Fuel use, GPH

[Libations Too]

Generally I have not kept a running calculation of fuel use on Libations. But on my two cruises to southern California, one in 2008 and one in 2013, I did compute fuel use rates....and they were identical at 0.35 gallons per hour use over a total of about 240 hours of engine run time. My engine is the Volvo MD11C and I turn the large 3-blade cruising prop. I generally cruise at about 1800 rpm and rarely run the engine over 2000 rpm for any length of time.

I am curious about the fuel use rates that others have experienced, especially those with other engine or prop combinations. What has your experience shown?

[selene]

Interesting.  I commend your accuracy!

I have a Westerbeke 30B3, fixed 3-blade prop (unsure of dia.).  My engine seems to be more comfortable at a higher RPM - around 2200-2400rpm.  I only use around 40 hours/year, so my numbers are not as accurate as yours (a lot of docking/leaving dock), but my current running estimate is a little below 0.4 gallons/hour.

[Dolce_Vita]

Just so us Atomic-4 owners don't feel left out.... 

I went to great lengths to improve my mileage, including installing an adjustable main jet, and an Air:Fuel ratio gauge to get the mixture tuned exactly right.

The net result has been that, on the last two cruises (totaling about 600 miles) I have averaged 0.76 gallons per hour.

[Libations Too]

Thanks Ed and Selene. Admittedly there are many variables that affect fuel consumption but with your two responses added to my own experience it seems as though the diesel/gasoline choice has significant affect. The diesel allows our 20 gallons of fuel to push us a little farther than the gasoline option.

I was starting to wonder about this because the MD11C is no longer a repower option while the A-4 is still available.

Ed, My first car was powered by a flat head 4-cylinder engine (Rambler...if anyone remembers those beauties). I fussed with that car a lot and one of my experiments was to fill the carburetor jet with solder and then redrill it at a slightly smaller diameter hoping that it would improve fuel economy. I did get an improvement of a couple MPG but after a year or so I had another problem....a cracked head. Whether the cracked head was from overheating or not I'll never know. But later when I was learning to fly I became very aware of how fuel/air mixture has an affect on manifold and engine temperature. Perhaps my experiment had an unintended consequence that caused the cracked head. I am curious...how does one determine the exact fuel to air mixture if one isn't monitoring manifold temperature?

[Dolce_Vita]

The gauge is driven by an oxygen sensor that I mounted in the exhaust manifold, on the opposite end from the exhaust stack, in order to keep it as far from the water injection point as possible, as moisture will degrade the sensor.  It's an unheated single-wire sensor.

I believe the ideal stoichiometric air/fuel ratio for gasoline is 14.7:1, and the gauge lets me achieve this....But...

The ratio tends to vary with rpm, and I'm very aware of the temperature going up with lean mixtures.  I noodled around a bit with an IR temperature gun, but never got anything conclusive.  In the end, I settled for setting the mixture to run just slightly on the rich side when at my cruise rpm range of 2000-2200.

Now, my plugs run clean, my idle doesn't stumble when throttling up.

Yes, diesels get better GPH than gas.  BUT, you say you have a 20 gal tank?  Interesting. Mine is 30 gal.  This gives me a theoretical runtime of almost 40 hours.  I've gone for 30 hours nonstop and had plenty of margin.  And I carry a few 5-gal jerry jugs of fuel on deck on longer or offshore trips.  Your 20 gals and 0.4 GPH should give you a range of about 50 hrs.  Life is full of compromises...

BTW, the first car I drove was a 1964 Rambler.

[Libations Too]

I was a bit loose with my comment about 20 gallons....actually Libations has the 22 gallon aluminum tank.

Ed, you are definitely the only person I know who has an oxygen sensor in their boat engine. You also obviously know a lot more about engines that I do...thanks for sharing here on the forum.

[Wandering_bill]

Good information here. I too have the Volvo diesel. I've always figured about a half gallon per hour for estimating purposes, good to know it could actually be a bit less. I have the 30 gallon tank and three blade fixed prop on Wanderer.  That's a lot of motor time, considering how good these boats do sail!

[Dave]

I have the MD11C and it varies between .35 and .4. I motor in the Gulf ICW and go with and against the current so my RPM is between 1600 and 1800. There are times I am at 1600 and making 7 knots with current and wind so I do not feel the need to increase the rpms. I also have a bigger 3 blade prop. We just finished a 5 day cruise covering 200 miles under power. We did some sailing also, but that is not always the best approach in the ditch of the ICW.