on the mast, and the turbulent reattachment occurs at around 10% of the mainsail chord. Between the leading-edge suction peak and the pressure recovery due

to the reattachment, the pressure is almost constant. The Cp trends over the mainsail are in agreement with the measurements performed on a 2D mast/mainsail section by Wilkinson (1989).

4.2 Pressures on the Mainsail Figure 4 shows the Cp over the three sections of the mainsail versus the position x along the chord c. The three curves are obtained with different trims of the mainsail sheet, which mainly changes the angle of incidence of the sail.

When the mainsail is tightened (trim #+1 in figure), the maximum differential pressure between the two sides is achieved. The Cp on the windward side is almost 1.0 over most of the chord, meaning it is a very low speed region. On the leeward side, the leading-edge suction peak reaches roughly Cp= -2 on the top section and Cp= -1 on the bottom section.

When the mainsail is eased (trim #0 in figure), the differential pressure between the windward and the leeward sides of the sails decreases. However, because easing the mainsail sheet decreases the angle of incidence but does not change the sail shape significantly, the second suction peak due to the sail curvature also does not change significantly. The windward Cp is lower showing a higher velocity on the windward side. The leading-edge suction on the leeward side decreases because of the decreased angle of incidence.

When the sail is eased even further (trim #-1 in figure), the angle of incidence becomes negative and the first 10% of the mainsail has negative Cp’s on the windward side and positive Cp’s on the leeward side.

The best trim depends on the wind conditions. In fact, from the figure it can be argued that by tightening the sheet, the pressure forces on the sail increase, resulting in higher thrust and side force components. In the tested condition, trim #0 allowed the highest boat speed. Trim #+1 resulted in excessive side force and hence, a higher heeling moment, which heeled the boat, increasing the hydrodynamic resistance.

Figure 5 shows the Cp over the three mainsail sections, for three mainsail twists. Twist was changed by tightening the leech, through the vang, the traveller and the sheet. On the top two sections, only the leeward-side pressure is plotted. The change in twist affects the Cp over only the highest sections, while the lowest section is insensitive to the twist. When the leech is tightened (from #-1 to #+1 in figure), the twist decreases and the angle of incidence of the highest sections increases. Therefore, on the highest section the leading- edge suction peak increases and trailing edge separation occurs, which causes the second suction peak to decrease. When the maximum leech tension is achieved (#+1), the flow is mainly separated, leading to an almost constant Cp. The low mean speed of the separated flow is unable to lift the tell-tails on the highest part of the mainsail leech,

From the figure, it can be argued that by increasing the leech tension the pressure force increases at the leading edge and decreases at the trailing edge, which rotates the direction of the resultant aerodynamic force further forward, leading to an increased thrust force and a reduced side force. However, increasing the leech tension also rotates the top sail-section backwards, which leads to the opposite effect. Therefore, the best trim is a medium trim, which maximizes the resultant thrust component. Moreover, by increasing

the leech tension, the aerodynamic load on the highest sections decreases which leads to a lower centre of effort.

Figure 6 shows the Cp over the three sections of the mainsail for three sail trims, where the effect of the size of the gap between sails is investigated. With regard to the reference trim (#0 in figure), firstly the mainsail was tightened and the genoa was eased, which increased the gap (#+1 in figure), then the mainsail was eased and the genoa was tightened, which decreased the gap (#-1 in figure). When the gap between the sails is increased, the Cp on the mainsail becomes constant along the three sections, showing a fully separated flow, which causes the leech tell-tails to collapse. In fact, easing the genoa leads to a lower downwash on the mainsail, which leads to a larger angle of incidence. Hence, when the mainsail was tightened and the genoa was eased, the mainsail stalled due to the excessive angle of incidence. Conversely, when the mainsail was tightened without easing the genoa (Figure 4, trim #+1), only the top section of the mainsail showed separated flow.

When the gap between sails is decreased (#-1), the lowest sections of the mainsail, where the genoa overlap is larger, show an inversion of pressure/suction between the windward/leeward sides of the sail. The mainsail stagnation point moves downstream on the leeward side of the mainsail. On the highest section, where the overlap is negligible, the Cp distribution shows the presence of a laminar separation bubble and hence, the stagnation point is at the leading edge and the angle of incidence is larger than the ideal angle of attack. Hence, the low angle of incidence at the lowest section is due to over-trim of the genoa and is not due to the mainsail trim.

Figure 7 shows the Cp over the three sections of the mainsail for three backstay tensions. Tightening the backstay (from #-1 to #+1 in figure), causes the mast to bend and flattens the mainsail. The lowest shrouds oppose the mast bending, hence the highest part of the mast bends more than the lowest part. The Cp on the lowest section is not affected by the backstay trim, while the Cp on the top section is affected the most. When the mainsail is flattened, the second suction peak, which is due to the sail curvature, decreases.

The best trim depends on the wind conditions. In fact, the second suction peak is located about 40% of the chord, and hence it generates a large side force component and a small thrust force component. When side force or heel moment has to be decreased, the backstay should be tightened. Of course this is common practice in yacht racing.

