Aqua Dynamics

[johnphilip]

wetted surface area

Probably as important as the shape of the bilge keel is the sheer additional surface area being dragged through the water. Pegasus yachts used to build the Prelude where the keels where short fore and aft to reduce rthe area but swept aft at the foot to give length to sit on and weight low down where it gives the best righting moment. We used to surprise people with the performance of this twin keel and I am surprised the logic has not been applied more often.

 

[DJE]

Seems simple to me. The bilge keels on my Sadler are splayed vertically, both are flat on the outboard face and convex on the inboard face so they generate lift forces which act towards the centreline and slightly downwards. The horizontal components of these forces cancel out leaving a resultant vertical force acting downwards through the centreline of the boat. Not great when the boat is vertical but when she heels this force heels and acts to windward cancelling our leeway.