Prop Calculator

Boat Propeller Calculator

Enter any 4 Variables and it Calculates the 5th
For Additional Info Read Instructions Below the Propeller Calculator

Copyright 1997 Polson Enterprises

You must have a browser capable of running JAVA to use this calculator. Almost all versions of Internet Explorer and Netscape have this capability. You may need to enable it (allow it to run) in some browsers.

When entering data, you can either click on the appropriate cell, or you can "tab" between the cells.

Engine RPM = rev/min
Prop Pitch = inches/rev (add 1 inch for cupped props)
Prop slip may be entered as either decimal or integer values (.15 or 15).
Prop slip may be entered as either decimal or integer values (.15 or 15).
Speed = Miles/hour Obtain speed data from a radar gun if possible, boat speedometers are notoriously wrong. If some of you want nautical miles/hr, knots, or Kilometers per hour, let us know and we will try to set it up.

Outboard Propeller
Information Center

Propeller Handbook : The Complete Reference for Choosing, Installing,
and Understanding Boat Propellers

by David Gerr 
International Marine/Ragged Mountain Press
1st edition (June 11, 2001
Dimensions (in inches): 10.9 x 8.6 x 0.5 inches 

This is THE great book on propellers. Billed as "the complete guide to choosing, understanding and installing boat propellers." Plenty of technical details, but still easy to understand. It can teach you what all those odd propeller terms mean and even have you calculating prop slip in a jiffy. It is now available in paperback for less than half what it used to cost in hardback! Order Yours Today!

Visit our Outboard Propeller Manufacturers and Propeller Industry Market Research Page

for a list of Propeller Manufacturers and information on the boat propeller market.

Mercury Marine Propeller Book

Mercury has an online Everything You Ever Need To Know About Propellers book that does a nice job covering propellers in general.

Other Prop Calculators on the Internet

Other "Prop Calculators" are online. Give them a try also. All of us are a little different and one or the other might fit your specific needs best. Obviously we are partial to ours, it is the only one that can calculate any of the 5 variables without having to set a selector switch of some type.

Boat Performance Analyzer from

Prop Calculator" from Continuous Wave. They recognize our calculator as the inspiration for theirs near the bottom of the page.

Surfbaud Freeware Propeller Calculator is an Excel spreadsheet program

Sizing Propellers

Those wanting to actually size a propeller for a boat (determine the best diameter and pitch) may find a free downloadable Propeller Calculator program from Castle Marine in the U.K. helpful.

We (Polson Enterprises) do not provide assistance in sizing propellers. We suggest you:


Prop slip is a measure of how efficiently the boat is going through the water. Prop slip is not the same thing as propeller efficiency.

If the prop did not slip at all as it screws through the water, each rev theoretically propels the boat the forward a distance equal to the prop pitch. (a 15 inch pitch prop theoretically propels the boat 15 inches in one revolution). The propeller revolution rate is the engine rpm divided by the gear ratio. This propeller rpm times the prop pitch determines the theoretical distance that the boat should have moved in one minute, which can be converted to a theoretical boat speed in miles per hour.

Due to prop slip the boat is actually going slower than the theoretical speed. Normally on the nature of 10 to 20% slower at top speed. This ratio is found by dividing the actual boat speed by the theoretical boat speed to get a ratio. Then, this ratio is subtracted from 1. The result is the propeller slip. In real life, you also have to do a number of unit conversions between hours, minutes, miles, feet, inches, etc. All of these calculations and conversions are done for you internally in the RBBI Prop Calculator.

See our Prop Folder for additional information about propellers.


If you find any problems or have any likes, dislikes or comments about our Prop Calculator, or other comments, please email us.

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Copyright © 1997, © 1998, © 1999, © 2000, © 2001, © 2002, © 2003, © 2004, © 2005, © 2006, © 2007, © 2008, and © 2009 by Polson Enterprises. All rights reserved.

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