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post #11 of Old 11-05-2003
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Taipan 28 - How well offshore?

Mr, Coles assertion that "The effects of large movements of the VCG on the propensity to capsize was surprising small." is extremely dated and represents the understanding of capsizing that lead to the Fastnet disaster. The current understanding of the mechanics of capsize that came out of studies of models, full sized boats, and studies of actual capsizes, suggests that in most cases almost nothing is as critical in preventing a capsize than a low vertical center of gravity. L/D is length to displacement. It is not a linear formula but it is useful as a rough predictor of drag.

Drag is actually much more critical to the cruiser than the racer. In order to over come drag and make way to windward in heavy air, a high drag boat must carry more sail area than a more easily driven hull. This means that to claw off a lee shore, the boat with the large L/D must carry a lot more sail. The problem is that high L/D, long keel cruising hulls generally have a lot less stability relative to their drag than modern race boats which have very high stability compared to drag. A racer has a large crew to deal with sail trim in heavy going but cruisers are usually undermanned making it harder to carry the sail needed to claw off a lee shore. The problem gets worse on boats that are fin keel (by the classic definition where a fin keel is any keel where the bottom of the keel is 50% or less of the length of the boat or the length of the sail plan which ever is greater) with attached rudders, as I believe this boat is. Fin keel boats with attached rudders are notoriously poor at tracking and tend to develop heavy weather helm in high wind conditions making sail trim and sail shortening highly critical.

I can easily assure you that you are completely mistaken in your assumption that you have a AVS (which is currently referred to as the LPS which stands for Limit of positive stability) of 183 degrees. The highest LPS that any boat can have is 180 degrees which means that the boat is unstable when completely inverted (i.e. rotated to a heel angle of 180 degrees from a vertical position of zero degrees) If a boat is in equilibrium right side up it can''t have a LPS of 180 degrees even.

I do not know what "formula for angle of vanishing stability" you found but the only accurate way to calculate the LPS is to calculate a curve of the center of buoyancy at varying heel angles and to calculate the center of gravity. Calculating the center of gravity is relatively easy. You calculate the weight of every part of the boat times the distance from some fixed point. Total them and then divide the answer by the sum of the weight.

Manually calculating the center of buoyancy at varying heel angles is quite difficult as most boats change in trim as they heel. There are often notches in the curve as the cabin and cockpit volumes hits the water which is why most manual righting curves ignore the effects of cabin and cockpits. I know of no surrogate formulas that produce even a reasonably accurate rough estimate of the LPS. Certainly the one you used is not accurates since it produced an LPS that is beyond vertical.

While I do not know what the actual LPS for your boat is, I do know that when the EU was attempting to develop a standard for rating the safety of various types of pleasure craft, the stability committee looked at the issue of LPS on a wide range of boats. RORC era boats of types similar to yours generally had LPS''s around 120 degrees but most RORC boats had lead ballast and aluminum spars which would mean a lower VCG than your boat with its low balalst to displacement ratio, shallow draft, low density ballast and wooden spars, etc.

Look, if you are not performance oriented, boats like yours are a good way to get out on the water. While I tend to own higher preformance boats these days, I still enjoy sailing on older designs. The companionship or solitude of sailing is just as pleasant and the sea is just as beautiful whether you are going a bit slower or not. In moderate 10-20 knot winds, boats like yours are in their element. They will not out perform a more modern design or be easier to handle but they sail at their best. It is really at the extremes, winds under 10 knots and winds over 20 that these older designs become a liability. I am not attempting to tell you that you bought the wrong boat, that is not for me to say, but I am attempting to give you my best understanding of the likely reality of the behavior of boats like yours in higher wind situations.


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