Prop Slip Calculator - Slip from Pitch, RPM, and Speed

A prop slip calculator measures how much of a propeller's theoretical advance is lost to the water giving way. The slip percentage tells you whether your propeller is matched to your hull, engine, and gear ratio, and it is the most useful diagnostic a boater can compute from engine RPM, prop pitch, gear ratio, and observed boat speed.

• • Diagnose an over-pitched or under-pitched propeller: Compare the slip from a sea trial against the 10 to 20 percent band for recreational outboards at top speed.
• • Validate a new propeller before the first install: Estimate the slip that a candidate prop will deliver with your engine, gear ratio, and target top speed.
• • Compare two props on the same hull: Compute the slip of a 19-inch prop against a 21-inch prop on the same speed and RPM, and see which one lands inside the band.
• • Convert a tachometer reading into propeller performance: Pair the boat's speed with the engine RPM at that speed and the gear ratio to expose the slip percentage in one number.

Slip is unavoidable on a displacement hull because water is not a solid. The printed pitch is a theoretical distance, and the real advance per revolution is shorter. The prop slip calculator turns that difference into a number you can compare against a reference band.

Most production outboards and sterndrives slip 10 to 20 percent at top speed. Numbers above that point to a fouled bottom, an overloaded boat, or an under-pitched propeller. Numbers below 10 percent can signal a propped-too-high condition where the engine never reaches rated RPM.

The same four inputs feed the related Prop Pitch Calculator, which inverts the formula and returns pitch in inches for a given top speed and slip.

The slip formula uses four measured inputs and one conversion constant that rescales boat speed into inches per minute so the relation lines up with prop pitch.

• boat_speed: Observed boat speed in the chosen unit (mph, kph, or knots).
• gear_ratio: Engine revolutions divided by propeller revolutions (2.0 means a 2:1 reduction).
• C: Unit conversion constant: 1056 (mph), 656 (kph), 1215.2 (knots).
• prop_pitch: Pitch in inches printed on the propeller hub.
• engine_rpm: Crankshaft revolutions per minute at the measured boat speed.
• prop_slip: Decimal fraction of theoretical advance lost to slip, rendered as a percent.

The constant C is what makes mph, kph, and knots interchangeable in the same formula. One mile equals 63360 inches and one hour equals 60 minutes, so C equals 1056 for miles per hour, 656 for kph, and 1215.2 for knots.

The prop slip calculator carries the unit constant for you, so the answer lands in percent of slip no matter which unit you start with.

According to Omni Calculator, prop slip equals one minus boat speed times gear ratio times C divided by prop pitch times engine RPM, with C equal to 1056 for mph, 656 for kph, and 1215.2 for knots.

The same engine RPM and boat speed feed the related Work, Energy & Power Calculator, which converts the propeller load into horsepower and torque at the measured RPM.

Four ideas come up every time you interpret a slip percentage, and each one changes the answer.

These four concepts feed the same five inputs the prop slip calculator asks for. Boat speed, engine RPM, and gear ratio come from your numbers, prop pitch from the propeller hub, and slip is the output you read against the 10 to 20 percent band.

If the slip is far outside the band, the prop is the first place to look. A small drop in pitch raises slip and lowers top speed; a small increase in pitch lowers slip and raises top speed, until the engine falls out of its rated RPM band.

If you are not sure the gear ratio you typed in is realistic for the drive, the Gear Ratio & RPM Calculator helps confirm the gearbox reduction against the engine's rated RPM band.

Five inputs feed the calculator and walk you through a sea trial.

1. 1 Pick the speed unit: Select mph for US boats, kph for European charts, or knots for marine charts. The unit sets constant C inside the formula.
2. 2 Enter the boat speed: Use a GPS or paddle wheel reading at steady wide-open throttle. A handheld GPS in calm water gives the cleanest slip number.
3. 3 Enter the propeller pitch: Read the second number stamped on the propeller hub, in inches. A 13 x 16 prop is 13 inches in diameter and 16 inches in pitch.
4. 4 Enter the gear ratio: Get the ratio from the engine or drive owner's manual. A 2.0 means the engine turns twice per propeller turn.
5. 5 Enter the engine RPM: Read the engine RPM from the tachometer at the same speed you used in step 2. The pair has to be measured together.
6. 6 Read the slip percentage: Compare the slip to the 10 to 20 percent band. A slip above 20 percent means the prop is under-pitched, fouled, or overloaded; a slip below 10 percent means the prop is over-pitched for the hull.

On a 17-foot boat with a 16-inch pitch aluminum prop, a 2:1 outboard gearcase, 37.5 mph from the GPS at 5500 RPM, the prop slip calculator returns about 10 percent slip, at the low end of the 10 to 20 percent band for recreational outboards at top speed.

After the slip percentage is in, the Torque, Power & Speed Calculator converts the same RPM into torque and horsepower to see whether the drivetrain can sustain the target top speed.

A slip number turns a tachometer reading and a GPS speed into a decision you can act on.

• • Diagnose engine loading: If slip is far above 20 percent, the engine is over-loaded and the prop is under-pitched. If slip is below 10 percent, the engine is under-loaded and the prop is over-pitched.
• • Pick a propeller before you order it: Run the candidate pitch through the prop slip calculator with the engine's rated RPM band and the hull's expected top speed before paying at the marine desk.
• • Compare two props side by side: Test a 19-inch and a 21-inch prop on the same speed and RPM, and see which one lands inside the band.
• • Use the unit you read: mph, kph, and knots are all first-class options. The unit constant C is applied automatically so the slip percentage is correct in any unit.
• • Plan a re-pitch or re-prop: Run the same RPM and speed through a 17-inch and a 19-inch pitch, and pick the prop whose slip sits inside the band at the engine's rated RPM range.

Slip links the propeller stamped numbers to the boat you own. The prop slip calculator is most useful before and after a propeller change, when engine RPM either peaks too high or never reaches the rated band.

The 10 to 20 percent band is a starting point for recreational outboards and sterndrives at top speed. Displacement hulls at lower speeds slip more, and the band widens as the hull gets heavier or the bottom gets fouled.

The same RPM and boat speed go into the Kinematics & Motion Calculator when you want to see the underlying linear and angular motion of the propeller and hull.

Slip is a single number, but it is sensitive to real-world factors you can adjust before trusting the result.

• • The formula treats water as a uniform solid with a fixed slip percentage, so it cannot detect ventilation, cavitation, or partial submersion at high RPM.
• • Slip is computed from a single speed and RPM pair, so read it at steady wide-open throttle on calm water for a number that reflects the propeller, not the chop.

Run the prop slip calculator with the slip your boat type typically shows, then re-run after a sea trial if RPM does not land in the engine's rated band. The two readings bracket the prop you need.

The 10 to 20 percent band is a starting point for recreational outboards at top speed. Planing hulls at low speed, displacement hulls, and loaded pontoons can all sit above or below it.

According to Wikipedia - Marine propeller, propeller slip is the difference between theoretical and actual advance and typically runs 10 to 20 percent at top speed for most recreational outboards and sterndrives.

According to boats.com, the practical way to measure prop slip is to record boat speed from GPS, engine RPM from the tachometer, and prop pitch and gear ratio from the owner's manual, then plug all four into the slip formula.

Slip rises with drag, and the Drag Equation Calculator applies the same v-squared scaling to a fouled bottom or an overloaded hull to show why slip climbs above the 10 to 20 percent band.