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prop wash

I have a 45 foot canal boat which is producing excessive prop wash mainly to stbd. How can I reduce it?

Asked by: ian  | 10.13am, Tuesday 20 May

WW says:

There are many factors that could produce excessive propeller wash; however the major question is 'has it always done it'?
Assuming that you have checked for any debris caught on the prop and shaft, maybe the propeller has been damaged due to hitting something hard. If one of the blades is slightly bent or distorted this would disrupt the even flow of water produced by the motion of the propeller.
Ensure that the stern of the boat is in the water, and the boat is ballasted so that the front of the swim is below water. If the boat is too high in the water, air will be sucked in to the propeller giving a very disturbed wash. Long swims allow the water to flow smoothly to the propeller; if the swim is very short this will disturb the water flow to the propeller. If the boat is shallow draught, the propeller is close to the water surface and the wash will be more apparent.
If the swim is quite wide where the propeller gland is fitted, and the prop is small; the hull will be obscuring the propeller. Fitting a slightly longer shaft to position the propeller further away from the hull will improve the water flow.
Small propellers rotating fast produce more water disturbance than larger props running slower, this is directly related to the blade tip speed. If this is the case, investigate fitting an increased reduction gearbox and a larger propeller.
You mention that the disturbed wash is on the starboard side, this effect is due to the direction of rotation and hence the 'hand' of the propeller.

Rupert Smedley  | 9.45AM, Wednesday 21 May

If you increase the gearbox reduction, this can exacerbate the prop effect (say going from a 2:1 to 3:1 reduction) and make initial movements astern or ahead have more of a swing. The increase in reduction effectively increases the torgue delivered via the prop, so can change things.
You might check that the propshaft has not moved forward or aft- sometimes the prop shaft moves closer to the sternpost, and this can affect the water flow- there should be at least a thumb width of prop shaft between the back boss of the sterntube and the prop boss itself. This can be felt down the weedhatch.
It is also worth check the rudder to ensure that it hasn't become dented below (or at) water level, as this can make things worse. Also, have a check that the prop is completely free of any debris and, as Rupert says, feel the blades of the prop for nicks and distortions.
Ultimately, you may find that a different design prop might have the effect of reducing the prop wash, but ultimately the design of the shell might be the underlying cause, if poor flow of water to the prop means that it might be hard to rectify. The bulbous finish, rather than a smooth taper, will always give poor flow to the prop- and if the swim terminates in a flat bar (where the prop shaft leaves the boat) rather than a sharp edge, then it will give very poor water flow to the prop.

Mark Langley  | 11.00AM, Wednesday 28 May

Readers say:

Thanks for your response. In reply to your 'has it always done it'? Yes, but sometimes the wash is normal, then to port and back to stbd without moving the tiller. The swim is about normal for size of boat but rather than taper uniformly it is bulbous

ian  | 3.01PM, Wednesday 21 May

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