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BRUNTONSAustralia

Symptom diagnosis

Boat engine won't reach max RPM under load

You push the throttle to the stop and the tacho gives up 200–500 rpm short of the plate figure. The engine is willing; something is holding it. Nine times out of ten, that something is the propeller.

  1. 1. Over-pitched or oversized propeller

    most likely

    A propeller with too much pitch (or diameter) demands more torque than the engine can deliver at rated revs — the diesel lugs, exactly like a car forced uphill in top gear. Most production boats leave the factory close to this condition.

    Do thisRun the WOT test to confirm, then size properly from power, RPM and ratio.

    Run the WOT test →

  2. 2. Fouled propeller or hull

    common

    Growth adds drag and effective blade loading. A fouled prop mimics over-pitching precisely — and it's free to rule out.

    Do thisDive or haul; clean the blades and the waterline first, then retest.

    The Australian fouling guide →

  3. 3. Tired engine — injectors, air filter, exhaust back-pressure

    worth checking

    Down on power looks identical to over-propped. Black smoke plus low revs after a clean-hull WOT test points at fuel delivery or breathing, not bronze.

    Do thisIf the prop and hull are clean and the numbers still fall short, service the engine before buying anything for the shaft end.

Before you spend money

If the WOT shortfall survives a clean hull and a serviced engine, the propeller is mis-sized — and correcting it transforms the boat. But do the free checks first; we'd rather size from a healthy baseline than sell bronze to fix an air filter.

Other symptoms

Black smoke under loadVibration at cruising revsNoisy aft cabin under power

Want the numbers checked properly?

Tell the Builder about this symptom — it screens for it, and an engineer reviews every recommendation before anything is supplied.

Build my prop spec0418 327 403