4.3 Pressures on the Genoa Figure 8 shows the Cp over the three genoa sections for five genoa-sheet trims. Only the Cp over the leeward side of the genoa is plotted. For all the trims, the boat was sailed at the same AWA=30°.

In the first trim (#0 in figure), the genoa was trimmed to the ideal angle of attack (Theodorsen 1931), which means that the stagnation point is on the leading edge and the suction is entirely due to the profile curvature. This condition is achieved when the genoa luff is close to collapse and it is sailed when the minimum AWA has to be achieved for tactical racing purposes. Tightening the sheet (#+1) causes leading-edge suction peak to occur and a minimum Cp=-5 was measured. The increased suction at the leading edge is correlated to a large thrust force component increase. This condition is achieved when the genoa tell-tails are aligned horizontally, and it is sailed when the maximum boat speed has

to be achieved. When the sheet is tightened further (from #+2), the trailing-edge separation point moves upstream until all the sail is fully stalled in trims #+3 and #+4 and all the tell-tails collapse.

Figure 9 shows the Cp over the three genoa sections for AWAs=27°, 30°, 45° and 70°. The genoa and the mainsail are trimmed according to the AWA.

On the lowest section, the trends of the leeward Cp curves are similar to the trim #+1 in Figure 8, which was the trim that allowed the maximum boat speed. Hence, the genoa was trimmed correctly on the lowest section. On the highest section, the trends of the leeward Cp curves are similar to the trim #+0 in Figure 8, which was the trim that allowed the minimum AWA to be sailed. Hence, the genoa was under-trimmed on the highest section. If the genoa twist was decreased, for instance by moving the car, the angle of attack of the highest section would have increased. Lower pressures would have been achieved at the leading edge followed by a pressure recovery, as shown by the lowest section. The systematic error in the genoa trim is partially due to the unfavorable sheeting angle at large AWAs. In fact when the AWA increases, the car should be moved forward and away from the centerline. On the yacht Aurelie the car can be moved only forward and backwards. Hence, it cannot be properly trimmed for large AWAs. At AWA=27° and 30°, the over-twist of the genoa is due to a wrong trim; the car should have been moved a little further. At AWA=45° and 60°, the over-twist of the genoa is due to the impossibility to properly trim the car at large AWAs. As a consequence, the larger the AWA the more the top section is under-trimmed and the higher is the pressure on the leeward side.

The leeward-side pressure also increases on the lower section when the AWA increases, because the sail section becomes deeper and deeper, and the trailing edge separation point moves forward.

In conclusion, the pressure load on the genoa decreases significantly when the AWA increases. This is due to the unfavorable sheeting angle which leads to an over-twist on the top sections and to excessively deep lower sections.

5. Conclusions

The present paper shows pressure distributions on a full-scale genoa and mainsail. The paper shows how the pressure distributions change with the sail trim and with the apparent wind angle. These trends can be used to validate the numerical codes that are commonly used in the sail design practice. Moreover, the value of pressure measurements to help understand and optimize sail trims is pointed out.

The pressure coefficients and the related flow field over a generic genoa and mainsail section are shown in the form of a schematic diagram. The pressure coefficient on the windward side of the sails is almost 1.0 over the whole section due to low speed of the flow. On the leeward side, the pressure coefficient shows a suction peak first, followed by a pressure recovery at around 10% of the sail chord, which is related to the turbulent reattachment after the laminar separation bubble. Then a second suction peak occurs due to the sail curvature, followed by a pressure recovery and, eventually, by a pressure plateau due to the trailing edge separation.

In particular, the following conclusions can be drawn from the pressure distributions on the mainsail:

- The mainsail sheet primarily affects the leading-edge suction peak. Tightening the sheet causes the leading-edge suction peak to increase, while the second suction peak due to the sail curvature is not affected significantly.

- The mainsail twist primarily affects the trailing edge separation. Decreasing the twist causes the angle of incidence of the highest sections to increase, which causes the suction to increases until the trailing edge separation becomes excessive.

- The gap between the genoa and the mainsail must be trimmed with care. In fact, a small gap increases the pressure on the leeward side of the mainsail, while a large gap causes the mainsail to stall.

- The camber of the mainsail (trimmed with the backstay tension) primarily affects the second suction peak, which is due to the section curvature. Hence, a larger camber allows a larger suction but it increases the side force component more than the thrust force component.

The pressure distribution on the genoa showed two possible genoa trims. When the genoa is trimmed at the ideal angle of attack, only one suction peak is present and the suction is all due to the sail curvature. This trim allows the lowest apparent wind angle to be sailed. When the genoa is slightly over-trimmed, a leading-edge suction peak occurs and the maximum drive force is achieved. This trim allows the maximum boat speed to be achieved.

When large AWAs are sailed, the unfavorable genoa sheeting angle leads to an excessive twist and to deeper bottom sections. As a consequence, the top sections are under- trimmed and the bottom sections show large trailing edge separation, which lead to a reduced suction on the sail.

6. Acknowledgments

The support of staff and students in the YRU is gratefully acknowledged in helping to carry out the full-scale pressure measurements and to sail Aurelie.

