fuel burn gas vs diesel

Discussion in 'Sport Yachts/Yachts' started by GJarrett, Jun 15, 2018.

  1. GJarrett

    GJarrett Member

    Oct 3, 2006
    Tampa, FL
    '02 260 Sundancer
    6.2 MPI
    I am sure the answer lies in several places here buried in some of the "thousand post" threads, but hopefully I can get just a quick general rule-of-thumb from the experts here.

    I have been offered a job near a beach city in South Carolina and am considering being a liveaboard on something like a 400 sedan bridge or possibly 390MY or something similar. Of course a trawler might be an option, but that's another topic.

    I can save a ton on the purchase price by getting gassers but know they will be thirsty pigs in boats this size. How much better fuel economy do the diesels really get in boats this size? For those of you that own either of those models, what do you get mpg? Looking for input from both gas and diesel owners to see the real world numbers and difference, thanks in advance.
  2. k9medic

    k9medic Active Member

    Mar 27, 2013
    1998 420 Aft Cabin
    twin 3116 CATs
    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.
  3. Stee6043

    Stee6043 Well-Known Member

    Jun 1, 2015
    West Michigan
    1997 Sundancer 400
    7.4L Gassers
    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.
  4. peterkvs

    peterkvs Active Member

    Nov 11, 2012
    Guntersville, Alabama
    400 Sundancer 1999
    3116 Cats, 1000 hours
    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.
  5. Stee6043

    Stee6043 Well-Known Member

    Jun 1, 2015
    West Michigan
    1997 Sundancer 400
    7.4L Gassers
    And you're probably paying less per gallon for diesel to boot? That's quite a result.
  6. fwebster

    fwebster Well-Known Member TECHNICAL Contributor PLATINUM Sponsor

    Oct 6, 2006
    Middle Tennessee ; Panama City Beach, FL
    1996 450DA
    3116 Caterpillars
    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.
  7. 2000SR380

    2000SR380 Member

    Feb 17, 2014
    Emeryville, CA
    2005 Sea Ray 280 Sundancer, Express Cruiser
    Two 5.0L Gas Mercruiser V8, B3 Outdrives
    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.
  8. northern

    northern Well-Known Member

    Jan 17, 2007
    Anacortes Washington
    380 Aft Cabin 1989 Charts Timezero radar Furuno
    Twin 454 strait shaft
    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.
  9. JVM225

    JVM225 Well-Known Member

    Apr 8, 2008
    Farmingdale, NY
    2002 410 Sundancer, Monaco Edition.
    3126 Cats.
    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.
  10. GJarrett

    GJarrett Member

    Oct 3, 2006
    Tampa, FL
    '02 260 Sundancer
    6.2 MPI
    Thank you for all of the replies. I read the consensus to mean that at trawler speeds there is not that much difference but at cruise it is huge; diesels get about twice the nmpg as gassers. I can trawl sometimes but greatly prefer cruising speeds. And I don't want to stay tied to a pole.... I could get gassers and trawl I suppose if it came down to that. I've considered a trawler but prefer the layouts of the cruisers, and trawlers are usually so much more expensive that the difference in purchase price would buy a LOT of fuel.

    I have a lot more research to do and wonder how accurate the 50 hour per year go/nogo diesel decision rule is... from what I understand, maintenance for diesels is a whole lot more expensive and offsets their better fuel economy to a large extent. I have never owned anything diesel and know little/nothing about them, but am a decent DIY'r and could learn to DIY some of the yearly maintenance to offset some of that cost, unless special tools or needed or other complications.
  11. ZZ13

    ZZ13 Well-Known Member

    Nov 25, 2009
    Lady's Island, SC
    2001 400 Sedan Bridge
    Cummins 450 Diamond
    If you can afford the diesel purchase price, then buy them. Forget all the micro analysis. They are just more fun to own.
    mjoplin, gmacd81 and FootballFan like this.
  12. bbwhitejr

    bbwhitejr Well-Known Member SILVER Sponsor

    Apr 14, 2013
    Lake Lanier GA
    2003 420DA, 6CTAs

    mjoplin likes this.
  13. Philo

    Philo New Member

    May 17, 2018
    Racine Wi
    2000 580 Sun Sport,
    Flexi Teak cockpit, walkways, loaded

    900 hours
    Detroit Diesel 8V-2000
    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
  14. midexp

    midexp Active Member

    Oct 5, 2016
    Harrison Township, Michigan Lake St.Clair
    1999 40' Sundancer
    454 merc
    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.
  15. RollerCoastr

    RollerCoastr Well-Known Member

    Nov 15, 2007
    Cedar Point, OH / Miami, FL / MacRay Harbor, MI
    1997 400DA
    340HP 7.4 Mercruiser Bluewaters
    Garmin 741, 742, 8212, 24HD, Intellian I2
    1999 280BR
    Twin 250HP Merc 350 Alpha Ones
    He could also have the wrong props, need a tune-up, have bad bottom paint...
    CaptRonn likes this.
  16. fwebster

    fwebster Well-Known Member TECHNICAL Contributor PLATINUM Sponsor

    Oct 6, 2006
    Middle Tennessee ; Panama City Beach, FL
    1996 450DA
    3116 Caterpillars
    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.
    northshore and FootballFan like this.
  17. Woody

    Woody Well-Known Member

    Nov 20, 2007
    N. Wisconsin/Lk Superior
    2005 420DA
    Cummins 6CTA8.3
    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.
  18. dtfeld

    dtfeld Water Contrails GOLD Sponsor

    Jun 5, 2016
    Milton, GA
    410 Sundancer
    12" Axiom and 9" Axiom+ MFD
    Cat 3126 V-Drives
    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

  19. Creekwood

    Creekwood Well-Known Member GOLD Sponsor

    Apr 26, 2009
    Oakville and Georgian Bay, Ontario
    '97 330 Sundancer, Raymarine C80 suite with radar, Mercury 310 Hypalon w/8hp Yammie 2stk
    2X 454 carbs w/ vDrives
    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.
  20. rondds

    rondds Well-Known Member

    Oct 3, 2006
    Jersey Shore
    2001 380DA
    Merc 8.1s (2008)...Hurth ZF 63 V-drives...WB 7.0 BCGD (2013), Garmin 8208 & 740 MFDs, GMR 24xHD dome
    helm.jpg 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.

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