The catastrophic drop in lift occurs when the separation bubbles meet and the entire suction side suddenly becomes separated.
The maximum lift coefficient for the four-digit and the 63 and 65-series sections is given as a function of thickness ratio in Fig 6.25. It can be seen that the highest lift may be achieved by sections with a thickness ratio in the range 12-15%. and that the four-digit series is the best one in this respect. The angle at which the maximum occurs for each section is also indicated. Note again that this' angle is approximately twice as large for keels of normal aspect ratio.
A possibility not discussed earlier is to divide the section into one fixed and one movable part. The mean line is then no longer straight, but exhibits a sharp corner at the hinge. This design may have several advantages, provided it is well done. An example of such a configuration is the trim tab behind a fixed keel. Fig 6.26 shows the principal effect of a deflection of the tab on the pressure distribution, as well as on the lift and drag. The stagnation point moves from its asymmetric position at the nose closer to the original symmetry line of the section, and therefore the large suction peak created by the sharp bend around the nose is reduced or even eliminated. The pressure distributions on both sides become closcr to the one at zero angle of attack, and the favourable pressure decrease can be maintained at higher angles of attack. This effect is substantiated by the shift of the drag bucket to the right in the lower part of the figure. Another important effect is that the lift curve is moved to the left, giving a lift force even at zero angle of attack. By proper adjustment of the trim tab enough side force to balance the sails may be generated without leeway, which is an advantage, since the hull will then move straight through the water and thus produce minimum resistance.
When the tab is deflected there is normally a knuckle in the section at the hinge, which causes pressure spikes on both sides. This inevitably causes a drag increase, which to some extent reduces the positive effect of the tab. A way to alleviate this problem was pointed out by the yacht designer G Heyman. Since the suction side is the most sensitive one, it is advantageous to design this to be smooth with the tab deflected. The suction sides of the tab and the main part of the section are thus integrated to yield a smooth curve from nose to tail, at the tab angle of interest. Of course, this means that when the tab is set to zero angle the section will not be smooth, but this may not be so serious, since the section is then normally unloaded.
Some practical We are now in a position to draw some practical conclusions from the conclusions regarding discussion above. We will have to consider the keel and the rudder section shape separately, since their function and operating conditions are different.
Thus the keel normally operates at small angles of attack and the speed of the yacht depends on the drag produced at these small angles. The rudder, on the other hand, may help the keel to produce the necessary side force, but its main task is to provide enough moment to manoeuvre the yacht under all conditions. Therefore the rudder has to be designed with emphasis on the maximum side force required.
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