cupped prop question

[jlh3rd]

I have an aluminum Merc. 4 blade 14x14 that gives me 5800 rpm @ WOT. I want 6000.
There is no 4-14x13 that fits my motor (4 3/4 gearcase), so I'm entertaining having a prop re-shaped.
Dans Props say he can take a 4-15x13 and make it a 14x13......but it's a cupped prop.
that's gonna give the same results, at least on paper, as the original, right?

 

[Scott Danforth]

cupping adds performance

so removing the cup will decrease performance

relieving the cup a bit should get you the 200 RPM you are looking for

 

[Chris1956]

Gee, most props are cupped. Is your 14P prop cupped? if so, changing the pitch to 13P cupped prop should give you the 200RPM you want.

Why are you set on 6K RPM? Your current prop would seem to be in the max RPM range already. You can likely get a 12P, which would, in theory, raise your RPMs to 6200.

 

[alldodge]

There is always a give take between max RPM achievable and max RPM performance increase. Gaining that last 200 RPM may not gain any speed because of the Torque curve. Max Torque starts to fall off as RPM increases past the Torque peek

 

[Jeff J]

Fuel efficiency takes a pretty big hit when max torque falls off as well.

 

[Scott Danforth]

No such thing as fuel efficiency. It's BSFC. Most 4-strokes burn 0.4#/HP/hr

 

[Jeff J]

If you go past the power curve you waste fuel. I see with my boat, my truck and my airplane. Staying on top of the power curve will cost speed but save fuel. I haven’t crunched the numbers for the Yamaha F70 but my truck gets 15-17 mpg at 75 mph, 20-21 at 65 mph. The boat acts the same way.