what size prop

[grpqueen]

I am building a motor cruiser in a mould that i have widened by 2 feet and extended by 4 feet, which now makes it 45 ft by 14 ft I am going to have to build a tunnel to reduce the pitch of the engine as i want to move the engine further back in the boat, it has been suggested that i might need a a 30 inch prop, which seems massive. any opinions on this would be welcomed.

[jehardiman]

I would suggest contracting a local Naval Architect. Selection of a prop is integrally linked to many other factors such as vessel mission, desired speed, motor hp, motor torque, availability of gearing, vessel tonnage, operational draft, etc...There is really too little information here to help you.

[Wynand N]

There are software for calculating prop & pitch sizes whereby you feed data of hull type etc, engine size, gear ratio, boat weight and rpm to name but a few and then will calculate a prop size and pitch for that RPM range.

I used a few of couple of these programs to calculate a prop for my cabin cruiser fishing boat and all came up within a 1/4" to a prop calculated for 13 diameter (engine spec) and 19" pitch.
I tested the boat with the calculated prop and what a disappointment - difficult to plane fast, could not even get it close to max RPM (5400) and the motor laboured badly.
Lucky to have access to props I down pitched and eventually the magic prop was a 15" pitch which is a great jump from 19" pitch.

Be aware that calculations are only a pointer and the only real way to find the right prop is to test props....