Mainsail shape in very light airs

[Stephen Weiss]

Last weekend we were racing in the biggest boat (45') in the field with next to no wind and it was embarrassing!

Upwind, the mainsail refused to conform. The draft stayed close to the mast just above the second batten and the sail formed an S shape abaft of the draft. I suspect there may have been a problem with one of the battens being jammed half in and half out of its housing, but I haven't had the chance to check this yet.

On the water, in a race with the benefit of 7 other opinions, all sorts of interventions were made with the result that no-one knew what was going on and the sail stubbornly maintained its ridiculous shape.

This is not our racing mainsail which we should be getting back in a week or so. It was a make-do sail from the cruising wardrobe but even so I'm confused and frustrated by my inability to trim it.

Next time I'll start with kicker off, backstay slack (going upwind), leach-line slack, halyard and out-haul "au-point" and the traveller to windward.

Any other suggestions?

 

[lw395]

Get the crew to shut up and sit to leeward. A bit of heel will help the sail fall into shape.
In light air, particularly upwind, the main needs to be fairly flat, and usually well twisted.
But a fair shape is
more important than the exact correct draft IMHO.

 

[TimBennet]

Get the crew to shut up and sit to leeward. A bit of heel will help the sail fall into shape.
In light air, particularly upwind, the main needs to be fairly flat, and usually well twisted. But a fair shape is more important than the exact correct draft IMHO.

All points are spot on. In very light airs the main is actually flatter than when it pipes up a little. You need to keep the air flow 'attached' and it can't do this if the sail has too much camber. Leech tell tales all flying.

The S shape sounds definitely like a batten problem. Are they full length?

 

[Stephen Weiss]

The S shape sounds definitely like a batten problem. Are they full length?

Yes, full length, circular cross section, with nylon housings at each end, I suspect there is a centimeter of unoccupied housing at the luff end, but I haven't had the chance to check this yet, and that particular sail is coming off probably next weekend. I'm more concerned about our inability to move the draft about the sail with the wind so light. It really was very light - max 5 kt.

 

[TimBennet]

Full length battens can limit the effectiveness of the usual sail controls in governing sail shape.

I would think that if you have an S-shape, then the batten has too much end loading on it. ie is slightly too long, not too short. If it was short, I would expect to see the sail be overly straight with a 'hinge' where the batten ends.

All full length batten systems have some way of controlling end loading. I would first check they were all just 'snug' and not cranked up too tight. Then in really light winds I would go with the 'flatness' induced by the battens and concentrate on achieving a constant shape with an open leach.