That noise from the aft cabin may be telling you something
You’re motoring along comfortably. Then the rpm comes up, and somewhere beneath the aft cabin the boat starts talking to you. Noise. Vibration. Something that sounds decidedly unhappy.
It’s tempting to blame the engine. But the problem may be several feet behind it, at the propeller.
One of the less glamorous but more important subjects in propeller selection is blade loading. Folding and feathering propellers face a geometric limit that fixed propellers don’t: their blades have to fold together or rotate past one another. That limits how much blade area the designer can use.
When a propeller has to put a lot of horsepower through limited blade area, blade loading rises. Push that far enough and you get cavitation, noise and vibration. It matters more as horsepower and displacement go up. A propeller that works beautifully on a moderate-displacement cruiser isn’t automatically right for a heavy yacht with a powerful engine.
That’s why “two blades, three or four?” doesn’t tell us enough. We need to know what those blades are being asked to do: how much power, at what shaft rpm, on what displacement, with how much room behind the hull, and how the boat is actually used.
Propeller engineering is a system problem. Engine, gearbox, shaft or saildrive, propeller and hull. Change one and you affect everything else.
And sometimes the right answer is a fixed propeller after all. Bruntons has been making propellers for 150 years, starting with commercial ships, and still designs and builds custom fixed propellers for boats where a standard catalogue size won’t do. When a boat needs that, we can design it, make it and fit it.
If your boat has noise or vibration under power, or you’re thinking about changing propellers, come and see us at the Annapolis Sailboat Show, Booth YB3, City Dock, October 15–18. Tell me your engine model and I’ll size your propeller while you’re there. You’ll walk away with your sizing and show pricing on a postcard. No numbers needed: just know your boat and how you use her.
Next: the best propeller? Wrong question.
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