How to Determine Tohatsu Outboard Motor Prop Rotations

When you want to calculate the number of rotations done by the prop on your Tohatsu, you can do so with some simple arithmetic and the outboard's manual. With the pitch of the prop -- the theoretical distance in inches that it moves the boat ahead with each rotation -- you can calculate the boat's approximate speed. The speed is only approximate because of "slippage," a catch-all term for the prop's failure to transfer the outboard's power to the water due to factors such as cavitation (where bubbles form underwater) and ventilation (where the blades are out of the water).

Instructions

1. • 1

     Record the rpm at which the motor is operating.

   • 2

     Locate the gear ratio for your motor. This is found in the operator's manual for the motor.

   • 3

     Divide the rpm by the larger of the two numbers in the gear ratio. The result is the number of prop rotations per minute. For example, if the recorded rpm is 2,500 and the gear ratio is 2.0-to-1, divide 2,500 by 2.0. The result, 1,250, is the number of propeller rotations at that engine speed.