Woody
Well-Known Member
What effect does increasing prop cup have on rpm? Is there a good rule of thumb? My props currently have a .4 cup, what rpm drop or other performance effects might be associated with increasing to .5, .6, and so on.
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The reason I ask is I'd like to drop my WOT rpm by about 40. I have seen general comments that one step in cup = 'X' rpm(of course I can't find that now). I was thinking to tweek for a few rpm that cup would be a simpler way vs repitch and it might come with benefits.Increasing the cup effectively increases pitch, up to a point. At some point, where depends upon the specific boat/engines/prop diameter, etc. the cup gets so great that the prop will ventilate and lose bite in turns.
This is one that quickly gets into the trial and error territory and is best left to the prop professionals.
I should have included this at the start...I'm turning 2,725rpm WOT. In your opinion too high or not?Honestly, I would not spend prop money to reduce rpms by 40. It you can turn slightly over rated rpm's the chances are very good that as the boat ages and you put more "stuff" on board (as we all seem to do) you will quickly end up propped correctly. Your condition is slightly under-loading your engines and is a better situation for engine longevity, as long as you don't run over rated rpms.....and who runs these boats at WOT for longer than a minute os so anyway.