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View Poll Results: How much true wind speed is needed to create and maintain air flow over sails?
1-2 kts 2 40.00%
2-3 kts 1 20.00%
3-4 kts 1 20.00%
4-5 kts 0 0%
5-6 kts 0 0%
more than 6 kts 1 20.00%
Voters: 5. You may not vote on this poll

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Old 30-09-2014, 08:20   #1
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Sail Draft in light air...

Most trimmers agree that in very light air one has to flatten the sails because without sufficient wind speed the air flow will separate if you have "too much" draft in the sail.

I know all boats are different but generally speaking with 32-44' cruising boats, at what level of true wind speed do you think there is enough wind to begin powering up the sails and increasing the draft?
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Old 30-09-2014, 08:49   #2
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Re: Sail Draft in light air...

The only draft issues I concern myself with on a cruiser is IPA versus lager.

Seriously though, I race and am very in tune with sail trim when racing higher performance boats but when cruising I take it down a notch. I cruise on a Beneteau 423. The genoa on this boat is the big driver so first order it to be sure it is trimmed right with proper jib car placement. Next I make sure that the main trim and leech tension are correct. I don't usually get to considering the main draft, partially because adjusting the outhaul is not very friendly on my boat.

The last thought is that the flatter sail works best in flat water. In most cruising conditions, you will need a fuller sail (or engine) to keep you moving.
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Old 30-09-2014, 08:55   #3
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Re: Sail Draft in light air...

it depends on the waves, the wind sheer, the type of sails, and the point of sail. There are other factors as well.
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Old 30-09-2014, 09:02   #4
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Re: Sail Draft in light air...

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Originally Posted by boat_alexandra View Post
it depends on the waves, the wind sheer, the type of sails, and the point of sail. There are other factors as well.
Tough crowd...as usual. Should have stipulated flat to 2 ft swells with Dacron sails.

Our crew had different opinions about this during our last race. Some said the air was too light and we needed to flatten the sails. Others said we had enough wind and needed to power up.

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Old 30-09-2014, 09:16   #5
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Re: Sail Draft in light air...

I believe multi hulls especially trimarans do much better in light air when there are waves on the beam because the rig doesn't shake around.
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Old 30-09-2014, 09:24   #6
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Re: Sail Draft in light air...

How heavy are the sails?
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Old 30-09-2014, 09:30   #7
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Re: Sail Draft in light air...

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How heavy are the sails?
I don't know how to answer that. Standard cruising boat manufacturer supplied Dacron sails. Nothing special. Catalinas, Hunters, Beneteaus, Jeanneaus, etc.

Sailing on a close reach and one guy says "loosen everything up we need more power" and another guy says "the winds are too light we need to keep everything flat".

Just curious what level of true wind would be necessary for sailors/racers here to start loosening everything up.
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Old 30-09-2014, 09:41   #8
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Re: Sail Draft in light air...

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Originally Posted by OldFrog75 View Post
Most trimmers agree that in very light air one has to flatten the sails because without sufficient wind speed the air flow will separate if you have "too much" draft in the sail.

I know all boats are different but generally speaking with 32-44' cruising boats, at what level of true wind speed do you think there is enough wind to begin powering up the sails and increasing the draft?
On many, many occasions with wind <5mph racing on a lake in Mississippi (even though in the book it will say to flatten the sails) I had much better speed with lots of draft in the sails.

Now the water was flat and sometimes what little wind there was was oscillating.
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Old 30-09-2014, 19:00   #9
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Re: Sail Draft in light air...

The solution is to do your own on-the-water testing. Record metrics (speed, course true wind angle/velocity, etc) using NavMonPC or something similar. Try different sail trims and in different conditions - make note of the times for each to compare back against recorder data.
On flattening sail depth – consider that if close hauled and wind angle is 35 to 45 degrees relative to the boat a flattened sail forces a harder bend, causing disattached flow. A headsail designed for very light air has more depth and camber profile with rounder shape to gently bend the air – more flow, less turbulence.
On wind velocity – the amount required depends very much on the boat and sail construction. Heavier Dacron sails require more wind velocity to push sail into design shape, light weight materials less. Healing the boat to leeward in light air is using gravity to help sails hang somewhat closer to resembling an efficient sail shape. Boats that are heavier and/or have much wetted surface area (friction) are obviously slower to get moving in light air, and so suffer for not being able to generate their own apparent wind.
Every boat/situation is different so I don’t believe that you poll will tell you much of anything– have a competition between your trimmers: flat vs fat. It’s also very helpful to try different people at the helm. Of course your boat, your helm, but subtle differences in driving techniques can yield helpful info on what makes the boat go best in light air.
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Old 30-09-2014, 19:30   #10
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Re: Sail Draft in light air...

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Originally Posted by svTOTEM View Post
The solution is to do your own on-the-water testing. Record metrics (speed, course true wind angle/velocity, etc) using NavMonPC or something similar. Try different sail trims and in different conditions - make note of the times for each to compare back against recorder data.
Would that we could. We're club racers which means we sail approximately 20 different boats a year without any practice in between.

We pick a boat, we're given a course, we get on the boat and go.

That's why I was looking for a general consensus for the interpretation of "very light wind" where sail trim authors recommend flat sails vs. "light wind" where you can start loosening things up to add some draft depth and power up - specifically for 32-42' cruising boats with Dacron sails.

One trimmer might say, "the wind is only 5 kts keep the sails flat." Another trimmer might say, "my god, man, the wind is up to 5 kts power up the sails."

I know what the various members of our team think. We all agree on the concepts; it's the inflection point we disagree about. I was hoping to get some outside opinions since I can't find any articles that state specific true wind speeds for "flat vs. fat" sails.
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Old 30-09-2014, 19:46   #11
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Re: Sail Draft in light air...

OldFrog75 – you mean you don’t have a support team of engineers and sailmakers eking out every 10000th of a knot possible?
Okay, well for what it’s worth (I used to be one of those sailmakers): 1) I never found that flat sail/light air approach to work –fuller shapes, eased sheets, and steering fatter angles get the boat moving. 2) At 2 knots or 6 knots - movement around the boat to change sail shape (halyard tension, outhaul, backstay, etc) and the shape change itself is far more disturbing to boat speed than picking one approach or the other and sticking with it. Subtle changes and subtle movements!
Good luck and sail fast!
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Old 16-10-2014, 14:39   #12
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Re: Sail Draft in light air...

Racing in Upper Narragansett bay in the summer runs into a lot of light air, the biggest issue with full shapes is keeping the sails from flopping.
WIth Dacron sails I used to have the crew sit on the low side rail to get the boat to heel and the sails would fall into shape.
this was in really light air with Dacron sails, the molded stuff s holds shape no matter what.
Basically your using gravity on a full shaped sail to get it to hang into the proper shape by heeling the boat to the low side, it seemed to work OK but that was on a lighter racer/cruiser, I don't think that would work as well on my current 43,000lb cruiser, my crew eighs more now but not enough proportionally to make a difference.
In that case I use the iron genny.
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