4th High Performance Yacht Design Conference Auckland, 12-14 March, 2012

PRESSURE DISTRIBUTION ON SAIL SURFACES IN REAL SAILING CONDITIONS

Miro Lozej1, [email protected] Damjan Golob 2, [email protected] , Bojan Vrtič 3, [email protected], Drago Bokal 4, [email protected]

Abstract. Simultaneous measuring of pressures in a number of measuring points on the sail surfaces while sailing in real conditions can give good representations of pressure distribution on the sail. Combined with sail profiles measured by a compass-based measuring system an approximation of the force vector field can be deduced. The measured data can be analyzed in real time and/or used for offline analysis. The analyses gave interesting insights on how the sail performs in actual conditions. Effects of boat pitching, sails interactions, distortions of sail surfaces and various unexpected pressure distributions were detected. Combining data from the system with other instruments data an empirical model can be deduced. Our goal is to develop a highly sensitive measuring and analytic tool for sail tailors and yacht designers and stripped down to essentials an accessible aid to non-professionals.

1. INTRODUCTION The system was tested on various types of yachts (TP52 in September 2007, X40 in May 2008, RC44 in Measuring pressure distributions on sail surfaces can September 2009, and other) showing operation stability give a lot of information about sail efficiency since they and producing data that opened many new questions. are closely related to forces that drive the boat. The forces computed out of measured pressures and Multiplication of measured pressure values with the using an approximation of sail profiles (sail makers data) representative surfaces (disjoint and covering all sail Answered seldom of them. surface) gives the scalars that multiplied with surface A new version of the system is under development to normal vectors represent the forces vectors at the incorporate compasses at each of the measuring points so measuring points. Summation of all the vectors gives the that sail shape can be dynamically measured and force approximation of resulting force vector that can be vectors computed in real time even with changing sail decomposed into the heeling and driving components. trim. Numerical modeling and wind tunnel tests are proven tools to understand the aerodynamics of the sails [1]. 2. MEASURING THE PRESSURES However these methods neglect or model relatively poorly the unsteadiness of the wind, the movement of the Differential pressure transducers are used to measure sails and the yacht. An on-water system with an pressures on many measuring points. The actual system instrumented sail can reveal many of these more complex has 72 measuring points or less depending on the interactions. mainsail dimensions. The system can easily be Warner and Ober reported on their pressure reconfigured to higher number of measuring points if measurements using U-tube manometers in the 1925 [2]. needed. Electronic instruments and availability of small pressure The windward and leeward side distributions are transducers with estimated accuracy about ±0.5 Pa measured separately at the same measuring point on the revived the interest in such measurements recently [3], sail simultaneously. Each transducer is connected by a [4], [5]. Extensive work has already been done to tube to the measuring point and by another tube to the compare the results of numerical modeling, wind tunnel common pressure reference point. Pressures are trials and on-water sail tests [6]. communicated with 2.5 mm PVC pressure tubes. This paper describes the prototype system and analysis Tests on 50 m long tubes of 2.5 mm bore with signals based on sea trials with several modern performance generated using oscilloscope and reproduced as sound by sailboats. The prototype system for measuring the a woofer gave good results. The tubes caused negligible pressure distributions on the sail surfaces was developed distortions of the pressure changes and virtually no delay. in 2006 at the COSYLAB Control System Laboratory However, distortions quickly deteriorated the signals (http://www.cosylab.si/). It was intended for high when tubes with smaller bore were tested. precision, high speed, real time data acquisition.

1 Business Developer, COSYLAB Control System Laboratory 2 Senior Hardware Developer, COSYLAB Control System Laboratory 3 Physics Researcher, COSYLAB Control System Laboratory 4 Senior Mathematics Researcher, COSYLAB Control System Laboratory and Faculty of Natural Sciences and Mathematics, University of Maribor. Figure 1. The schematic presentation of the prototype system.

