Bad Gas Mileage

[Levin]

Hello all...

So I have a question about "gas mileage" (or gallons-per-hour as I realize these things are actually calculated) on my 3GM30F Yanmar. It seems that in the last few years I've been getting less bang for my buck when it comes to the diesel I put in the tank. I haven't kept precise numbers, but it seems that I need to fill the diesel tank more often and I seem to run through what I'm putting in the tank faster. This could be that I'm using my boat more (maybe motoring more?) although that doesn't seem all that logical to me as I don't feel like I've change my type of use any.

Does anyone know what might cause a loss of efficiency in the fuel department of this type of engine? I recently moved from a fixed 2 blade prop to a Kiwi Prop... but I would have imagined this made things more efficient not less. Otherwise I tend to keep up on oil changes, change out the fuel filters often and otherwise keep up on maintenance.

Any ideas?

Thanks,
-Levin

 

[Ross]

If you don't have hard data we can only speculate as to the fuel consumption rate.

 

[sailaway78]

Ross is correct

More data needed. It's pretty easy to do. Just keep up with gallons burned and engine hours. Your engine should probably burn around 1/2 gal. per hour, or just a tad more. My 3GMD burns right at 1/2 gallon per hour on a seasonal basis.

 

[kloudie1]

The gross things that can change ove time and increase fuel consumption are exhaust restriction.. (elbow) intake restriction (air strainer) and dirty prop and bottom (barnacles and slime).. Some other info as the other guys have mentioned would be good.. do ya go for a constant rpm or are ya setting a speed thru the water as a target?? Lots of other stuff, obviously, but these are the usual suspects..

 

[Joe]

Hello all...

Does anyone know what might cause a loss of efficiency in the fuel department of this type of engine? I recently moved from a fixed 2 blade prop to a Kiwi Prop... but I would have imagined this made things more efficient not less. Otherwise I tend to keep up on oil changes, change out the fuel filters often and otherwise keep up on maintenance.

Any ideas?

Thanks,
-Levin

Yeah..... no jackrabbit starts at the intersection, don't use cruise control and put it in neutral on the downhill sections.

 

[NYSail]

How much are you burning? My Yanmar 3HM35F burns about .8 gallons per hour.

 

[Bill Roosa]

I suspect you have the dreaded "diesel gremlin." They usually travel in groups with the equally dreaded "light wind gremlin" and "gota get back for work gremlin" That last being a bane of sailors everywhere.
What we are really talking about here is commitment to sailing. Once the "gota get back gremlins" realize that you are not going to go to work anymore they will leave and you will notice that the wind and diesel gremlins leave with them.

Seriously, when was the last time you had your bottom cleaned?

 

[kloudie1]

Bill, Laughing.. Yup those guys are related to the Underwear Gnomes who steal socks and shrink clothes in the dead of the night.. I have noticed the Diesel Gremlins more in the summer when the wind is very light.. The wind must be able to blow them away from a sailboat !! ??

 

[Ross]

It is amazing how much extra fuel it takes to increase your speed from four kts to five kts. The rate changes exponentially.

 

[Bill Roosa]

All I know is my boat has an infestation of various species of grimlins:
teak maintance
bedding maintance
water leak (both fresh and salt sub species)
electrical
head

Apparently the electrical and head grimlins are mating as the GPS located next to the head is now acting up.

Still better than going to work though.

 

[Benny17441]

Fuel consumption is all about engine size, RPM and running time. The ultimate test is verifying your actual burn rate in gallons per hour. You should be between 1/2 and 3/4 GPH depending on your preffered RPM range. Hull condition, prop size, currents and wind all may affect speed but by themselves they do not affect fuel consumption other than they may cause you to run at higher RPM to maintain speed.

 

[Ross]

On my engine fuel comsumption is specified in grams per horse power hour. About 285 I believe. So if I am using 4 HP then I am using about 1.2 quarts per hour. You can use this in both directions if you know your gross fuel consumption rate then you can figure how many HP you are using. If you know the distance traveled and the fuel used you can figure your MPG .

