Diesel VS 4 stroke outboard fuel consumption

Discussion in 'Propulsion' started by DennisRB, Jul 20, 2010.

  1. goodwilltoall
    Joined: Jul 2010
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    goodwilltoall Senior Member

    CDK,

    Did the calculations and if yours are correct in regards to the kubota generator, it will be 25% percent more efficient at 1/2 load and 63% more at full load.

    The EU2000 will produce 5818 watts per gallon and the Kubota 9500 watts per gallon.

    Would still like to see same boat with same horsepowers comparing fuel consumption of both types.
     
  2. CatBuilder

    CatBuilder Previous Member

    It's a difficult calculation in real world marine use because of losses due to transmissions and prop sizing and pitch. Maybe comparing a diesel outboards to a gas/petrol outboards would at least make it apples to apples?


     
  3. WestVanHan
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    WestVanHan Not a Senior Member

    The diesel and gas props will be optimized for themselves.
    Only way is to swap engines and test...and the diesel will come out on top.

    Oh- and good will- my calcs are correct,it's all been posted b4
     
  4. Eric Odle
    Joined: Jul 2010
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    Eric Odle Tugboat Mariner

    My mid-90's 4-stroke Yamaha 9.9 carbureted outboard came in at about .85 gph open throttle, which moved my 33 foot sailboat about 6 kts. I wasn't very impressed by that, nor did I like having to deal with gasoline issues on a boat. Some of the diesels I was looking into to replace it were getting closer to .5 gph with additional output going to alternators and other necessary functions.

    More importantly IMHO the diesel is a safer powerplant. Gasoline outboards are way better than gasoline inboards, but you still have to deal with a substance that is creating explosive vapors on your boat.
     
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    That figures. Consumption of a diesel engine is fairly constant over a wide rpm range while gas engines tend to get very thirsty at the end of the graph (injected gas engines do better than carbs).
     
  6. CatBuilder

    CatBuilder Previous Member

    Apples to Apples:

    A Yanmar 27HP diesel outboard burns 1.86 gallons per hour, as per the manufacturer:

    http://lists.samurai.com/pipermail/power-catamaran/2005-December/000754.html

    A real life fuel metering system used by a guy on a fishing forum says the Yamaha T25 gets 2.0 gallons per hour at full throttle.

    http://www.ifish.net/board/showpost.php?p=1763151&postcount=24

    Now, manufacturers often "over estimate" positive figures, but let's discard that fact for now.

    If you look purely at how many gallons per hour are consumed by the Yanmar Diesel outboard as compared to the Yamaha outboard, the newer 4 stroke engines do indeed rival the diesels for fuel efficiency, which is what I had said earlier.

    In fact, based on the figures above (real figures, provided by a manufacturer and by a real time fuel metering system), the gasoline outboard is 93% as efficient as the diesel outboard.


    Ok, so as I was saying, modern 4 stroke outboards rival the fuel efficiency of a diesel... but wait! There's more!

    In the States (but I don't know about elsewhere), the average price for gas/petrol last week was $2.73 per gallon. At the same time, the average price for diesel was $2.93 per gallon.

    SO... here are the *real* figures:

    Hourly cost of running diesel outboard: $5.45 per hour

    Hourly cost of running gas/petrol outboard: $5.46 per hour

    Math is only as good as the numbers you put in. Your math skillz are fantastic, but garbage in = garbage out! :p These two very equivalent engines cost the same (to within 1 cent! a rounding error!) to run. My assumptions included that the Yanmar factory likely overstated the GPH a smidge and that makes up for the 2hp difference between the two engine models.

    This is a true apples to apples diesel to petrol/gas comparison between two outboards.

    Concerns about gasoline on your boat? Only if you're a monohull. Catamarans keep gasoline outside where it belongs.
     
  7. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    I think your GPMiles will be dependant on the prop efficency and an inboard diesel where you have more control over the gear ratio and diameter is the reason it will beat an outboard when in displacemnet mode.
    99% of outboard gear ratios's and prop diameters are designed for planning hulls and in most cases you cant change them.
    Dont fall into the trap of comparing GPH you need to use GPMile

    PS in most cases you get more petrol from a barrel of crude than diesel
    http://www.txoga.org/articles/308/1/WHAT-A-BARREL-OF-CRUDE-OIL-MAKES
    http://ezinearticles.com/?What-Comes-From-a-Barrel-of-Oil?&id=3785911
    http://wiki.answers.com/Q/How_much_gasoline_can_be_made_from_one_barrel_of_crude_oil
    Diesel is usually only cheaper as countries are set up up sell it tax free to boats and they all feel sorry for farmers....
    Just look what is subsidised in the 3rd world..kerosene and diesel so the 3rd world is a great place to have a private jet and a 60' Bertram
     
  8. CatBuilder

    CatBuilder Previous Member

    Powerabout: What about "high thrust" and "Powerthrust" models with displacement-matched gear ratios and full size props? The T25 by Yamaha I used above as an example is properly geared with correct props for displacement boats.