System was used on yachts with masts not higher than 25 m – so all tests stayed within the 50 m distortion testing limit. The reference measuring point was located in a calm place in the cabin wrapped in porous foam so that the airflow there was negligible. On the X40 catamaran the location was in a box at the mast foot behind the beam, connecting the hulls. The measuring points are arranged in horizontal lines. In our case up to six measuring points (Figure 2.). The Figure 2. RC44 example of measuring points arrangement transducers corresponding to such a line are concentrated on the mainsail. in small boxes (130 mm by 130 mm by 35 mm) packed with the sampling and communication electronics (Figure 3). Standard cables and connectors are used for power and communication on the prototype and shall be specifically designed in future versions. The boxes are located 200 mm from the luff not to interfere with the mast in downwind conditions. The first measuring points are located another 300 mm away from the boxes. Pairs of boxes from opposite sides are attached to each other through the sail fabric by two small screws. The cables and tubes are attached to the sail with adhesive strips. This arrangement makes the prototype portable from sail to sail. (In future versions, the dimensions of the boxes shall be minimized and other methods shall be used to attach the Figure 3. The prototype box with 7 pressure transducers, components on the sail.) electronics and connectors. Special care was taken to prevent unwanted aerodynamic Horizontal and vertical tangential directions could also effects at the measuring point tubes endings. The be derived and used to determine the flying shape of the pressure variations caused by small turbulences are sail and the cord line directions relative to the longitude averaged using a cover of permeable foam. The tube axis. ending is covered with foam and wrapped with adhesive plastic sheet leaving two openings where the tube is in pneumatic contact with the environment. The tips are 120 mm apart (Figure 4.). Silicone was used to prevent direct contact at the tube surface. To install the system on the sail 100 mm wide adhesive strips are used so as not to not cover the pressure tip endings.

Figure 5. The sail profile reconstruction out of the Figure 4. Pressure tap (left) and pressure tap mounted on measured normal vectors angles. the sail using the adhesive tape.

A dedicated processor and disk is needed for data A good approximation of shape and cord lines angels collection, computing and storage. The components are could be numerically recorded thus providing essential mounted in a system box that also houses the data to make the on-water tests more repeatable. There inclinometer, GPS system and the wireless would be no need to record the marks on the sheets and communication to connect the system with other portable deck except to quickly find the sail trim that reproduce computers on the boat or on chase boats in wireless the same flying shape. range. The system box is mounted in the yacht cabin so The earth’s magnetic field is used for these that it is level and aligned with the yacht longitude axis measurements. It can be assumed, that at the level of on in order to obtain accurate inclination measurements. board distances the magnetic field is homogeneous and Connection to other measurement systems shall be the global magnetic field anomalies don’t produce errors implemented using the standard shipboard protocols. in computing the angle differences. Testing will be required to root out on-board sources of magnetic errors however. 3. MEASURING THE NORMAL VECTORS

While several techniques exist to determine the sail 4. DATA PRESENTATION surface’s normal vector, in the tests presented in this paper, sail profiles were derived from sail maker’s data. Pressure distribution measurement produces many time Sail trim was carefully recorded so that this data could be series where values vary in space and time. A spatial adjusted in later analysis. display of isobars on the sail can be reproduced on A new idea is to use tiny electronic compasses at the different screens in real-time. measuring points on the sails coupled with the referential For offline analysis MS Excel computation and one in the system box (the system is patent pending). presentations capabilities are used for simplicity despite The horizontal and vertical angle differences obtained by the limit that only equidistant surface diagrams are the compasses at the measuring points and the referential possible. This causes some distortion of isobar shapes but compass located in the system box would be measured. the measured values are correctly presented. This is another reason why the box must be level and Figure 6. presents an example sequence of six snapshots aligned with the yacht longitude axis. Differences of measured on RC44 yacht in windward conditions every 3 measured angles would give the directions of the normal tenth of a second. vectors at the measuring points needed to compute the forces vectors. Pa 52,0 However the sea trials were not long enough for racers to 48,0 44,0 2 1 40,0 combine the “sail thrust” values with their existing skills. 36,0 32,0 28,0 It is reasonable to think that such a fast acting signal 24,0 20,0 16,0 could amplify the racers feedback. Another strong 12,0 8,0 4,0 potential for improvement would be to use the data 0,0 -4,0 -8,0 through maneuvers (rather than steady state speed -12,0 -16,0 -20,0 optimization). Here the additional high speed signal -24,0 -28,0 -32,0 could help teams identify which elements of a particular -36,0 -40,0 -44,0 maneuver (1/10 of second by 1/10 of second) is near -48,0 -52,0 optimal and when driving force is effectively lost so that attention could be directed to other aspects of the 3 4 maneuver other than sail shape and position. It was expected that the influences of the systems components on the sail would mask minor differences caused by deformities in the sail - in practice this was not the case. Anomalies in the sail profiles were spotted in real time (Figure 7.).

