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Lay the sail out on the floor and measure from the luff to the clew, perpendicular to the luff. That dimension will be your "LP" (should be nearly 18 feet [17.85]) which is 155% of your J measurement (ie forestay to mast along the deck - 11.5 ft)
So the 100% will be rolled up til there's still 11.5 feet of sail between the rolled portion and the clew (again, perpendicular to the luff). Just do the ratios for any other position you might want. Mark the position on that 'line' you measured, and extend it to the foot parallel to the luff.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Last edited by Faster; 06-15-2011 at 12:14 AM.