    My concern with the inboards (aside from poor performance while under sail) is there are transmission losses to subtract which we may not have accounted for. That's why I used outboard to outboard for comparison: Apples to apples. Comparing outboards to inboards opens this discussion up to a lot more than just fuel efficiency.

    I'd love to have that discussion (outboards vs inboards), because while I think I've decided on outboards, it's great to find problems before you buy something. :)
     
  9. powerabout
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    powerabout Senior Member

    Yes High thrust models are better but I doubt there is a model with the optimal ratio and diameter.
    The thread was diesel V 4s I assume they meant inboards?
    How about a diesel outboard?
    BRP and MERC make outboards that will run on kerosene so if its cheap it will be better $/mile than petrol/gas
     
  10. CatBuilder

    CatBuilder Previous Member

    If I missed that, I apologize. I thought it was any diesel vs. any petrol/gas, including outboards.

    Big difference there. I would agree that absolutely no inboard gas/petrol engine could ever touch an inboard diesel for efficiency.
     
  11. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Not quite true.
    These pages give actual figures from US refineries and say something about the US market for oil products only.
    The process to make diesel fuel from crude oil is rather straightforward and cheap, but then the gasoline yield would be very low. Breaking down the long chains can fulfill the demand for gasoline, making the production process more complicated and wasteful.

    Diesel in Europe is 5-20% less expensive than gasoline (we pay 5-8 US$ for a gallon of gas here!). A red dye turns diesel into heating oil which is taxed differently. To the best of my knowledge the only source of tax free (aviation) fuel for private use is Amsterdam airport. Tax free fuel for boating once did exist, at least 30 years ago.
     
  12. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    This thread has a lot of good info in it now. Thanks to all who replied and keep the discussion coming.

    My original question was aimed at real word comparisons. That would be a diesel inboard VS a petrol outboard. These are by far the most common 2 choices for aux power for a sailing boat. I would not consider a petrol powered inboard.
     
  13. CatBuilder

    CatBuilder Previous Member

    Let's add a diesel inboard of relatively the same size to the data then. Take a look at the PDF file in this Yanmar link:

    http://shop.torresen.com/utility/includes/content/yanmar/sales_lit/3YM30.pdf

    Now scroll down the PDF to find the graph (on the right hand side) entitled "fuel consumption." Looks suspiciously like the Yanmar outboard comparison I put up, doesn't it? Oddly, this 3GMY30 puts out 27HP at the prop at 3489RPM and uses (from the graph) about 1.8 gallons per hour to do so. What do you know?!! Same engine they use in the outboard, I guess.

    So, again... for the same power, the diesel inboard (Yanmar) uses a comparable amount of fuel per hour. What does it cost to run this Yanmar 3GMY30 at 27HP at the shaft?

    Inboard 3GMY30 = $5.45 per hour, just like the outboard producing the same HP at the shaft.

    **Note, as I said way way back... it's comparable to a modern gasoline/petrol 4 stroke outboard in running cost (in the States).

    **Outside the States (as well as in), a modern 4 stroke outboard is 93% as efficient as a brand new Yanmar inboard diesel engine for the same HP at the prop. Only thing I don't know is the price of petrol vs diesel in Brisbane. Maybe you can fill in the operating costs for all three of these engines in terms of how many Australian dollars per hour it takes?

    Of course, outboard or inboard in general is a whole different question. :)
     
  14. Eric Odle
    Joined: Jul 2010
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    Location: Prince William Sound, Alaska

    Eric Odle Tugboat Mariner

    I installed my gasoline outboard as a replacement to my Atomic 4 gasoline inboard because I was unwilling to spend another dollar on that inboard. The 9.9 was just liftable by one person so I was able to take it to the shop for service rather than have them come to me - i.e. cheaper to service.

    Another drawback was the lack of an alternator to recharge the batteries. Some 9.9 outboards had one, but they were low power and added unwanted weight. I ended up using a Honda EU1000 attached plugged in as shore power to use my 40 amp battery charger, functional but somewhat cumbersome. A proper diesel inboard can have a hefty alternator if you want one.

    Also, boathandling was a challenge with the prop aft of the rudder, I made it work by rotating the outboard and the tiller at the same time. Functional but cumbersome! It may just be a matter of design, but handling is greatly improved when you have prop wash moving over the rudder. This little bit could save your marriage!
     

  15. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    Interesting Catbuilder. When I started this thread I had a feeling that it would be close. Seems like the outboard diesel and inboard diesel consumption differences are close enough to ignore. The findings are all at WOT. I wonder how the cruise consumption compares in the real world?

    Here in Brisbane diesel is usually more expensive than petrol. Sometimes the same sometimes cheaper. Usually diesel would be 5-15% more expensive than petrol which would make it cheaper to run the petrol motor (in fuel costs at least) if the figures quoted are as valid as they seem. The gearing/prop combinations of the inboard vs outboard argument may have some merit unless you can find a high thrust outboard of the power you require.

    Eric you also raise some good points, re handling and power generation. Although this thread started as just comparing consumption, all matters need consideration for real world applications.
     
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