Prop Slip Calculator

1. Enter the propeller pitch in inches, as stamped on the hub, and the engine RPM from your tachometer.
2. Enter the gear ratio of your lower unit or gearbox. If you are not sure, check the engine manual; a blank field is treated as 1.00.
3. Enter the actual boat speed in mph from a GPS, read at the same moment as the RPM. The slip, prop RPM, theoretical speed and rating update at once.

Here is what each result means:

Propeller slip is the difference between how far a propeller would move forward if it turned like a screw in wood and how far the boat actually moves. Pitch sets that ideal distance: a 21-inch pitch prop would advance 21 inches per turn through a soft solid, as Mercury Marine puts it.

Water is not a solid, so every working prop slips. Mercury explains that slip comes from the blade angle of attack, and that without some slip there is no thrust at all. So a low number is not automatically better, and a zero or negative number is a warning sign rather than a win.

Boaters, dealers and prop shops use slip as a quick diagnostic. It helps when you change props, add weight, repower, or notice the boat running slower than it used to. It is not a direct measure of propeller efficiency, and it is not a substitute for a proper sea trial.

The calculator turns pitch and prop RPM into a theoretical speed, then compares your real speed with it.

1. Divide engine RPM by the gear ratio to get prop RPM. A 1.00 ratio means the prop turns at engine speed.
2. Multiply pitch by prop RPM to get inches per minute, then by 60 for inches per hour.
3. Divide by 63,360, the number of inches in a mile (5,280 ft x 12 in), to get theoretical mph.
4. Subtract the actual speed, divide by the theoretical speed and multiply by 100 for slip.

Many guides fold the unit constants into one number: 63,360 / 60 = 1,056, so theoretical mph = pitch x prop RPM / 1,056. Both forms give the same answer. The knots figure divides mph by 1.1508, because a nautical mile is 1,852 m and a statute mile is 1,609.344 m (NIST SP 811). For other speed units, the mph to km/h converter handles the change.

Suppose a boat runs a 17-inch pitch prop at 3,000 engine RPM and the GPS shows 28.5 mph.

Direct Drive (ratio 1.00)

Prop RPM = 3,000 / 1.00 = 3,000. Theoretical speed = 17 x 3,000 x 60 / 63,360 = 48.3 mph. Slip = (48.3 - 28.5) / 48.3 x 100 = 41.0%. On our rough guide that is poor, and the boat is losing about 19.8 mph to slip.

Same Boat with a 1.50:1 Gearcase

Now the prop turns slower: 3,000 / 1.5 = 2,000 prop RPM. Theoretical speed = 17 x 2,000 x 60 / 63,360 = 32.2 mph. Slip = (32.2 - 28.5) / 32.2 x 100 = 11.5%, which rates good.

The inputs for pitch, RPM and speed are identical, yet the slip drops from 41.0% to 11.5%. That is why the gear ratio matters so much. Leaving it at 1.00 on a geared drive makes a healthy setup look badly broken.

Slip is a ratio of two speeds, so anything that moves either speed moves the result.

Gear Ratio and Tachometer Accuracy

Prop RPM comes straight from engine RPM and the ratio. Mercury Racing notes that a negative result usually means the gear ratio is wrong or the tachometer is not dialed in. A tach that reads 5% high inflates theoretical speed and slip together.

How You Measure Speed

Mercury Racing asks for a GPS-verified top speed. A dash speedometer can drift, and wind or current can add or remove speed on a single run.

Engine Height and Ventilation

Mercury Marine lists ventilation from an outdrive or outboard mounted too high on the transom as one cause of very high slip. Trim, load and hull condition also change how cleanly the prop grips the water.

Prop Condition and Design

A bent, chipped or repitched blade no longer matches the stamped pitch. Mercury also names blade diameter, shape, area, rake and cup as design choices that shape slip. So two props with the same pitch number can slip differently on the same boat.

These three are easy to mix up, but they answer different questions.

