This section deals solely with the various formulae required and used to calculate the ideal propeller characteristics from other known data. While these various formulae are dealt with individually, this is for the purposes of explanation only. In practice not only do you find that you have to include the result from one formula in the data for another formula, but you must integrate the data from all the formulae to arrive at a propeller specification.

As it says on the Introduction page, all units of measurement are Imperial, so if all you have is metric data you MUST convert it to Imperial first.

For the purposes of these calculations we will use a theoretical boat not dissimilar from the Golden Hind / Waterwich type of sailing cruiser of approximately 30 foot length overall. This is just to help you visualise the example vessel while you work through the calculations. We will assume this vessel has the following significant characteristics. Obviously this vessel has many other characteristics such as length overall (LOA) and chine but these are irellevant to propeller selection so we omit them completely.

• 10,000 lbs displacement
• 27 foot max length at waterline LWL
• 8.5 foot (8 feet 6 inches) max beam at waterline.
• 2.3 feet (2 feet 4 inches) max hull draft excluding keel and deadwood.
• 7 knots required hull speed. Note that you cannot exceed “hull speed”.
• 150 “C” for hull. A constant that defines the type of hull, eg cruiser.
• 16 inch maximum permissible propeller diameter.
• 1 engine.(single screw)
• 45 BHP engine power at maximum continuous RPM.
• 2,000 RPM maximum continuous RPM
• 1 gearbox and  / or vee drive / reduction box.
• 2 shaft bearings (one cutless, one inboard thrust bearing)
• 1:1 gearbox reduction ratio.

This is sufficient data to proceed and calculate the ideal propeller specification.