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Wilkinson S. 1990 Boundary–Layer Explorations Over a Two–Dimensional Mast/Sail Geometry, Journal of Marine Technology, Vol. 27, pp. 250-256.

802 I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807

3. Full-scale pressure measurement experimental setup and hence, the sampling frequency was reduced to 100 Hz to reduce the amount of stored data. It was also found that a 3.1. Description of yacht recording duration of 120 s contained several periods of the lowest frequency fluctuations and hence, the subsequent data The yacht Aurelie was kindly provided for the experiment by acquisitions were performed for 120 s. Madame Emanuela Molinari Viola. The yacht is a Sparkman and This pressure system has also been used for the measurement Stephens 24-foot cruising yacht with a Bermuda rig, which was of the complete map of the sail pressures on a 1/15th scale model designed in the late 1960s by Sparkman and Stephens. She was of a AC33-class yacht sailing with mainsail and asymmetric built in New Zealand in 1978 in fibreglass and polyester resin, spinnaker (Viola and Flay, 2009, 2010). with an alloy mast and boom. Table 1 summarizes the main The reference pressure was measured in a locker inside the specifications of the Sparkman and Stephens 24-foot. yacht cabin, which was connected with a 10 m length tube to the box. The locker helped avoid that the static pressure measurement being affected by cabin ventilation. Each transducer was 3.2. Sails connected to a pressure tap through a tube, which was stuck onto the sails with adhesive tape. The pressure taps were truncated A mainsail with low roach (almost triangular) and a genoa very flat cones with a base diameter of 20 mm and a height of with overlap were used. The fully-batten mainsail was brand new, 5 mm. The pressure tap on the top of the cone was connected to a whilst the genoa was older but in reasonable condition and stainless steel tube lying flat against the sail, to which the PVC without battens. Mean values of the sail section shapes were pressure tube was connected. The pressure taps and the tubes measured from photos taken during the experiments. The main were expected to be sufficiently small not to affect the sail dimensions of the sails are summarized in Table 2. boundary layer. In fact, the diameter of the pressure tube was of the same order of magnitude as the roughness of the sail, due to 3.3. Pressure system the cloth panel seams. However, to minimize the interference with the sail boundary layer, the tubes were stuck to the sail along Surface pressures were measured on the sails with a system the main flow direction in the region near the pressure taps. Fig. 1 developed by the YRU. In particular, Nick Velychko developed the shows a schematic drawing of the acquisition system and a concept, the electrical circuit, the PCB layout, the enclosure design photograph of the pressure tap. and the LabVIEW control software, while David Le Pelley Pressure taps were placed on 3 horizontal sections of the two developed the LabVIEW data collection and manipulation soft- sails. To simplify the experiment, one sail face at a time was ware. The system has 512 channels, and each of them can acquire measured. Each test condition was repeated on both tacks. Hence, up to 3900 samples per channel per second. The transducers have the same sail face was firstly the windward face and then the a pressure range of 7450 Pa and a resolution of 9.25 mV/Pa. All leeward face. To verify the correspondence of the test condition the transducers were pneumatically connected to a reference from one tack to the other, while measuring the pressure on one static pressure. The system is made of 4 boxes, which contain 64 sail face, a few pressure taps were used to measure the pressure transducers each. To simplify the measurement setup, only 42 on the other face. Moreover, for each test, 4 pressure taps transducers were used and hence, only one box was necessary. measured the pressures around the mast, two on the windward The box was connected to a laptop, which ran the acquisition side and two on the leeward side, respectively. software. A standard marine battery supplied 12 V DC to the The measured genoa sections were at 1/4, 1/2 and 3/4 of the pressure system. The pressure measurements were initially luff, respectively, whilst the mainsail sections were at 1/4, 1/2 and acquired at 1000 Hz for 5 min. Subsequent analysis of those data 800 mm below 3/4 of the luff, respectively. The lowest section of showed that high frequency signals were damped by long tubes the mainsail was chosen in order to be able to reach the pressure

taps from the deck when the main was hoisted. On the top, Table 1 middle and bottom sections, 6, 9 and 16 pressure taps were used, Speciﬁcations of Sparkman and Stephens 24-foot. respectively. Fig. 2 shows the sailplan of the yacht Aurelie. The ﬁgure shows the two sails with the three measurement sections, Length overall 7.3 m and the terminology adopted in the following description. In Length at waterline 5.8 m Max beam 2.3 m particular, the leading edge of the sail is also named the luff, while Max draft 1.3 m the trailing edge is named the leech; the aft (back) corner of the Displacement 2105 kg sails is connected to a rope named a sheet and is used to trim Lead keel 862 kg the sail; the vang is a rope used to increase the tension on the Mainsail area 13 m2 mainsail leech, and the backstay is a metal cable used to bend the Genoa area 13 m2 top of the mast back by pulling on the top.

Table 2 Speciﬁcations of the sails.

