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JimOppy 04-28-2003 01:24 PM

Sail trim question
 
Here''s the question: at what point does a sail cease acting as a foil, thereby deriving it''s drive from high and low pressure on either side of the sail, to acting as a parachute, deriving its drive from the force of the wind pushing against its windward side? I know that point happens somewhere between a beam reach and a broad reach. I guess the point is, when on a beam reach....I mean a dead beam reach, with the wind coming directly from the side, is it better to let the main sheet out so the sail acts as a foil or keep the line drawn from the foot to the clew of the sail perpendicular to the wind and use the sail as a parachute?

I''m not asking so much about sail shape as how to position the boom. Should it be perpendicular to the wind on a beam reach or let out to act as a foil. Seems to me like letting it out would just spill wind and de-power the sail, but a longtime sailor says otherwise, and becasue I know sails get power as a foil when pointing, there may be something to his claim. His point is, on all points of sail, the sheets should allow a straight line drawn from the foot to the clew the most obtuse angle possible without luffing. Is this right?

My instinct is to keep the boom perpendicular.

End our debate!

Jim

Sailormon6 04-29-2003 12:38 PM

Sail trim question
 
I can''t tell you what others do, but I have had good success racing, and, as I bear off downwind, I continue to trim my sails so that they act as a foil (generating lift on both sides) for as long as the cut of the sails, the rigging of the boat, and the direction of the wind will allow. It seems logical to me that it is always better to have both sides of the sail generating drive than to have only one side of the sail generating drive. (Unless, of course, you are intentionally depowering the sails.)

JimOppy 04-30-2003 08:14 AM

Sail trim question
 
Yup, that helps. Makes sense too. Thanks. Feel free to tell me what to do. Being a married man, I am quite used to it.

Jim

geohan 05-01-2003 01:32 PM

Sail trim question
 
A sail no longer acts as a foil the moment it stalls, ie., the air flow is no longer attached to the lee side. This can be detected by telltales (yarns) attached to the sail near the luff on a jib and at the leach of the main. Trim so as to keep the telltales streaming as long as possible. When the boom is out to the max, come up into the wind enough to keep the telltales streaming. In light air,under 10kts, tacking downwind is a good strategy as it allows you to maintain the lee side lift on the sail. If it is blowing hard even a stalled sail will have all the power one can use and a direct downwind course is best. There is a lot more to it than this of course. Good condensed references are the North Sails Performance Racing manuals.
Regards, George

couzensj 05-16-2003 07:30 AM

Sail trim question
 
The above comments are good the only thing I would add is to note boat speed before and after each sail adjustment you make. Remember the objective to to increase your VMG''s.


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