Questions about bottom strakes on a small run-about

[woodwingdoc]

I'm building a small run-about and the plans call for a fin on the keel at the CG to prevent sliding in turns. It is a shallow V going flat at the transom. I was thinking about using several bottom strakes in lue of the fin but looking at many production ski boats at the lake this weekend, I noticed that the vertical face of the strakes is toward the chine in every case. To my novice, untrained thinking, it seems that the vertical face should be toward the keel thereby allowing the loaded half of the bottom to bite as it rolls and goes around the corner. Are strakes used for tracking or are they just stiffeners for the bottom or what?

Any advice from an engineer along with real world experience with bottom strakes/fins would be appreciated.

[Eric Sponberg]

Strakes on the bottom of a planing boat are technically "lifting strakes". They get their name because they cause the bottom flow along the hull to change direction, and in changing direction, there is change of moment of the water flow which pushes up on the hull--lifts the hull--therefore, "lifting strakes." The purpose of this lift is to push the boat higher out of the water so as to create less drag--drag due to wetted surface, and drag due to displacement. In the end, the boat is effectively "lighter", higher out of the water, than it would be without the strakes.

There may be some directional stability component to lifting strakes, making the boat a little more stable in turns, but this is not their primary function. The primary function is hull lift.

There is also additional stiffness in the hull bottom if it has lifting strakes, but again, this is a side benefit, not the primary purpose.

I hope that helps.

Eric

[woodwingdoc]

Thanks Eric! Makes perfect sense. Don't think I'm going to have to lift this little hull at all!

Pete