light winds and older sails ...

mikehoyt · #1 (**permalink**) 06-29-2004

After purchasing a new 2+2 dacron main last season I have come to suspect that in very light winds the old bagged out main it replaced was actually working reasonably well. In higher winds the new main is unquestionably better as I can leave more sail up and the draft is farther forward ...

So here is my question....

Would it be worthwhile to run an old bagged out main in very light winds?

Regards

Mike
Full Tilt 2

paulk · #2 (**permalink**) 06-29-2004

There is a theory that having baggier sails in light air is better for two reasons. One, the baggier sails make for a less abrupt change in direction for the wind, so the moving air stays attached to the curved foil longer, and is thus more efficient. (Think of wind coming to a flat sheet of plywood - or a flat sail -- the flow becomes detached and turbulent, instead of being able to follow a gently curving route. ) Secondly, the baggier sails create more drive for cutting through light-air chop and slop. A newer, more finely tuned sail with less draft will be more efficient in smooth water and a steady breeze, but without those conditions, it may tend to stall out. Think of a jet fighter''s wing, with hardly any depth of chord (draft) in it. It has to be going 300 mph before it starts to fly. Then look at a DC3''s wing. It looks almost like a camel in comarison, but the DC3 starts to lift at closer to 60 mph. At slower wind and boat speeds, more draft is desirable. From here, however, we can''t tell what your sails actually look like, so it''s all just a guess. Try changing and see if it works. It may psych out the competition to have them think you''ve got a "secret weapon" in light air.

RichH · #3 (**permalink**) 06-30-2004

A full (baggy) sail is OK for ''acceleration'' and ''power-up'' settings .... needed for sailing in ''left-over'' chop, power boat chop, etc. in very light winds. However once the boat begins to reach its max. speed (for the at-the-time-conditions), a flatter shape in light winds usually is best to prevent flow separation - especially on the lee side and especially near the leech. Once a boat completes its acceleration, the apparent wind and the stream flow across the sail becomes faster and then usually the fuller shape is bad. Once the acceleration phase is complete usuallly a much flatter shape is needed to prevent the deadly ''flow separation''. My racing was/is primarily on ILYA scows and those boats do best to ''maintain speed with board-flat sails in extra light winds once you''ve accelerated out of a tack / gybe. If you apply tell tales, expecially in ''Gentry-Tuft'' fashion - at the luff, leech and foot - you can SEE the effect.

So typically you need a fuller draft to accelerate and get ''moving'' but then need to ''shift'' to a flatter shape to gain and maintain max. speed. A short course that involves LOTS of tacks and gybes maybe would benefit from keeping a ''bagged'' shape .... but not usually as you have to ''keep shifting gears'' to consistantly win. Just keeping a baggy shape will let you accelerate quickly but usually wont be able to reach the max. speed potential for the conditions - due to flow separation. Your tell-tales will show you.

Sailormon6 · #4 (**permalink**) 06-30-2004

I''ve been shifting gears at the point when the boat begins to become overpowered. You say that we should shift gears before that point, when the boat has fully accelerated with the full-shaped sail, whether the boat is overpowered or not.

Your explanation is very logical and persuasive, Rich, but before I change my practice, I''d like to hear from others as to whether the general consensus agrees with with Rich''s thinking.

Jeff_H · #5 (**permalink**) 06-30-2004

Rich has it right. That said, ideally you should be watching the speedo as you do this to see if you experience a drop in speed. At some point around 3 or 4 knots you can end up over blading killing speed rather than staying powered up and get more speed.

Jeff

Sailormon6 · #6 (**permalink**) 06-30-2004

Thanks Rich and Jeff! It''s a good day when you learn how to squeeze another .026 kt out of the boat. I''ll put that to good use next weekend!

mikehoyt · #7 (**permalink**) 07-06-2004

My sailmaker has suggested that I would get all the same benefits of the "full" old sail by easing the halyard and easing the outhaul. Have been playing with this and it seems to help a lot ...

Mike