Calculate Propeller Slip

Posted by John Doe 18/05/2018 0 Comment(s)


  • Calculate Propeller Slip: RPM div by GEAR RATIO x PITCH div by 1056 = Theoretical Speed. Then you divide the customers actual (gps) speed by the Theoretical speed to get propeller efficiency. For Example:  5500 div by 2.07 x 20 div by 1056 = 50.23mph. If customer speed is 44.00mph the propeller efficiency is 87.59% or 12.41% propeller slip.

  • Pitch Selection: customer is under reving (5000 rpms) with a 19 pitch prop on a 4/stroke motor, short 1000 rpms from achieving 6000 rpms. Divide 5000 by 6000 and you get 83.33% or 17% short of wide open throttle range at 6000. Decrease your current pitch prop (19) by 17% and you get a 15.80 pitch prop to achieve 6000 rpms. So that translates into a 16 pitch prop of the same style and manufacture.

  • A general rule: one-inch pitch change equates to about 150 to 200 RPM change.  This rule is pretty solid when applied to the same blade style by the same manufacture.

  • A general rule: 1/4 inch of diameter change equates to about 150 to 200 RPM change.

  • Boats with a tunnel generally (i.e. bay & flats style boats) perform better with a 4-blade prop due to the disturbed water flowing to the foot of the motor.

  • To convert Knots to MPH: KNOTS x 1.15 = MPH

  • V-bottom heavy offshore boats (3000lbs and up) need diameter (more blade surface) to perform at their best with a 4-blade being the best overall.  (The OFS4R blade style)

  • Flat bottom boats (Jon boats & flats boats) need a stern lifting propeller. (The SCB, SCD or SCE blade style)
  • High performance bass boat like the TRO4R, VMX or VMS blade style.

  • High performance boats (Donzi, Baja, etc) like the OFX blade style.

  • Pontoon/barge style boats like the WBA, WBH or the WBB blade style.

  • Stern drive boats like the MQS & OFS blade style.

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