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BRUNTONSAustralia

Symptom diagnosis

Black smoke from boat diesel under load

Black smoke is a diesel's way of saying it's being over-worked: fuel is going in faster than the air charge can burn it. The question is what's demanding the torque.

  1. 1. Over-propped engine lugging below rated RPM

    most likely

    Excess pitch loads the engine at low revs where the turbo (if any) and airflow can't keep up — unburnt diesel exits black. Long-term, lugging glazes bores and shortens engine life.

    Do thisConfirm with the WOT test; if max RPM falls short, the propeller is the fix.

    Run the WOT test →

  2. 2. Fouling — hull or propeller

    common

    Extra drag is extra load. The smoke often appears at the end of a season for exactly this reason.

    Do thisClean first, retest second.

  3. 3. Air and fuel system faults

    worth checking

    Choked air filter, clogged mixing elbow, tired injectors — all produce smoke at load regardless of propeller.

    Do thisIf smoke persists at correct WOT revs with a clean bottom, it's an engine service, not a propeller.

Before you spend money

Smoke plus full rated RPM = engine problem. Smoke plus RPM shortfall = propeller problem. The tacho tells you which cheque to write.

Other symptoms

Engine won't reach max RPMVibration 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