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.

Choose the wrong anode and your prop and shaft pay for it. Zinc, aluminium or magnesium — here’s how to decide.

What an anode does

A sacrificial anode is a block of less-noble metal electrically bonded to the vessel’s underwater metalwork. In seawater, the anode corrodes preferentially — being consumed to protect the shaft, propeller, rudder and engine.

Zinc — the saltwater standard

Pure zinc anodes are the historical standard for seawater. They work. The downside is that they stop working in brackish or fresh water (the oxide layer insulates them). If your vessel moves between saltwater and freshwater, think about aluminium.

Aluminium — the modern all-rounder

Modern aluminium alloy anodes (containing indium) work in salt, brackish and fresh water. They last roughly 50% longer than zinc on an equivalent-mass basis. The main caution: don’t use aluminium anodes in a diesel engine’s heat exchanger unless the manufacturer approves it.

Magnesium — freshwater only

Magnesium anodes are strongly electronegative — too strong for seawater (they’ll alkaline-corrode aluminium hulls). Use them only in freshwater: lakes, rivers.

Sizing and placement

Shaft: one anode within 100mm of a bearing. Rudder: one on the trailing edge or stock. Hull: one or two bolted to a dedicated plate. Engine: one in the raw-water circuit. Replace at 50% consumption.

Checking they work

Half-cell reference voltage: for a steel/aluminium hull in seawater, read −800 to −1050 mV vs. silver/silver-chloride. For a GRP hull with bronze gear: −550 to −650 mV. More negative, over-protected (hydrogen on bronze alloy). Less negative, under-protected.

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.

If your boat is wearing the wrong propeller, you’re burning diesel for no return. Here’s how to see it in numbers.

Why fuel burn depends on propeller

Fuel burn depends on two things in the drive train: how much power the engine makes, and how efficiently that power is converted to thrust. The propeller is the second half of that equation — and small changes in propeller design produce measurable fuel differences.

Over-propped boats cost fuel

An over-propped engine can’t reach rated RPM and labours at every throttle setting. Specific fuel consumption (grams per kWh) is 10–20% worse than at rated RPM. Correct the propeller and the fuel savings pay for the work in a season of hard use.

Under-propped boats cost fuel too

Under-propped engines over-rev. Mechanical friction losses increase with the square of RPM; cooling fan draw increases; the engine never develops its best torque. Fuel per nautical mile climbs.

Blade area and cavitation

Too little blade area = cavitation (micro-boiling on the blade back). Cavitating propellers are inefficient and noisy. Adding blade area — or going to one more blade — cures the cavitation and the fuel bill.

Real-world example

A 42 ft prawn trawler we re-propped: original 3-blade 36×26 cavitated at tow. New 4-blade 38×22 with higher BAR eliminated the cavitation. Fuel burn at tow dropped from 25 L/h to 22 L/h — 12% saving on an 8 hr tow night, every night. Pays for the new propeller in a quarter.

What to check

• At WOT, does your engine reach rated RPM? If not, under-propped.
• At WOT, does the engine over-rev? Over-propped.
• At cruise, does the boat vibrate or sound "cavitationy"? Blade area too small.

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.