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Sail Twist |
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ellistine
Really should get out more Joined: 06 Mar 08 Online Status: Offline Posts: 762 |
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Topic: Sail Twist Posted: 21 Oct 08 at 3:05pm |
I like the idea of the crew working the jib sheet. Why should I have to do all the hard work I'll give that a go too. |
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G.R.F.
Really should get out more Joined: 10 Aug 08 Location: United Kingdom Online Status: Offline Posts: 4028 |
Post Options Quote Reply Posted: 21 Oct 08 at 3:45pm |
Sails should twist to work efficiently.
Now before y'all start citing this class and that class, all boats do something else, they heel and therefore spill wind off the head. Whereas in that more sail efficient world that I have always existed and I note will come to pass in the future with foiling moths, the rig is canted over to weather, so the sail must work automatically and efficiently to ensure continued power and no stalling action so twisting off at the head or what became known as 'floppy leech' worked out to be very very much faster than sails that didn't twist and remained tight leeched and more stable. The reason is exactly the same as having a tuned exhaust on a race car, get the fuel in extract the power and get it out, so it is with air, use it and exhaust it before it turns into something else. I've also been told down the years by everybody that knew anything about dinghies, that 'twist' and the way twist occurs is very important, so if you can adjust your rig so the lower half remains powerful with a tightish leech, then the further up the leech the more it 'twists' off in the puffs, the better it is. But equally I've also found that the way the jib reacts and the gap that brings a venturi effect of speeding the airflow over the main, without backing the leading edge of the main is also of paramount importance to an efficient and fast rig, that bit I found out for myself, the hard way and the jib itself is very inefficient in that it has a propensity for stalling the flow rather than twisting off at the head, which in turn adds to the turbulence over the main, another reason not to have it sheeted too hard in a breeze. Edited by G.R.F. |
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ellistine
Really should get out more Joined: 06 Mar 08 Online Status: Offline Posts: 762 |
Post Options Quote Reply Posted: 21 Oct 08 at 3:54pm |
We've always sailed with the jib hard on when going up wind pretty much regardless of wind strength. It sounds like it's more sensible to let it off a little to depower and reduce the main backing when it too is let off a little? All these new things to try. I'll have to right myself a list to take out with us. |
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Chew my RS
Really should get out more Joined: 05 Oct 05 Location: United Kingdom Online Status: Offline Posts: 790 |
Post Options Quote Reply Posted: 21 Oct 08 at 4:09pm |
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. |
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http://www.sailns14.org - The ultimate family raceboat now available in the UK
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tgruitt
Really should get out more Joined: 02 Dec 04 Online Status: Offline Posts: 2479 |
Post Options Quote Reply Posted: 21 Oct 08 at 4:39pm |
Are you sure? I was led to believe it was the venturi effect? Edited by tgruitt |
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Needs to sail more...
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G.R.F.
Really should get out more Joined: 10 Aug 08 Location: United Kingdom Online Status: Offline Posts: 4028 |
Post Options Quote Reply Posted: 21 Oct 08 at 4:56pm |
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. |
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ellistine
Really should get out more Joined: 06 Mar 08 Online Status: Offline Posts: 762 |
Post Options Quote Reply Posted: 21 Oct 08 at 5:18pm |
Could you break that down a little? Our shrouds and lowers are still on factory settings. The lowers are if anything quite loose. What effect does the lowers tension have on sail shape? I was as interested as you with the explanations of the adjustments made on some of the boats in the Endeavor Trophy. Somebody mentioned they moved the spreaders foreward and out. I can't work out in my head what that would do. It's all interesting stuff. Keep it coming! |
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AlexM
Really should get out more Joined: 10 Jan 06 Online Status: Offline Posts: 857 |
Post Options Quote Reply Posted: 21 Oct 08 at 5:33pm |
Sail twist (wind 25-30knts)
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ellistine
Really should get out more Joined: 06 Mar 08 Online Status: Offline Posts: 762 |
Post Options Quote Reply Posted: 21 Oct 08 at 5:41pm |
So was that by accident or intentional? |
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AlexM
Really should get out more Joined: 10 Jan 06 Online Status: Offline Posts: 857 |
Post Options Quote Reply Posted: 21 Oct 08 at 5:46pm |
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 |
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