Most Efficient RPM?

[timinfla2]

1 Engine year: 2005, make: 2005 Meridian 381, type: Flybridge Yacht
2 type of engine: Mercruiser 8.1 410 HO


My question: is there a place where I can locate my boat's most efficient cruising RPM? Boattest did one, http://www.boattest.com/oem/test-results.aspx?ID=387&lp_id=395&from_gi=yes, but as you can see it was for the 370HP 8.1's.

Should I be concerned about the HP difference and if so, in which direction is my most efficient RPM expected to be found? Higher or Lower than that of the 370 HP model?

Thank you for your help.

-Tim

 

[Haut Medoc]

Re: Most Efficient RPM?

Welcome to iboats.......
I think the 370's numbers will be comparable to the 410......
You could spend the bucks on a flowscan to dial that in.......​

 

[45Auto]

Re: Most Efficient RPM?

I think Haut is right, the power curve of the 410 vs the 370 will be so close that you won't see any difference.

Doesn't look like it much matters what speed and RPM you run at once you're up on plane. The total MPG difference between 3000 and 4600 RPM is .02 (.59 high, .57 low) MPG. Somebody walking back and forth on the boat could probably make that much difference!

You probably have some kind of GPS already. A flow-meter tied into it will give you instantaneous efficiency (MPG) readings.

 

[rodbolt]

Re: Most Efficient RPM?

MPG on the water is worthless due to load,wind and currents. industry standard is actually gallons per Hour.
your mercs are smart craft compatible so there may be a plug in stand alone flow meter or you can run it with the diagnostics hooked to it to determine most efficient RPM and trim tab settings.
the smart craft and scan tool readings will be based on an ECU mathmatical computation not actual flow.
but RPM makes a big difference, on my 22 sea pro with an F150 3900 gives me 30 MPH at 6.2 gallons per hour, 4800 or so gives me 40 MPH at about 14 GPH.
I spent a lot of time at 3900
changing the jack plate height or tabbing up or down makes minor differences as well.

 

[45Auto]

Re: Most Efficient RPM?

If MPG is worthless then why do you include your speed with your flow rates??????

Is it maybe because you can then figure out that you're going 4.8 MPG (30/6.2) at 3900 RPM, and only 2.9 MPG (40/14) at 4800 RPM? Doesn't matter what load, wind or currents are: your boat is STILL more efficient at 3900 RPM.

Or are you just assuming that the lower flow rate is "better"???

 

[Silvertip]

Re: Most Efficient RPM?

I'm watching!!!!!!

 

[bruceb58]

Re: Most Efficient RPM?

I have a Lowrance GPS with a fuel flow meter. Since it knows speed and flow rate, it calculates MPG on the fly. My boat is a 24' boat with 7.4L carbed Volvo duo-prop. I get my best MPG in the 2800-3200 RPM range. Surprisingly, it is fairly constant within that range. I adjust both my throttle and trim to maximise the MPG.

Every hull/engine combo is going to be different. Likewise various loading will also have an effect.