X40

Pa 5 6 35,1 32,4 29,7 27,0 24,3 21,6 18,9 16,2 13,5 10,8 8,1 5,4 2,7 0,0 -2,7 -5,4 -8,1 -10,8 -13,5 -16,2 -18,9 Figure 6. A time sequence of isobar diagrams three tenths -21,6 -24,3 of a second apart is representing the pressure dynamics. -27,0 -29,7 -32,4 -35,1 Each snapshot contains isobar presentations of the pressure distribution at the starboard and port side of the sail (left and middle graphic). An isomorphic presentation of the RC44 mainsail is added (far right Figure 7. Distorted batten caused abnormal pressure graphic) with exact presentation of measuring point distribution at the windward surface on X40 catamaran in locations in Figure 6. Three centers of effort are also windward condition. displayed. The green point for the starboard side forces, the red point for the port side forces and the white one Racers reported that sail performance was not for the forces, computed out of differences of pressures significantly altered by the addition of the measurement from the two sail sides. system. This suggests that it might be possible to build systems that could remain installed during racing. 5. ONBOARD USE OF THE SYSTEM It is expected that changes of isobar patterns on different It was hoped that the system would provide real-time tacks could reveal the presence of wind shear which is feedback to skippers that would allow them to maximize often very difficult to detect by other means. Further tests the driving force of the sail when desired by are needed to catch such conditions since this was not the decomposing the forces vectors into longitudinal and case yet. orthogonal components. It was possible to measure “sail The pressure distribution on the mainsail during trust”. When wind and sea conditions are bounded, on- downwind conditions shows that the mainsail is trim and suboptimal trim were clearly distinguishable in producing very little “sail thrust”. By contrast, the X40 this “sail thrust” value and as expected, these catamaran data indicates that the upper part of the measurement had far lower latency than direct mainsail (the part higher than the gennaker head) still measurement of boat speed. contributes some pressure and likely sensitive to improper trimming (Figure 8.). X40 TP52

Pa Pa 35,1 35,1 32,4 32,4 29,7 29,7 27,0 27,0 24,3 24,3 21,6 21,6 18,9 18,9 16,2 16,2 13,5 13,5 10,8 10,8 8,1 8,1 5,4 5,4 2,7 2,7 0,0 0,0 -2,7 -2,7 -5,4 -5,4 -8,1 -8,1 -10,8 -10,8 -13,5 -13,5 -16,2 -16,2 -18,9 -18,9 -21,6 -21,6 -24,3 -24,3 -27,0 -27,0 -29,7 -29,7 -32,4 -32,4 -35,1 -35,1

Figure 8. The upper leeward surface of X40 mainsail is still Figure 10. The influence of the jib on the TP52 mainsail in producing good suction in downwind conditions. upwind conditions.

The leeward area in the bottom trailing edge is often bearing a positive pressure in windward conditions (Figure 11.). Analysis showed that pitching movement of In upwind condition the influence of jib to the leeward the yacht causes synchronous oscillations of the side of the mainsail leading edge is recognizable (Figures pressures (Figures 12, 15. and 16.). The leeward 9. and 10.). pressures values at the mainsail trailing edge are negative (suction) but small. If the oscillations amplitudes are big enough those values can become positive (Figure 11. locations LA6, LB6).

