At full revs, which seems to be about 2600, she will only do about 3 to 4 knots. I have a 4 blade prop, bronze Porter 14 x 11. Could I change props to increase speed but lower revs.
Steve Leonard advises :-
The 14x11 is correct for the engine, the low revs sounds like a throttle adjustment on the engine, worth a try.
The HP isn't developed to drive the boat faster unless you gets more revs., ie. 3200 rpm is 18 hp.
Porters will re-pitch that prop to 14x10 if you want, may help.
Also the engine may be down on power from a variety of reasons, generally dirty injectors.
How are you measuring the speed? Must be a GPS "speed over the ground".