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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #11  
Old 09-21-2011
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When reaching, particularly at wide angles, because of the point at which the head-sail sheet makes contact with the leeward fairlead, the headsail develops a large horizontal curve unless the clew is held outboard with whisker pole or by a twig to the end of the main boom. Such curvature causes flow to separate early on the headsail, making it inefficient, but, also, directs the air-flow on the weather side of the sail back and into the leeward side of the main, i.e. "back winding" the main. Depending upon the wind speed and apparent wind, as little as 25% of the aft end of the main may be generating power, which is far aft of the center of lateral resistance, hence weather helm. Either figure out how to rig a twig for you headsail sheet from the end of the main boom or simply roll in some of your head sail and move you lee fairlead forward to the widest portion of the yacht.

FWIW...
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  #12  
Old 09-21-2011
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I don't know your rig. It sounds like your vang was causing the mainsail to be too powerful.

Generally lots of tell tails to indicate how and where the air is travelling over the sail are a good idea.

The jib determines the course you can maintain so get the forward tell tails drafting nicely then work back. On the mainsail, the top 1/3 of the sail does the most work so always trim the sails starting with the top tell tails then work down.

Also start with things loose and gradually tighten the adjustments to figure out what is affecting air flow over different parts of the sail.

When in doubt, let things out. Selectively of course, but the natural instinct to tighten things is not always productive, tighten something, look at the sails then loosen and check that what happened was logical.

Of course when the wind is increasing you need to tighten thing but its also easy to overtighten so loosening off to check trim is a good check while you are figuring out what works for your rig.

Do this long enough and you will find you get the tell tails on the mainsail and headsail moving in tandem as the flows change over the set sails.

Don't think about power, think about efficiency, keeping the air flowing all over the sail and keeping the forces as low as possible in the sails and the wetted surfaces.

Its nice to have everything set so that when a gust comes through you can take the lift heading up a bit then resume coarse when the gust has gone. Its easy on the crew and easy on the rig.
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  #13  
Old 09-21-2011
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I would presume that without a traveller, and using vang sheeting (with stretchy vang) as a substitute, when going to a beam reach .... that the main became overtrimmed at the foot, and grossly powered-up in the mid panel sections due to boom-rise. This characteristically happens to jib/genoas (and most staysails) as they are not set to travelers.

I used to have the same problem with vang sheeting coupled with a lousy 'pin stop' traveller. A proper max. length traveller helped solved all this 'power-up' on a reach ... I wanted fast not powered-up; a rising boom / stretchy vang can cause such a 'power-up'

On my sport boat I have a full-width of beam radial traveler for the same reason .. and the 8:1 vang simply could not do the job of keeping the boom at a constant angle to the deck when the boat was 'screaming' on a reach.
Next time out in such conditions, walk forward and measure the boom to mast angle when the helm becomes heavy ... if much less than 90 deg., you'll need a good traveller to correct the situation.

:-)
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  #14  
Old 09-22-2011
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Quote:
Originally Posted by GeorgeB View Post
Mark, out of mild curiosity, what was your heel angle when you were experiencing excessive weather helm? As said earlier, try easing the vang to depower the main. This raises the boom tip and puts twist in the sail. On boats with travelers, you would be lowering the traveler instead, which changes the angle of attack instead. You will definitely want to route the vang back to cockpit. May I suggest you terminate it in a cam cleat instead of a clutch or Spinlock? I find that the line runs through a clutch much too slowly. There will be a time (like broaching) when you want to release tension immediately. Part of the joy of new sails and rigging is “dialing in” the boat. Great excuse to go joy riding around the Bay. We’ve been laid up most of the summer with a broken stem fitting so I am jealous – looking for another winch monkey?
Hi George, PM sent.
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  #15  
Old 09-26-2011
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Quote:
Originally Posted by MarkSF View Post
I have heard contradictory things : that use of boomvang on a beam reach flattens the sail, reducing drag and weather helm, and the opposite - let the boom rise to spill out some air.
Everything you describe makes me agree with svHyLyte. I describe it as, your main is creating "lift" from the wind being directed around it by the Genoa.

Quote:
Originally Posted by svHyLyte
directs the air-flow on the weather side of the sail back and into the leeward side of the main, i.e. "back winding" the main. Depending upon the wind speed and apparent wind, as little as 25% of the aft end of the main may be generating power
The "lift" is created by the main toward the aft end of the main. The leading edge of the wing (luff) is producing little force.

Quote:
Originally Posted by svHyLyte
which is far aft of the center of lateral resistance, hence weather helm. Either figure out how to rig a twig for you headsail sheet from the end of the main boom or simply roll in some of your head sail and move you lee fairlead forward to the widest portion of the yacht
The easiest way to reduce the main force is (as stated by svHyLyte), move the genoa car much more forward to reduce the genoa effect on the main. This has another more direct effect. That is, it reduces the twist in the genoa creating more gen power which counteracts the weather helm.

You should also tighten the vang to flatten the main. This flattening of the main does two things. First, it reduces the curvature of the main so that "lift" effects caused by the genoa air flow are dramatically reduced. Secondly, it moves the windward side main forces more forward toward the luff changing the point of force on the main. This flattening gives you back the power you lose when the genoa "lift" forces are taken off the main.

Thank you svHyLyte for the idea to use a whisker pole to move the genoa out to further reduce these "lift" effects. I now have another reason to get a whisker pole.

I was quite certain what was happening when I first read your post Mark. Today, I had the oppotunity to verify everything I have posted. I carefully adjusted the main and Genoa to first create huge weather helm by moving the genoa car back to enhance the "lift" effect.

Loosening the main sheet had little affect because it caused the "lift" effect on the main to increase even while the windward main forces reduced.

Moving the car forward reduced the weather helm immensely at the expense of speed. I was able to get that speed back by tightening the vang and adjusting the traveller (in my case) to get optimum speed with only a bit of weather helm.

Hopefully this helps

Last edited by BryceGTX; 09-26-2011 at 12:42 AM.
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