The Sea Bee multi-chine is a 13 footer that will carry over 200 lbs and can
easily be transported atop a small car. See attached frame pic for one under
construction in Quebec, Canada.

The link below show my old Celica with a very narrow roofline carrying a 14ft
kayak. The racks have been extended by using aluminum tubing. There are
several pages showing this rack setup in the "Transport" section of the Wood
Frame Kayak Manual.

I think you are making some incorrect assumptions about sizing and
transporting an SOF. Initially, you may be better served to simply build
an existing design, and go paddling. Worry about designing or modifying an
existing design until later when you better understand the design process.
You will more than enough problems to keep you busy while building

Regards, Tom

http://yostwerks.com/GTSKayak3.jpg - Kayak Racks