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post #1 of 11 Old 07-30-2013 Thread Starter
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Main and Jib Trailing Edge Flapping

What can be done to stop the trailing edges of the main and jib from flapping? When sailing in 25 knots or more of wind the trailing edge of my jib and the leech of the main start to flap (the jib somtimes significantly). I have tried different tensions on main outhaul, cunningham and halyard but no real improvement. With the jib I have tried different sheet tensions and placement of the lead block to change sheet angles, again with no significant improvement. I have also tried different tensions on the string that runs down the trailing edge of these sails, again with no real improvement. Could the flapping be an indication the sails are old and blown out? Even with reefing of the main and roller reefing of the jib the flapping continues. Both the main and jib have more "belly" than I would like and this may contribute to the flapping, but nothing I do can remove this belly.
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post #2 of 11 Old 07-30-2013
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Re: Main and Jib Trailing Edge Flapping

They don't "flap" under 25 knots? If they did, I'd say they may be blown out (depending on age) and you should go see your sailmaker. But if you see a lot of "belly", that could be an indication to do the same. Normally you'd tighten halyards and outhaul (main) to flatten a baggy sail, but that will only accomplish minor flattening.

Have you moved jib leads forward some? that could tighten the leech, though possibly too much if indeed the leech is all stretched out.

Personally, in 25 knots, just about all of my main and jib would "flap", I'd be feathering just to keep the boat on her feet. Although, typically in heavy air it's the forward part of the sails you carry the luff in, especially the main.

The more I type, the more i think you may have blown-out sails with too much draft and stretch for breeze of any strength. But a sailmaker, I ain't. Invite one to come out for a sail and listen.
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post #3 of 11 Old 07-30-2013
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Re: Main and Jib Trailing Edge Flapping

Do you have a topping lift? Have you eased off of it so the boom takes the weight of the main? Do your jib sheets run through blocks or the stanchions? Could that be playing with the shape of the sail, especially at the foot?

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post #4 of 11 Old 07-30-2013 Thread Starter
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Re: Main and Jib Trailing Edge Flapping

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They don't "flap" under 25 knots? If they did, I'd say they may be blown out (depending on age) and you should go see your sailmaker. But if you see a lot of "belly", that could be an indication to do the same. Normally you'd tighten halyards and outhaul (main) to flatten a baggy sail, but that will only accomplish minor flattening.

Have you moved jib leads forward some? that could tighten the leech, though possibly too much if indeed the leech is all stretched out.

Personally, in 25 knots, just about all of my main and jib would "flap", I'd be feathering just to keep the boat on her feet. Although, typically in heavy air it's the forward part of the sails you carry the luff in, especially the main.

The more I type, the more i think you may have blown-out sails with too much draft and stretch for breeze of any strength. But a sailmaker, I ain't. Invite one to come out for a sail and listen.
In winds below about 20 knots I have no flapping. Between 20- 25 I can get a minor amount. Above 25 knots I get constant flapping on the main and jib whenever I try to point into the wind, even after bearing off the wind (say 55 deg) I still get some flapping. I have tried moving the jib leads fore and aft and still get a flap. I have tried significantly reefing the roller jib and still get a flap at the trailing edge.
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post #5 of 11 Old 07-30-2013 Thread Starter
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Re: Main and Jib Trailing Edge Flapping

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Do you have a topping lift? Have you eased off of it so the boom takes the weight of the main? Do your jib sheets run through blocks or the stanchions? Could that be playing with the shape of the sail, especially at the foot?
Topping lift is eased and slack so boom is loaded by sail and main sheet only. The jib sheets run through blocks that are on track slides so I can adjust sheet angle. I have made adjustments of the block slide with no real change of the flapping. Also changing jib sheet tension cannot get rid of the flapping. Even with the jib reefed and sheeted in tight I still get a flapping.
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post #6 of 11 Old 07-30-2013
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Re: Main and Jib Trailing Edge Flapping

Do you have leech adjustment lines on your sails? If so, pull those until the flapping stops assuming everything else is trimmed correctly. Read the top section called "Reducing power by reefing" here Sails tuning recommendations - Sparcraft masts
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post #7 of 11 Old 07-30-2013
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Re: Main and Jib Trailing Edge Flapping

I second FarCry's idea. This was the first thing that came to mind. Every sail (both main and jib) that I've ever seen on 30+ foot boats have leech lines. These are very easy to miss unless you are looking for them.

On some sails, they look like little shoelaces sticking out of the leech of the sail anywhere between 1 and 3 feet above the clew.

On other sails, they could be webbing type straps completely hidden underneath velcro flaps in the same locations as I just mentioned. You cannot see these lines unless you open the velcro flaps.

At any rate, just a little snugging of the leech lines will stop the vibrations in the leech.

Good luck!
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post #8 of 11 Old 07-30-2013 Thread Starter
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Re: Main and Jib Trailing Edge Flapping

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Do you have leech adjustment lines on your sails? If so, pull those until the flapping stops assuming everything else is trimmed correctly. Read the top section called "Reducing power by reefing" here Sails tuning recommendations - Sparcraft masts
I'll do some reading. I do have the leech lines on main and jib. I have tried different tension on the lines but cannot get rid of all the flapping. The lines do help some.
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post #9 of 11 Old 07-30-2013
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Re: Main and Jib Trailing Edge Flapping

At 25 knots, you should most certainly be reefed. Reefed or not, making certain the leech lines are tightened will help, but also be sure to tighten the outhaul to help flatten the sail (and the belly) and tighten the cunningham to move the draft forward. The combination of the three will give you the best possible sail shape. If you are reefed, make certain you don't have any scalloping in the luff by having the reefed portion secured tightly to the boom. It's sometimes difficult to get a really efficient sail shape when you're reefed.

A reefed headsail is even more difficult to attain a good shape.
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post #10 of 11 Old 07-31-2013 Thread Starter
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Re: Main and Jib Trailing Edge Flapping

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Originally Posted by CLucas View Post
At 25 knots, you should most certainly be reefed. Reefed or not, making certain the leech lines are tightened will help, but also be sure to tighten the outhaul to help flatten the sail (and the belly) and tighten the cunningham to move the draft forward. The combination of the three will give you the best possible sail shape. If you are reefed, make certain you don't have any scalloping in the luff by having the reefed portion secured tightly to the boom. It's sometimes difficult to get a really efficient sail shape when you're reefed.

A reefed headsail is even more difficult to attain a good shape.
Good points, I will give it a try.
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