Thank you very much for your comments.  It does seem that wind sheer is less over 10 knots and really only very much of a factor under 6 knots.  I calculated less than 4 degrees change in wind direction from the bottom of the mast to the top.  I guess twist would reduce induced drag but also at the cost of lift.  Very interesting that twist could be a function of triangular shape.

Ron

If I understand the experts’ texts correctly, we are actually sailing in a quite thick (20 - 100m) boundary layer of wind. The wind is weaker near the earth’s surface, and gets stronger and stronger higher up.

Firstly, we sheet our sails to the relative, apparent wind direction, not the true wind. If we are sailing on a (true) beam reach, doing 4 knots in  8knots wind, the resultant wind will appear to hit us at 63° from the bow, not 90°.

Secondly, since we, as said, are sailing in that thick boundary layer, the wind strength will be higher at the top of our sail than near the boom. Let ‘s say the 8kts wind above was measured at boom level. If we imagine that the wind is 11kts near the top, the apparent wind direction would be 70° from the bow up there. This alone is enough to call for some (7°) twist in the sail.

I agree with David that camber could well be used all the way to the top panel. However, I have chosen to make my sails with a fanned top with just a little camber in the top panels, and that for three reasons:

·         This planform lets one have the tallest and biggest sail for a given mast length. I want all the area I can pile on for the downwind leg.

·         The flatter top panels make up my ‘storm sail’. Since I cannot alter the camber under way, I have to compromise and cut these panels to suit stronger winds.

·         Actually, just by chance (no planning), I have found that when my sails are deeply reefed, the top three panels get a bit extra twist in them, and this seems to add a bit to the camber. I have found the top section to be remarkably powerful. The tacking angle is wider than with seven panels up, but that could well be due to increased parasitic drag from the masts etc., not only due to the lower AR or efficiency of the reefed sail. We surely sail to windward with only three panels up.

Arne

Thanks Arne,

I wonder if anyone else has any further thoughts on sail twist.  Twist at the top of the sail should shift the force vector forward, reducing heel and reduce induced drag.

Ron

I apologize if this has been covered elsewhere.  I read that some junk rigs should be set with less twist or no twist.  This seems a departure from the world of BM rigs.  Does anyone have a thought on why less twist in the Junk rig is preferable?