Aerodynamic Lift and Drag versus App. Wind Angle

Discussion in 'Hydrodynamics and Aerodynamics' started by farjoe, Nov 14, 2014.

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farjoeSenior Member

The attached is taken from from the book "Principles of Yacht Design" and describes the variation of sail lift and drag as well as hull drag in relation to the apparent wind angle.

I find it hard to understand the implications and the usefulness of this chart since whilst sailing one is always striving to align the sails correctly to the apparent wind angle except perhaps for angles above 150 degrees.

Conversely the drag created by the hull and the mast is shown to be constant whatever the angle yet surely is must be a lot higher when the wind is coming from Broadside than when it comes on the nose.

What am I missing?

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RurudyneSenior Member

How does the text differentiate between induced v viscous drag?

It looks like induced may be a function of heeling while viscous drag is that of the surrounding water the boat's wave system is pulling along with it. If that were the case I'm not sure why values at all below 30 save for trying to deal with tacking through the wind.

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farjoeSenior Member

This is purely an aerodynamic issue. Waves are not related. Neither is Heel since its effect is added later. Induced drag drag is related to the aspect ratio of the sails.

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jehardimanSenior Member

I think you need to re-read the text to get you head around the vector directions of apparent wind, lift, and drag. While a x-y graph like what you show is useful to pick off engineering values, you really need to draw out the free-body-diagram in polar ( like in Marchaj's "Aero-hydrodynamics of Sailing") to get the big picture on the forces.

A quick Google turned up the following site that has a good diagram of upwind, downwind, and a polar in Sections 1.2 and 3.2.

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farjoeSenior Member

Can you forward the link?

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jehardimanSenior Member

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