Use this prop slip and boat speed calculator to find the theoretical speed of your propeller and, if you enter your actual GPS speed, the percentage of slip. Theoretical speed is how fast the boat would go if the propeller moved forward its full pitch on every turn, like a screw in wood.
How the calculation works
Propeller shaft speed is engine RPM divided by the lower unit gear ratio. Each turn moves the boat forward the pitch of the propeller in inches, and 1056 converts inches per minute into miles per hour (63,360 inches in a mile divided by 60 minutes):
- Theoretical speed (mph) = engine RPM ÷ gear ratio × pitch ÷ 1056
- Prop slip (%) = (1 − actual speed ÷ theoretical speed) × 100
Example: 5,500 RPM with a 2.08:1 gear ratio and an 11-inch pitch prop gives 5,500 ÷ 2.08 × 11 ÷ 1056 = 27.5 mph theoretical. If the GPS shows 24 mph, slip is (1 − 24 ÷ 27.5) × 100 = about 13%.
What does prop slip tell you?
Every propeller slips, because water is not solid. Slip depends on the hull, load, propeller design and trim, so there is no single correct figure. What matters is how slip changes: a sudden increase can point to a damaged or spun propeller hub, ventilation, or a propeller that does not suit the boat. Compare readings taken under similar conditions, with the engine reaching the full-throttle RPM range in its owner’s manual.
Where to find your numbers
- Gear ratio: in the owner’s manual or the outboard’s specifications, written like 2.08:1.
- Pitch: stamped on the propeller hub, usually as diameter × pitch (for example 9.25 × 11).
- RPM: from the tachometer at wide-open throttle with a normal load.
Choosing a new propeller? Read the boat propeller pitch and sizing guide and stainless steel vs aluminum props, or browse boat propellers and outboard motors by horsepower.
More guides: how fast a 9.9 vs 15 HP will go.
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