fuel burn gas vs diesel

At cruise I get about .6nmpg in my diesel powered boat. A friend of mine has a Carver with twin 454's and also gets .6nmpg.

At slower hull speeds we both run around 7.5 knots. At the fuel dock is where we see the difference.

 

The most interesting and seemingly reasonable suggestion I've found on this topic is this - if you use (run) your boat for 50 hours or less per year gas may not be a bad decision. Cheaper to buy, cheaper to maintain. If you're running more than 50 hours per year you're far better off investing in diesel.

On the same boat I've seen folks suggest gas is anywhere from 25%-50% less efficient than diesel. The diesel guys are also generally going to be running a bit faster from what I gather.

 

I recently did a 200 mile trip with a friend that had a boat with twin 454 Magnums. His boat was a 38' mine 40'. I burned 160 gallons of diesel round trip and he burned 320 gallons of gasoline. We followed each other at the same speed the entire way. Speeds ranged from 8 knots to 24 kts.

 

And you're probably paying less per gallon for diesel to boot? That's quite a result.

 

I've got the same engines as k9medic, and I moved from a similar sized boat with twin 7.4's. The old boat burned 35 GPH at 18 kts.....the diesel powered boat burns 18 GPH at 20 kts. That is a huge difference in terms of range and cost, however, the biggest difference for us has been the "will you go" question. I can run about 40 miles up the coast for lunch and not worry about the $190 fuel cost....the same trip in my old gas boat would cost $665 at today's fuel prices. That is one very expensive hamburger and the cost differential for fuel will keep your boat tied to a pole rather than using and enjoying it.

In my view, if you are not going to use a 40 ft boat enough to warrant diesel engines, you should probably be looking at Airstreams rather than a liveaboard boats.

 

Our 380 Sundancer had 8.1 Fuel injected V8's and was right on spec at 33-gph cruise (19-20 knots).
Bad weather, funky wave conditions, or more throttle could punch you up to more like 65-gph. An off-shore run from San Francisco to Monterey took 5.5 hours at cruise in moderate conditions. About $726.00 each way.
Do not be fooled by the great mileage quoted at lower RPM's. In my opinion these boats are designed to run at cruise. Slower speeds for longer times load up plugs fowl injectors and generally create a maintenance head ache.
if you want to run slow, get a Trawler. A Trawler will take 11-12 hours to do the same trip and the fuel bill will be less than $200 each way. Plus trawler engines and hulls are designed to run at those speeds.

 

We have gas and burn 1US gallon a knot at 6 knot speed. We have an 8KW gas generator and it burns 3/4 of a gallon an hour. We started out in 2005 with this boat and went 100 hours a year it had 650 hours on it. We now do 200 to 300 hours a year. Engines have 3100 hours on them. When they fail the cost to replace will be half that of diesel so in the end it all seems to work out to be about the same cost per knot. If we had single engine diesel we would be far ahead re cost per knot. I have thought of replacing these engines with diesels but it is 100K the boat is worth 50K.

 

My 410 with Caterpillar 3126’s and V drives burns about the same amount of fuel at cruising speed as my old 300 Sundancer with 5.7 I/O’s even though it weighs roughly twice as much.
The 5.7’s were installed brand new (not rebuilt) carb to pan, circulator pump to flywheel by me in 2012 and I had fuel flow monitors on them to help me maximize fuel economy in real time while cruising. It’s amazing how much those things can save you.
The Cat’s are original from 2002 and have about 900 hours on them. I don’t have any way to monitor fuel flow in real time to maximize burn rate as I’m cruising, and compute fuel based on hours and amount I take on when I fill up.

 

If you can afford the diesel purchase price, then buy them. Forget all the micro analysis. They are just more fun to own.

 

+1!

Bennett

 

A mechanic once told me, drop the ladt number for the hp and this will give you an idea how much fuel you will burn full throttle.

400 hp= 40 gph

I have twin 8v2000 detroits at 900 hp each.

Let’s change the subject

 

Your friend burned that much fuel because he cruised at a speed way higher than his optimal fuel efficient speed. His is about 21 knots and 3300 RPM. That will get him about 0.7 MPG. No way gas burns twice your diesel.

 

He could also have the wrong props, need a tune-up, have bad bottom paint...

 

Maintenance is not more on diesels than on gas engines........I owned a Sea Ray powered by gas 7.4's for 10 years. I've owner a Cat powered diesel boat for 21 years, so I have a reasonable frame of reference. I spend a fraction of the money I spent on the 7.4 Merc's in maintaining my diesels. The only facet of maintenance that costs more is the amount of oil you have to buy because the diesels hold 26 quarts compared to 7 quarts for the 7.4's. But the oil change interval is 2.5X greater if you run the boat........anually or 100 hours for t he 7.4's and annually or 250 hours for the diesels.


Repairs are a different story.....if you do not properly maintain and care for diesels engines, they can cost a lot of money when you get a diesel mechanic involved in the repairs.


As far as being a diesel DIY'er..? I had never owned a diesel anything when I bought my first diesel car in the 1970's. Now I own the 450DA, a dozer, a backhoe, a Bobcat, 3 tractors, an ATV, and 2 trucks that are all diesel powered, and I do all the routine maintenance myself. So, we all start at the same place, do some reading, get the factory maintenance schedule, put your big boy pants on and get some diesel oil on your hands. It just isn't that hard.

 

Look at it this way.....peterkvs burned 160 gallons, even using your .7mpg number you'd burn 200/.7= 286 gallons. You'd have burned 126 gallons more than peterkvs. Your right, it's not twice as much, it's only 79% more but I think that's plenty enough to make a case for diesels.

Work out Frank W's numbers, his actually indicate the 'twice as much' position. His fuel sippin' 3116's would be burning .79gal/mile and his previous gas boat was burning 1.69gal/mile in his example.

I sometimes run with a 40+ft Carver aft cabin with 454's...sorta run with him, he's either leaving early or pullin' in late because he's slow. We haven't put any effort into detailed comparisons but I know this for a fact....including the fact in my area diesel is cheaper than gas when we fuel up his bill will double mine.

 

You can estimate you WOT fuel consumptuous using the following formula. If you cruise at 75% power, fuel consumption with be roughly proportional.

The boat will move at a set speed at a given rpm, so you can estimate your mpg from that.

You actual will depend on a lot of factors but this will allow you to budget fuel expense

 

If you are buying used I think you get your money back when you sell so the price difference to buy diesel now is not a cost except for the future depreciation and carrying cost on that difference. That will be less than the fuel cost savings. And it will be much easier to sell a diesel powered large boat. That’s why they are harder to find. They sell faster. Go diesel and enjoy.

 

Here's some numbers from my boat. I travel with a 2003 410DA and a 1999 420AC, both powered by Cat 3126s. We run at the same speed very comfortably. You see my consumption. They do not have a readout but at this speed they claim 20gph consumption.

I think the tipping point in the gas/diesel argument is right at about 40 ft. Anything larger you are asking a lot of a gas engine. It starts getting voracious and the wear and tear will take it's toll over time.

For my usage pattern, I'm very happy with these numbers, but if I were to go any larger, I'd go diesel or go home.