In the vast majority of cases we will be dealing with a vessel that has an existing propeller fitted. If we are dealing with specifying a propeller for a brand new vessel or a vessel that has had a brand new engine / geabox fitted then we simply calculate the ideal propeller.

The vessel that already has a propeller presents two methods of “tuning”, the obvious one being to replace the propeller with a properly specified one, but this does not address the reasonably common scenario where the correct propeller is fitted, but is being driven by the wrong engine / gearbox combination.

We need to know the following data about your propeller;

• Diameter in inches
• Pitch in inches
• Disk Area Ratio (DAR)
• Mean Width Ratio (MWR)
• Number of blades.

Diameter in inches and pitch _should_ be stamped on the boss, if not, diameter is quite easy to measure. The other measurements are harder to make, and the links to the left of the page go into each in more detail with illustrations.