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post #3 of Old 08-01-2002
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Hull Speed

Oh I am so sorry Dallas,I am afraid that is not the correct answer, Bert, tell him what he would have won!....OK seriously, Dallas has it mostly right. I am quoting here:

"Waterline''s affect on hull speed is theoretical and not absolute. As a hull goes faster, the bow wave stretches to the point where the bow and stern wave become on wave cycle, whose wavelength is equal to the waterline length. This brings us to wave theory. "

"The speed of a wave (in knots) is equal to the square root of the wavelength (in feet) multiplied by 1.34. If your boat has a waterline length of 32 feet, the theoretical hull speed is 7.6 knots. The waterline length is thought to limit the hull speed because if the boat goes any faster the stern waves has to move further back taking the trough between it and the bow wave along with it. As the trough moves aft, it causes the stern to drop, making the boat sail uphill."

"Except for planning designs, sailboats typically can''t generate enough power to go any faster and climb their own bow wave. But a boat with extra volume in the stern can exceed its theoretical hull speed because the extra bouyancy prevents the stern from dropping into the trough. By the same token, a fine-ended design might not achieve its theoretical hull speed if buoyancy in the stern is insufficient." (Written by Steve Killing and Doug Hunter)

In looking at the more recent data it is not all that unusual to achieve speeds that are 1.5 times the square root of the waterline. It is harder to achieve higher speeds upwind since there are generally less drive upwind than when reaching. My boat generally goes upwind in windspeeds over 10 knots at 1.5 to 1.6 times the square root of her waterline at wnd angles as close a s50 degrees true. Newer more efficient designs will often do better than that.


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