RC44 RC44 Pa Pa 35,1 52,0 32,4 48,0 29,7 44,0 27,0 40,0 24,3 36,0 21,6 32,0 18,9 28,0 16,2 24,0 13,5 20,0 10,8 16,0 8,1 12,0 5,4 8,0 2,7 4,0 0,0 0,0 -2,7 -4,0 -5,4 -8,0 -8,1 -12,0 -10,8 -16,0 -13,5 -20,0 -16,2 -24,0 -18,9 -28,0 -21,6 -32,0 -24,3 -36,0 -27,0 -40,0 -29,7 -44,0 -32,4 -48,0 -35,1 -52,0

Figure 9. The influence of the jib on the RC44 mainsail in Figure 11. Locations of measuring points where pressures upwind conditions. presented in Figure 12. were measured. 40 40 30 30 20

) 20 a 10 P ( e r )

u 0 10 a s P

s Time (1/10 sec) ( e r -10 e r

P 0 u s

-20 s e r -10 P -30 Time (1/10 sec) -20 -40 5 6 9 2 3 8 0 3 4 7 0 2 9 7 4 1 8 6 3 0 8 5 , , , , , , , , , , , 7 4 5 7 4 8 0 2 9 1 2 -30 3 3 4 4 4 4 4 4 5 5 5 : : : : : : : : : : : 6 6 6 6 6 6 6 6 6 6 6 5 5 5 5 5 5 5 5 5 5 5 : : : : : : : : : : : 2 2 2 2 2 2 2 2 2 2 2

1 1 1 1 1 1 1 1 1 1 1 -40 0 6 3 9 6 2 8 5 1 8 4 0 7 3 0 6 4 0 7 6 3 2 1 9 8 7 5 6 3 1 9 5

LA6 LB6 LC3 RA6 RB6 RC3 now , , , , , , , , , , , , , , , , 0 3 4 5 7 8 9 9 0 1 2 9 1 2 3 6 0 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 : : : : : : : : : : : : : : : : 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 : : : : : : : : : : : : : : : : 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Figure 12. Sections of time pressure series at the locations 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 LA3 LB3 LC3 LD3 LE2 LF2 LG1 on Figure 11. (Green spike shows when the snapshot in Figure 11. was taken). Figure 15. Time series measured at the leeward side at the measuring points indicated on the Figure 13. 6. DATA ANALYSIS Figures 14. and 15. show that the amplitudes of pressure In all conditions, signals have a strong periodic oscillations also on grow with height on both sail sides. component. The force on the sail is a vector sum of the windward and leeward surface contributions.

RC44 1400

Pa 1300 35,1 32,4 1200 29,7 1500 27,0 1100 24,3 1400 21,6 1000 18,9 1300 900 16,2 13,5 s 1200 e 800 c

10,8 r o f 8,1 700 1100 5,4 g n i 2,7 l 600 e 1000 0,0 e H -2,7 500 900 -5,4 400 -8,1 e 800 c

-10,8 r

300 o f

-13,5 700 -16,2 g n

200 i -18,9 l e 600 -21,6 e

100 H -24,3 500 -27,0 0 -29,7 -32,4 Port heeling force Time 400 -35,1 Starboard heeling f. Total heeling force 300

200

400 100

300 s 0 e c

r 200 o

f -100

g -100 0 100 200 100

Figure 13. Positions of the measuring points where the time n i v i

r Driving force series in Figures 14. and 15. were measured. 0 D

-100 Port driving force Time 40 Starboard driving f. Total driving force

30 Figure 16. The force decomposition into windward and 20 leeward components. To the left the heeling and driving

) 10

a component decomposition, to the right the vectors are P (

e

r 0 displayed. u s

s Tim e (1/10 s e c) e r

P -10

-20 Measurements show that the fractions of the total force produced by each side have similar averaged amplitudes. -30 In time they can oscillate quite violently but canceling -40 0 6 3 9 6 2 8 5 1 8 4 0 7 3 0 6 6 4 3 1 1 9 8 7 5 0 9 7 6 5 3 2

