Join Date: Jun 2005
Location: New Orleans
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I'll take an educated guess and say that the clew of a 150 genny is halfway back from the mast, and assuming there's no intervening headstay (i.e. this is your typical sloop, not a cutter), that you want at least a boat-length of sheet, plus 10 or 20 percent.
So if you loop it at the clew, two boatlengths plus 10 or 20 percent, or around 70 feet?. When you buy it, err on the long side, it's lots easier to shorten a sheet than to lengthen it..;-)