Main sail setting

PabloPicasso

PabloPicasso

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Hi all

I was single handing in a race recently, as various crew were dispersed around the globe ( the troubles of modern life). On a beat the mainsail was not setting well and was baggy near the mast. Due to my shorthandedness I wasn't able to play with sheet positions to try and improve things.

What affect would jib car position have on the mainsail, I was thinking afterwards moving the jib car aft a bit might open the jib and aid the main to set, does that sound right?

Fractional rigged cruiser/racer with large overlapping furling jib.

Sails are not that old and in good nic, and perhaps we can comment on this assumption. Thanks
 
Some boats, particularly older ones with large overlapping genoas have a tendency for the genoa, if over sheeted or with the car in a position that causes the leech to be hooked, to backwind the mainsail. It can be perplexing to see a supposedly perfectly set up mainsail luffing mysteriously.
You are on the right track. Experiment with the setup of the genoa.
 
A light luff-bubble shows you are not quite sheeted in enough, however, it is no bad thing. It is better to be under-sheeted with a bit of a luff bubble than it is to be over-sheeted.

I suspect just sheeting in a little would have improved it, but then you never really know how far you have gone. A spot of cunningham may have helped, or a little more halyard tension, but it really depends on so many things. I doubt your jib car would have been the first thing to change.

I would prioritise setting the jib car to get the correct jib shape and synch the upper and lower telltales when hard to wind. When the jib is flowing correctly, you have more than enough controls on the main to trim it ... perhaps you had just closed the slot a little too much?
 
Bit more info required!

What wind mode were you in i.e. looking for power, perfect power/design wind, or overpowered? Was the backing towards the head, or uniform?

Your main might back at the luff because of the following; these adjustments are all consistent with pointing higher.
- too full, in which case add pre-bend, tighten outhaul
- too loose, in which case sheet harder, raise track
- too twisted, in which case sheet harder, maybe dropping track, or use kicker if overpowered

...before considering the effect of the headsail. You could very likely move your cars back or out and/or ease jibsheet to reduce the main backing; might be a nice way to sail fast and be wayyy below close-hauled!

Typically, at the design wind in flat water (which is where tuning starts) I would be looking to make adjustments consistent with pointing higher, up to the limit where the helm feels dead and speed-through-water drops off a cliff. Then ease off a bit.
 
Having the luff of the main lifting a bit is not necessarily a bad thing. Especially when you have a large overlapping genoa. Often referred to as a "speed bubble" when the main trimmer wishes to assure the skipper that he knows what he's talking about...

But as with all things, a little is ok, a lot isn't. And it is also rig dependant. For example a masthead rig with a big genoa and small main would almost always have a fairly sizeable speed bubble, basically created by the high pressure on the leeward side of the main by the effect of the slot.
With a smaller genoa and a larger main, then less, and then with a non overlapping jib and large main, only a very small one would ever be tolerated, if at all.
You are correct that if it is big, and you cannot get rid of it by sheeting the main in, possibly because the boat is then overpowered, then easing the genoa car back will help. Especially as you may well then be able to sheet the main in a bit harder.
However, determining if this is the actual cause, or the best solution, is not something that I think it is possible to do over the internet...
 
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