But to hold down a lee hull by putting downforce on the rudder? Insanity!
And yet this is what existing cats are doing with T foil rudders.
Right, but only because they are not using positive lift foils forward. As you already pointed, negative lift rudder foils make more sense then pumping water ballast into the hull: same displacement with less mass. It does increase stability.
The big tris alternate between using their rudder foils to keep the bow up or to help lift the central hull out of the water, according to the conditions. I am sure that at least Hydroptere and Geant can do both things.
