# shape of sail at head of square top

Discussion in 'Hydrodynamics and Aerodynamics' started by Vincent DePilli, Jul 17, 2013.

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### Vincent DePilliJunior Member

Hi all—

I posted on sailing anarchy about techniques for shaping the head of square head sails, Based on a complete lack of knowledge, I assumed that you would strive to keep the depth of the sail (in percentage terms) constant over the span of the sail.

I received two responses (quoted below), both posters being of the opinion that the head of a square head should be very flat. Any of you aero-geeks have any opinions on this?

Response 1: “My belief is that there should not be any shape in the squarehead. The advantage of the squarehead is that increases the apparent aspect ratio of the sail, to increase upwind efficiency, and to create zero lift (no shaping) to decrease vortex drag (no lift = no induced drag).”

Response 2: “You definitely need slight shape in the head of a square top to allow you to stand the leech up. Only a tiny bit though. I'm talking about mains and above the top batten not the very head...”

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### DCockeySenior Member

Induced drag is a function of the lift distribution, not whether the sectional lift is zero at some location. The lift can go to zero at a particular location with trailing vorticity causing induced drag from the same location.

"No shaping" only means "zero lift" if the local angle of attack is zero. Otherwise lift will be generated.

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### Vincent DePilliJunior Member

So- Response 1 ignored angle of attack.

But what do you think about the underlying issue-- would the ideal square head shape have thesame camber (in percentage terms) as the middle of the sail-- or should we be seeking to make the head as flat as possible consistent with structural requirements?

(Certainly I can see the benefit of a flat head in practical terms-- it can be hard to get the gaff battens to "pop" in light air. The shallower the camber in the head, the easier the pop.

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### philSweetSenior Member

Very flat compared to what?

In terms of percent camber, I believe it usually increases the further up the span you go. The sail bounces around more higher up and also operates at a higher Reynolds number and also experiences a lot of up-flow. The up-flow will lessen the effective camber because the leach is twisting off as the airflow climbs the sail near the peak. So it ends up depending on the specific boat and mostly on how high performance it is and the range of twist the leach will take. I suppose it would be flat compared to a pinhead though.

We have a couple really good sail guys around here. I expect they'll chime in soon.

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### lewisboatsObsessed Member

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### ErwanSenior Member

I bought my first suare top sail for my A-Cat in early 90's.

When rigged , and the boat put at hrizontal on the parking of the sailing club, it was striking to see how little force with the finger on the tip of the square top batten affected the whole leech profile.

IMHO Square top is primarily to manage the twist automatically in the gusts,
It provides also a taper ratio (tip chord/ boom chord) within 0.4 to 0.6 which is considered to be induced-drag optimum for glider with rectangular wing.
Best regards

EK

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