North Sails One Design

Span Squared?

Article by Paul Bogataj

When a wing (like a sail or a keel or centerboard) produces lift, an accompanying drag force is generated called induced drag. This is related to pressure that is lost around the wingtip and the formation of vorticity. A wing is considered to be more efficient if it produces lift with less induced drag.

A common misconception is that a centerboard or keel with shorter chord but the same span is more efficient because it has higher aspect ratio. When we recognize that when span is the same, induced drag is therefore the same, we can properly attribute any performance differences experienced to the difference in area. These differences relate to the viscous drag instead, which is the friction of the fluid flowing over the surface and is dependent on the area, and also to changes in angle of attack (leeway and apparent wind) that occur as area is changed.

An example of the affect of span on sails that is common for many one-designs is that as speed increases, raking the rig aft has been found to be beneficial to performance. This can be explained through span squared.

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