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BRUNTONSAustralia

Symptom diagnosis

Aft cabin noise under engine — causes and fixes

The berth over the propeller gets the concert: every blade pass sends a pressure pulse into the plating above. Whether it's a hum or a headache depends on clearance, blade design and what the driveline adds.

  1. 1. Pressure pulses from tight tip clearance

    most likely

    Below ~10% of diameter, each blade pass thumps the hull. Skewed, tip-unloaded blades (the Varifold design signature) soften the pulse dramatically — an Oyster 72's master-cabin problem vanished this way.

    Do thisMeasure the clearance, then look at blade design rather than more headlining foam.

    The quiet-blade design story →

  2. 2. Driveline vibration through the bearers

    common

    Misalignment noise conducts through the hull as a body — the aft cabin just sits closest to the source.

    Do thisTypical result from correcting it: noise and vibration down around 50%.

    SigmaDrive before/after →

  3. 3. Freewheeling shaft under sail

    worth checking

    If the noise is under sail rather than power, it's the shaft spinning — a hydraulic gearbox can't lock its own shaft.

    Do thisAn automatic shaft brake ends it.

    Autolock shaft brake →

Before you spend money

Before any purchase: confirm where the noise is loudest (over the prop vs over the engine) and whether it tracks shaft or engine RPM. Two minutes with the rev counter saves a mis-aimed fix.

Other symptoms

Engine won't reach max RPMBlack smoke under loadVibration at cruising revs

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