Mainsail Genoa

1/8th (of the luff) Girth 580 350 mm 3/4th (of the luff) Girth 1000 660 mm 1/2th (of the luff) Girth 1620 1395 mm 1/4th (of the luff) Girth 2160 2285 mm Foot length 2550 3280 mm Luff length 8060 8800 mm Leach length 8400 8560 mm Draft (indicative) 40 48% of the chord Max camber (indicative) 12 18% of the chord Fig. 1. Schematic diagram of the data acquisition system.

802 I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807

Table 2 Speciﬁcations of the sails.

Mainsail Genoa

1/8th (of the luff) Girth 580 350 mm 3/4th (of the luff) Girth 1000 660 mm 1/2th (of the luff) Girth 1620 1395 mm 1/4th (of the luff) Girth 2160 2285 mm Foot length 2550 3280 mm Luff length 8060 8800 mm Leach length 8400 8560 mm Draft (indicative) 40 48% of the chord Max camber (indicative) 12 18% of the chord I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807 803 Fig. 1. Schematic diagram of the data acquisition system.

reversed in the following ﬁgures, showing negative values above and positive values below.

4.1. General pressure distribution trend

A general trend of Cp along a sail section can be described. Fig. 3 shows a schematic diagram of Cp along a generic section of the genoa and the mainsail. The correlated ﬂow ﬁeld is also

shown. The apparent wind velocity Va is determined by the sum of the boat velocity Vb and the atmospheric true wind Vt. The windward side of the two sails is a low speed region.

Hence, Cp is near 1.0 over the whole section. The leeward side of the genoa shows a suction peak at the leading edge, which is followed by a quick pressure recovery with a local minimum suction at around 10% of the chord. The pressure recovery is related to the laminar separation bubble formed behind the sharp leading edge of the luff, which reattach after the turbulent transition (Abbot and Von Doenhoff, 1949; Crompton and Barret, 2000). The reattachment location is just in front of the local maximum pressure location (Crompton and Barret, 2000). Down- stream of the reattachment, the pressure decreases again due to the section curvature, showing a second suction peak at a location between 20% and 40% of the curve length. After the pressure recovery, the pressure becomes constant due to the trailing edge

separation (Thwaites, 1969). The Cp variation over the genoa is Fig. 2. Sailplan of Sparkman and Stephens 24-foot. similar to the Cp measured over the asymmetric spinnaker by the

same authors in previous research (Viola and Flay, 2009, 2010). 3.4. Reference dynamic pressure The leeward side of the mainsail shows similar trends to the genoa, but the laminar separation occurs on the mast, and the Static and dynamic pressures were measured with three Pitot turbulent reattachment occurs at around 10% of the mainsail static tubes located on the windward side of the yacht. Two of chord. Between the leading-edge suction peak and the pressure

them were fixed to the shrouds at two different heights. These recovery due to the reattachment, the pressure is almost constant. were oriented approximately 351 to windward of the boat’s The Cp trends over the mainsail are in agreement with the heading. The third one was fixed on a pole, which held the probe measurements performed on a 2D mast/mainsail section by roughly 1.5 m away from the boat stern on the windward side. Wilkinson (1989). This latter probe was free to pivot into the wind direction and was found to be the most reliable of the three of them. The differential 4.2. Pressures on the mainsail pressure between the total probe on the pole and the cabin reference static pressure was taken to be the reference dynamic Fig. 4 shows the Cp over the three sections of the mainsail pressure. versus the position x along the chord c. The three curves are The measurements were performed in September 2009 in the obtained with different trims of the mainsail sheet, which mainly Hauraki Gulf (Auckland, NZ), in roughly 4 m/s of breeze. All the changes the angle of incidence of the sail. tests were performed sailing upwind, with AWAs between 251 When the mainsail is tightened (trim #+1 in the figure), the and 451. maximum differential pressure between the two sides is

achieved. The Cp on the windward side is almost 1.0 over most of the chord, meaning it is a very low speed region. On the 4. Results and discussion leeward side, the leading-edge suction peak reaches roughly C 2 on the top section and C 1 on the bottom section. p ¼À p ¼À The pressures on the mainsail were measured for different When the mainsail is eased (trim #0 in the ﬁgure), the mainsail and genoa trims. For each trim, the boat was sailed on differential pressure between the windward and the leeward the genoa trim using tell-tails, which means that the helmsman sides of the sails decreases. However, because easing the mainsail helmed the boat to keep the optimum course for the genoa trim. sheet decreases the angle of incidence but does not change the In this way, only optimum trims were tested. Hence, the AWA sail shape signiﬁcantly, the second suction peak due to the sail

depended on the genoa trim. curvature also does not change significantly. The windward Cp is The pressures on the genoa were investigated for different lower showing a higher velocity on the windward side. The genoa trims. The effect of changing the angle of incidence of the leading-edge suction on the leeward side decreases because of the genoa was investigated by sailing at a fixed AWA. This investiga- decreased angle of incidence. tion allowed the optimum pressure distribution for a fixed AWA When the sail is eased even further (trim # 1 in the figure), À to be determined. Then, different AWAs were sailed and the genoa the angle of incidence becomes negative and the first 10% of the

was trimmed according to each AWA. mainsail has negative Cps on the windward side and positive Cps The following results are shown in terms of pressure on the leeward side.

coefficients Cp, defined as the difference between the pressure The best trim depends on the wind conditions. In fact, from the measured on the sail surface p and the reference pressure inside figure it can be argued that by tightening the sheet, the pressure the cabin, divided by the reference dynamic pressure q measured forces on the sail increase, resulting in higher thrust and side force by the poled Pitot static tube. Note that the vertical axis is components. In the tested condition, trim #0 allowed the highest 804 I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807

804 I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807

Fig. 3. Schematic diagram of the pressure distributions over the genoa and the mainsail, and the corresponding ﬂow ﬁeld.

