However the tip foil might create structural issues in that the rudder would have to be kept strong so it would not distort when under load. (probably not an issue)
The other thing is that the added rudder tips would probably be susceptible to damage and dragging weeds.
This issue could be pacified by placing the foils up from the bottom of the rudder say a few inches
When foils are added to the rudder, an extra vertical load stresses the gudgeons. Those are usually loaded mostly in the horizontal plane. The new load is equal to the rudder foils lift, I guess about 1/3 of the boat weight plus a safety margin in a Rave - probably more in a cat. This is not an issue only if all is calculated beforehands.
If it is a big boat designed to use an outboard, the engine's weight can be used to REDUCE the maximum vertical load in the gudgeons.
The foils can also be raised to work closer to the boat's bottom. The flow there is moving slightly up, so even horizontal symetrical foils will generate lift, at the expense of the back wave height.
The only problem is that higher foils are easier to take out of the water, in which case their lift would disappear. I guess they can be placed there when the boat is not supposed to fly that specific hull or when taking the foil out of the water is desirable (ex: if the windward hull is flying and its rudder foil is better out of the water then generating drag).
Cheers,
Luiz