How Published Slip Ranges Compare

Our bands are deliberately stricter, so a result in our fair band may still be normal for your boat. Use them as a prompt to check, not as a pass or fail.

Testing a New or Different Prop

Run each prop at wide-open throttle, log RPM and GPS speed, and compare slip and top speed side by side. The prop that gives the best speed within the engine RPM range usually wins.

Diagnosing a Speed Drop

If the boat is slower than last season at the same RPM, a jump in slip points to the prop, ventilation or extra load rather than the engine.

After a Repower or Gearcase Change

A new engine or ratio changes prop RPM. Recalculate with the new ratio before you decide the pitch is wrong. The ratio calculator helps if you only know tooth counts or turns.

1. Leaving the Gear Ratio at 1.00

Many outboards and sterndrives use a reduction gear, so the prop turns slower than the engine. Ignoring the ratio overstates prop RPM and makes slip look far too high, as the 41.0% versus 11.5% example shows.

2. Mixing Knots and Mph

The formula gives mph. Entering a knots reading as mph understates your speed by about 13% and inflates slip.

3. Reading RPM and Speed at Different Moments

Speed and RPM must come from the same steady run. A peak RPM paired with an average speed gives a meaningless slip.

4. Trusting a Single Run

Wind and current push the boat along or hold it back. Run both directions and average the speeds before you judge the prop. If you time a known distance instead of using GPS, the velocity calculator turns distance and time into speed.

5. Treating Low Slip as the Goal

Slip is needed for thrust. Chasing the lowest number can lead you away from the prop that gives the best real speed and handling.

6. Using the Stamped Pitch on a Damaged Prop

A bent or worn blade no longer matches its marking, so the theoretical speed is wrong before you start.

The math is exact for the numbers you enter, so the result is only as good as your pitch, RPM, ratio and speed readings.

It depends on the boat, but many setups land in a band of roughly 5 to 20 percent at wide-open throttle. Mercury Racing says most setups are 5 to 20 percent, while Mercury Marine calls 5 to 25 percent typical. This calculator uses a stricter rough guide, with 15 percent or less rated good.

A negative result means the boat went faster than the prop should allow, which is not physically normal. It almost always points to bad data: a wrong gear ratio, a tachometer reading low, or a speed reading that is too high. Recheck each input.

Look in the engine owner manual, the manufacturer specification sheet, or ask a dealer with your model and serial number. The ratio is written as engine turns per prop turn, such as 2.00:1, and you enter the first number.

It is 63,360 inches per mile divided by 60 minutes per hour. Dividing pitch x prop RPM by 1,056 converts inches per minute into miles per hour, which is the same as the longer formula shown on this page.

Enter speed in mph for the slip math. To convert a knots reading, multiply it by 1.1508, since one nautical mile is 1,852 m and one statute mile is 1,609.344 m. The calculator also shows theoretical speed in knots for reference.

No. More pitch raises the theoretical speed per turn, but it also loads the engine and can pull RPM down. If RPM falls out of the recommended range, real speed may not rise and slip can change.

Use a steady wide-open-throttle run for comparisons, because that is how prop makers usually quote it. Slip at lower speeds is often higher, so comparing a cruise reading with a top-speed reading is not fair.

Most gaps come from the gear ratio, rounding or the speed unit. Some tools also use different rating bands, and ours are stricter than the ranges Mercury publishes. With the same inputs and ratio, the slip itself should match.

Not always. High slip can come from ventilation, an engine mounted too high, a heavy load, a fouled hull or wrong inputs. Check the setup and the readings before you replace the propeller.

No. The calculation runs in your browser and nothing is sent to our servers. If you press Save, the result is stored only in this browser.

• Prop Bite: Understanding Propeller Slip (Mercury Marine).
• Prop Slip Calculator and result guide (Mercury Racing).
• NIST Guide to the SI, Appendix B.9 conversion factors (NIST SP 811).