Fig. 4. Cp over the mainsail for 3 mainsail sheet trims. Fig. 5. Cp over the mainsail for 3 mainsail leech tensions.

boat speed. Trim #+1 resulted in excessive side force and hence, a tightened (from # 1 to #+1 in the ﬁgure), the twist decreases Fig. 4. Cp over the mainsail for 3 mainsail sheetÀ trims. Fig. 5. Cp over the mainsail for 3 mainsail leech tensions. higher heeling moment, which heeled the boat, increasing the and the angle of incidence of the highest sections increases. hydrodynamic resistance. Therefore, on the highest section the leading-edge suction peak

Fig. 5 shows the Cp over the three mainsail sections, for three increases and trailing edge separation occurs, which causes the mainsail twists. Twist was changed by tightening the leech, second suction peak to decrease. When the maximum leech boat speed. Trim #+1 resulted in excessive side force and hence, a tightened (from # 1 to #+1 in the figure), the twist decreases through the vang, the traveller and the sheet. On the top two tension is achieved (#+1), the flow is mainly separated, leading to À sections, only the leewardhigher side pressure heeling is plotted. moment, The change which in heeledan almost the constant boat,C increasingp. The low mean the speed ofand the separated the angle flow of incidence of the highest sections increases. twist affects the Cp overhydrodynamic only the highest resistance.sections, while the is unable to lift the tell-tails on the highestTherefore, part of the mainsail on the highest section the leading-edge suction peak lowest section is insensitive to the twist. When the leech is leech. Fig. 5 shows the Cp over the three mainsail sections, for three increases and trailing edge separation occurs, which causes the mainsail twists. Twist was changed by tightening the leech, second suction peak to decrease. When the maximum leech through the vang, the traveller and the sheet. On the top two tension is achieved (#+1), the flow is mainly separated, leading to

sections, only the leeward side pressure is plotted. The change in an almost constant Cp. The low mean speed of the separated ﬂow twist affects the Cp over only the highest sections, while the is unable to lift the tell-tails on the highest part of the mainsail lowest section is insensitive to the twist. When the leech is leech. 804 I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807

I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807 805

From the ﬁgure, it can be argued that by increasing the leech tension the pressure force increases at the leading edge and decreases at the trailing edge, which rotates the direction of the resultant aerodynamic force further forward, leading to an increased thrust force and a reduced side force. However, increase in the leech tension also rotates the top sail section backwards, which leads to the opposite effect. Therefore, the best trim is a medium trim, which maximizes the resultant thrust component. Moreover, by increasing the leech tension, the aerodynamic load on the highest sections decreases, which leads to a lower centre of effort.

Fig. 6 shows the Cp over the three sections of the mainsail for three sail trims, where the effect of the size of the gap between sails is investigated. With regard to the reference trim (#0 in the figure), firstly the mainsail was tightened and the genoa was eased, which increased the gap (#+1 in the figure), then the mainsail was eased and the genoa was tightened, which Fig. 7. Cp over the mainsail for 3 backstay trims. decreased the gap (# 1 in the figure). When the gap between À the sails is increased, the Cp on the mainsail becomes constant along the three sections, showing a fully separated flow, which causes the leech tell-tails to collapse. In fact, easing the genoa leads to a lower downwash on the mainsail, which leads to a larger angle of incidence. Hence, when the mainsail was tightened and the genoa was eased, the mainsail stalled due to the excessive angle of incidence. Conversely, when the mainsail was tightened without easing the genoa (Fig. 4, trim #+1), only the top section of the mainsail showed separated flow. When the gap between sails is decreased (# 1), the lowest À Fig. 3. Schematic diagram of the pressure distributions over thesections genoa and of the the mainsail, mainsail, and the where corresponding the genoa flow overlap field. is larger, show

an inversion of pressure/suction between the windward/leeward sides of the sail. The mainsail stagnation point moves downstream on the leeward side of the mainsail. On the highest section,

where the overlap is negligible, the Cp distribution shows the presence of a laminar separation bubble and hence, the stagnation point is at the leading edge and the angle of incidence is larger than the ideal angle of attack. Hence, the low angle of incidence at Fig. 8. C over the genoa for 5 genoa-sheet trims. the lowest section is due to over-trim of the genoa and is not due p to the mainsail trim. The best trim depends on the wind conditions. In fact, the Fig. 7 shows the Cp over the three sections of the mainsail for three backstay tensions. Tightening the backstay (from # 1 to second suction peak is located about 40% of the chord, and hence À #+1 in the ﬁgure), causes the mast to bend and ﬂattens the it generates a large side force component and a small thrust force mainsail. The lowest shrouds oppose the mast bending, hence the component. When side force or heel moment has to be decreased, the backstay should be tightened. Of course this is common highest part of the mast bends more than the lowest part. The Cp on the lowest section is not affected by the backstay trim, while practice in yacht racing.

the Cp on the top section is affected the most. When the mainsail is ﬂattened, the second suction peak, which is due to the sail 4.3. Pressures on the genoa curvature, decreases.

