Flying Cuben sails

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I took some sail pictures of Cuben fibre sails for my IOM when rigged on the pond-side, and then when flying on the water, using the mast camera.  The resulting comparison of static shape and flying shape is interesting.  Wind speed was around 2.5 m/sec, pretty much middle of the No.1 rig wind range.  Sheeting angles were about 3 or 4 degrees for the main boom, 12 or 13 degrees for the jib boom.  The following graphs derive from the revised sail analysis spreadsheet.


As can be seen, the jib was cut with around 9% draft, the main with around 6%.  On the pond-side, the foot of both jib and main was trimmed relatively flat, particularly so for the jib.  When flying, both sails showed an average increase in draft of around 1%, but differed in how the head of the sail reacted.  The head of the main did not show increased draft, while the head of the jib showed significantly increased draft.  Part of the reason for the head of the main not showing any increase is that, when the pictures were taken, the main's head had distorted and stretched already, so that its static shape was the same as its flying shape.

Position of draft

On the pond-side, maximum draft for the middle of the main was around 40% of chord, about 35% at the foot, and 32% at the head.  The jib had less consistent placement of maximum draft on the pond-side, 34% in the middle but up at 45% at the foot and 40% at the head.  When flying, draft moved forward.  This forward movement was moderate but systematic for the main, about 2 or 3% of chord.  Forward movement of draft was more pronounced for the jib, around 6%, except at the foot, where draft moved aft!  Now that was a surprise.


When trimmed at the pond-side, the main had "average" twist ending with about 10 degrees at the head, while the jib had somewhat more twist, ending with about 13 degrees at the head.  The interesting finding was that, when flying, jib twist hardly changed at all, while the head and upper part of the main twisted off very significantly.

My rig tension was sufficient to hold the topping lift in place, but allowed the head of the main to twist off.  I think that the sail cloth must have been a little stretched as well to permit such a significant increase in twist.

Entry angle

This graph shows the "effective" entry of the sail luff to the wind, and includes the entry angle of the sail itself, added to the sail twist at that point, and the sheeting angle of the boom.  At the pond-side, the foot of the jib and most of the lower main was trimmed quite tightly, with the rest of the jib pretty free.  From a theoretical perspective, we would want to see entry increasing from deck level up to the head of the sails to accommodate both the wind gradient and the induced upwash due to lift, while the jib should have an entry some 10 to 15 degrees more open than the main.  When flying, the forward movement of the jib's draft increased its entry angle 10 degrees, a very significant (and almost certainly undesirable) increase.  Similarly, the twist-off of the head of the main also caused the entry at the head to increase by 10 degrees.  No way was this boat going to point!

Exit angle

The final graph shows the "effective" exit angle of the leech, which includes the actual exit angle of the sail leech less the twist at that point on the sail.  The exit angle measures how closed or open the leech is.  A common rule of thumb is that the upper batten should be parallel with the boom, and this is how the sails were trimmed at the pond-side.  As can be seen from the graph, the "effective" exit angle was around 0 degrees for the top of the jib and main.  When flying the exit angle changed largely as the sail twist changes, and so the leech opened up towards the head of the sails.  Interestingly the leech closed at the foot when flying, again a somewhat unexpected finding.


2022 Lester Gilbert