PA.WOODCHUCK

I'd like to get about 100-150 more RPM to get 496HO at 5000... with Hydromotive quad 4x 24P. Hydromotive suggested removing some cup VS repitch. Does reducing cup increase slip especially running mid RPM?
Currebtly truning 4800rpm WOT
I'm going to call them again, but looking for feedback here.

thanks

Helmwurst

Likely blow out more with no cup.

105Fountain

Go to a prop guy more RPM may not increase speed?

Brad Christy

PA,

My direct experience with modifying props is only with high performance model boats. That experience is that reducing cup will allow for higher RPM, but probably a net loss in speed. Cup also greatly affect lifting properties, so reducing or eliminating can also be a benefit or detriment to overall performance, depending on specifics.

The key to prop efficiency is in pitch progression from leading edge to trailing edge. Cup is just aggressive progression applied specifically at or toward the TE. Slip robs the prop of thrust. Pitch progression regains that slip, but increases engine load, up to or near the point where the forward travel of the LE pitch is reached, at which point increasing progression renders little more than load (and probably other undesirable effects). Cup and full blade progression work differently, and impart different effects, of which I'm not going to pretend to really understand, other than my own empirical observations. Cup can also be applied around the tips of the blades, rolling it around to the diameter, also imparting different effects, sometimes including an increase in efficiency. I also have no idea if our full-sized props employ full blade progression or just TE cup. I'd be interested in picking the brains of the prop gurus like Brett, to compare how these features relate between the two disciplines. My guess is that they are fairly tight lipped with their "secrets", though. I know that model boaters can be downright neurotic about it.

Thanks. Brad.

PA.WOODCHUCK

Thanks for the feedback... that said, best to leave it alone since the results may not net more speed.

thanks

SecondWind

This is a better way to increase rpm and speed without modifying your prop.

Sydwayz

Give BBlades a call, and run it by them. You can change a few things: pitch, cup, and diameter. Sometimes reducing diameter can increase RPM; but the answer is always: "It depends."