Guillaume sent me this article from scuttlebutt about mainsail trim.
I used to try to keep my leech tighter and straighter than I do now. Cliff
has taught us that by adding twist you lose a lot of weather helm and
therefore drag. What do you think about this article? What is the latest
thinking in mainsail design. What advice can you give the fleet for trimming
the main on a Santana 30/30. I also don't understand what they mean by
"lower third should be flat to reduce drag under the boom". What drag is
there under a boom?
MAINSAIL TRIM
(Pete Colby distills mainsail trim in a story published on the SailNet
website. This excerpt explains what the main should look lie.)
The maximum draft (shape) should be located at 38 to 48 percent back from
the luff.
The lower third should be flat to reduce drag under the boom.
The middle third needs a bit more shape. This section promotes lift with its
longer chord length (the luff to leech straight-line measurement). The added
area accelerates flow, thus increasing the lifting force.
The top third is a bit trickier. It is designed with as much depth as it can
take. The added depth increases surface area, giving the airflow a longer
runway on which it can accelerate. Depth also reduces the effects of drag,
by giving the airflow more time and area to generate lift before it reaches
the turbulent leech. Further, depth creates a twist. In other words, under a
given sheet tension a main with more depth up high will fall off, or twist,
to leeward. Seen from behind, the sail looks like a fan blade. The twist
flattens the curvature, reducing the chance of separation. And twist also
combats surface friction. The air moves faster the higher it is, and a
faster-moving breeze approaches the sail at an angle aft of that of a slower
breeze. In order to maintain a uniform angle of attack (the angle where the
apparent wind direction intersects with a straight line from the leech to
the luff), you need to turn off the leech. A greater angle of attack creates
more lift. - Pete Colby, SailNet website
Received on Tue Oct 9 15:45:49 2001