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post #4 of Old 10-14-2003
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sails well in light air AND heavy weather?

There is a lot that I agree with in Paul''s post regarding the selection of a boat with good performance at both ends of the wind range and I do think that Paul comments open up a lot more choices, and more signoficantly a lot more reasonably priced choices.

I wanted to touch on the issues that Paul raised regarding L/D and SA/D. Most of the sailing texts that deal with the topic of ideal L/D and SA/D based those numbers on earlier designs. If we compare a very typical old style performance 41 footer we might have seen numbers like a WL length of 31.5 feet, 17000 lbs of displacement, 700 s.f of working sail area. That would have produced a boat with an L/D around 240 and a SA/D around 17. In sailing mode these boats used very large genoas to get help them perform in lighter going which would often take these boats up into the L/D in the mid-20 range.

If we look at boats derived from the IMS type form (or boats like the J''s mentioned above which are closer to stretched 1980''s era MORC type forms)the numbers would shift some. For a newer 41 footer we might have seen numbers like a WL length of 36 feet, 16000 lbs of displacement, 850 s.f of working sail area. That would have produced a boat with an L/D around 150 and a SA/D around 21.5. The more more modern design would typically have more ballast carried lower and so would have an easier time standing to its sailplan. The lower D/L comes predominantly in the lengthening of the waterline and not from excessive lightening of the boat. In most studies of factors involved in safety at sea, all other things being equal, nothing succeeds like waterline length in determining how well a boat handles heavy weather. (The second most critical factor seems to be vertical center of gravity [VCG} as compared to VCB [vertical center of buoyancy.)Longer waterline lengths generally mean less pitching and more comfortable pitching motion. They also mean a lot more speed. (While older style boats do pick up some speed while heeled, it comes at the price of sailing at a greater heel angle and greater drag. Current studies suggest that the incremental gain in speed due to heeling in these older designs loses something to increased heel induced drag and so is nowhere near as large as would be expected with a boat whose vertical waterline equals the heeled waterline of an older boat.

The big advantage to a rig whose standing SA/D of 22 is the ability to use of smaller non-overlapping or minimally overlapping jibs even in light air. That actually means smaller easier to tack headsails which is a great advantage to short handed family type crews. Modern rigs permit the quick depowering without needing to reef as soon and that ability to shift gears on the fly also works well on a short-handed boat and allows a much greater control with a larger standing sail plan. On both the older style (large genoa/small mainsail) and on newer style rigs, somewhere before 20 knots something needs to happen. In the case of the older style rig, it often means a change to a smaller headsail (or living with a poorly shaped partially roller furled genoa) and perhaps a reef. In the case of the more modern rig, this generally means a reef in the mainsail. Properly rigged this is very easy for a single person to do on the fly with a two line reef system lead back to the cockpit. This is especially ideal since most boats develop more weather helm as the windspeeds increase and so reefing the mainsail helps move the CE forward and reduces weather helm. Most Fractionally rigged boats that I have sailed with L/D''s in the 22 range can sail quite well with the small jib into wind ranges up approaching 30 or so knots. This ability to maintain balance without a sail changes really helps with both shorthanding especially at the heavy air end of things and is something given up with a standing sail plan with an L/D down around 17.


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