The Effects of Staysails on Yacht Perfomance
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3rd High Performance Yacht Design Conference Auckland, 2-4 December, 2008 THE EFFECTS OF STAYSAILS ON YACHT PERFOMANCE D.J. Le Pelley1, [email protected] L. Kjellberg2, [email protected] R.G.J. Flay3, r.fl[email protected] Abstract. A staysail can be defined as any supplementary sail flown between the mast and primary genoa or spinnaker to enhance aerodynamic performance. Much of the time, huge expense and effort is put into improving the performance of the flying sails with little thought to staysail design and position. Wind tunnel experiments were carried out in the University of Auckland's Twisted Flow Wind Tunnel on a range of staysails operating under typical flying sails. Flow visualisation of interesting concepts was carried out. The analogy of the staysail as a turning vane that improves the flow over the other sails was investigated. Independent supports were used to determine the load contribution of the staysail to the overall loading of the sail combination. A range of clew heights of the staysail were investigated. A significant gain in driving force of around 10% on average and up to 17% has been demonstrated by correct selection and placement of staysails. Staysail area is not a key factor, with the smallest staysail testing giving one of the largest improvements. Masthead staysails and sailing triple-headed were both found to have a significant performance benefit over a conventional medium sized staysail. NOMENCLATURE In order to investigate the effects of staysails, a number AWA Apparent wind angle of wind tunnel tests were carried out at the Twisted Flow CFX Drive force coefficient Wind Tunnel (TFWT) at the University of Auckland. CFY Side force coefficient Here a real-time velocity prediction program (RT-VPP) qio Dynamic pressure at 10m height [2] was used to allow the model to heel to the correct SA Sail Area angle for a certain sailing condition. TWS True wind speed For the testing, a model of the proposed America's Cup 90; rule (AC90) class was used. However, the sails, 1. INTRODUCTION combinations, attachment points, etc. that were investigated in this paper are not constrained to the AC90 A staysail can be defined as any supplementary sail rules - the model was simply used as a convenient test- flown between the mast and primary genoa or spinnaker bed. In particular, a number of staysail combinations to enhance aerodynamic performance. Staysails have more relevant to the VO70s and other offshore classes been in use on sailing ships since the 16th century. In the were looked at. high performance yachting sector they are commonly used in ocean-going classes such as the Open 60s and the 2. TEST SETUP Volvo Ocean 70s (VO70) and have more recently started making a more significant appearance in inshore events 2.1. Model such as the America's Cup. Much of the time, huge The tests were carried out in the University of expense and effort is put into improving the performance of the flying sails with little thought for staysail design Auckland's Twisted Flow Wind Tunnel [3]. For all the and position. Some work has been carried out on tests, a model of the proposed AC90 at 1:17 scale was evaluating their performance [1] although mainly under used. The model was equipped with remote controlled winches to trim the sails while the wind tunnel was genoas at relatively tight angles where flow separation is not significant. Experimental and numerical studies are running. The model was set up with hydrodynamic data commonly performed without a staysail present even allowing the RT-VPP to be used. This test method enables the rolling moment produced by the sails to be though it is accepted that it will be used at full scale for that particular sailing condition. This is in spite of the balanced by the righting moment produced by the keel and form stability. This is simulated in the wind tunnel significant gains in performance that staysails can provide. The ability to substitute un-rated sails as by using an electric motor to adjust the heel angle staysails under certain rules, such as storm jibs, increases dynamically. Consequently, accurate and efficient testing the potential benefits of staysails and makes it even more of a sail configuration through a range of simulated true wind speeds becomes possible; and this makes it important to understand their effects. 