This section deals with the formulae for the various gear, specifically;

• Ideal Prop Diameter.
• Torque & BHP available at propeller.
• Minimum Diameter of propeller.
• Shaft diameter, material, and bearing spacing.

The Minimum propeller diameter is calculated from the shape of the hull, and is able to drive the hull in all conditions, particularly adverse conditions with short steep waves being met head on, when you need the motor most to make way against the conditions.

Torque and BHP is important as the propeller must be chosen so as to _just_ absorb the maximum power available to it. If it absorbs too much power the engine will never reach full RPM / power output, and if it cannot absob it all the motor will over rev. In both these scenarios the power being applied to moving the hull is significantly less that the maximum available motive power.

There is also a certain Ideal diameter that can drive a hull efficiently, which is why, when there are outboard motors available exceeding 300 BHP, nobody uses them to drive displacement hulls. OK, a seagull will drive our example boat on or off a calm mooring, but in a steep chop and F5 winds it will be useless.

The shaft itself is also important, you do not want to fit a shaft that will break or shear when full power is applied, or that will warp because the bearings are spaced too far apart.