Work out your slip
Use numbers from one full-throttle run with your normal load.
Calculators
Boat Propeller Slip Calculator
Prop slip is the gap between how far your prop would push the boat in theory and how far the boat actually goes. Enter four numbers from one full-throttle run.
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How to calculate prop slip
Find your gear ratio
It is in your engine's specifications. Mercury's 150 FourStroke manual, for example, lists 1.92:1.
Read the pitch on the prop
Mercury says diameter and pitch are stamped into the side or end of the prop hub, diameter first. A 14 x 19 prop has 19 inches of pitch.
Make a full-throttle run
On calm water with your normal load, hold full throttle and note the engine RPM. Mercury says an accurate tachometer is important when choosing a prop.
Note the GPS speed
Read the speed at the same time as the RPM.
Enter the four numbers
The calculator gives the theoretical speed and your slip. Pick your boat type to compare it with Mercury's figure.
The prop slip formula
Pitch is the distance a prop would travel forward in one turn if nothing slipped. The prop turns at engine RPM divided by the gear ratio, so each minute it would travel that many turns times the pitch, in inches. A mile is 63,360 inches and an hour is 60 minutes, so dividing by 1,056 (63,360 ÷ 60) turns inches per minute into miles per hour.
- Theoretical speed (mph) = RPM ÷ gear ratio × pitch ÷ 1,056
- Slip (%) = (theoretical speed − actual speed) ÷ theoretical speed × 100
- For knots, divide the theoretical mph by 1.151. For km/h, multiply it by 1.609.
A worked example
A Mercury 150 FourStroke has a 1.92:1 gear ratio and a full-throttle range of 5,000 to 5,800 RPM. At 5,600 RPM with a 19-inch pitch, the prop turns about 2,917 times a minute and would push the boat 52.48 mph if nothing slipped. If the GPS reads 46 mph, slip is (52.48 − 46) ÷ 52.48, or 12.3%.
What prop slip tells you
Mercury's Propeller Guide says slip is not a measure of propeller efficiency. A blade needs some angle of attack to make thrust, and that angle means the prop advances less than its pitch. In Mercury's example, a 10-inch-pitch prop that advances 8 1/2 inches in one turn has 15% slip. The guide puts the right angle of attack at about 4 degrees, give or take one.
- Too little slip: too much diameter for the engine, Mercury says, so power goes into blade friction instead of thrust.
- Too much slip: a diameter too small for the power and load, or too much cavitation or ventilation.
- Above 20%: Mercury says slip generally jumps from its normal 10% to 15% to more than 20% when a prop is partly blown out, on an average-to-lightweight boat.
- Zero slip only happens when the prop is windmilling, as when the boat coasts or is towed.
Common prop slip by boat type
Mercury's Propeller Guide lists these as common slip figures. Compare your result with your own boat type.
| Boat type | Common slip |
|---|---|
| Fiberglass fishing | 10.3% |
| Bass | 10.5% |
| Aluminum fishing | 12.5% |
| Fish and ski | 9.5% |
| Offshore fishing | 9.5% |
| Tritoon | 12% |
| Pontoon | 20% |
| Runabout | 9.6% |
| Cuddy cabin | 11% |
| SD performance | 7.0% |
| Cruiser | 13.8% |
| Flats | 10.1% |
| Utility | 16.1% |
| Deck | 10.7% |
| RIB | 10.3% |
Common questions
Mercury's Propeller Guide calls 10% to 15% normal for an average-to-lightweight boat, and its common figures by boat type run from 7.0% for SD performance boats to 20% for pontoons.
Mercury points to a prop diameter too small for the power and load, or too much cavitation or ventilation. A partly blown-out prop can push slip past 20%. Check your entries too, since a wrong gear ratio or pitch changes the answer.
Mercury says too much diameter or blade area will lower slip, but it also lowers propeller efficiency and so performance. The aim is the right amount of slip, not the least.
Not under power. Mercury says zero slip only happens when the prop is windmilling, as when the boat coasts or is towed, so a result at or below zero means one of the entries is off.
Theoretical speed in mph is RPM divided by the gear ratio, times the pitch in inches, divided by 1,056. Slip is the theoretical speed minus the actual speed, divided by the theoretical speed.
In your engine's specifications, in the owner's manual. Mercury's 150 FourStroke manual lists 1.92:1, for example.
Sources
Mercury, Propeller Guide and 150 FourStroke Owner's Manual.