Light wind rigging quandary

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Light wind rigging quandary

Postby ittiandro » Thu Jan 22, 2015 12:39 pm

I am mostly subplaning because of the prevailing conditions here and my aim is to maximise the lightwind capability of my gear , by compensating, if at all feasible, the inherent bias of modern equipment towards strong winds sailing, speed and, ultimately, planing.

Regarding the sail, I had always thought that, for maximum power,( needed for 10-12 knts light winds), it should have a fuller profile ( a deeper pocket) in its the lower part and forward of it, towards the mast.. Here is supposedly where the center of effort of the sail should be located for optimal (light wind) performance and this fuller profile would be attained, as far as I knew, by increasing the DH tension, in combination, of course, with the OUTHAUL adjustment. .

I am however a bit puzzled, because ... TVScPDt.97 gives a different story. It is said there that in order to give a fulller ( deeper shape ) to the sail, the DH should be slightly DECREASED by 1 cm , not increased!

I also read in one of the forums that one way to deepen the profile in the lower sail (and to shift the center of effort even more forward, closer to the mast ) would be to stiffen the batten above the boom) by inserting a 2nd shorter half batten in the pocket, at the leech end. The flattening of the sail on the leech side would amplify and shift even more forward the full profile of the sail.

I wonder if you or anybody can comment on all this.



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Re: Light wind rigging quandary

Postby JayTurcot » Thu Jan 22, 2015 6:22 pm

I agree with boardseeker. Less downhaul gives you more pocket in the sail.

Incoming poor man's mental model!
[insert standard disclaimers about rigging, each sail is different etc.]

Generally speaking when you apply more downhaul tension, you bend the mast more.

This has a couple of effects:
  • The leech (non-mast/trailing edge of the sail) becomes floppy, aka wrinkles in the sail at the top. This looseness translates to twist, which allows the sail to spill power in gusts. I set my Goya sails downhaul by looking for this floppiness. This is the stronger effect, and the one mentioned most often WRT downhaul tension, hence me listing it first.
  • The clew moves further away from the mast, along with the battens. When I MAX my downhaul, my outhaul is usually ~2cm longer (to achieve equal outhaul tension). I picture if the clew is closer to the mast, it can bag out more, i.e. achieve a fuller shape when you press on the sail area. I set my Loco's downhaul by looking at how much the batten sticks past the leading edge of my mast (advice from my friend for the Loco). More less downhaul, battens stick past the mast more = more power.
The second point (clew movement) is the one that applies to your question.

That being said:
The outhaul as a much more pronounced effect on that aspect of the sail shape. When I'm really trying to get power, I'll pull it maybe +1cm from neutral, or sometimes just engage the cleat with outhaul neutral. When you press on the sail, you'll see that it sags more under hand pressure. My Eclipse, fully bagged out will be touching the far side of the boom. It's a heavy pig rigged like this, but does give a bit more grunt.
Jay Turcot
"It's a crazy addiction we have isn't it?"

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Re: Light wind rigging quandary

Postby chris » Sun Jan 25, 2015 9:54 am

Boardseeker is right. All sails begin with more draft pocket when you first rig up. You trade some of this full draft for sail tension when you DH the sail. You probably lose some low end power but also reduce drag when you downhaul the sail. I disagree that all modern gear is biased for strong wind. It's designed for strong apparent wind, ie. the forces experienced on a lightweight board that is planing. The solution for light winds in this paradigm is almost always a larger sail. There's a few sail models that are good for displacement boards. Demon sails, ezzy zephyr, Severne, starboard race sail... high aspect ratio sails with less twist, deep draft.

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