Prop Slip Calculator

Propeller pitch is the distance a propeller would move forward in one turn if it were screwing through a solid. Water is not solid, so the boat always covers less distance than the pitch suggests. The difference, as a percentage, is prop slip. It is a quick way to compare propellers, spot a problem and check whether a boat is set up well.

Prop Slip Formula

The constants convert inches per minute into speed. There are 63,360 inches in a mile, and dividing by 60 minutes gives 1,056. A nautical mile is 72,913 inches, giving 1,215.2, and a kilometer is 39,370 inches, giving 656.17. The gear ratio divides engine RPM down to propeller RPM. A 2.00:1 gearcase turns the prop once for every two turns of the engine.

Getting a Measurement You Can Trust

Slip is calculated from four numbers, and an error in any of them goes straight into the result. The most common sources of error are easy to avoid:

• Speed. Use GPS, not a pitot speedometer, which can read high or low. Run the same course in both directions and average the two speeds to cancel out wind and current. Against a 3 knot current a boat that really does 33 knots may show 30 one way and 36 the other.
• RPM. Analog tachometers can be off by a few percent. A digital gauge or engine diagnostic reading is more reliable.
• Gear ratio. Take it from the owner's manual or the engine's specification sheet for your exact model and gearcase. Two otherwise similar outboards can have different ratios.
• Pitch. Use the number stamped on the propeller. Cupping and blade repairs change how the propeller behaves, so a cupped propeller can act like one with more pitch than the stamp shows.

Take the reading at steady wide-open throttle, with the trim and load you normally run. Slip changes with speed, so readings at different throttle settings are not directly comparable.

What Slip Actually Means

Slip is not wasted motion in the way the name suggests. A propeller blade is a wing turning in the water. Like a wing, it only produces thrust when it meets the water at an angle of attack, and that angle exists because the boat moves forward less than the pitch. A propeller with zero slip would produce no thrust at all.

What matters is how much slip, compared with similar boats and other propellers on the same boat. Prop calculators and propeller suppliers commonly describe something like 10 to 20% at top speed as normal for recreational boats, with well-matched, high-performance setups running lower. There is no single correct figure. Use slip mainly to compare: if one propeller shows less slip at the same RPM on the same boat, it is converting more of its rotation into forward motion.

Slip rises when the propeller is working harder for the speed it delivers, for example with a heavier load, a dirty hull or the wrong trim. A sudden jump in slip with RPM rising and speed falling can point to ventilation, where the propeller draws in air from the surface, or to a slipping hub.

Negative Slip

A negative result means the boat appears to move faster than the propeller's pitch allows. For an ordinary setup that almost always means one of the inputs is wrong. Typical culprits are a speedometer reading high, a tachometer reading low, the wrong gear ratio, or a propeller whose effective pitch is higher than its stamp because of cupping or rework. Check each number before drawing conclusions.

Theoretical Speed by Pitch and RPM

The table gives theoretical speed in mph for a 2.00:1 gear ratio. Actual speed will be lower by the slip percentage. For a different gear ratio, multiply by 2.00 and divide by your ratio.

Worked Examples

Bass boat: 19 inch pitch, 5,800 RPM, 1.85:1 gear ratio, 48 mph on GPS. Theoretical speed is 5,800 × 19 ÷ (1.85 × 1,056) = 56.4 mph, so slip is 14.9%.

Small runabout: 15 inch pitch, 5,500 RPM, 2.00:1, 33 mph. Theoretical speed is 39.1 mph, so slip is 15.5%. If a different propeller holds the same RPM at 35 mph, its slip drops to about 10%, and it is the better match.

Slip in Knots and km/h

Slip is a percentage, so it comes out the same in any unit as long as theoretical and actual speed use the same one. The same 25 inch, 6,000 RPM, 2.00:1 setup has a theoretical speed of 6,000 × 25 ÷ (2.00 × 1,215.2) = 61.7 knots, or 6,000 × 25 ÷ (2.00 × 656.17) = 114.3 km/h. The 65 mph GPS reading equals 56.5 knots or 104.6 km/h, and both give the same 8.5% slip. Pick the unit your GPS shows and set the calculator to match.

Using Slip When Choosing a Propeller

Slip is only one part of propeller selection. The engine must also reach its recommended wide-open-throttle RPM range, which the manufacturer lists in the owner's manual. A propeller with low slip that keeps the engine below that range is too much pitch. Change pitch to bring RPM into range first, then compare slip between propellers that do. To estimate the speed a given engine and hull can reach, use the horsepower to knots calculator. To check the legal power limit for a small boat, use the maximum boat horsepower calculator.