: That means a boat like this |__________| can be
: as fast as a boat like this \_______________/ ...isn't it?

: Or maybe a boat like this |__________| can be only
: as fast as a boat like this \___________/ (that is still longer but
: only ?

: In other words, is a matter of the "plumb end" or a
: matter of keelson lenght, length of the underbody of the boat?

: (I talk about "fast" but actually I mean "easy to
: paddle around 4,5 knots)

The length that matters, speed wise, is the length at the waterline (called Length Water Line, LWL). The ends above the water do nothing for speed, they may have quite a bit to do with how nice the boat looks and how dry it'll be while paddling though.

Bill H.