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.

 

[Scott Danforth]

Factor in wind load and other things and your truck motor is pushing a lot more power to hold 75 vs 65. BSFC for a motor is almost flat. It literally is the Specific Fuel Consumption for the motor at output Brake HP

 

[jlh3rd]

the 6000 rpm is at the max recommended range. I want that with me on board only. Now it's 5800.
It's a pontoon.
Not looking for more speed or mpg.
I had a merc. 3-16x12. At WOT (by myself) it hit 6150+. It let me pull teens on toons. The best I could do prop wise for that. But solo at 6000 rpm wasn't optimum. Two other 3 blades, one a solas, gave same results as the 4-14x14.
Propping gets expensive even when understanding "propping".
Every prop sold might be cupped, but then why are there props sold labeled cupped...or double cupped. None of the mercs. or solas were labeled cupped. The one being offered is advertised as cupped. So if a prop is not marketed as cupped.....then it's not cupped to me.
So, back to the original point.
The aluminum Merc. 4-14x14 gives me 5800 rpm, by myself, at WOT.
I want 6000. I want to try and need an aluminum 4-14x13. That's what "on paper" says I need. But there is no aluminum 4-14x13. So I'm stuck with re-shaping one.
It'l be my last try to fine tune my rig for what I want.

that's all propping is....fine tuning.

 

[jlh3rd]

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.

it's in the range but I want to be at the max by myself.

 

[Texasmark]

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

.....and the peak of the torque curve is usually much lower than the max RPM point.....at least that's the way it is on 4 stroke diesels and gassers....one of the things that I always found interesting is that OEM outboard mfgrs. never posted their engine torque curves......except for one instance.....I think it was OMC publishing the specks on its new Loop Charged 3 cylinder 2 stroke.

 

[Texasmark]

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.

I guess I missed something over the years. Other than Turning Point Hustler aluminum prop mfgr. the only cupped props of which I was aware were "most" SS props.

My experience with cupping is better bite at high trim angles and in sharper turns it helps to prevent "prop ventilation"......the process of sucking up air rather than water for reduced thrust performance.

 

[Chris1956]

Gee, it is pretty easy to tell if a prop is cupped. Put a straight edge in the prop blade. If it sits flat on the whole blade, there is no cup. The cup will usually be at the trailing edge of the prop blade.

As for raising the RPM to 6000, I think you are changing happy to glad. I do not think that 200RPM will make any difference in speed or efficiency, especially on a semi-displacement hull like a 'toon.