Center of effort on mainsail moves forward with speed??

[Thomas Wilson]

In Geoffrey Nightingale's classic book "Dinghy ownership ..." (1956) (page 84), he states that the CE on the mainsail moves forward with motion. This has me puzzled, unless one can argue that the Bernoulli pressure gradient has a larger forward component due to its more curved shape as it catches more wind with increasing boat speed. Another explanation might be that one is on a reach with the sail at a larger angle towards windward. Nightingale is considering helm balance between the CE and the Center of Lateral Resistance (from the hull/daggerboard combo). Excessive forward motion of the CE would then lead to lee-helm it seems to me.

 

[fhhuber]

Its an effect of most highly under-chambered wings near stall.

When stalled its essentially equal to a flat plate deflecting the wind.
When it begins producing aerodynamic lift the wing's center of lift moves based on the airfoil shape.

A sail is essentially a flexible wing operating in low airspeeds.

 

[Thomas Wilson]

Thanks for the reply. Yes, under-cambered (or flatter) at low speed. Bernoulli Law's (conservation of energy for incompressible, laminar fluid flow) explains it all. There is a larger forward of the net force component, averaged over the entire foil with the asymmetrical air foil that develops with speed, then with a flatter sail There must be tomes written on the topic.