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Propeller size & distance

Hello, we are about to take our trip boat out of the water for hull inspection and possible blacking. Whilst out we want to look at the configuration of prop shaft and propeller. Is there any guidance on propeller distance from back plate and size of prop. Our narrow boat is 42 feet long. Thank you

Asked by: Roger Tapping  | 12.54pm, Monday 20 June

WW says:

I seem to recall a rule of thumb that says that the front of the propeller boss should be the prop shaft diameter from the stern post but how critical this is, I am not sure.
As for the size and pitch of the propeller, this depends on a number of factors including the weight of the boat, the torque of the engine and the gearbox ratio. Propeller manufacturers like Crowther have tables and computer programs which will give you the best propeller size for your particular set-up.
Beta Marine's website has advice on propeller sizes for its engine range which you may be able to interpret for your needs. Their information sheet on http://betamarine.co.uk/wp-content/uploads/downloads/propellers_guidelines/PROP-SGL-1011.pdf also has some useful information.

Graham Booth  | 10.55AM, Monday 27 June

One of the most important (and overlooked) is the distance between the tip of the blade and the lower skeg- and upper uxterplate (or base of weedhatch). It should be at least 10% of the blade diameter- partly to avoid debris being trapped and also to give a better flow to the rudder- narrowboats are not very hydrodynamic, so any improvements can be helpful!

Mark Langley  | 1.49PM, Tuesday 28 June

Hello Roger,
I did work with George in my previous (architectural) existence. If you see him, please give him my regards.

Graham Booth  | 4.21PM, Tuesday 28 June

Readers say:

Thanks Graham, it seems the problem is not too serious. Did you ever work in Birmingham with a gent called George Coombey-Jones?

Roger Tapping  | 12.01PM, Tuesday 28 June


Roger Tapping  | 2.05PM, Tuesday 28 June

Thanks Mark

Roger Tapping  | 2.36PM, Tuesday 28 June

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