Physics and Sail Shape

A great tutorial on sail shape and trimming from Adam Walsh.

Quick note on the physics of sail shape.  The power (pull) and direction of that power generated by the sail is correlated to the max depth of the sails cord length. Meaning drawing a straight line from tack to clew.   At the point that has the max distance is known as max draft . Find that point and draw a line perpendicular to the sail  That is going to be the most power generated and it’s direction.    A full round sail will produce a large amount of power but the boat will have a wide angle of attack.   A flat sail will generate a smaller amount of pull but will have a very narrow angle of attack.   This is illustrated here.

Literal back of an envelope drawings.

Now with the asymmetrical kite. It is even more pronounced to rotate and move the max draft as far forward as possible. This will put the max effort of the sail closer into the direction of the boat.  With an over trimmed kite, the pull is more sideways that induces healing and gives a feeling of power but it’s power in the wrong direction.  So the key to the Asym is to constantly be easing the kite and working a robust curl at all times.  With a grinder it’s vital to work the sail constantly. The draft of the sail moving forward even for a few seconds gets the pull in line with the direction of the boat  This will increase speed dramatically.   But you have to stay on top of the trimming and not let the curl break too much. 

Bonus point. When you see a huge puff coming, if you can do a massive ease right when it hits it will move the draft way forward and the boat will leap forward.   But the grinder Must be ready to quickly grind in the eased sheet and more. As the boat accelerates the apparent wind will move even further forward so the trim will be much greater after the initial ease  That’s why it’s helpful to have the kite on a large drum winch in really breezy conditions 

Additionally. In big breeze if you can cross sheet to the weather winch it moves weight of grinder to a more optimal place.

Drawings are crude and questions welcomed.


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