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

Originally posted by Chew my RS 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: .pdf"> http://www.arvelgentry.com/magaz/Another_Look_at_Slot_Effect .pdf %20Sail%20Theory.pdf"> http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20 Sail%20Theory.pdf

Lot of interesting stuff there and probably relevant at the time, but things
have long moved on since then, and, the jibs and mains I have been
exposed to react differently. Now, this could get very very technical, a lot
of what that theory debunking the original venturi theory I believe was
probably correct, but it doesn't explain, nor take into account the fact
that inorder to function, the main HASto have a faster airflow over
the leeside in order to function. And it's my theory it doesn't slow the
airflow over the main, it deflects it and the balance is drawn through the
gap which evens out the pressure, but obviously speeds and 'heads' the
main (Drawing needed here).

Back then they were still tied up in Bernouilli, fretting about boundary
layer separation, AND they were stuck with traditional cloths that by
their very nature had tight trailing edges that induced massive tip
vortices, and the speeds and apparent wind effect was a much much less
part of the equation.

I accept the point you and they are making, and agree my view was
couched in the original theory debunked in that article, but it has always
been a black art, with theory countering theory.

I'd agree with the jib sheeted in, there would be no increased airflow
between the two foils, particularly with full jibs with overlap (Our current
jib is a self tacking affair so the gap is bigger)but, ease the sheet increase
the gap and that airflow will increase and so will the efficiency of the
main.

I'm minded to set up a bit of a test with those hand held anemometers, it
would be an interesting experiment.

We know, from sailing tandem windsurf boards, that the aft sail
experiences huge pressure drops if the lead sail is sheeted in, both sails
have to be balanced exactly so there is an airflow between them before
the sailors can for example hook into their harnesses.

So it is only when the airflow between the sails is restored that they work
anything like their maximum, but it is also true that neither sail feels or
works like it would if sailed singly, so the whole becomes less than the
sum of its parts. Which kind of makes both theories true.

Edited by G.R.F.

I'll try and keep it simple.  Ignoring drifting conditions you sail in 2 basic wind conditions - you are either under-powered or over-powered. 

Under-powered requires a rig set-up that produces maximum force and if it produces a lot of heeling force that is of little consequence because you can always hike a bit harder.  You therefore want full sails sheeted as close to the centre line as they will go.  Too close and they will stall especially at the top so some twist is good and you control that with mainsheet tension.  No kicker or downhaul will be required with a conventional rig.

Over-powered requires a rig that minimises heeling force (you still want as much forward force as possible).  Flat sails produce less power and less drag so apply kicker to bend the mast but this hooks the leach so apply downhaul which opens the leach.  Open the slot too by moving the jib fairleads out or back.

You could of course lose power by setting the sail up badly by not appling any kicker or downhaul but you then have a sail which flogs.  A flogging sail produces a lot of drag and is "snatchy" (either on or off) which is very difficult to control.

So in strong winds you want flat sails and if you apply a lot of kicker and downhaul you'll see the top batten go almost straight meaning that the top of the sail is producing hardly any power (and hence hardly any heeling force).  And because the sail moves in and out like a barn door it is easy to modulate hence you can easily increase and decrease the heeling forces that fluctuate so much in the turbulent air of strong winds.  Incidentally a bit of twist in inevitable in strong conditions.

Well you can't quite get the sail on the centre line.  On a 4000 we use a strop to get the boom as near as we can - which proably puts the lower leach more or less parallel to the boat axis.  You have to remember that a fast boat like a 4000 can be beating at 8 or 9 Knots in a 10 Knot breeze so that'll mean the apparent wind is only about 25 degrees off the bow - so the sheeting has to be narrow compared to say a Laser which is not going to get above 5 Knots.

It very important not to hook the leach at the top which is why the kicker isn't used and its controlled by the mainsheet tension which can be continually hardened or eased acording to pressure.  If the top does hook the sail stalls and the rudder suddenly goes dead.

I might just mention something which at first seems counter intuative.  On faster boats the increased speed which come from increased pressure is worth taking advantage of.  So unlike slower boats which luff when over powered and thus gain hieght.  The faster boats drive off in the gusts and just go faster.  We actually have to ease the jib a bit.

But I was trying to keep it simple.

I can tell you, from very bitter experience beating with no kicker in strong
wind is a very unpleasant experience especially when you have to do it for
the best part of 10 mile or so. Everything redback said is true.