Prop slip is the difference between how far a propeller would move the boat in theory, based on its pitch and shaft speed, and how far it actually moves it. To calculate it, work out theoretical speed in mph as engine rpm divided by the gear ratio, times pitch in inches, divided by 1056. Then slip percent is theoretical speed minus actual GPS speed, divided by theoretical speed, times 100. Many outboard boats run somewhere around 10 to 20 percent; lower generally means a more efficient setup.
The prop slip formula
- Theoretical speed (mph) = (engine rpm / gear ratio) x pitch (inches) / 1056
- Slip (%) = (theoretical speed – actual speed) / theoretical speed x 100
The 1056 converts inches per minute into miles per hour. Use the lower unit gear ratio from your engine’s manual, rpm from an accurate tachometer, and speed from GPS rather than a pitot speedometer.
A worked example
| Input | Value |
|---|---|
| Engine rpm | 5,600 |
| Gear ratio | 2.00:1 |
| Prop pitch | 19 inches |
| Theoretical speed | 5,600 / 2.00 x 19 / 1056 = 50.4 mph |
| GPS speed | 44 mph |
| Slip | (50.4 – 44) / 50.4 x 100 = about 12.7% |
What affects slip
- Hull and load. Heavier, slower hulls slip more than light, fast ones.
- Engine height and trim. Too low adds drag; too high or over-trimmed lets the prop ventilate.
- Prop design and condition. Blade count, cup, rake and diameter all matter, and bent or chipped blades raise slip.
- A spun hub. A sudden jump in slip, with rpm rising but speed not, often means the prop’s hub is slipping; see spun prop hub.
Using slip to choose a prop
Slip is most useful for comparing props on the same boat. Run each prop with the same load and conditions, record full-throttle rpm and GPS speed, and compare. Make sure full-throttle rpm stays within the engine maker’s range; see outboard tachometers and boat propeller pitch and sizing.
Material and blade count
Stainless props flex less than aluminum and often run with less slip at speed; four-blade props usually grip better in turns and rough water but can cost some top speed. See stainless steel vs aluminum props and 3 blade vs 4 blade props.
Props we list
Examples include the Yamaha M/T Series stainless propeller, 15-1/8 x 25 pitch ($1,088.67). Browse the rest on boat propellers, and if the boat struggles to plane, see boat won’t get on plane.
More guides: how to tell if a prop is bent · how to remove an inboard prop.
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