'Wind Tunnel Manager Yacht Research Unit, University of Auckland 2Dept of Naval Architecture, Chalmers University of Technology 3 Director Yacht Research Unit, Mechanical Engineering Department, University of Auckland 247 straightforward for the trimmer to depower the sails location is just out of the photograph to the left and the correctly at the higher wind speeds. forestay position is just out of the photograph to the right. The spinnakers were flown from the end of a bowsprit al A vertical velocity gradient was generated by adding approximately 150% J. roughness elements to the upstream section of the wind tunnel. The profile was such that it was an adequate simulation for both an AC90 and a VO70 sailing in about 12 knots true wind speed, and it was kept constant for all of the simulated wind speeds and angles. The horizontal twist angle was adjusted by twisting the wind tunnel's flexible vanes to the calculated values for the AC90 in 12 knots true wind speed and was corrected for each change in apparent wind angle in the wind tunnel. The model was mounted in a cradle that was attached to a force balance underneath a turntable. Water was used to fill the gap in between the model and the floor to prevent pressure leakage under the hull. CL1 CL2 2.2. Sails The staysails were tested in combination with a Figure 1. Tack positions on foredeck conventional mainsail and two different generic flat and deep masthead asymmetric spinnakers, referred to as the 2.4. Test procedure A2 and A4 respectively. In total, eleven staysails of different shapes were tested in this investigation. The Each staysail combination was tested beneath the A2 and different staysails are listed in Table 1. the A4 where appropriate. The A2 was tested over a range of apparent wind angles (AWA) from 30° to 70°. The A4 was run from 40° to 90° AWA. At each angle, Table 1. Table of the different staysails. three true wind speeds (TWS) were chosen to give heel Sail Name Properties angles of approximately 0°, 10° and 20°. Small "small Tack @ 52% J overlapping Foot length 75% J The change in roll moment, and hence heel angle, Medium between two different sail configurations has two main "med Tack @ 71% J overlapping Foot length 82% J effects. The first is the aerodynamic change which is Large Slrg Tack @ 90% J dealt with implicitly by the RT-VPP, as the yacht is overlapping Foot length 108% J heeled in the same manner in the wind tunnel as at full Masthead S,\1H Masthead version of Sm;J scale. The second is the change in hydrodynamic Small non- Ssmall-NOL Non-overlapping version resistance with heel angle. The RT-VPP allows for this in overlapping of S>nuu its calculation of boat speed but, in order for this research Medium non- Smed-NOL Non-overlapping version to be independent of yacht type, the drive force is overlapping of SmcJ presented in all cases instead of the boat speed and so Extreme high "clew ' ' High clew version of S^-j this change in hydrodynamic resistance is ignored. This clew approach is justifiable in this case as the addition of a High clew Sclew+ High clew version of Smcd staysail only makes a very small change to the heel Low clew version of SmcJ angle, as will be shown in the following section. This Low clew "clew" means that the hydrodynamic resistance will not vary Extreme low "clew Low clew version of S,„,.j significantly between staysail configurations tested at the clew same condition, and that the drive force is a valid Modified small Ssmall-NOL- Cut-down Ssmj||.N0L for indicator of performance. non-overlap improved sheeting After optimising the trim for the best possible boat speed, 2.3. Tack positions the 3 forces and 3 moments were recorded and coefficients were deduced as: Many current rules do not prohibit tacking sails off the centre line. There is conceivably an advantage by tacking Force a staysail to the windward edge of the deck so that it acts cForce (1) as an extension to the mainsail to some degree. This was ?>o„, • SA investigated in this research by altering the tack position. There were four tack positions to windward and three Mom tack positions on the centerline. The locations of these cMom 3/2 (2) tack positions are illustrated in Figure 1. The mast '10/M SA 248 where qi0m is the dynamic pressure at a 10m full scale the point at which the yacht cannot de-power any further. height and SA is the sail area of the mainsail and This is a very useful way of presenting the large amount spinnaker combination. None of the analysis in this paper of data recorded and enables a comparison of different takes into account the area of the staysail as, under most sail configurations. It can instantly show the conditions rules, the individual sail areas are rated but not the total where cross-overs occur between different sails. area of sail carried at any one time. Hence, a staysail can effectively provide bonus boat speed. In the same way, no attempt has been made to equalise the areas of the different staysails tested.