Prop Slip Calculator

Prop Slip

What is Prop Slip?

Prop Slip is simply the difference between the theoretical and actual distance traveled with one revolution of your propeller.  For example, if you have a 24″ pitch prop, your boat might actually only travel 21″ in one revolution resulting in a 12.5% prop slip.

Most boats have between 10% and 20% prop slip and some slip is actually necessary to thrust your boat forward.  But too much slip is problematic too.  The main causes of excess prop slip are a damaged or faulty propeller, suboptimal engine mounting height (too low creates too much drag while too could cause the propeller to ventilate), propeller type (aluminum propellers have much more flex and so tend to slip more), hull design and hull condition (e.g., marine growth will cause excess resistance).

How to Use the Prop Slip Calculator

Using the Prop Slip Calculator is very easy.  You just need to enter four things:

  • Propeller Pitch.  Should be stamped on the side of your propeller.
  • Engine RPMs.  The speed in which you want to check your prop slip.  Prop slip will typically be lowest at wide-open-throttle (W.O.T.) so that is what most people use.
  • Gear Ratio.  This is critical and can greatly impact your results.  If you don’t know it, do an online search for your engine brand, model, size and year.
  • Maximum Speed.  The maximum speed achieved at the RPM level you entered.  Note that this should be GPS-based (from a fish finder or phone app) since boat speedometer gauges are often inaccurate.

By default, the prop slip calculator assumes that your Maximum Speed input will be in MPH.  However, if you measure speed in Kilometers, be sure to select that setting at the top of the calculator.

Should I Try a Different Propeller?

If you think you might have too much prop slip, check for prop damage, inspect your hull, and research the recommended engine mounting height for your boat before you start experimenting with different sizes and types of propellers.

Assuming your prop slip is not excessive, you can estimate how a change in PROP PITCH will impact your performance by first estimating your prop slip using the prop slip calculator on this page.  Then, with that value along with knowledge of your maximum RPMs and gear ratio, you can estimate how going to a different pitch propeller will impact your top speed by entering that information in my prop pitch calculator.

Stainless steel propellers will also tend to outperform aluminum propellers in many applications.  To help understand the advantages of each type, check out this article on Aluminum vs. Stainless Steel propellers.


Prop Slip Calculator
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