Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec The pressure profiles indicate either leading edge


separation with following reattachment and trailing edge stall (AWA 90° & 120°) or fully separated flow (AWA 150°). Comparison with measured pressure distributions on an asymmetric spinnaker, given by Viola [11], shows good qualitative agreement.


] - f


n t [


eicif


e c


s Figure 15: Streamlines around boat and sails


e s


o e


u r


p r


-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5


0.0 0.2 0.4 90° 0.6 120°


curve length [-] 0.8


150°


Figure 18: Pressure distributions along sail girth on ½ mitre height at different AWAs


Figure 19 shows the distributions of principal stresses (1 and 2) within the spinnaker. It is clearly observable that the largest stresses (areas of large 1) run up the luff. 2 is zero in large parts of the sail, yet, as the sail’s size is not visibly reduced, the cloth in these areas appears to be right at the verge of wrinkling. Near the head 2 is larger than zero, indicating that the behaviour of the sail near the head will be quite stable. This is supported by observations during trimming in the TFWT.


1.0


Figure 16: Pressure distribution on spinnaker ] - f


n t [


eicif


e c


s


e s


u r


p r


o e


-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5


0.0 0.2 0.4 cP Leeward 0.6


Figure 19: Principal stresses in directions one and two


curve length [-] 0.8


cP Windward


Figure 17: Typical pressure distribution on both sides of spinnaker with partially detached flow


Figure 17 gives a typical pressure distribution along a girth line for both sides of the sail. While the pressure distribution on the suction (leeward) side is highly dependent on the actual flow conditions, it is always virtually


the same on the pressure (windward) side.


Figure 18 exemplarily gives the suction side pressure distributions on a girth at half height at different AWAs.


B-78 1.0 ]


m ²


y [


A,]


A x [


m ²


-0.5 0.0 0.5 1.0 1.5 2.0


90


120 Driving Force Area (Ax)


150


AWA [deg] Side Force Area (Ay) Figure 20: Force areas of sail set ©2011: The Royal Institution of Naval Architects


180


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