Fuel useage at 4000 vs 4500 RPM

[marcortez]

My 22' Mexican built panga, with a F75 TLR motor (2010) seems to run easy at 4000 rpm with a 17" pitch prop and tops out at 5200 @ 33mph.
WOT range is listed at 5000 to 6000.

I bought another prop to be used as a spare....a different manufacturer and with "cleaver" style trailing edges....3 blade.......whereas the 17" prop has rounded "ears" on the back side.

Just for grins, I put on the 15" to feel the difference.
Hole shot was great but to get at an "easy" cruising RPM, I had to advance the RPM's to 4500....and the top speed was down by a few MPH.
And....at 4000 RPM, it seemed to be lugging just a bit, compared to the 4500.

I am more concerned with fuel burn than top speed. I never bury the throttle and am quite comfortable with 4000/4500 cruise @ mid 20MPH speeds.

Seems to me that by advancing the throttle by 500 RPM, to achieve the same "easy" running speed and "listening" to the hum of the engine, the motor would use more fuel.
IE: why push the throttle another 500 RPM, to get what I can get at 4000 RPM?

Lots of chatter on here about lugging the motor and so-forth....however, with the motor spinning at 4000 RPM with the 17", seems to be the sweet spot vs a sweet spot with the 15" at 4500.

For reference.....the 17" prop is a Solas Amita aluminum 14 1/4" X 17" and the 15" is a Michigan Wheel "Match" at 14 1/2"....with a cleaver style back edge.

 

[QC]

Re: Fuel useage at 4000 vs 4500 RPM

The fact that it takes more RPM for the same speed usually equates to poorer fuel efficiency. The fact that she is slower top speed also suggest that the 15 combo is not as efficient although that could be due to the higher WOT RPM being outside of peak horsepower. Generally though the difference will be very small and although you might think you are advancing the throttle to achieve the same speed you really have only advanced RPM. Oddly enough the throttle is actually more closed at 4500 to achieve similar horsepower to move the boat the same speed. This is an extremely complicated comparison to make with the info we have to work from, but it does sound like the 17 is better on fuel.

 

[steelespike]

Re: Fuel useage at 4000 vs 4500 RPM

Really the only way to be sure which is working best is to fill her up and make a run at the ideal cruising speed to a specific point and back.top it up and make a run with the other prop.
top it up and note any difference.An easier way would be a flow meter.
If your making 5200 with the 17 your not really lugging but obviously on the edge
especially if you regularly carry a heavy load.The right way to comepare props accurately would be to run both Solas Amita or both Michigan cleaver.
When you change brands and styles you introduce more variables.
The ideal cruising speed can be just on plane up to about 1,000 rpm higher.
Another way to watch fuel use is a vacume gauge. Vacume is highest at idle
and decreases as you open the throttle.With a little instruction it can also tell you the condition of your 4 stroke motor if the timeing is right,condition of rings valves etc.

 

[Leardriver]

Re: Fuel useage at 4000 vs 4500 RPM

I just looked at a boat manufacturers' web site, and it lists fuel flow for a 5.0 carb engine at varying RPM's. 4,000 RPM's burns 12 GPH, and 4500 burns 17 GPH and went 10% faster. That is just for comparison purposes, but it sounds to me like you are really pushing fuel through the vessel at 4500. The most efficient range is 3,000 to 3500.

 

[QC]

Re: Fuel useage at 4000 vs 4500 RPM

It is important to understand that the fuel use numbers above are with one prop pitch. There would be a similar fuel rate change for less pitch on a heavier boat and a lighter boat with higher pitch. Basically any engine/boat that was propped properly will have a steadily increasing fuel rate with increased RPM. But the OPs question is about the same boat with less pitch vs. more pitch . . . So it is possible that the fuel rate is actually lower at 4500 RPM with the 15 inch clever than 4000 RPM with the 17, buuuut if speed is also lower then efficiency has degraded.

If you stare at data long enough you come to realize that the 3000 - 3500 RPM sweet spot (I/O 4 cycle) is more about hull efficiency than engine efficiency. If 3000 - 3500 was the most efficient point for these engines our cars (very similar engines) would be geared to cruise there too, but they are geared for cruise at 1500 - 2000, so why? Because at road load cruise, say 30 - 70 horsepower depending on vehicle size, efficiency is best at much lower RPM.

These are not engine fuel use curves, they are an engine coupled with a propeller use curve. So we must be careful to glean much from a different engine with a different pitch on a different boat etc. Plus the OPs engine is a Yamaha OB that is designed for higher RPM use.

With that said, propeller load "curves" are actually pretty dang linear and so is the fuel rate increase to correspond with it . . . you turn the prop faster, which pushes the boat faster, which requires more fuel, and the resulting rate increase line is pretty dang straight. As long as you do not insert a variable such as a different pitch.

 

[marcortez]

Re: Fuel useage at 4000 vs 4500 RPM

I can get 25 mph @ 4000rpm with the 17" and the same speed with the 15" takes 4500.

I hate numbers as much as I hate eating live cockroaches and when all is said and done, it appears to me, why turn 500 more rpm for the same mph effect?
Unless.......there is some fuel savings to be had.

Swinging 500 more rpm over the life of an outboard equals a ton of rpm's and would certainly yield a shorter life......unless...and big unless....some loading issue is apparent.

 

[steelespike]

Re: Fuel useage at 4000 vs 4500 RPM

I can get 25 mph @ 4000rpm with the 17" and the same speed with the 15" takes 4500.

I hate numbers as much as I hate eating live cockroaches and when all is said and done, it appears to me, why turn 500 more rpm for the same mph effect?
Unless.......there is some fuel savings to be had.

Swinging 500 more rpm over the life of an outboard equals a ton of rpm's and would certainly yield a shorter life......unless...and big unless....some loading issue is apparent.

There may be something in what you say;but there are other considerations.
A marine engine is working like its going uphill all the time if the max rpm is too close to the bottom of the wot rpm rating it is working even harder and of course as you say if often running a heavy load that could be an issue as well.
All that said; if the 500 rpm we refer to doesn't take you out of the rpm range we are probably beating are gums for nothing.