Look up aspect ratio in a good aerodynamics book (Anderson is my personal favorite). Structural concerns do limit how high of an aspect ratio can be achieved, but a narrow deep rudder will always be more effective than a wide shallow rudder (of same airfoil).

The reason fast nimble planes have short wings is not aerodynamic. The moment of inertia causes resistance to sudden rolls (dynamics, not aerodynamics). The forces generated during abrupt maneuvers would tear off long wings due to increased moments. Transonic flight is also hard on structure.

If engineers could keep a high speed plane together with higher aspect ratios it would be one of the greatest performance breakthroughs of modern aircraft. By pivoting the wings at the root the higher moment of inertia could be overcome.

Ultimately it comes down to structural concerns that keep us from going to higher aspect ratios.

> The U2 was also a slow plane. And wasn't even faintly nimble.
> That's why fast planes or nimble planes have much shorter wings. A
> kayak, even a sea kayak, is very nimble.

> If it was so simple for rudders, you wouldn't see the rudder
> designs on many of the sail boats you do see, which is not a long
> blade drafting deeply into the water, but is squatter, nnot extending
> below the keel. Just as airplanes do not have long thin rudders or
> elevators, but have more paddle like shapes. A significant effect
> off hand that I can see for boat rudders is the need for drastic action
> on the part of the helmsman, which a tall thin rudder would not take
> well. A large wide blade produces much more side thrust to turn the
> boat with minimal angle change then a long foil, especially at the
> rather slow speeds a sailboat works in.

> A kayak rudder is more to handle boat cocking, not ruddering
> the boat around like a sailboat. If you were to try to sail a kayak
> as a sailboat with those tiny rudders, you would be deeply disapointed.
> At least that's is what I've read and heard on every occassion from
> those who do sail kayaks and canoes.

> Nor would a rudder that is wide fore and aft through the water
> be hard to turn. As long as it's forces are balanced along the axis
> of rotation, it would be just as neutral as could be.

> Myself, I tend to suspect kayak rudders are shaped the way they
> typically are because (1) they aren't really rudders in the steering
> sense, and (2) a small flat blade flips up onto the back of the deck
> easily and more esthetically then a sailing rudder would.