Fig. 4. Cp over the mainsail for 3 mainsail sheet trims. Fig. 5. Cp over the mainsail for 3 mainsail leech tensions. Fig. 8 shows the Cp over the three genoa sections for ﬁve

genoa-sheet trims. Only the Cp over the leeward side of the genoa is plotted. For all the trims, the boat was sailed at the same AWA 301. boat speed. Trim #+1 resulted in excessive side force and hence, a tightened (from # 1 to #+1 in the figure), the twist decreases ¼ À In the first trim (#0 in the figure), the genoa was trimmed to higher heeling moment, which heeled the boat, increasing the and the angle of incidence of the highest sections increases. the ideal angle of attack (Theodorsen, 1931), which means that the hydrodynamic resistance. Therefore, on the highest section the leading-edge suction peak stagnation point is on the leading edge and the suction is entirely Fig. 5 shows the Cp over the three mainsail sections, for three increases and trailing edge separation occurs, which causes the due to the profile curvature. This condition is achieved when the mainsail twists. Twist was changed by tightening the leech, second suction peak to decrease. When the maximum leech genoa luff is close to collapse and it is sailed when the minimum through the vang, the traveller and the sheet. On the top two tension is achieved (#+1), the flow is mainly separated, leading to AWA has to be achieved for tactical racing purposes. Tightening sections, only the leeward side pressure is plotted. The change in an almost constant Cp. The low mean speed of the separated flow the sheet (#+1) causes leading-edge suction peak to occur and a twist affects the Cp over only the highest sections, while the is unable to lift the tell-tails on the highest part of the mainsail minimum C 5 was measured. The increased suction at the p¼À lowest section is insensitive to the twist. When the leech is leech. leading edge is correlated to a large thrust force component increase. This condition is achieved when the genoa tell-tails are aligned horizontally, and it is sailed when the maximum boat speed has to be achieved. When the sheet is tightened further (from #+2), the trailing edge separation point moves upstream until all the sail is fully stalled in trims #+3 and #+4 and all the

Fig. 6. Cp over the mainsail for 3 genoa/mainsail gap amplitudes. tell-tails collapse.

I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807 805 I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807 805

FromFrom the the ﬁgure, ﬁgure, it can be argued that by increasing the the leech leech tensiontension the the pressure pressure force increases at the leading edge edge and and decreasesdecreases at at the the trailing trailing edge, which rotates the direction of of the the resultantresultant aerodynamic aerodynamic force further forward, leading to to an an increasedincreased thrust thrust force force and and a reduced side force. However, increase increase inin the the leech leech tension tension also also rotates the top sail section backwards, backwards, whichwhich leads leads to to the the opposite effect. Therefore, the best trim trim is is a a mediummedium trim, trim, which which maximizes the resultant thrust component. Moreover,Moreover, by by increasing increasing the leech tension, the aerodynamic load load onon the the highest highest sections sections decreases, which leads to a lower centre centre of of effort.effort.