 

[Stu Jackson]

All of this advice is very good. I have found that keeping track of an AVERAGE consumption, just that an average, can be very helpful - clean bottom just painted, dirty bottom at end of bottom paint life, and everything in between.

http://c34.org/bbs/index.php/topic,3841.0.html

I did have one hiccup a few years ago, where the average jumped up (from 0.423 g/h to a tad over 0.5). I attributed that to the need for another bottom cleaning.

You really do need to give us some real numbers.

 

[Levin]

Hello all,

Actually in my experience it is very difficult to accurate calculate gallons per hour for a diesel engine, mostly because we never actually completely fill our tanks. In a car getting gas mileage is very easy because you almost always fill the tank, but in a boat I never completely fill the tank because of worry about it spilling into the water and therefore just fill it "nearly full". This means that I never know how much I have used for a given time period, and thus never know my gallons-per-hour. If anyone has suggestions on how to figure this out without filling the tank (and risking spilling diesel everywhere) I would love to hear about it.

As for the bottom of my boat... it is very clean. Actually it is probably cleaner recently than in times past because I just had the bottom re-done about 3-4 months ago. I also have a diver clean it monthly and believe he does a very good job. So having a dirty bottom isn't the reason. Could be my mixing elbow I guess... although being that I only have about 500 hours on the engine I wouldn't expect this to be a problem yet... and because I always run the engine at about 2900 RPMs when I’m out to prevent major build-up… but I guess it could be a problem. I've been meaning to take that off and look at it but have been lazy because it looks like one nasty job.

Any other suggestions besides the mixing elbow and a dirty bottom?

Thanks,
-Levin

BTW... Stu, your fuel log looks like an intersting solution to figuring out some of how much I use (as you say an estimate).... but reading over your sample is very depressing... I mean when was the last time I could even dream of getting 4gallons of diesel for 6 bucks?

 

[FastOlson]

Hello all...

So I have a question about "gas mileage" (or gallons-per-hour as I realize these things are actually calculated) on my 3GM30F Yanmar. It seems that in the last few years I've been getting less bang for my buck when it comes to the diesel I put in the tank. I haven't kept precise numbers, but it seems that I need to fill the diesel tank more often and I seem to run through what I'm putting in the tank faster. This could be that I'm using my boat more (maybe motoring more?) although that doesn't seem all that logical to me as I don't feel like I've change my type of use any.

Does anyone know what might cause a loss of efficiency in the fuel department of this type of engine? I recently moved from a fixed 2 blade prop to a Kiwi Prop... but I would have imagined this made things more efficient not less. Otherwise I tend to keep up on oil changes, change out the fuel filters often and otherwise keep up on maintenance.

Any ideas?

Thanks,
-Levin

I looked at your bio to see what kind of boat & model you have. Nothing there. How about helping us help you by putting in some info about your boat and normal sailing area?

Other than making a change in your motoring habits, maybe you have put enough hours on the engine that it needs some regular maint.
How long's it been since valves were adjusted and injectors tested? Someone else already suggested checking the exhaust elbow for carbon buildup.

Good luck,

LB

 

[Ross]

If you have a 15 gallon tank and you motor 200 hours and you have put 40 gallons of fuel in during that time The you will have used no more than 55 gallons and no less than 30 presuming that you don't run the tank nearly dry. So your rate of consumption would be less than .27 gallons per hour and more than .15 gallons per hour. The longer your observation period the closer you will get to the exact average. If the tank has as much as you started with then you use .2 gallons per hour

 

[Ken13559]

Levin, I've a plastic tank so I mark the existing fuel level, then fill in 10litres of diesel, I run my engine normally over several weekends and check the fuel level and engine hours. When I hit the 10 litre mark, I calculate my fuel consumption using the engine run hours. If you have steel tank then this wouldn't do. My 2GM20F average 1.5Lt/hour.

 

[Joe]

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[Joe]

Actually in my experience it is very difficult to accurate calculate gallons per hour for a diesel engine, mostly because we never actually completely fill our tanks.

Have you tried using a dipstick to measure the fuel level in your tank?

You'll have to take a few readings during the fill process to roughly calibrate the stick. A piece of stiff wire might make a good stick.... like a coathanger