twist in the main sail ?

[simonfraser]

the top part of the main is supposed to twist at low to medium wind speeds, sugesting that higher up the wind direction is further aft

but the wind speed is supposed to be greater further up as there is less drag from the water surface, so that would suggesd no twist at all ?????

please, someone explain this to me

 

[gandy]

Assume you're sailing upwind. The wind onto the sail is made up of the true wind, plus the effect of the boat's speed through the water. This Apparent wind is further forward than the true wind.

At the top of the sail, the true wind will be a little stronger, but the boat speed is the same (obvoiusly) so the apparent wind is not so far forward.

That's the theory.

 

[ytd]

also the effect of swell means the top part of the sail oscillates through the wind more than the bottom part. Having twist means the top part of the sail doesn't stall when the boat rolls into the wind direction. Anyway it seems to work in practice

 

[john_morris_uk]

Not only the theory, but the fact. Watch the tell tales as you adjust twist and it becomes obvious that its very easy to stall out the top of the main by hauling too tight on the kicker, and/or having the mainsheet and traveller set incorrectly.

 

[wooslehunter]

No-one's mentioned the most important factor yet: your foresail.

This has the effect of creating a slot between the foresail leach and the mainsail luff. Since air is accelerated through the slot, the pressure is lowered even further than just a single sail and Mr Bernouli pulls your boat foreward. The foresail is far wider at the foot and therefore has a far greater effect of directing air over the main.

 

[simonfraser]

thanks

 

[john_morris_uk]

Hmm - I think that sail makers and theorists have mixed opinions regarding the idea of the boat being 'sucked' forward....

 

[mikejames]

I really hate it when the crew put a 360 degree twist in the main sail before placing the luff rope in the groove and then try to hoist it

 

[dulcibella]

Just can't get the staff, can we?

 

[john_morris_uk]

This post in the finest tradition of thread drift

Nearly as bad a crew on boat I was in company with hanked the headsail on upside down with the Princess Royal on board. She thought it was very funny, they were very embarrassed.