Fig.Fig. 6 6 showsshows the the Cp over the three sections of the mainsail for for threethree sail sail trims, trims, where where the effect of the size of the gap between between sailssails is is investigated. investigated. With regard to the reference trim (#0 in in the the figure),figure), firstly firstly the the mainsail was tightened and the genoa was was eased,eased, which which increased increased the gap (#+1 in the figure), then then the the mainsailmainsail was was eased eased and the genoa was tightened, which which Fig.Fig. 7. 7. CCppoverover the the mainsail mainsail for for 3 3 backstay backstay trims. trims. decreaseddecreased the the gap gap (# 1 in the figure). When the gap between between À À C thethe sails sails is is increased, increased, the Cp on the mainsail becomes constant constant alongalong the the three three sections, sections, showing a fully separated flow, which which causescauses the the leech leech tell-tails tell-tails to collapse. In fact, easing the genoa genoa leadsleads to to a a lower lower downwash downwash on the mainsail, which leads leads to to a a largerlarger angle angle of of incidence. incidence. Hence, when the mainsail was was tightened tightened andand the the genoa genoa was was eased, the mainsail stalled due to the excessive angleangle of of incidence. incidence. Conversely, when the mainsail was tightened withoutwithout easing easing the the genoa (Fig. 4, trim #+1), only the the top top section section of of the mainsail showed separated flow. the mainsail showed separated flow. When the gap between sails is decreased (# 1), the lowest When the gap between sails is decreased (#À 1), the lowest sections of the mainsail, where the genoa overlapÀ is larger, show sections of the mainsail, where the genoa overlap is larger, show an inversion of pressure/suction between the windward/leeward an inversion of pressure/suction between the windward/leeward sides of the sail. The mainsail stagnation point moves down- sides of the sail. The mainsail stagnation point moves downstream on the leeward side of the mainsail. On the highest section, stream on the leeward side of the mainsail. On the highest section, where the overlap is negligible, the C distribution shows the where the overlap is negligible, the Cp distribution shows the presence of a laminar separation bubblep and hence, the stagnation presence of a laminar separation bubble and hence, the stagnation point is at the leading edge and the angle of incidence is larger pointthan the is at ideal the angle leading of attack. edge and Hence, the the angle low of angle incidence of incidence is larger at Fig. 8. C over the genoa for 5 genoa-sheet trims. than the ideal angle of attack. Hence, the low angle of incidence at p the lowest section is due to over-trim of the genoa and is not due Fig. 8. Cp over the genoa for 5 genoa-sheet trims. theto the lowest mainsail section trim. is due to over-trim of the genoa and is not due to the mainsail trim. The best trim depends on the wind conditions. In fact, the Fig. 7 shows the Cp over the three sections of the mainsail for secondThe suction best trim peak depends is located on about the wind 40% of conditions. the chord, In and fact, hence the threeFig. backstay 7 shows tensions. the Cp over Tightening the three the sections backstay of the (from mainsail # 1 for to À second suction peak is located about 40% of the chord, and hence three#+1 in backstay the figure), tensions. causes Tightening the mast the to backstay bend and (from flattens # 1 the to it generates a large side force component and a small thrust force À it generates a large side force component and a small thrust force #+1mainsail. in the The figure), lowest shroudscauses the oppose mast the to mast bend bending, and flattens hence the the component. When side force or heel moment has to be decreased, thecomponent. backstay When should side be force tightened. or heel moment Of course has this to be is decreased, common mainsail.highest part The of lowest the mast shrouds bends oppose more thethan mast the lowestbending, part. hence The theCp practicethe backstay in yacht should racing. be tightened. Of course this is common higheston the lowest part of section the mast is bendsnot affected more than by the the backstay lowest part. trim, The whileCp practice in yacht racing. onthe theCp on lowest the top section section is not is affected affected the by most. the backstay When the trim, mainsail while theis flattened,Cp on the the top secondsection issuction affected peak, the which most. When is due the to mainsail the sail 4.3. Pressures on the genoa iscurvature, flattened, decreases. the second suction peak, which is due to the sail 4.3. Pressures on the genoa curvature, decreases. Fig. 8 shows the Cp over the three genoa sections for five genoa-sheetFig. 8 shows trims. the OnlyCp theoverCp over the three the leeward genoa side sections of the for genoa five isgenoa-sheet plotted. For trims. all the Only trims, the Cp theover boat theleeward was sailed side at of the the same genoa AWAis plotted.301. For all the trims, the boat was sailed at the same ¼ AWAIn the30 first1. trim (#0 in the figure), the genoa was trimmed to ¼ theInideal the angle first of trim attack (#0(Theodorsen, in the figure), 1931 the), genoa which was means trimmed that the to stagnationthe ideal angle point of is attack on the(Theodorsen, leading edge 1931 and), the which suction means is entirely that the duestagnation to the profile point is curvature. on the leading This condition edge and is the achieved suction when is entirely the genoadue to luff the is profile close tocurvature. collapse This and itcondition is sailed is when achieved the minimum when the AWAgenoa has luff to is be close achieved to collapse for tactical and it isracing sailed purposes. when the Tightening minimum theAWA sheet has (#+1) to be achievedcauses leading-edge for tactical suction racing purposes. peak to occur Tightening and a minimum C 5 was measured. The increased suction at the the sheet (#+1)p¼À causes leading-edge suction peak to occur and a leadingminimum edgeC is correlated5 was measured. to a large The thrust increased force suction component at the p¼À increase.leading edge This condition is correlated is achieved to a large when thrust the genoa force tell-tails component are alignedincrease. horizontally, This condition and is it achieved is sailed when when the the genoa maximum tell-tails boat are speedaligned has horizontally, to be achieved. and it When is sailed the sheet when is the tightened maximum further boat (from #+2), the trailing edge separation point moves upstream speed has to be achieved. When the sheet is tightened further until all the sail is fully stalled in trims #+3 and #+4 and all the (from #+2), the trailing edge separation point moves upstream Fig. 6. Cp over the mainsail for 3 genoa/mainsail gap amplitudes. tell-tails collapse. until all the sail is fully stalled in trims #+3 and #+4 and all the

Fig. 6. Cp over the mainsail for 3 genoa/mainsail gap amplitudes. tell-tails collapse. 806 I.M. Viola, R.G.J. Flay / J. Wind Eng. Ind. Aerodyn. 98 (2010) 800–807

of the sails is almost 1.0 over the whole section due to low speed of the flow. On the leeward side, the pressure coefficient shows a suction peak first, followed by a pressure recovery at around 10% of the sail chord, which is related to the turbulent reattachment after the laminar separation bubble. Then a second suction peak occurs due to the sail curvature, followed by a pressure recovery and, eventually, by a pressure plateau due to the trailing edge separation. In particular, the following conclusions can be drawn from the pressure distributions on the mainsail:

