The Calculator

The Mark's Marine Prop Calculator is capable of calculating any of the 5 values based on the remaining 4 values. This tool is useful for calculating how effectively your prop is operating based on the theoretical performance.

If your current prop has more than 15-20% slip you would probably benefit from a different prop on your boat.



The calculator is based on a rather simple mathematical formula that relates the variables.

          RPM     PITCH       
SPEED =  ----- X  ----- X ( 1- (SLIP/100) )
         RATIO      C        


  • RPM the crankcase speed in revolutions-per-minute.
  • RATIO lower unit gear reduction ratio; the number of revolutions of the crankshaft to produce one revolution of the prop shaft.
  • PITCH blade pitch of prop in inches.
  • SLIP a percentage of propeller performance. Entered either as a decimal or a whole number
  • SPEED the boat speed in MPH.
  • C A constant to convert MPH to inches

What is Slip

Slip is the deviation in actual distance traveled from the theoretical. For example if you had a 20 pitch prop then that prop should cover 20 inches in distance per revolution. If you actually only covered 18 inches then your slip would be 10%.

In general as your speed increases, the slip value decreases. Typically this value will range from 6% - 10% at full throttle and may be as high as 50% at lower speeds.

Setting the Slip to 0 will show the maximum theoretical speed for a particular prop

Choosing a prop

Picking a perfect prop for an individual boat depends on a large number of factors including hull design, weight, altitude, engine, and usage patterns. It is impossible to pick a perfect prop for every hull and situation. Fine tuning a boat's performance involves a great deal of trial and error though the expert's at Mark's Marine and Precision propeller can get you close.

What we stock here at Mark's

Mark's Marine stocks a number of OEM props from Yamaha and Mercury as well as some older OMC props

We also stock a large number of different sizes of Michigan Propellers in the Vortex aluminum and Apollo Stainless Steel serries. These props feature interchangeable Hub Kits allowing them to fit a wide variety of different motors.

Prop Calculators on the web

  • Mercury's Prop Selector Mercury's prop selector is based off a complex formula to calculate the appropriate prop based on an aproximation of your hull's performance charecteristics.
  • Michigan Propellor's Selector Michigan's inhouse prop selector is based on a table of props and should be used as a starting point for finding the prop that best suits you and your boat.