Speed from RPM, or the pitch you need
Speed (mph) = RPM × pitch × (1 − slip) ÷ (gear ratio × 1,056). The gear ratio and the full-throttle RPM range are in your engine’s owner’s manual. To measure your slip from a GPS speed, use the Prop Slip Calculator.
Boat speed (mph) = engine RPM × propeller pitch (inches) × (1 − slip) ÷ (gear ratio × 1,056). Enter the engine RPM, the propeller pitch, the gear ratio and the slip to estimate your speed, or switch to Pitch to find the pitch that gives a target speed at the full-throttle RPM you want. For example, 5,280 RPM with a 20-inch propeller, a 2.00:1 gear ratio and 10% slip gives 45 mph (39.1 knots).
RPM to Speed Formula
Speed (mph) = RPM × pitch × (1 − slip) ÷ (gear ratio × 1,056)
Speed (knots) = RPM × pitch × (1 − slip) ÷ (gear ratio × 1,215.2)
Pitch is the distance, in inches, that a propeller would move forward in one revolution if it did not slip, like a screw turning into wood. The engine turns the propeller through a reduction gear: with a 2.00:1 gear ratio, the engine turns twice for each turn of the propeller. So the propeller RPM is the engine RPM divided by the gear ratio, and each minute the propeller would advance its RPM × pitch inches. The 1,056 turns inches per minute into miles per hour: a mile is 63,360 inches, and 63,360 ÷ 60 minutes = 1,056. For knots, the constant is 1,215.2, the 72,913 inches in a nautical mile divided by 60.
Worked example: at 5,280 engine RPM with a 2.00:1 gear ratio, the propeller turns 2,640 times a minute. With 20 inches of pitch it would move 2,640 × 20 = 52,800 inches a minute, and 52,800 × 60 ÷ 63,360 = 50 mph with no slip. With 10% slip the boat makes 90% of that: 45 mph, or 39.1 knots and 72.4 km/h.
How to Calculate the Pitch You Need
Pitch (inches) = speed (mph) × gear ratio × 1,056 ÷ (RPM × (1 − slip))
Use the full-throttle RPM you want the engine to reach, not the RPM it reaches now. Mercury’s owner’s manual says that at full throttle with a normal load, the engine RPM should be in the upper half of the recommended full-throttle RPM range. For example, to run 45 mph at 5,500 RPM with a 2.00:1 gear ratio and 10% slip, you need 45 × 2.00 × 1,056 ÷ (5,500 × 0.90) = 19.2 inches of pitch.
The result is a starting point. Fit the nearest pitch available for your engine, then check the full-throttle RPM on the water with a normal load; Mercury notes that an accurate tachometer is important in choosing the correct propeller.
How Pitch Changes Full-Throttle RPM
Mercury’s outboard owner’s manual gives these guidelines:
- Adding 1 inch of pitch reduces full-throttle RPM by 150 to 200.
- Removing 1 inch of pitch raises full-throttle RPM by 150 to 200.
- Going from a 3-blade to a 4-blade propeller generally lowers full-throttle RPM by 50 to 100.
Mercury’s propeller guide adds that cupping usually lowers full-throttle RPM by about 150 to 300 compared with an uncupped propeller of the same pitch. For example, if your manual gives a full-throttle range of 5,000 to 6,000 RPM and the engine reaches only 5,200 with a 21-inch propeller, a 19-inch propeller should add about 300 to 400 RPM, bringing it to roughly 5,500 to 5,600, in the upper half of the range. Mercury also warns never to use a propeller that lets the engine exceed the recommended full-throttle RPM range.
Choosing a Slip Value
Slip is the difference between how far a propeller would move in theory and how far it actually moves, and the calculator needs a value for it. Your own measured slip is best: compare your GPS speed with the speed at zero slip, which the Prop Slip Calculator does for you. Without a measurement, start from Mercury’s common value for your type of boat in the table below.
Mercury’s propeller guide explains that a blade needs some angle of attack to produce thrust, so some slip is necessary: in its example, a propeller with 10 inches of pitch advances 8.5 inches per revolution, a slip of 15%. Slip is not a measure of propeller efficiency. Mercury adds that a partially blown-out propeller can push slip from the normal 10% to 15% to more than 20% on an average to lightweight boat.
Typical Slip by Boat Type
Mercury lists these common slip values by type of boat:
| 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% |
| Sterndrive performance | 7.0% |
| Cruiser | 13.8% |
| Flats | 10.1% |
| Utility | 16.1% |
| Deck boat | 10.7% |
| RIB | 10.3% |
Where to Find Your Gear Ratio
The gear ratio is listed in the specifications of your engine’s owner’s manual, together with the recommended full-throttle RPM range. Ratios differ between engine models, even within one manufacturer’s manual, so use the figure for your engine rather than a typical value.
To estimate top speed from horsepower and weight instead, use the Boat Speed Calculator. To convert between knots and mph, use the Knots to MPH Converter, and to turn a speed into trip time, the Speed, Distance and Time Calculator.
Frequently Asked Questions
How do you calculate boat speed from RPM and pitch?
Multiply the engine RPM by the propeller pitch in inches and by (1 − slip), then divide by the gear ratio times 1,056. For example, 5,280 RPM with a 20-inch prop, a 2.00:1 gear ratio and 10% slip gives 45 mph.
How do I calculate the propeller pitch I need?
Pitch = target speed in mph × gear ratio × 1,056 ÷ (full-throttle RPM × (1 − slip)). For 45 mph at 5,500 RPM with a 2.00:1 gear ratio and 10% slip, that is 19.2 inches. Then check the full-throttle RPM on the water.
How much does 1 inch of pitch change RPM?
Mercury’s owner’s manual says that adding 1 inch of pitch reduces full-throttle RPM by 150 to 200, and removing 1 inch raises it by 150 to 200.
What is normal propeller slip?
It depends on the boat. Mercury lists common slip from 7.0% for performance sterndrive boats to 20% for pontoons, with runabouts at 9.6%. Slip is not a measure of propeller efficiency: a propeller needs some slip to make thrust.
Where do I find my gear ratio?
In the specifications of your engine’s owner’s manual, next to the recommended full-throttle RPM range. Ratios differ between engine models, so use the figure for your engine.
Will a higher pitch make my boat faster?
Often, as long as the engine still reaches its recommended full-throttle RPM range. Mercury says a higher pitch usually makes a boat faster but slows acceleration, and that full-throttle RPM with a normal load should be in the upper half of the range.