If the proa's rudders rotated 180˚ when shunted it would be trivial to set up NACA GOE611 airfoils angled so that the aft foil generated less lift than the forward foil.

Let me explain it a little better.  Instead of the rudder running vertically it should be angled up two to three degrees towards the end it is closest to.  The GOE611 lifting foil portion should be perpendicular to the rudder.  The GOE611 generates lift at negative angles of attack.  Basically what would happen is that the forward foil would create more lift than the aft foil  .3 CL aft vs .7 CL forward, lifting the bow. It is important to note that both foils would be creating lift in either direction, just that the forward foil would be providing more lift.

It is important that this be foil assist and not actually designed to lift the boat out of the water. It should be designed to take most of the load of the boat at  20 knots or so.  This foiling design is unstable if the hull actually comes out of the water, especially if the foils come out of the water.

Foils are four times more efficient than hulls at reducing wave drag.  My calculations indicate that at speeds less than 5 knots, the foils create more drag than lift. Up to 10 knots or so the foils are break even.  After 10 knots the foils just about eliminate wave drag (dramatically reduce it anyway). That means that there would be a decrease in total drag once the foils start creating substantial lift.

Then there would be a gradual increase in drag as the speed increased, the limit then becomes the speed at which the hull lifts out of the water and the foils become unstable.

There is another thing to consider. A long slender hull is already pretty good at reducing wave drag and the benefit of foils will not be as significant for that hull shape.  In other words if the Proa can already go 15 knots adding foils is going to reduce light air performance and only add a few knots at the top end, but it will make it a lot easier to get to the top end too.

It is not clear cut at all, the correct decision depends on the wind and the waves.

Talador

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