, , , , , , , , , , , , , , , , each other in a way, that the total sum stays quite calm. 9 0 1 2 9 9 0 1 2 3 3 4 5 6 7 8 0 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 : : : : : : : : : : : : : : : : 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 Figure 16. shows the time series and vector : : : : : : : : : : : : : : : : 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 RA3 RB3 RC3 RD3 RE2 RF2 RG1 representation of forces measured in windward conditions on RC44 yacht. Figure 14. Time series measured at the windward side at Grasping the dynamics out of still pictures is difficult. the measuring points indicated in Figure 13. Showing the snapshots like a film is needed (and possible in MS Excel) but not presentable in printed form. that these oscillations were induced by the sail itself Analyzing the dynamics in time domain is often difficult (natural frequencies) and are slightly different (higher too. The Fourier transforms can be used to shift the time frequency) from boat pitching. series from time domain to static spectra in frequency 18 domain forth and back. The frequency spectra show which frequencies are 16 strongly present in the signals. Figure 21. presents 14 spectra of all pressure time series, arranged in two groups – the leeward (bottom half) and windward ones (top 12 half). Spectra of the time series measured in horizontal 10 Pa lines are put together in subgroups which are ordered 15,6 14,4 8 13,2 12,0 from bottom to top of the sail. Presented data was 10,8 9,6 8,4 measured on the RC44 trials in upwind conditions. 6 7,2 6,0 4,8 3,6 In Figure 21. it is evident that frequencies near 0.602 Hz 2,4 4 1,2 0,0 are strongly present in all pressure signals increasingly -1,2 -2,4 -3,6 2 -4,8 from bottom to top of the sail and similar on both sides. -6,0 -7,2 -8,4 (The average of spectra of all pressure signals is a good -9,6 0 -10,8 -12,0 representative of what is happening since in general they -6 -4 -2 0 -13,2 -14,4 are very similar.) -15,6 To further investigate the pressure oscillations an Figure 18. The sail area influenced by frequencies in the inclinometer was installed in the system box. This made range from 0.703 to 0.903 Hz indicated on Figure 17. it possible to compare the spectra of the pressure signals with the spectra of the pitching signal. Figure 17. shows Interestingly, the pitching effects in windward and the result. Pitching spectra (the blue line) is presented leeward sail surface cancel each other. Faster relative multiplied by 16 to make the comparison with the velocity on the leeward side makes more suction when averaged spectra of the pressure signals (the orange line) the mast is moving forward and less, when backward more evident. Good match confirms the assumption that movement causes a drop in relative velocity. On the yacht pitching is reflected in pressure oscillations. windward side the effect is opposite. Forward mast

2000 movement causes lowering of the positive pressure and 1800 increases it while mast is moving backward. 1600

1400 RC44 e 1200 d u

t Pa i l 1000

p 52,0

M 48,0

A 800 44,0 600 40,0 36,0 400 32,0 28,0 200 24,0 20,0 0 16,0 0 0 1 1 1 2 2 2 3 3 4 4 4 4 0 0 1 1 1 1 2 2 2 3 3 3 3 3 4 4 4 4 8 8 2 6 6 0 4 0 4 6 0 4 8 0 2 6 0 4 0 4 8 2 8 2 6 8 2 2 6 0 12,0 0 0 0 1 1 2 2 2 3 3 4 4 4 5 5 6 6 6 7 7 8 8 8 9 9 0 0 0 1 1 2 , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 8,0 Pitching signal spectra multiplied by 16 Frequencies 4,0 Averaged pressure signals spectra 0,0 -4,0 -8,0 Figure 17. The average spectra over all pressure time series -12,0 -16,0 compared to spectra of the pitching signal. -20,0 -24,0 -28,0 -32,0 -36,0 Successful use of wave-piercing hulls on catamarans -40,0 -44,0 might have the same background – minimizing the -48,0 -52,0 pitching. Unfortunately measurements in calm and rough sea have not yet been done to verify if steady pressure signals can lead to greater performance or not. Figure 17. shows also some mismatch in the frequency Figure 19. The positions of the centers of effort: green – range from 0.703 to 0.903 Hz (marked by the red circle). starboard side, red – port side, white – difference starboard Taking just these frequencies and reconstructing the minus port side. corresponding time series gives us an insight about which part of the sail is mostly affected by these The windward and leeward centers of effort change their secondary oscillations. position quite violently in opposite directions (Figures 6. The source of additional oscillations has been found in and 19.) but the centre of effort of difference of the the upper trailing edge. As seen in the picture sequence forces at windward and leeward side (the white dot) stays (figure 18.) that part is relatively free to move from side in a limited region. Therefore the dynamics of pressures to side depending on the leech tension. It was concluded and forces on the two sail surfaces does not result in torques and no additional force is felt on the rudder There are methods that can be used for “parameters hiding it from our experience. fitting” and are an area of investigation by the authors. Ideally then, the knowledge base could be continuously 7. THE EMPIRICAL MODELS improved using data from different yacht configurations and environment conditions. The same model could also By storing the results in a database together with be adapted to other yachts and “parameter fitted” to the computed values and meta data a knowledgebase could specifics. be constructed that could empirically reveal, among other things, a new type polar diagram. Instead of showing the 8. CURRENT LIMITATIONS possible velocities, target “sail thrust” might prove more optimal and for sure would be faster responding. The presented system is a working prototype with A complete set of data from the instruments on board the potentials for improvements especially in the field of TP52 describing the 1 hour test in upwind and downwind data presentation, miniaturization, ease of installation conditions permitted the team to attempt the ambitious and durability. project to empirically model the event. Having the A simpler version of the system is being developed. essential time series of aerodynamic forces at hand and Miniaturized version of electronic is already developed using the data of other instrumented devices on the boat so, that together with the pressure transducer it can be including rudder angle, speed, etc. the effort lead to good placed directly at the measuring point. Measuring just the results. pressure difference between the two sides of the sail over Related to the system some time series are input and a small hole only one pressure transducer per measuring other time series are dependent to them. Each dependent point is needed and no tubing and pressure reference time series is represented by a parameterized formula point is required. The communication and power lines involving past values of other time series in order to could be eliminated by wireless data exchange powered generate a new set of computed values. Known formulas by photovoltaic units possibly combined by batteries for are used wherever possible. When relationships were not night sailing. The components for measuring the sail so obvious, a linear polynomial was used. The measured profiles shall also be fitted in the units. values can then be substituted one by one with computed Currently, the data acquisition frequency is 10 Hz. This values. By trial and error optimal parameters were can easily be set to higher values for more investigations selected to produce the minimal difference between into higher frequency events such as rig shake. The measured and computed values. MS Excel proved to be resolution of the system is 0.1 Pa and can still be the tool choice since immediate graphic presentation of improved. computed and measured time series made the trial cycles Using the presumed sail profiles can lead to some errors fast and effective. in forces computation. Sail shape must be measured If the formulas are not correct the process doesn’t directly. The system with compasses shall be included in converge to a solution. A good example is the need to next versions. include the time series describing the lee and weather The major part of data analysis and helm. Without that the relation between the rudder angle data presentation was done using MS and course over ground could not be modeled with Excel. The distributions are presented wanted accuracy. Appropriate delays have to be taken in with the surface diagrams. This implies account when present value is computed using past that the distributions are slightly values of other time series. Sometimes interpolation is distorted but still informative. needed to smooth out small fluctuations that in reality don’t affect other values due to masses and other For better display resolution, damping effects. horizontal and vertical interpolation is