Diagnostic
Slip is really efficiency. Measure yours.
How far your propeller would travel through a solid versus how far the boat actually goes — that gap is slip. Some is necessary (it's how blades make thrust); too much is fuel churned into wake. Four numbers tell you where you stand.
Reading your number
- 20–35% — well matched at displacement cruise.
- 35–50% — common; watch it with the WOT test and a clean bottom.
- Over 55% — power is going into churned water: fouling, damage or mis-sizing.
Traditionally people call it slip; we call it efficiency, because that's what it budgets. A well-matched propeller closes this gap — which is exactly where the documented fuel savings live, and why the pitch on the stamp is only the start of the story.
Pitch is on the hub stamp — decode it here — and the ratio is on the gearbox plate; the sizing walkthrough shows where to find it by brand.
Slip ≈ 49% — theoretical 11.8 kn vs your 6.0 kn
Slip looking high?
Send the numbers through the Builder — sizing from power, RPM and ratio is precisely the fix, checked by an engineer first.