George Hansen started this with his article in February. Jon Knudson added his two cents in April.
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 under body. 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, in High Performance Sailing, page 252, claims to have measured a 2% drag increase due to an invisible but just detectable to the touch, road film over the polished bottom of a high performance 18 foot dingy towed at 2 kts. But how much should that decrease 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, a 19 ˝ -foot, 2300# displ., 400# outside ballast keel, round bilge, was test towed with a rough bottom condition: no marine growth but “…paint ridges in her rudder, grooves in her iron keel, and caked paint along her topsides”, between one and six miles per hour to develop a speed vs. drag curve. A similar curve was developed after the 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 1.4 min. they will have diverged 1000 feet1 with both on a Line of Equal Position 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 kts this amounts to a 44 seconds 3 of 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 Hansen, Vela
Notes: 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-sec
I read George Hansen’s letter in the February issue, and would like to add my 2 cents worth. I think the issue of a smooth bottom is 1/3 of a good raceboat, the other 2/3 being sails and the crew. With older sails, the other 2/3 become really important.
Whenever we get passed in light air by some older smaller boat, and it can happen, we like to think it must have a really fast bottom. We have seen the bottom on Morphine: smoothness and perfection. Perfect Ten’s is not perfect; big chunks of paint have fallen off here and there, but the rest of it is quite smooth, and the keel and rudder are pretty fair.
We like George’ idea, incidentally, that if we cross tacks, then one of the boats is going the wrong way! In the recent Toliva Shoals race, we basically drifted in tide toward the finish line in extremely light air, until just before the finish, and we wondered if a little more traction against the water might not be a good thing!
Seriously, we run a VC Offshore Teflon bottom, which is murder to maintain, but you can easily feel the difference between clean and dirty. And then don’t forget the famous college racing coach: “I firmly believe that even a fly on the bow affects performance!” I think I have read, don’t ask where, that a smooth bottom is even more important in heavy air than in light air, because of the exponential effect of the drag resistance as speed increases.
All I can say in this regard is that we were passed by Morphine going way faster than that little boat had any right to, during extremely heavy air off Ketron Island. In that case, the fastest boat on the course had the cleanest bottom.
Jon Knudson, Perfect Ten