Depowering the sails and twist

[DuaneIsing]

jbr,

Stalling and luffing are the opposite conditions. Luffing is where the angle of attack of the airflow over the sail is too shallow. Stalling occurs when the angle is too steep. For example, if you are perfectly trimmed on a close reach and you trim the sail in, you will put it into a stalled condition. Ease it out and it luffs.

A lot of beginners overtrim sails and have them partially stalled. The sail cloth takes a full shape with all the dynamic pressure on the windward side, so it can look ok, but it isn''t trimmed properly. That''s why the mantra remains, "when in doubt, let it out."

Duane

[Jeff_H]

While gradient effect is a littel less of a problem on small boats than on larger boats (shorter masts and so less difference in windspeed over the height of the mast but a slower speed through the water and therefore a greater apparent wind impact) but gradient affect is still an important consideration in making desicions about twist. Sails are not cut with twist. They are cut to permit twist but the decisions that you make about vang and mainsheet tension as well as traveller position really control mainsail twist. In most conditions you want the upper batten parallel to the boom. In light air it may sag slightly to leeward.

As was explained above, luffing is exactly the opposite of stalling. Of the two, a little bit of stalling is generally slower than a little bit of luffing. Beginners often get sucked into overtrimming when that makes control harder and results in slower speeds.

Twist is one of those fine points of sailing that is hardest to learn. In some conditions it can yield big changes in the amount of helm and the speed that the boat is going. At other times it can have very small affects.

Next week I will be trimming mainsail on a 40 foot Farr design in the SORC. The boat that I am on has a very large mainsail. It is pretty easy to get within a half knot of the ideal speed at any given windspeed. Getting that last half knot is really a matter of getting angle of attack, twist and sail draft just right for the conditions. I am literally making two or three adjustments each minute on the upwind leg, watching the sail, the wheel and the instruments for clues about the changing dynamics. Small boats can be more forgiving in some ways but also lack the tools for that kind of precise sail trim.

Regards
Jeff

[jbr]

Thanks, Jeff, Duane, and Sailormon6 for your clarifications and great advice.