Propeller slip calculator
Enter the propeller pitch in inches (the second number on the prop, as in 14 × 19), the engine rpm, the lower-unit or gearbox ratio and your GPS speed at that rpm. The calculator shows the speed the prop would make with no slip and the slip in per cent. Switch to Speed to estimate how fast a new prop would go.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Choose what to find and the speed unit.
- Enter the pitch, the engine rpm and the gear ratio.
- For slip, enter the GPS speed at that rpm; for speed, enter the slip you expect.
How it works
- propeller rpm = engine rpm ÷ gear ratio;
- speed with no slip = pitch (in) × propeller rpm × 60 ÷ inches per mile (or km): 63,360 for a statute mile, 72,913 for a nautical mile, 39,370 for a kilometre;
- slip = 1 − actual speed ÷ speed with no slip;
- expected speed = speed with no slip × (1 − slip).
Example
A 19-inch pitch prop, 5,000 rpm, 2.00:1 gear ratio, 38 mph on the GPS:
- prop speed 5,000 ÷ 2 = 2,500 rpm;
- 19 × 2,500 = 47,500 inches a minute; × 60 ÷ 63,360 = 45 mph with no slip;
- slip = 1 − 38 ÷ 45 = 15.5 %.
At 10 % slip the same setup would reach about 40.5 mph.
Tips and common mistakes
- Use GPS speed over ground on calm water, averaged over runs in both directions to cancel current and wind.
- Read rpm from a checked tachometer: a 5 % rpm error moves slip by about 5 points.
- Pitch stamped on the prop is nominal; cupped or worn blades act differently.
Frequently asked questions
What is propeller slip?
The difference between the distance a propeller would travel in one turn if it moved like a screw in wood (its pitch) and the distance the boat actually moves. Some slip is normal: a prop needs it to make thrust.
How do you calculate prop slip?
Speed with no slip in mph = pitch (in) × engine rpm ÷ gear ratio ÷ 1,056; for knots divide by 1,215.2 instead. Then slip = 1 − actual speed ÷ speed with no slip.
What is a normal slip?
Figures of around 10 % for light, fast boats to 20 % or more for heavy or slower boats are often quoted. Very low or negative slip usually means a wrong speed or rpm reading; very high slip can mean the wrong prop, ventilation or a damaged blade.
Where do I find the gear ratio?
In the outboard or sterndrive manual or on the manufacturer's spec sheet, often written as 2.00:1 or 1.86:1. Inboards with a gearbox list the reduction on the gearbox plate.