The reason they won't work on "conventional" rudders is that you wouldn't be able to use the rudder in anything other than the fully down position - there would be too much drag otherwise when you have the rudders down just enough to sail out of shallow water.
But why would there be water drag if they are in the fully "up" position? Maybe a little air drag.
And being able to flip them up briefly would help to get grass off the rudder blades. Grass is a major problem in many areas of the United States.
I thought maybe the reason for needing a casing so the rudder can go straight up and down was because the foils cause increased stresses on the transom?? Or maybe too much stress or "drag" for existing rudder systems to keep from kicking up??
