Where the 1056 comes from
Pitch is the distance a propeller would advance in one revolution if it were turning in something solid, and it is stamped in inches. To turn revolutions per minute and inches per revolution into miles per hour you divide by 63,360 inches in a mile and multiply by 60 minutes in an hour, which is a division by 1,056. Engine RPM has to be divided by the gearcase ratio first, because the prop shaft turns slower than the crankshaft. That gives the whole formula: theoretical speed in miles per hour equals RPM times pitch, divided by gear ratio times 1,056.
At 5,200 rpm through a 1.86:1 gearcase on a 19 inch prop, that is 5,200 times 19 divided by 1,964, or 50.3 mph. Nobody makes 50.3 mph on that setup, and that gap is the entire subject.
Slip is not waste
A propeller generates thrust by accelerating water astern. That accelerated water is, by definition, moving relative to the blades, so the prop cannot advance the full pitch distance per revolution. Zero slip means no water was accelerated, which means no thrust, which means the boat is not moving under its own power. The useful question is never how to get slip to zero. It is whether the slip you have is in the range that a matched, healthy setup produces.
Roughly, a light planing boat with a well matched prop, clean bottom and correct trim often sits somewhere around 10 to 15 percent. Heavy displacement boats and heavily loaded hulls run higher and always have. Numbers under 8 percent tend to belong to fast, light, efficient hulls, or to a measurement error. Numbers over 25 percent almost always mean a specific fault worth finding rather than a general inefficiency worth accepting.
What high slip is usually telling you
| Cause | How it shows |
|---|---|
| Spun or slipping hub | RPM climbs freely, speed does not follow, often appears suddenly after a strike |
| Damaged or eroded blades | Gradual, gets worse over seasons, visible as edge damage or cupping loss |
| Engine mounted too high | Ventilation, especially in turns and when accelerating; blowing out at hole shot |
| Never fully on plane | High slip together with poor speed and a bow-up attitude |
| Overloaded or badly trimmed | Slip rises with the load aboard and improves when weight moves forward |
| Foul bottom | Everything degrades together, and the owner is the last to notice |
Two of those are prop problems and four of them are not, which is why slip is a diagnostic rather than a verdict.
Pitch and RPM trade against each other
An engine has a wide open throttle operating range specified by its manufacturer, and the prop is the gearing that puts the engine inside it. Too much pitch and the engine cannot reach the bottom of its range, which is the loaded, lugging condition that engines dislike most. Too little and it runs into the limiter with speed left on the table. The adjustment is pitch, and the working rule people use is that one inch of pitch is worth somewhere around 150 to 200 rpm at wide open throttle. That rule has a wide spread across engines and hulls, and it is a way to choose which prop to borrow, not a prediction.
The arithmetic on this page takes a cleaner line: at a fixed boat speed and slip, pitch and RPM are inversely proportional, so the pitch that reaches your target RPM is the current pitch times current RPM over target RPM. Both approaches point the same way, and neither substitutes for hanging a different prop on the boat and running it with the load you actually carry. For the gearing side of a wheeled vehicle, the same idea appears in the gear ratio and speed calculator.
Getting numbers worth calculating with
Read RPM and speed in the same moment on a steady run, not on the way up. Use GPS speed over ground, and run the same leg in both directions and average, because a current that flatters you one way punishes you the other. Note the load, because a boat with four adults and full tanks gives a different answer from the same boat with one person and a quarter tank, and someone comparing props needs those runs done alike. The load itself is worth totalling on the boat weight and capacity planner before you conclude the prop is the problem.
Questions people ask
What is a normal prop slip percentage?
There is no single figure, because slip depends on the hull as much as the propeller. A light, clean planing boat with a well matched prop commonly lands somewhere in the region of 10 to 15 percent. Fast, efficient hulls with good propellers can be under 10. Heavy boats, displacement hulls, sailboat auxiliaries and anything pushing a load run considerably higher and always have, and that is not a fault. What is worth investigating is a number that has changed. If the same boat with the same load on the same water used to show 12 percent and now shows 22, something happened, and the number told you before the speedometer did.
How do I calculate theoretical speed from RPM and pitch?
Multiply engine RPM by pitch in inches, then divide by the gear ratio times 1,056. The 1,056 converts inches per minute into miles per hour, since there are 63,360 inches in a mile and 60 minutes in an hour. So 5,200 rpm with a 19 inch prop through a 1.86:1 gearcase is 5,200 times 19 divided by 1,964, which is 50.3 mph. For knots, divide the result by 1.15078. That is the speed the prop would make if the water were solid, and the boat will always be slower.
Will a lower pitch prop make my boat faster?
It will make the engine turn faster, and whether that makes the boat faster depends on where the engine currently sits in its wide open throttle range. If the engine cannot reach the bottom of the manufacturer range with the load you carry, it is over-propped, and dropping pitch usually improves acceleration, hole shot and often top speed as well, because the engine finally gets to make its power. If the engine is already at the top of its range, less pitch just hits the limiter sooner with no speed gained and more fuel burned. Get the specified range from the engine manual, measure where you actually are with a real load aboard, and prop to that.
Why is my calculated slip negative?
Negative slip means the measured speed exceeded what the pitch could theoretically deliver, which cannot happen, so one of the inputs is wrong. In order of likelihood: the gear ratio is wrong, since people frequently use a figure from a different model year or a different gearcase; the pitch is wrong, either misread or from a prop that was repitched; the RPM reading is low, which some tachometers are; or the speed is not speed over ground, which a paddlewheel in a following current definitely is not. Check the ratio first, because it is the input most often taken from the internet rather than from the engine.
Does prop slip change with load and trim?
Considerably, and that is the useful part. Slip rises with weight aboard, with a bow-heavy or stern-heavy attitude that stops the hull running level, with a fouled bottom, and with sea state. Trim tabs and engine trim change it measurably on the same run, which is why a boat feels like it found another gear when the trim is right. If you want to compare two propellers honestly, run them with the same people, the same fuel level and the same trim on the same stretch of water, both directions, and average.