: The Nikumi really charms me a lot. What about the 17' version?
: The SeaTour is also nice and I saw there is a 15' version.

: Actually I am seeking a roomy, fast and good tracking boat but not
: much longer than 15' .

Sometime people confuse "fast" with "easy to paddle at the speed that mere mortals can get up to".

If your idea of "fast" is just a bit better than the plastic bathtubs some people paddle, then a well designed 15' kayak might be just what you are looking for. 4 knots is realistic over an extended period in such a kayak.

If, on the other hand "fast" means 6 knots, then you aren't going to even get close for an extended period unless you have something closer to 20' and can sustain the sort of effort for hours on end that I can only manage for a few minutes.

If the "fast" speed you are trying to maintain is less than about 4.5 knots, then you can actually do it more easily in a good 15' kayak than an 18' kayak because up to that speed, the shorter kayak can have less drag. The problem is if you paddle harder, the 15' kayak won't go much faster, while the 18' kayak has the potential to pick up a bit more speed.

"Fast" is one of those terms like "stable" that is a bit subjective and means different things to different people.