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Propellor spec

I have a 36' Fernie boat built ca. 1973 with an SR2 engine, which I have owned since the late '70s. It has always been rather slow and dreadful in reverse, so my ears pricked up when I was told very recently that these boats were often supplied with a propellor that was deliberately 'under-powered' so that novice hirers wouldn't get into trouble! The boat had a 'spare' propellor when I bought it, which I have always assumed to be just an eccentric purchase by the previous owner. However I now wonder (after all these years!) whether it was a better performance prop that the previous owner had bought but not got round to fitting!
My recent informant told me that the improved prop would be stamped '12 by 17'. However the 'spare' prop only has the numbers '360 02550' stamped on it.
Can anyone:
a) Confirm the story about 'under-powered props' for hire boats
b) Tell me what '12 by 17' means for a prop and how I could measure it.
c) Explain the significance of the '360 02550' - unless of course it is just a serial number.
Many thanks

Asked by: John Martin  | 12.52pm, Wednesday 22 July

WW says:

I can't confirm the 'under-powered prop' theory but it could be true. On the other hand, if the hirer ventured onto a river and got near to a weir, he might get into more trouble with an under-propped boat than an over-propped one.
Propellers are usually referred to by two dimensions similar to the ones you mention. The first is the overall diameter in inches. The second is the distance the propeller would travel forwards in one revolution assuming that it was 100% efficient with no slippage. This is called the pitch. The higher the number, the coarser the pitch and the greater the angle of the blades.
Normally, the diameter is greater than the pitch so it could be that your informant has got the two dimensions round the wrong way. If this is true and it is a 17 x 12 propeller, this would be fairly normal on a 45ft to 55ft boat but possibly a bit big for a 36 footer. It would be interesting to compare the angle of the blades on the two propellers if this is possible.
I am afraid I can't explain the number on the spare propeller either and agree with you that it it possibly a serial number.

Graham Booth  | 2.28PM, Wednesday 22 July

Readers say:

Many thanks, Graham - a very helpful response.
Since posting my message I've been viewing various youtube videos on propellers, trying to find out how to measure pitch. Youtube video https://www.youtube.com/watch?v=lMD7lpdFVeQ includes some useful pictures of pitch measurement. It uses a workshop gadget, but with a bit of ingenuity the principle can be used without the gadget.
It does indeed look as if you may be right - my 'spare' prop is 16.5" in diameter and my (rather crude) attempts to measure pitch suggest ca. 11/12". That would leave my 'spare' prop as essentially 17x(11-12)as you suggest.
I'll have to compare it to the prop currently fitted when the boat is next out of the water - trying to measure pitch under water through the weed hatch is perhaps pushing my luck!
Many thanks.

John Martin  | 7.13PM, Wednesday 22 July

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