Trans RINA, Vol 1521, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun


aerodynamics of the sail presents a significant challenge to most current CFD codes. The thin profile can easily lead to a leading edge laminar separation that is followed by transition and turbulent reattachment [9],


addition, the high curvature causes a strong adverse pressure gradient, which leads to separation Transition,


reattachment and separation, which don’t


occur around surfaces with sharp curvatures, are still a challenge for CFD codes. Two-dimensional high grid- resolution simulations have been successfully performed matching experimental data, for instance [12], but the computational effort does not allow computations to be performed with the same grid accuracy for a complex 3D geometry. Moreover, the more we understand about sail aerodynamics, the more questionable it appears to be to model the sails in 2D.


In the last 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 to overcome the problems associated with using the wired pressure system developed in 2006 for full-scale sail testing [13]. Similar systems were also developed by an Italian research team [14] and by an American team collaborating with the AC challenger BMW Oracle Racing [15].


At the current state of the art, these systems have provided pressure measurements at only a few points and haven’t yet been able to provide a complete pressure map on the sails. They have been used on upwind sails where the rigidity of sails allows the weight of the devices to be carried, and the stability of the sail leads to a more reliable measurement. In the near future, full-scale pressure measurements will hopefully allow complete pressure maps of both upwind and downwind sails.


Finally, in the past year at the YRU’s wind tunnel, double-surface rigid sails with pressure tubes inside were tested and the pressure


distributions on both the


windward and leeward sides of a fully 3D symmetrical spinnaker were measured


[16]. The pressure was


measured with 8 pressure taps at each of 7 horizontal sections of a 1/25th model-scale International America’s Cup Class (IACC) spinnaker sailing at an apparent wind angle (AWA) of 120°.


1/25th to 1/15th, which led to a larger model, and also by increasing the dynamic pressure from 4.7Pa to 7.5Pa.


The experiment presented in this paper was performed with the aim of improving the accuracy of the previous experiment


Two new class rules were developed for potential use in the 33rd AC: the AC90 and the AC33 classes. Although it appears at the present time that the format of the 33rd AC will be a “Dead of Gift” (DOG) match in multi-hulls,


[16] by increasing the model-scale from


significant design work has been conducted on the AC90 and AC33 classes.


[10]. In [11].


Both of these rules would lead to much faster boats than the previous IACC design.


They were to have long


bowsprits and to sail with asymmetric spinnakers only. Hence the new tests had the aim of investigating the more recent downwind sails designed for the AC33 rule and the closer AWAs which would be sailed by these faster designs than the former IACC rule which they were to replace.


3. 3.1


YRU EXPERIMENTS TEST CONFIGURATIONS


Three asymmetric spinnakers were tested with the same mainsail. The sails had different shapes and sail areas. In fact, as for an airfoil, a flatter sail performs better at smaller angles of attack, while a more cambered sail performs better at larger angle of attacks. The angle of attack of a sail section (the angle between the wind direction and the chord of the sail section) increases with the AWA (the angle between the wind direction and the boat heading). Hence, a flatter sail performs better at smaller AWAs and a more cambered sail performs better at larger AWAs. In a windward-leeward course as


the


AC course is, to reach the lower mark in the shortest time, a yacht sails at smaller AWAs in light wind and at larger AWAs with more breeze. Hence, in light wind conditions a flatter sail has to be flown. In very light conditions, a large spinnaker might be too heavy to fly and hence, a smaller sail area (SA) is desirable. The three asymmetric spinnakers tested were labelled A1, A2 and A3, respectively. The A1 had the smallest SA and flat sections and was designed for light wind and small AWAs; the A2 had an intermediate SA and was a general purpose sail; the A3 had the largest SA and deep sections and was designed for stronger winds and larger AWAs. The main dimensions are summarised in [1]. Each sail was tested in 5 configurations: at 40°, 55°, 70° AWA with 10° heel; and at 0°, 10°, 20° heel with 55° AWA. Conventional cloth sails were used so as to be able to trim the sail for the maximum drive force. For each condition, two spinnaker sheeting trims were considered: the trim maximising the drive force which can lead the luff to flap in some cases, and a tighter trim required to stabilise the luff and stop it flapping. In the present paper only the trims corresponding to a stable luff are considered.


The tests were performed in uniform flow (without twisting vanes) but all of the configurations measured with the A3 were re-measured with the twisted flow to investigate how the twisted flow changed the pressure distribution on the spinnaker. The reference wind speed was roughly 3.5m/s giving a Reynolds number based on the model height h equal to 6x105. This is less than the full-scale Re by a factor of about 20. In uniform flow, the turbulence intensity was lower than 3%.


B-42


©2010: The Royal Institution of Naval Architects


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