Propeller Slip & Pitch Calculator
Theoretical speed from RPM, pitch and gear ratio, your actual slip against GPS speed, and what an unusual figure is telling you.
Use GPS speed, not the boat's speedometer — a paddlewheel or pitot reads high or low depending on how it is mounted, and slip calculated from it is meaningless.
What pitch to try instead
Two inches of pitch is worth roughly 300 to 400 RPM at wide-open throttle. The goal is to reach the top of the engine's recommended range with a normal load — an engine that cannot get there is lugging, which is considerably harder on it than revving freely.
What slip figures mean
| Slip | Reading | Notes |
|---|---|---|
| under 5% | Suspiciously low | Under about 5% usually means a measurement is wrong rather than a miraculous prop — check the GPS speed, the tachometer and the gear ratio before believing it. |
| 5–10% | Very efficient | A light, well-trimmed boat on a clean bottom with the right prop. Typical of a fast bass boat running flat. |
| 10–15% | Normal | The band most planing hulls live in. Nothing to fix. |
| 15–25% | High | Common on heavy or deep-V hulls and pontoons, and normal for them. On a light hull it points at too much pitch, a fouled bottom, or being over-loaded. |
| over 25% | Very high | Either a heavily loaded displacement hull, which is expected, or something wrong: a damaged blade, ventilation, the engine mounted too high, or a badly mismatched prop. |
Why the formula has a 1056 in it
A propeller is a screw. In a solid it would advance one pitch-length per revolution, so theoretical speed is simply RPM times pitch, divided by the gear ratio to get prop RPM from engine RPM. The 1056 converts inches per minute into miles per hour — 63,360 inches in a mile, 60 minutes in an hour.
Water is not a solid, so the prop slips. That is not a fault. Slip is how a propeller generates thrust at all — a figure of zero would mean no thrust and no movement. What matters is whether your number is normal for your hull, and a heavy displacement boat legitimately slips far more than a light planing one.
Choosing a prop
- Pitch is the theoretical distance travelled in one revolution. More pitch is a taller gear: higher top speed, slower acceleration, and lower RPM at wide-open throttle.
- Set pitch so the engine reaches the TOP of its recommended wide-open-throttle range with a normal load. An engine that cannot reach its range is lugging, which is far harder on it than over-revving.
- Two inches of pitch is worth roughly 300 to 400 RPM at wide-open throttle. That is the usual unit of adjustment when a prop is close but not right.
- Slip is not waste. Some slip is how a propeller generates thrust at all — a figure of zero would mean no thrust. What matters is whether it is normal for your hull.
How to use
- Enter engine RPM, propeller pitch and gear ratio.
- Read the theoretical speed.
- Enter your real GPS speed to get slip.
- Compare against typical figures for your hull type.
Frequently asked questions
What is propeller slip?
The difference between how far the propeller would advance in one revolution if it were a screw in a solid, and how far the boat actually travels. Some slip is unavoidable and necessary — a propeller with zero slip would produce no thrust at all.
What slip figure is normal?
It varies widely by hull type. Light planing hulls commonly run in the low teens as a percentage; heavier boats and displacement hulls run considerably higher. There is no universally correct figure, which is why comparing against similar boats is more useful than against a general number.
What does unusually high slip indicate?
Often a propeller with too little pitch, a damaged or fouled propeller, ventilation from the propeller drawing air, a drive mounted too high, or simply an overloaded boat. High slip means energy going into churning water rather than moving the boat, and the cause is usually mechanical rather than mysterious.
What does very low slip mean?
Sometimes an accurate, well-matched setup — and sometimes a measurement error. A pitch figure taken from the wrong source, an inaccurate tachometer, or GPS speed over ground in a following current will all produce a flattering number. Slip below the plausible range for your hull is a reason to check the inputs.
Should I change pitch to reach my rated RPM?
That is the usual reason to change. An engine that cannot reach its rated wide-open-throttle range is over-propped and will be lugged, which is genuinely damaging; one that exceeds it is under-propped and loses efficiency. Roughly, each inch of pitch is worth a few hundred RPM, in the opposite direction.
Does current affect this?
Yes — GPS measures speed over ground, so a current makes slip look better or worse than it is. Running the test in both directions and averaging cancels most of it, which is standard practice for any speed testing worth trusting.
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