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


the two trims show a force increase when measured in the twisted flow.


in the upright condition than in the heeled condition. The twisted flow can increase the drive force by up to 30% when upright but by only 5% at 20° of heel. This is largely due to the angle of attack difference between the lowest and highest sections of the sail, being smaller when the boat is heeled than when it is upright.


The force measurements show that the drive force increases with the increased angle of attack at higher sections and decreases at lower sections due to the twisted flow. This is not unexpected as it is designed to sail in the apparent wind which is twisted when sailing a real yacht in a breeze.


The pressure measurements allow an investigation of how the pressure on the low and high sections change if tested in uniform flow or in twisted flow.


Figure 9: Deflection angle due to the twisted vanes. Figure 12 shows the pressure measured on three


horizontal sections of the A3 at 55° AWA and 10° heel, both with and without the twisted flow. The sections are measured at ¾, ½ and ¼ of the sail, from top to bottom in the figure,


respectively. The pressure distribution


measured on the leeward side of the sail with the twisted flow shows higher suctions both on the highest sections and on the lowest sections. The suction increase at the highest section was expected because of the positive flow deflection due to the onset twisted flow. However, the suction increased at the lowest section even though the twisted flow causes a negative flow deflection. Hence a significant 3-dimensional flow effect must occur.


Figure 10: Cx versus the AWA with and without twisted flow for the A3 at 10° heel.


Figure 11: Cx versus the heel angle with and without twisted flow for the A3 at 55° AWA.


Figure 11 shows the drive force coefficient variation with the heel angle at 55° AWA and the same results are achieved. The drive forces measured with the twisted flow are higher than the forces measured in uniform flow. The force increase due to the twisted flow is larger


Figure 12: Cp with and without twisted flow versus the curve length, measured at ¾, ½ and ¼ of the mitre (from top to bottom), respectively, for A3 at 55° AWA and 10° heel.


B-48 ©2010: The Royal Institution of Naval Architects


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