Seems to me that you can best steal all the idea's that are good about the catri and then have you own boat designed by an trustworthy (marine) engineer. Forget about the patent, that only applied when you are building the boat for commericial exploitation. If you only build one boat for personal use then no patent rights can be brought against you.

Patent law in Europe only protects commercial exploitation of an idea, not the (private) use of it.

Wouter