Sail Twist

Originally posted by tgruitt Originally posted by Chew my RS BTW GRF, the jib slows down the air flow over the main, not speed it up. Are you sure? I was led to believe it was the venturi effect?

Apologies for taking this of topic, but yes, I'm absolutely certain.  Take a look at these for a thorough explanation of the process:

http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect .pdf

http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20 Sail%20Theory.pdf

intentional. (i hope)

It was very windy and some big waves, so the rig tension was let off making the jib slot open up.   we had max kicker and loads of cunningham opening up the top of the main sail but notice the main was still brought to the middle.  Made us go very fast that day

Sail twist (wind 25-30knts)

 

The tendency of the jib is very similar to the lee bow effect you get from
another boat ahead and to leeward of you.

Too tight and it will 'head' the main which backs it. Even I know that
much, but still made that mistake early on because at the end of the day
the crew operates it and when it's windy we were more intent on
surviving, we're past all that now.

But it is a fact that the slot between the jib and the main, and this intel
goes back to Scorpion times, actually venturis the airflow as well as
slightly heading the main sail hence why you do all your pointing with the
jib leading edge and it those tel tales you watch.

Balanced properly a two sail dinghy rig is really acting as one unit as far
as propulsion is concerned, but if you get the balance wrong you are
effectively creating an air brake, which done wrong produces all those
parasitic drag things I crack on about.

We most of us these days with these modern boats are over canvassed
more often than not, so getting the depower right is very important and I
wish I could sort the magic rig formula, the controls open to us are so
over complicatedly useless compared with what i"m used to.

Bang the Gnav on, it will tighten the leech, but it also now bends the
mast forward and flattens the sail unless you tighten the lowers which
then prevents the cunningham from bending the mast to twist the top off
because the jib tension will bend the top of the mast ahead of the bottom
unless you then tighten the upper shrouds which then rakes the mast
back and it doesn't point as well... it goes on.

We've got a new main now, a good metre and a half smaller, but with very
little twist at the head, I've not used it in anger yet, but it'll be ages
before I work out what to do with it and when, life's a lot simpler for a
windsurfer with just two controls, downhaul and outhaul.

On some classes (RS400 for example) the boat is EXTREMELY sensitive to jib sheet tension.  Always let the jib out if you let the main out. 

BTW GRF, the jib slows down the air flow over the main, not speed it up.