Re: Tohatsu 4 to 6 hp Propeller change?
Proper propping will depend on load and speed. There are 4 props for the 4/5/6, plus the EE (elephant ear) slow-speed sailboat prop. More pitch is for lighter loads at higher boat speeds, less pitch for better takeoff (or hole shot), with less top end. Small OB's are very sensitive to small changes in propping if they are heavily loaded, but not so much when lightly loaded.
In all cases, you will need to re-prop and test to see the results. Use a GPS. Do speed testing in the same conditions for both props -- conditions that you will likely encounter when using the boat normally. Every boat is different. You likely have the 7" pitch, if you have the factory 4 hp prop. See if your dealer or a friend will loan you a prop for testing. You will want a spare anyway, so if no one will loan you one, buy a spare that's a different (say 9") pitch, and test that for comparison.
I would recommend testing with a tach attached as well, since you want the WOT to be in the correct range to develop the most HP from the motor. Too much pitch (and too much load) will slow the motor to the point of lugging, which is hard on 4-stroke exhaust valves. Too little pitch, and you will over-rev, which is hard on the connecting rod and crank. So a GPS and tach are called for -- to tune in the best prop for your rig.
FWIW, I have seen some small, lightly-loaded RIB's perform best with the big EE props.
