Outboard mounting on a CAT can be tricky, and the Livingston design hull design can be even more tricky. The best test is that the anti-cavitation plate shoud be right at the water surface, or just slightly below at WOT (Given all the conditions stated by Homdamech, above.) If the anti-cavitation plate is too far below the water surface, raise the motor before changing the prop. Then, at WOT, on smooth water, play with the trim until you max out the RPM's. If you are still significantly below 6000 rpm's (say less than 5700), then consider going to a 1/4" or 1/2" smaller prop with the same pitch, or drop the pitch to 14, keeping the 13.75 diameter. As a rough rule of thumb, on heavier boats, a 1" drop on pitch will get you about 200 - 300 rpm increase. But I don't have any experience with anything like a Livingston hull, so you will need to experiment once you are assured that the anti-cavitation plate is in the right place, and you have optimized the trim. Your ideal should be to get as close to 6000 rpm as possible.
