Correct Main Sail Tensions

[foolproof]

Don:

I have a 1976 Catalina 30, and have some questions about sail tensions. First, a little about my boat.

It is a tall rig and has a flat mid-boom traveler mounted on the coach roof. The goose neck of the boom "floats" as it is not secured to the mast. On the boom are 3 bales with blocks for the main sheet. This weekend I moved the aft bale further aft, from about 60% aft to approximately 75% aft on the boom, in hopes of being able to really flatten the main. It worked splendidly, which leads to my question(s).

The following assumes close hauled beating upwind as closely as possible.

In very light air, say 3 knots, would you pull the boom down hard to flatten the main, thereby reducing twist, or would you ease the mainsheet a bit and add a little twist. I understand how to use the traveler...that's not an issue, but I'm not really sure how flat to make the sail.

I know in heavier winds of 15 to 20 knots that it is OK to ease the mainsheet a bit and still keep the boom centered...I just pull the traveler car a little more to windward. This helps keep the boat a bit more on her feet and reduces heeling some, while maintaining a reasonable tack angle.

Am I on the right track with my thinking? If so, I should pull the boom down really tight in light air so that I'm not dumping air out the top of the sail, right?

I appreciate your opinion, thank you.

 

[Joe]

I'm not Don........ but I will tell you that pulling the mainsheet down hard, thereby closing the leech, doesn't "flatten" the sail... if by flattening the sail you mean reducing draft depth. You use the outhaul for that. The mainsheet controls twist, and you use your leech tell tales to guide you when making that adjustment.

You might try to think in terms of powering up or de-powering the sail.

Analyze each sail trim control in that context and you may see your example in a different light. Refer to Don't guide if needed.

For instance, in your light air example, closing down the leech with the mainsheet would indeed power up the sail.... but since your primary concern should be to keep the airflow attached then closing the leech will be detrimental.... instead, set the outhaul around half way, then open the leech, using the telltales to get the correct twist.... easing the halyard a bit will keep the draft position at mid chord. Adding some twist will also flatten the top of the sail (althought there has been some controversy about that in this forum) to help the air stay attached.

You don't mention whether your sliding gooseneck is rigged as a down haul like the original factory configuration.... most of us have gone to a cunningham set up which is a more effecient way of controlling draft position. Whatever, in light air you don't really want to pull the draft position too far forward.... so I'd leave it off.

The most important thing I can tell you, when sailing up wind, is to trust your tell tales.

 

[Alan]

Sailing in very light air is the most difficult to do and get right. Boat speed is very low and anything that is not dead on will cost more speed which can ill afford to be lost. Very light breeze has difficulty turning and staying attached to the sail. In these conditions you need a very flat foil that can maintain attachment as far back along the sail as is possible. Joe is correct that mainsheet does not control draft depth, outhaul does. Your mainsheet is used to control twist. Here again, with very low wind speed the wind angle doesn't vary that much (wind shear) with mast height so little twist is needed to maintain the correct angle of attack at all points.
I do have to disagree with your assertion of the correct use of your traveler. In heavier air a low twist/low draft profile needs to be maintained as long as possible. The traveler is the primary control for puffs and keeping the boat on her lines. Dumping the traveler to leeward to de-power then bringing it back up to approach centerline. Dumping mainsheet induces twist in the sail which only adds power which is counter productive.