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Launches, Yachts & Commercial

Gearbox Ratios

Does your boat have the best gearbox ratio to get the best economy from your power plant?

Far too many boat owners are concerned with how much horse power they require to drive the boat where in actual fact it is the propeller doing all the work. If you think of your boat being a car; if it was a mini with 10" wheels and you fitted it with a 300hp V8 what would happen..... the wheels would spin and no hp would really be put through to the road. Same thing can happen with a boat. Just because a larger engine is fitted does not mean it will go faster and sometimes all that is required is a different gearbox ratio and newer designed propeller.

Basic rule of thumb: Fit the largest propeller the Vessel can fit (including the 10-15% clearance required), take the horse power of the engine at it rated max rpm and then you can work out the correct gearbox ratio required to drive the propeller.

Very Important - "Over propping" any boat is wrong DO NOT DO THIS- it is a myth that it is the right thing to do and simply not correct – displacement, semi-displacement or planning. All engines (especially diesel) are designed to produce their hp at a given rated rpm. For example - take an engine everybody knows - Ford 120; the 120hp Ford produces 120hp @2450rpm, if the engine does not reach it's max rpm at wide open throttle (underload) it will not produce its hp, be over loaded because the fuel pump is still delivering the correct fuel into the engine, in turn the engine can run hot, will wear quicker, burn more fuel than it should, black smoke and so on.

Sea Trail - The very first thing to do before anything else is: check that your tachometer is reading correct. Once a vessel has been re-prop’d a sea trial needs to be carried out- for a fair seatrial the vessel needs to have a clean hull, Full of fuel, water and in normal cruising trim. The engine must reach full rated RPM or +50 rpm- this then allows for growth on the hull and extra weight that can be added to the vessel over time.

For new propellers and advice we recommend either of the below:

Craftmaster Marine – 09 4122251
Bri-Ski Propellers