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Hi, when stopping or reversing the sound from the prop changes. It sounds like gravel hitting the boat or maybe air bubbles. Is this normal!
I have a 18x15 prop with around 1 1/4" top clearance using a 50 hp shire 150 gearbox. I did have a 18x10 prop and that did the same, I changed it because it was under proped and I bent the old one. Many thanks

Asked by: anthony spanswick  | 7.28pm, Wednesday 12 August

WW says:

There are two potential causes- ventilation and cavitation. The latter is less likely, as you would find the engine speed increases, as the blade spin, they lower the pressure around the blade tips until the water vapourises (or dissolved gases come out) so the prop is effectively spinning in gas rather than water. This normally happens with high speed props, though it can happen sometimes to narrowboats.
A more likely issue is ventilation, where air is drawn into the prop. This can come from the surface- often along the swim (sometimes you see little whirlpools drawing air down) or from around the rudder. The shape of the hull, length of swim, taper to the ends of the swim, rudder shape and hull curves all play a factor in determining if air is drawn in. As it also happened with your previous prop, it would suggest that the issues lie with the hull, rather than the prop as such. however, a prop specialist may be able to advise- for instance, a four-bladed prop might enable a reduction in diameter, which in turn reduces the velocity of the blade tip, which can help reduce both ventilation and the possibility of cavitation.

Mark Langley  | 11.10AM, Thursday 13 August

Also, if you let us know the type and size of boat (and manufacturer, if possible) then we might be able to provide more advice.

Mark Langley  | 11.51AM, Thursday 13 August

It sounds as if the swim is too short, which will impede the water flow when in reverse causing more noise.

Rupert Smedley  | 5.11PM, Tuesday 18 August

Readers say:

Thankyou for your help. The boat is 65 foot and was made by an engineering company "Dark Steel" which are based in Stilton, cambridgeshire

anthony spanswick  | 3.07PM, Thursday 13 August

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