What is propeller slip?
Pitch is how far a propeller would move forward in one turn if it were screwing through a solid. In water it moves less. Slip is the percentage difference between that theoretical distance and the boat's real speed.
Example
5,500 RPM, 2.0:1 gearcase, 19 in pitch: (5,500 ÷ 2) × 19 ÷ 1,056 = 49.5 mph theoretical. At 38 mph on GPS, slip is (49.5 − 38) ÷ 49.5 = 23%, which is high and worth checking.
Normal slip ranges
| Boat type | Typical slip |
|---|---|
| Performance boats, bass boats | 5–10% |
| Runabouts, center consoles | 10–15% |
| Pontoons | 15–25% |
| Heavy cruisers, workboats | 20–30% |
Choosing pitch
Your engine should reach the top of its recommended wide-open-throttle RPM range. Each inch of pitch changes WOT RPM by about 150–200 RPM: lower pitch raises RPM, higher pitch lowers it.
Frequently asked questions
What causes high prop slip?
A prop that is too small or damaged, ventilation, an engine mounted too high, a spun hub, or a heavily loaded boat.
Can prop slip be negative?
No. A negative number means one of the inputs is off, usually the gear ratio or a speedometer reading instead of GPS.
Where do I find my gear ratio?
In the engine's owner's manual or spec sheet. Many 150–300 hp outboards use about 1.75:1 to 2.0:1.