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?
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.
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?
