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Old 19 April 2010, 16:09   #1
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Country: UK - Wales
Town: west wales
Make: humber destroyer 5m
Length: 5m +
Engine: 90 yamaha
Join Date: Apr 2010
Posts: 202
prop advice please

I have a 5m humber assault with an ex rnli 70hp evinrude two stroke,I have several props that came with the engine,all stainless,with a 17'' prop she will just about achieve 34 Knots on gps at 5800 rpm,with a 16.5 pitch (repitched) she will run slightly slower at 6200 rpm.
Is it worth me trying to find a 18'' pitch prop? I have raised the engine on the transom so that the cavitation plate is about 3/4 inch higher than the bottom of the transom.
the props that I have are 15'' ,16.5 (repitched) x 2,and the 17'' that is on it now.
Or as I suspect, is this as good as I'm likely to get?
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Old 19 April 2010, 17:31   #2
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Country: UK - Wales
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Make: Vipermax 5.8, SR4.7
Length: 5m +
Engine: 150 Opti, F50EFi
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Posts: 6,298
The difference in max RPM is surprisingly large for only 1/2" pitch difference.
34 knots sounds about right for that set-up. I don't know the max RPM for your motor, but 6200 sounds a lot, so I'd be inclined to stick with the 17" pitch/5800RPM.


EDIT:
I've just read on some yank site that the max RPM on 1986-2002 Evinrood 70 is 6000RPM, so the 17" prop seems to be the right one for the job.
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Old 20 April 2010, 03:41   #3
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Country: UK - Wales
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Thanks for that,I'd been told that the rpm difference would be in the region of 200rpm per inch of pitch but the 16 1/2 has been repitched to that (its engraved on the side of the hub,the hub is marked 18 inside) so perhaps its not exactly 16 1/2? I've no way of checking.
Its not meant to be a quick boat anyway I know but its nice to have it going as well as it can do.
Does anybody know what other engines my spare props will fit?
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Old 20 April 2010, 07:38   #4
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Soozooks: 75-140hp 2-strokes and 60-70hp 4-strokes
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