Diesel VS 4 stroke outboard fuel consumption

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

  1. DennisRB
    Joined: Sep 2004
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    DennisRB Senior Member

    For aux power in a sailing boat how does a modern outboard 4 stroke motor compare to a diesel inboard in fuel consumption?
     
  2. CDK
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    CDK retired engineer

    The diesel takes you 30-50% further for the same $$.
    But there's more to it than just fuel consumption, like reliability and life expectancy.

    And how about theft?
    I know several people who had their engine stolen and people who bought a stolen engine, but I never heard about the theft of a diesel from a sailing boat.
     
    powerabout likes this.
  3. DennisRB
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    DennisRB Senior Member

    Thanks. The purchases price and installation of a diesel is much higher though right? Theft can be a worry. Another thing about an outboard is if it dies you can always get a cheap one anywhere and fit it yourself and be on your way again. What got me thinking is an add for a searunner I saw with a 60hp outboard in the back. It was mounted in a way it can not be stolen easily. But the speeds they claimed were what I would expect from diesel with much less power. The weight savings would be quite beneficial on trimaran wouldn't they? I would still rather diesel though.

    Here is the boat in question. http://au.yachtworld.com/boats/2008/Custom-Searunner-Trimaran-2161936/United-States

    [​IMG]
     
  4. FAST FRED
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    FAST FRED Senior Member

    The first question is would the tri installation pitch the drive out of the water in rougher going.

    Second problem is todays gasoline ,poisoned with alcohol has a safe storage life of about one month.

    3rd hassle is outside the US many places do not have gasoline at docks,

    4th hassle is most cruisers use the aux as their home power plant.

    Refrigeration is usually the biggest load , but auto pilot , nav gear and lights and the rest of the toys are also a concer.

    Go small inboard diesel with HUGE DC alternator , and 3-4 stage V regulator , for fastest batt set re fill..

    On a tri a folding or feathering prop is a must.

    FF
     
  5. DennisRB
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    DennisRB Senior Member

    Thanks. Not to mention the danger of a gasoline fire...
     
  6. jonr
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    jonr Senior Member

    I would say diesel is ~25-30% more efficient in terms of gallons - in terms of $ depends on pricing. Also consider if you are going to put enough hours on it to matter.
     
  7. Easy Rider
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    Easy Rider Senior Member

    I believe this to be true.
    Because outboards are sea water cooled they need to be run much cooler than other gasoline engines. As a result they probably are injected or jetted to run rich all the time. They are a bit like a regular engine still on choke. Any time an internal combustion engine looses heat it looses efficiency since they are heat engines. Cold cylinder walls are a heat sink.
    However as CDK says there is more to it. Propellers, reduction drives, induction systems, exhaust systems and much else is involved in the efficiency of an engine and drive system. In short ..the engine may be less efficient itself but the whole system may be actually more efficient.
    There are so many expenses involved in operating a boat that I believe in the long run you will find fuel consumption is not as important as you feel it is now. I was bent on getting a boat w 1gph fuel burn and now that I have it I think I'd be just as happy burning twice as much.

    Easy Rider
     
  8. DennisRB
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    DennisRB Senior Member

    I cant see why an outboard could not be thermostatically controlled like any other engine.
     
  9. CDK
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    CDK retired engineer

    It can, but that's not the point. Raw water cooled stern drives also have a thermostat, 9 out of 10 do not close anymore after a few years in seawater due to marine growth, but they have one.

    In order to survive the warranty period without corrosion damage claims (!) the water temperature is kept at 70 C., while indirectly cooled engines run at 95-105 C., a much more efficient temperature.
     
  10. goodwilltoall
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    goodwilltoall Senior Member

    CDK, where have you come across the information that diesel is 30-50% more efficient than gasoline.

    Not doubting, just curious. I was thinking of a head to head comparison of say a 15hp 4stroke Yamaha high thrust (which might be the most efficient outboard for displacement type hulls) vs. 15hp marinized Kubota.

    How would you go about determining how many miles per gallon you would use say for a 5000# - 25' LWL hull doing 1.2VL. which would be 6Kts?

    It doesnt have to be the above boat but something similiar with both types of engines on same type of displacement hull doing under planing speed.
     
  11. CatBuilder

    CatBuilder Previous Member

    I'd be interested in this information as well.

    Just from experience, I know the new 4 strokes compete well with diesels. Given that gas/petrol is much less expensive than diesel and that outboards cost less, I see them as more economical in terms of total $ per gallon(liter).


     
  12. apex1

    apex1 Guest

    Petrol is more expensive than Diesel in almost all countries worldwide, to start with. (as usual the US are the exemption)

    A fair cost comparison compares the lifetime cost of a propulsion system and there the outboard is out of the race before it starts........
    BTW. this way one should calculate every system on board to have some eyeopening encounters!
    Most people are too poor to afford cheap stuff!

    A performance comparison between a 15 hp outboard and a 15 hp inboard Diesel is as sensible as the comparison between a 400hp Porsche and a 400hp truck.

    Regards
    Richard
     
  13. WestVanHan
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    WestVanHan Not a Senior Member

    IIRC gasoline has 125,000 btu per gallon and diesel around 148,000.

    So that's ~25% right at the start plus the pumping losses (no throttle) on a diesel are much less,combined with higher compression,higher combustion temps,and lower friction losses (not revving so high) and you get 30% (naturally aspirated) to 45% (turbocharged) higher efficiencies.
     
  14. goodwilltoall
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    goodwilltoall Senior Member

    Apex,

    There is no reason to distort the discussion (400hp porsch vs. 400hp dump truck).

    We are trying to compare apples to apples, as mentioned previously same type of hull using a diesel engine vs. gasoline outboard engine or to put it in your terms 400hp diesel engine dumptruck vs 400hp gasoline engine dumptruck and of course it is assumed same type propeller, reduction gears, tranny, etc.
     

  15. apex1

    apex1 Guest

    Well, then leave it!

    The Yamaha outboard mentioned versus the Kubota inboard is not apples and apples! It is a nonsense.

    And fuel consumption (your original concern) is only one part of lifetime cost. And lifetime cost is what you will pay in the end!...........

    That is what counts, but play your game as YOU want.........
     
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