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Author Topic:   Calculating most efficient speed/rpm?
will posted 11-05-2002 04:19 PM ET (US)   Profile for will  
Without a GPS and with guages that long ago stopped working, is there a way that I can calculate the most efficent speed/RPM at which to operate my Whaler? It is a 1991 '19 Outrage with a 1991 130 HP Yamaha. Thanks for any feedback.--Will
triblet posted 11-05-2002 10:25 PM ET (US)     Profile for triblet  Send Email to triblet     
Best gas mileage is generally the speed at
which you are fully on plane, but no more.

It's easier to push the boat over the top
of the water than through the water.

But once it's on top, power required goes
up as the square of the speed.

Worst gas mileage is when it's half on plane
(nose up, huge wake).


jimh posted 11-05-2002 10:58 PM ET (US)     Profile for jimh  Send Email to jimh     
By "most efficient" I make the assumption that you mean the highest fuel economy.

Without flow meters to record the rate of fuel usage and without accurate speed indicating devices (GPS or RADAR), I can't see how you will be able to measure either of the two parameters needed to deduce your fuel economy: fuel consumption and boat speed.

Bigshot posted 11-06-2002 10:50 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Depends on the boat and engine as well. My Baja will get the same MPG at 2500 all the way through 3500 rpms. It sucks up more gas but is relevent to the increase in speed. So like I said if I go 2700(where I normally cruise) or 3500 I will burn the same gallons of gas between point A and point B.
Arch Autenreith posted 11-06-2002 10:59 AM ET (US)     Profile for Arch Autenreith  Send Email to Arch Autenreith     
I remember hearing numerous times that the most efficient speed (generally, but certainly not always) was WOT then back off a little. My tach hasn't worked for 3 or more years so I guess it's about 500 or more rpms.

Bigshot posted 11-06-2002 11:09 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Sorry Arch.....that is only true in some diesels where you run at 90% throttle like on a Trawler. On an outboard your most efficient (MPG) range is "usually" 3300-4000 rpms. Actually it is usually 1000prms but that is a bit tough to cruise at 6mph or so.
dauntlass 18 posted 11-06-2002 11:50 AM ET (US)     Profile for dauntlass 18  Send Email to dauntlass 18     
I find bringing boat up on plane and to listen to engine and adjust throttle to get a smooth non laboring sound from engine gives me great gph results.My rpm per tach is 3200 to 3500 rpm.I go by sound and than look at tach.
Marco Whalo posted 11-06-2002 01:55 PM ET (US)     Profile for Marco Whalo  Send Email to Marco Whalo     
BS is right. 1000 RPMs gets the most MPG. Usually ~3500 RPMs get the second most MPG on a correctly powered boat. This is confirmed by just about every boat review I've seen. Triblet is right also. The best MPG is about 3-4 MPH faster than the minimum planing speed.
will posted 11-06-2002 04:33 PM ET (US)     Profile for will    
Thanks for the responses guys. Jim, I realize that this question would be tough to answer without accurate guages or a GPS. 3700 rpm seems to be a good cruising level for my boat but I like the low to mid 4's a little better :).
jimh posted 11-07-2002 12:00 AM ET (US)     Profile for jimh  Send Email to jimh     
I think the sonic approach is a good one. I notice on my boat that at 3900-4000 RPM the engines run very quietly. Advancing the throttle to 4100-4200 RPM produces an abrupt change in the pitch of the engines. They start to growl more. Therefore, I tend to use 3900 RPM as my cruising speed.

Now I don't have a scientific proof that that speed is the best fuel economy, but it sure sounds like it.

Monnas Rock posted 11-07-2002 11:58 AM ET (US)     Profile for Monnas Rock  Send Email to Monnas Rock     
jimh's "sonic" idea is probably more fact then fiction. While I'm having trouble nailing down performance on my newly aquired outboard Revenge, I have a fair share of experience with big block I/O's. Those engines will let you know, when there doing their best work. You can feel it and hear it. You don't even need to look at the tach. I suspect outboards are no different although the higher rpm's and sound pitch are admittedly, taking some time to get used to.
Bigshot posted 11-07-2002 01:38 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I always used the sonic approach as well. My 19 revenge with a 140 zuki liked 4100 the best. my 13' i just sold had no tach so sonic was it. I think it was running about 3700-4200.
rubadub555 posted 11-10-2002 01:10 PM ET (US)     Profile for rubadub555  Send Email to rubadub555     
Can anyone clearly state the relationship b/w RPM changes and altered trim angles of the motor while planing?
Logic suggests that as I'm planing in my Montauk, and adjust the motor trim to minimize prop-induced steering torque and maximize speed/fuel usage, the RPM variations that result from trim changes OUGHT to tell me something..
I'm guessing tha lower RPM's suggest more efficient planing i.e. less resistance, less motor work to move the boat.
Who knows?
triblet posted 11-10-2002 04:18 PM ET (US)     Profile for triblet  Send Email to triblet     
It seems to me that an interesting
enhancement to GPH meters would be to accept
a speed onput from your GPS, and display


John from Madison CT posted 11-10-2002 06:51 PM ET (US)     Profile for John from Madison CT  Send Email to John from Madison CT     
You need one of those flowmeters. I installed the Navman $145.00 unit and it works great. Gives me GPH very accurately on my 22' Whaler.

My most efficient cruise speed is basically 3700RPM's, at around 30mph, burning around 9.5gph or over 3mpg.

John from Madison, CT

Bigshot posted 11-11-2002 12:43 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Damn....they got cheap. Last time I looke dthey were in the $500 range. I'd get one but my 70 flows so little it might not register:)
lhg posted 11-11-2002 04:57 PM ET (US)     Profile for lhg    
All of the boating mags give this kind of report when they "test" a boat/engine package. You can generally look at one of these and see that almost all outboards have similar RPM/Speed ranges where they give the highest mileage. Yours won't be far off. I think it's generally between 3000 - 3500 RPMs if a boat is powered to it's full rating.

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