Pitch, engine RPM, gear ratio, speed and slip: give it any three and the gear ratio, and it works out the fourth — then says whether full throttle lands where your manual wants it.
19″ pitch · 5,600 rpm · 2.07:1 · 42 mph by GPS
13.7% slipThat prop would travel 48.7 mph through a solid; the boat made 42 mph. Most setups are in the 5-20% slip range, and of course, every boat is different.
- Theoretical speed
- 48.7 mph
- Propeller RPM
- 2,705 rpm
- Slip is not a measure of propeller efficiency. Rather, slip is the difference between actual and theoretical travel resulting from a necessary propeller blade angle of attack.
Your manual’s full-throttle range
It is in the Specifications of the owner’s manual. With it, the tool says which way the pitch should go, and roughly how far.
Which parts are arithmetic, which are published, and which are rules of thumb
The arithmetic is exact; the boat is not. Weight, trim, engine height, water and the prop’s design all move slip, so treat a pitch from this page as the one to try first — and never run an engine past the top of its full-throttle range.
Published guidance checked 2026-09-26.
Quick answer: prop slip is how far short of its pitch a propeller actually travels. Work out the theoretical speed as pitch × engine RPM ÷ (gear ratio × 1056) in mph, then slip = (theoretical − GPS speed) ÷ theoretical. A 19-inch prop at 5,600 rpm through a 2.07:1 gearcase would travel 48.7 mph; a boat that makes 42 mph by GPS is running 13.7% slip. Mercury Racing says most setups are in the 5–20% range, and slip is not a measure of efficiency. To pick a pitch, the target is full-throttle RPM in the upper half of the range in your manual: more pitch lowers RPM, less pitch raises it.
The four numbers, and where each comes from
- Pitch is the second number stamped on the prop: a 14 × 19 has a 14-inch diameter and 19 inches of pitch — the distance it would move in one turn through something solid.
- Engine RPM at full throttle, off the tachometer, with the boat loaded the way you normally run it.
- Gear ratio is in the Specifications of the owner’s manual, written like 2.07:1. It turns engine RPM into propeller RPM, and getting it wrong is the commonest reason a slip figure comes out negative.
- Speed from a GPS. A pitot-tube speedometer is not good enough for this — it reads the water pressure, not the ground covered.
Give the calculator any three plus the gear ratio and it works out the fourth. “Pitch I need” is the one people are usually after: the speed you want at the RPM your engine should turn, with the slip your current prop runs.
Theoretical speed at a glance
| Pitch | 5,000 rpm | 5,500 rpm | 6,000 rpm |
|---|---|---|---|
| 13″ | 29.7 | 32.7 | 35.7 |
| 15″ | 34.3 | 37.7 | 41.2 |
| 17″ | 38.9 | 42.8 | 46.7 |
| 19″ | 43.5 | 47.8 | 52.2 |
| 21″ | 48 | 52.8 | 57.6 |
| 23″ | 52.6 | 57.9 | 63.1 |
Take your slip off these to get a real speed: 15% slip on 47.8 mph is about 40.6 mph. A different gear ratio moves every number — which is why the calculator asks for yours.
Slip is not loss
This is the part most slip calculators get wrong by implication. A propeller blade has to meet the water at an angle to push on it at all, and that angle is what makes the prop travel less than its pitch. Mercury Racing’s own words: slip “is not a measure of propeller efficiency”. Zero slip would mean zero thrust.
So a slip figure is a diagnostic, not a score. Mercury Racing reads it like this: most setups sit between 5 and 20%; a negative number means an input is wrong — usually the gear ratio, or a tachometer that reads high; and over 20% there is typically room for improvement with a different prop or the engine height. A displacement hull is a different animal again: its top speed is set by its waterline length, not by its prop, and this calculator is built for boats that plane.
Choosing a pitch: where full throttle should land
Mercury’s owner’s manual is plain about it: at full throttle, with a normal load, the engine should run in the upper half of its recommended full-throttle range. Above the range, fit more pitch to bring RPM down; below it, less pitch to let it rev. Running outside the range, Mercury warns, “can cause severe damage to your engine and may void the warranty”. A Yamaha dealer puts the target a little higher still, in the top 30% of the range.
How far one inch of pitch moves the RPM is a rule of thumb, and the two sources we read disagree: one says about 150 rpm, give or take 50; the other says about 200. The calculator shows the change as a range and says so. Props come in one-inch or two-inch steps depending on the model, so the answer is always “try this one, then test it on the water with a tachometer”.
Engine height changes all of this too — how high the cavitation plate sits against the bottom of the hull. That is its own job: What is the Proper Height to Mount an Outboard Motor? And if you are wondering whether the speedometer is lying to you, How Does a Boat Speedometer Work? explains why it often is.