The mainsail sheet primarily affects the leading-edge suction peak. Tightening the sheet causes the leading-edge suction peak to increase, while the second suction peak due to the sail curvature is not affected signiﬁcantly. The mainsail twist primarily affects the trailing edge separation. Decrease in the twist causes the angle of incidence of the Fig. 9. Cp over the genoa for 4 AWAs. highest sections to increase, which causes the suction to increase until the trailing edge separation becomes excessive. Fig. 9 shows the Cp over the three genoa sections for AWAs The gap between the genoa and the mainsail must be trimmed ¼ 271, 301, 451 and 701. The genoa and the mainsail are trimmed with care. In fact, a small gap increases the pressure on the according to the AWA. leeward side of the mainsail, while a large gap causes the On the lowest section, the trends of the leeward Cp curves are mainsail to stall. similar to the trim #+1 in Fig. 8, which was the trim that allowed The camber of the mainsail (trimmed with the backstay the maximum boat speed. Hence, the genoa was trimmed tension) primarily affects the second suction peak, which is correctly on the lowest section. On the highest section, the trends due to the section curvature. Hence, a larger camber allows a of the leeward Cp curves are similar to the trim #+0 in Fig. 8, larger suction but it increases the side force component more which was the trim that allowed the minimum AWA to be sailed. than the thrust force component. Hence, the genoa was under-trimmed on the highest section. If the genoa twist was decreased, for instance by moving the car, the The pressure distribution on the genoa showed two possible angle of attack of the highest section would have increased. Lower genoa trims. When the genoa is trimmed at the ideal angle of pressures would have been achieved at the leading edge followed attack, only one suction peak is present and the suction is all due by a pressure recovery, as shown by the lowest section. The to the sail curvature. This trim allows the lowest apparent wind systematic error in the genoa trim is partially due to the angle to be sailed. When the genoa is slightly over-trimmed, a unfavorable sheeting angle at large AWAs. In fact when leading-edge suction peak occurs and the maximum drive force is the AWA increases, the car should be moved forward and away achieved. This trim allows the maximum boat speed to be from the centerline. On the yacht Aurelie the car can be moved achieved. only forward and backwards. Hence, it cannot be properly When large AWAs are sailed, the unfavorable genoa sheeting trimmed for large AWAs. At AWA 271 and 301, the over-twist ¼ angle leads to an excessive twist and to deeper bottom sections. of the genoa is due to a wrong trim; the car should have been As a consequence, the top sections are under-trimmed and the moved a little further. At AWA 451 and 601, the over-twist of the ¼ bottom sections show large trailing edge separation, which lead to genoa is due to the impossibility to properly trim the car at large a reduced suction on the sail. AWAs. As a consequence, the larger the AWA the more the top section is under-trimmed and the higher is the pressure on the leeward side. Acknowledgment The leeward side pressure also increases on the lower section when the AWA increases, because the sail section becomes deeper The support of staff and students in the YRU is gratefully and deeper, and the trailing edge separation point moves forward. acknowledged in helping to carry out the full-scale pressure In conclusion, the pressure load on the genoa decreases measurements and to sail Aurelie. signiﬁcantly when the AWA increases. This is due to the unfavorable sheeting angle, which leads to an over-twist on the top sections and to excessively deep lower sections. References

Abbot, I.H., Von Doenhoff, A.E., 1949. Theory of wing section. Dover Publications 5. Conclusions Inc, New York, ISBN: 0-486-60586-8. Crompton, M.J., Barret, R.V., 2000. Investigation of the separation bubble formed behind the sharp leading edge of a flat plate at incidence. In the Proceedings of The present paper shows pressure distributions on a full-scale the Institution of Mechanical Engineers, Part G: Journal of Aerospace genoa and mainsail. The paper shows how the pressure distribu- Engineering 214 (3), 157–176. Flay, R. G. J., Millar S., 2006. Experimental consideration concerning measurements tions change with the sail trim and with the apparent wind angle. in sails: wind tunnel and full scale. In: Proceedings of the Second High These trends can be used to validate the numerical codes that are Performance Yacht Design Conference (HPYDC2), 14–16 February, Auckland, commonly used in the sail design practice. Moreover, the value of New Zealand. Gaves, W., Barbera, T., Broun, J. B., Imas, L., 2008. Measurements and simulation of pressure measurements to help understand and optimize sail pressure distribution on full size scales. In: Proceedings of the Third High trims is pointed out. Performance Yacht Design Conference (HPYDC3), 2–4 December, Auckland, The pressure coefficients and the related flow field over a New Zealand. Gentry, A., 1971. The aerodynamics of sail interaction. In: Proceedings of the Third generic genoa and mainsail section are shown in the form of a AIAA Symposium on the Aero/Hydronautics of Sailing, 20th November, schematic diagram. The pressure coefficient on the windward side Redondo Beach, CA, USA.