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 benifit from a diffrent prop on your boat.

HOW IT WORKS

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

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

Where:

• 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.

We've written a great introductory article about choosing the right prop. You can read it HERE.

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.