used. The drawback is that the regions After quite a lengthy search for suitable formulas and with uniform pressure that are not parameters the model can predict the boat speed and vertical or horizontal are presented as a course over ground within 5% margin. series of spots instead of being a Running the model with a hypothetical rudder time series homogeneous surface. (the tacking time interval values taken from real measurements) it reproduced the yacht behavior during Figure 20. The distorted presentation of a compact area. consecutive tacks revealing realistic time needed for the boat to resume the normal velocity after tacking. Well documented trim and environment changes are essential for the analysis. Synchronization of the data and Different setups of the yacht and different tactics lead to video recordings is necessary to exactly determine the different optimal parameter sets thus revealing the effects relations between the trim configuration changes and the of changes with numerical values. This might be useful measured response. The synchronization process takes a in testing and training phases when small details can be lot of time and appropriate tools to simplify this will be decisive for the end results in regattas if the parameter used in the future. finding method is fast enough. The system used a standard GPS to determine the 10. CONCLUSIONS locations and velocities. As with most unfiltered GPS signals, it often contains unwanted spikes that need to be Measuring pressures on the sail surfaces and sail shape in cleaned before the analysis. Kalman filter shall be used real time provides crucial insight into relations between in future tests to wipe out the spikes. trimming and sail efficiency that would result in greater driving force on the ship. In this contribution such The data from other sources are often sampled at lower system and some results of its application in real sailing rate and need to be interpolated to catch the sampling conditions is presented. Use of compasses for sail rate of 10 Hz at the pressure time series. The opposite reconstruction and the empirical model derived from (discarding the pressure time series down to 1 Hz) leads measured data are to our knowledge innovative to significant loss of information. contributions. Data analysis revealed several interesting facts: 9. THE POTENTIALS  The pitching frequencies of the boat are reproduced Many possible modes of operations of the system can be in pressure time series, confirming that the pitching envisaged. For the top level teams, the system is a good of the boat is a cause of extensive pressure research tool for sail and boat design as well as training. fluctuations. On such systems the pressure distributions on the two  In windward conditions the pitching can temporarily sail sides would be measured independently. The lead to positive pressure values on leeward trailing measuring points network would be denser to capture the edge of the mainsail. smaller fluctuations. There is no limit to use the system  To a large extent, the effects on both side cancel, thus also on the hard wing sails. the overall centre of the effort and force is relatively Such a system could be used in training runs to find the stable. best setting and update polar diagrams. The sail sets  Different sail parts react to the pitching movement could be evaluated and fitted to the mast with fewer with their own natural frequencies influencing the trials. pressure distributions on the surfaces. Special configurations can be developed for monitoring Overall, the test runs of the system demonstrated its the sails during long day and night regattas. ability to serve as an experimental data gathering and Low level system shall be incorporated directly into the analysis tool, as well as its potentials in aiding both sail fabric and the differences between pressures across a experienced and novice sailing teams to exert the small hole in the sail could be measured. Some research maximum driving force from their sails. is still needed to define the minimal number and optimal positions of measuring points that provide reasonably reliable measurements for minimum price. Different software modules for online data presentation, data analysis and modeling could be developed, Figure 21. Spectra of all pressure time series measured on the RC44 yacht in upwind conditions. BIBLIOGRAPHY 1. Viola I.M., Pilate J., Flay R.G.J. (2011), “Upwind sail aerodynamics: a pressure distribution database 5. Viola I.M., Flay R.G.J. (2010), “On-water pressure for the validation of numerical codes”, Trans RINA, measurements on a modern asymmetric spinnaker”, vol. 153, Part B1, Intl J Small Craft Tech. Proceedings of the 21st International HISWA 2. Warner E.P.,Ober S. (1925), “The aerodynamics of Symposium on Yacht Design and Yacht yacht sails”, in the proceedings of 3rd General Construction, 15th -16th November, Amsterdam, The Meeting of the Society of Naval Architects and Nederlands. th th Marine Engineers, vol. 33, pp. 207-232, 12 -13 6. Viola I.M., Flay R.G.J. (2011), “Sail pressures from November, New York. full-scale, wind-tunnel and numeric investigations”, 3. Puddu P., Erriu N., Nurzia F., Pistidda A., Mura A., Ocean engineering, vol. 38, pp. 1733-1743. (2006), “Full Scale Investigation of One-Design nd Class Catamaran Sails”, in the proceeding of The 2 7. Viola I.M., Flay R.G.J. (2011), “Sail aerodynamics: High Performance Yacht Design Conference understanding pressure distributions on upwind th th (HPYDC2), 14 -16 February, Auckland, New sails”, Experimental Thermal and Fluid Science, vol. Zealand. 35, pp. 1497–1504. 4. Graves, W., Barbera, T., Broun, J. B., Imas, L., (2008), “Measurements and simulation of pressure distribution on full size scales”, in the proceedings of The 3rdHigh Performance Yacht Design Conference (HPYDC3), 2nd–4th December, Auckland, New Zealand.