Prop Slip Calculator

What Is Propeller Slip?

Prop slip is the difference between theoretical boat speed (based on pitch and RPM) and actual speed on the water. Water is not a solid medium, so the propeller loses grip and moves less than its geometric pitch per revolution.

Prop Slip Formula

$$V_t = \frac{\text{pitch} \times \text{rpm}}{\text{gear ratio} \times C}$$ $$\text{slip} = \frac{V_t - V_a}{V_t} \times 100\%$$

Here $V_t$ is theoretical speed, $V_a$ is actual speed, and $C$ is 1056 for mph, 656 for kph, or 1215.2 for knots.

Interpreting Results

Some slip is normal. At top speed, 10% to 20% is typical. Very high slip may mean the wrong pitch, fouled prop, or heavy load. Very low slip can indicate an over-propped setup.

Related tool: Prop Pitch Calculator.

Frequently Asked Questions

What is theoretical boat speed?

It is the speed calculated from prop pitch, engine RPM, and gear ratio assuming the prop moves through water at its stamped pitch each revolution.

Is lower slip always better?

Not always. Some slip is needed for thrust. Extremely low slip at low RPM can mean the prop is too large or pitched too high for the engine.

Does slip change with speed?

Yes. Slip is usually lowest near wide open throttle and higher at idle or slow planing speeds.

What inputs do I need?

Enter engine RPM, gear ratio, prop pitch in inches, and measured actual boat speed from a GPS or speedometer.

Can weather affect slip?

Yes. Wind, current, and rough seas change measured speed and can make slip readings vary between runs.

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What Is Propeller Slip?

Prop Slip Formula

Interpreting Results

Frequently Asked Questions

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