Darryl,

After five years of construction, ACID is a rather appropriate word… for my stomach! I knew nothing about foils before purchasing the boat's project and learned what I could during construction. It is still going on, so there’s time to learn more. Hopefully not much (I hope).

All remarks are based on what the designer said, not on research or first hand experience. In other words, I am just forwarding information and elaborating on it.

My boat is designed to reach maximum speed with the central hull just skimming the surface. It is not intended to fly two hulls - which is good for cruising.

The rudder foils are asymmetrical. They are placed close to the surface so that lift is maximized by the back wave flow and goes to zero suddenly, when the central hull leaves the water. Note that there is a “step” in the lift function: from very high lift to zero lift. In comparison, your rudder tip foil generates positive lift most of the time and becomes slower - eventually negative - as the bow points down. But the lift function is continuous, without steps, meaning that they work quite differently from mine.

Analyzing the differences:
- High asymmetric rudder foils are meant to maximize lift until the central hull is raised from the water, when their lift contribution becomes unnecessary. They never generate negative lift. The lift x angle of attack function is discontinuous. They are draggy in light airs.
- Your foils provide a smaller positive lift most of the time that gradually reduces and becomes negative as the pitch attitude requires more and more correction. There is a lot less drag in light air, but extra wetted surface in heavy weather conditions due to the negative lift sinking the stern. This means more drag AND more wetted surface. In other words, it slows down the boat to keep it from pitchpoling. Sacrificing speed for stability is a good trade whenever fore and aft stability is what limits speed. For this reason, it is reasonable to say that your type of foils is a great addition for any boat with more transversal stability than fore and aft stability.

I ignore the rationale behind the choice of each foil shape used in my boat. There are four different formats:

- Symmetric section rudder blade with elliptical tip
- Asymmetric section Bruce foils with elliptical tips.
- Asymmetric section delta shaped rudder's foils with straight tips.
- Swept back asymmetric section rear float foils with straight tips.

All asymmetric foils feature one flat surface and one curved surface. As far as I know, the choice of section was for constructive reasons, convex surfaces being difficult to build both for the plug and to laminate in the mould (for serial construction). I wonder if the tips of the smaller foils are the only straight ones for the same reason (constructive), or if it is because those are the foils that go in and out of the water to keep the boat with a bow up attitude.

More later. Or after tests <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />