Great information Harry. We all appreciate it. Thanks.
-----Original Message-----
From: owner-santana3030@sailpix.com
[mailto:owner-santana3030@sailpix.com]On Behalf Of Harry Pattison
Sent: Wednesday, October 10, 2001 8:41 AM
To: innove8@pacbell.net
Cc: Santana 30-30 Mailserver
Subject: RE: mainsail trim
Mike,
This is a pretty good article in general sort of terms, and pretty true for
boats such as a Santana 30/30. The drag he is referring to under the boom is
aerodynamic drag from eddies that form of the end of any airfoil. The
flatter the section is, the less the eddies will form. Of course like
anything else you can always overdo it. In light air conditions where you
need power you will need the bottom of the sail fuller than you do in
windier conditions.
As far as trimming the main you should be looking to have the boom at or near centerline with enough twist so that the back end of the top batten is parallel to the centerline. You will find that in under 8 knots of wind the top telltale will not flow back all the time, but it should be streaming back about 1/2 of the time. The lighter the wind, the less it will stream. As the wind increases the top telltale will stream more and more. By the time you get to about 11 knots of wind it should be streaming all the time. If you can't get the top telltale to stream it means you either have the sail sheeted too tight (too little twist) or the sail is too full. That is why even in light air you need to sail with at least some backstay on.
Once you get the top telltale streaming while the boom is trimmed to centerline and the top batten parallels the centerline you have about the right combination of twist and depth. After that as the wind increase you will need to flatten the main to reduce healing. As you increase mastbend you will need to sheet the main tighter and tighter because as the main gets flatter the upper leech becomes more and more open. As long as you can keep the boat from healing too much you want to keep the twist in the main about the same. When you reach the point where the main is about as flat as you can get it you will need to start letting the leech twist even more to depower the boat.
It is hard to talk about trimming the main without also talking about headstay tension and checkstay tension. You should always think of the checkstays as a control of headstay tension, NOT as a control of mast bend. If all you want to do is change the mast bend you don't need checkstays. Just put on more backstay to bend the mast and let if off to straighten the mast.
Checkstays are for controlling headstay tension. If you are sailing along and have the main trimmed just the way you want, but find you are not pointing because the genoa entry is too round, what do you do? Put on more backstay which will increase headstay tension and make the jib flatter. This will also make the main flatter so now you have to pull the checkstays tighter to reduce mastbend back to its original amount. Now the main will be the same shape as before, but the headstay will be tighter and the jib entry will be flatter.
Harry Pattison
Elliott / Pattison Sailmakers
-----Original Message-----
From: Mike Guccione [mailto:innove8@pacbell.net]
Sent: Tuesday, October 09, 2001 3:46 PM
To: Harry Pattison
Cc: Santana 30-30 Mailserver
Subject: mainsail trim
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 Thu Oct 11 07:13:58 2001