Join Date: Mar 2000
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Not Very Smooth Racing Bottom
I wonder if the detrimental effect of an "orange peel" bottom surface isn''t a bit overrated in any racing competition less than one-design boats, dry-sailed in the national or perhaps regional championships. I don''t know, I''m just asking but my thinking is that the effect may be small compared to missing a wind shift, or a puff or misplaying the currents. Perhaps I''m just trying to ease my conscience for not having a 600-grit-sanded and burnished underbody. (My wife seems to have lost her interest in sanding boat bottoms and I never really had the urge in the first place).
Do any of you know of or have quantitative data of the speed differences between orange peel and polished bottoms? Bethwaite claims (High Performance Sailing, page 252) to have measured a 2% drag increase due to an invisible but just-detectible-to-the-touch, road film over the polished bottom of a high performance 18-foot dingy towed at 2-kts but doesn''t say how much it decreased its speed. Can one get there from here?
Smith in "How Sailboats Win or Lose Races" reports his towing experiments. In one case a Small Point One Design, (19 1/2-foot, 2300# displ., 400# outside ballast keel, round bilge) was test towed with a rough bottom condition (No marine growth but"...paint ridges on her rudder, grooves in her iron keel, and caked paint along her topsides) between one and six miles per hour to develop a speed(y) vs. drag(x) curve. A similar curve was developed after she was "...carefully sanded, all cracks and keel gouges filled, and her entire bottom given two coats of clear epoxy paint". The bottom was then further smoothed and polished. The smooth bottom was faster by an average of 200-feet per mile. That equates to 3.8% faster for a gain of 227-feet per NM or 24-seconds per NM at 5-knots.
Now consider this: Two boats on a beat cross closely and continue on opposite tacks at 5-knots boat speed and 45-degrees off the true wind direction. In only 1.4-minutes they will have diverged 1000-feet(1) with both on a Line of Equal Postion which is perpendicular to the true wind. Now a 15-degree wind shift will cause the boat on the side to which the wind shifted, to gain 366-feet(2) sailing distance at the other''s expense. At 5-knots this amounts to a 44-second(3) gain or loss.
One lesson here is that when two boats on a beat cross on opposite tacks, ONE OF THEM IS GOING THE WRONG WAY! The other lesson is that it is better to have even a grossly rough bottom than to miss a 15-degree wind shift more than once every two miles. If you have a rough bottom AND miss the shifts, you should probably reevaluate your program, but a little bit of orange peel shouldn''t be catastrophic.
Ok guys, rip it up! Cheers, George
(1) (1.4-min)(500-fpm)(cos45)(2-boats) = 1000-feet
(2) (1000-ft)(sin15)(cos45) = 366-feet
(3) (60-spm)(366-ft)(500-fpm) = 44-seconds