Monday, March 30, 2015

Main Trim in Light Air

by Mike Ingham

For the 2015 (40th) Orange Peel in Jacksonville we held a well attended clinic on Friday and PRO Hal Gilreath ran 3 good races on Saturday. But the speed story for both the clinic and racing was trim in light air. Coaching from a motor boat on Friday, I had a good perspective to see how most boats handled the light air, and almost without exception, teams overt-rimmed.

I think the problem is the main needs to be more twisted in light air than people think. For example, in 6kts, the top batten can hook in a few degrees tighter than the boom, in 3kts it needs to be parallel to the boom. There are several reasons:

  • The mast is stiff up top: When we ease the main sheet, the mast straightens and makes the top full and hooked. So we need to ease even more to get any twist.
  • Wind shear: At the top of the sail, the the apparent wind is further aft in light air than in heavy. That is because there is a bigger % difference in wind speed up top. Therefore there is a bigger component of the apparent wind due to the wind speed than boat speed, moving the wind aft. To compensate, we need to twist more.
  • Wind attachment. The wind has trouble staying attached to a sail in light air. If the main is hooked in, the wind can get around it and stay attached well in 6kts and up, but down around 3kts it cant make it around.
  • Pressure: to add to the problem, the leach does not blow open with wind pressure, so it naturally wants to hook in.
Note how much more eased the main is from 'normal' trim when there is more breeze

So, the question is “How much sheet is right?”

The short answer is that in light air I trim so the top telltale is just flowing 10% of the time. I find to get it flowing even 50% of the time, it is too twisted.

When it is really light, I look at both the leach telltale and the top luff of the main above where the jib overlaps. If to get the telltale flowing at all, I need to ease so that the top leading edge starts to bubble (luff), it is too eased and I pull it in until it the bubble goes away. When it is very light, the top tale may stall 100%. But I am very careful to pull it in only until the bubble goes away.

I find with the Proctor cut, I can trim a little harder because the top is a little flatter than the Fisher cut. With the Fisher cut, I have to pay extra attention so I don’t over trim.

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