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.