For the purposes of selecting a propeller we are only interested in the hull as an object that has to be pushed through the water against the resistance of the water by the propeller.

To this end we can make a satisfactory description of the hull from only a few simple pieces of data. This data is easy to obtain and “concrete” in nature.

• Length (max) at waterline in feet.
• Beam (max) at waterline in feet.
• Hull draft in feet excluding keel or deadwood.
• Maximum displacement in pounds. (2240lb = 1ton)
• Maximum permissible propeller diameter to physically fit.

All these figures are easy to obtain, and there are only two points to watch. The hull draft means the HULL, not to the bottom of the keel or rudder skeg, and the maximum prop diameter is usually specified by the manufacturer. If it is not measure from the centre of the prop shaft vertically up and down to see how big a propeller can be before it will hit either the bottom of the hull or the deadwood / skeg, and take the maximum diameter as 7/8ths of this figure.

These 5 simple pieces of data are enough to determine how “hard” it is to push your hull under motor power (not heeled over) at any given speed up to and including hull speed.