Remember that the rudder has it’s own wave making and creates it’s own wake. If you include the volume of the centerboard and rudder in the curve of areas, you realize that you have extra bumps and the water thinks the hull isn’t fair. In the limit, the boat thinks it is pulling a dinghy. This is another merit of small skinny rudders, and or moving them forward under the boat where they can “hide” in a place with more volume.I couldn't find a decent photo (the one below was the best) but when watching the 18'ers on YouTube you often see quite a large rostertail between the trailing edge of the hull and leading edge of the rudder. I assumed that little endplate tucked and faired into the hull was to quieten that down...
View attachment 556436
Edit: and then this photo shows up in my Facebook feed to show what I mean!
View attachment 556440
The down side is that the shorter the distance between the CP of the board and the CP of the rudder steering gets twitcher. This can be unpleasant at high speed. Hanging the rudder out the back gives you a longer tiller which also helps.
ICs all had inboard rudders, but they have moved to transom hung because we pull our seats really far aft, and that really limits how long the tillers can be.
The diameter of the shaft ends up being the limiting factor on how thin you can make the foil, which in turn kind of limits the chord length.
I went through a period of making really deep really skinny rudder blades and have concluded that they were really to big. I had been convinced that rudder area and thus absolute “never stall” control was pretty cheap. I have revised my thinking and rudder blades are getting smaller again.