Slip is the single most misunderstood number in propeller selection. Here’s what it really means — and why 30% slip isn’t a problem for a trawler.

Introduction

Walk the dock on any Saturday and you’ll hear boat owners talking about prop slip. Most of them have one number in their head — usually a low one — and treat it as a measure of propeller quality. That’s not quite right. Slip is a theoretical quantity, and interpreting it correctly is the first step to specifying the right propeller for your vessel.

In this article we’ll break down what slip actually is, how it is calculated, and what reasonable slip percentages look like for different vessel types.

What is propeller slip?

A propeller has a pitch — the theoretical distance it would move forward in one revolution if it were turning in a solid medium (like a wood screw turning in wood). In water, which is a fluid, the propeller does not move the full pitch distance per revolution. The difference, expressed as a percentage of theoretical pitch, is slip.

The formula is:

Slip (%) = (Theoretical speed − Actual speed) / Theoretical speed × 100

Where theoretical speed is RPM × pitch / gear reduction, with the correct units — typically knots.

Worked example

Consider a vessel with:

• Engine RPM: 2,100
• Gearbox reduction: 2.5:1
• Propeller pitch: 20 inches
• Measured boat speed: 15 knots

Theoretical speed = (2100 ÷ 2.5) × 20 ÷ 1013 × 60 = 19.9 knots. (The 1013 is the conversion from inches/min to knots.)

Slip = (19.9 − 15) / 19.9 × 100 = 24.6%.

Is 24.6% slip a problem? Depends entirely on the hull.

Healthy slip percentages

A properly matched propeller usually falls in these ranges:

• Displacement cruisers / sailboats under power: 30–45% slip is normal.
• Semi-displacement work boats: 20–30% slip.
• Planing hulls at cruise: 10–20% slip.
• Prawn trawlers at tow: 50–70% slip (because the boat is being deliberately dragged by its net).

The highest-slip application is not a problem — a trawler at tow is intentionally working against the propeller. The slip number simply reflects that physical reality.

Why slip matters (and when it doesn’t)

Slip tells you one thing clearly: whether the propeller is loading the engine correctly.

Too little slip (say, 5% on a displacement cruiser) often means the propeller is under-pitched or too small — the engine over-revs and never builds its rated torque.

Too much slip (say, 50% on a planing hull at WOT) usually means the propeller is cavitating, the blades are damaged, or the engine isn’t producing rated power.

Common slip problems

If your slip number has changed over time, check for these:

• Blade damage — nicks, bent tips, erosion.
• Growth on the hull or propeller — barnacles especially.
• Change in load — are you now routinely carrying more weight?
• Engine not making rated power — fuel system, compression, turbocharger.

Conclusion

Slip is a diagnostic number, not a merit badge. The right propeller for your boat might have 30% slip or 5% — what matters is that the engine reaches rated RPM at wide-open throttle with margin to spare, and that your cruise point is in the engine’s happy zone.

If your current propeller doesn’t do that, we can help — a measurement, a bit of maths, and a re-pitch or new design will put the engine and the hull back on speaking terms.

Need advice on your boat’s stern gear or propeller? Call Brian directly on 0413 771 090 or use the contact form — we respond the same day.