Converting an Outboard to Diesel.

Discussion in 'Outboards' started by rustybarge, Aug 26, 2014.

  1. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Hi All,

    Just thought I would start a new thread on this subject rather than hijack Hastings Harry's diesel OB blog.

    The Concept:
    To convert petrol/gas OB to diesel fuel as a standby engine on a single screw 40' diesel trawler with only one engine; no petrol needs to be stored onboard and standby OB can use the main fuel tank.

    About 60-70hp approx output to push at hull speed.

    Large propellor to with fine pitch to give good grip in the water.

    I previously proposed attaching a saildrive to an outboard leg, which mr Efficency quite rightly pointed out wouldn't work so well:

    Then an outboard diesel conversion occurred to me:

    [/QUOTE]

    What sort of problems do you think this project might face?
    What size of petrol OB engine would be big enough to take strain of diesel conversion?
    What sort of diesel pump and injectors would be the easiest to use?
    Run a standard capacity at slow speed, or re-sleave to make smaller capacity?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously you will be using a large 4-stroke petrol outboard, which will be expensive to purchase before you do the conversion. Beyond my ken what the hurdles would be, but the high compression may be difficult to contain safely.
     
  3. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    I was thinking that an early fuel injected OB from about the late 1990's would do nicely as it is going to be totally rebuilt., as It will have pockets machined in the head to take injectors.

    Wild guess: 150hp Yamaha or Mercury might work at 70hp on diesel; seems like a good choice as there are readily available spares.

    Smaller capacity, or lower revs on diesel, to stop the block cracking?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Fuel injected 4-stroke ? Two stroke won't be any good !
     
  5. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Yep, seem to remember fuel injected 4str came in in the late 1990's /early 2000's....?

    When I was growing up in the 70's I had a 1600cc ford cross flow engine in an Escort; the same block came in 1100cc, 1300cc and 1600cc and is still a favourite tuners engine with an 1700cc overbore....

    ....I am wondering if there's an outboard which had a standard Block that came in differing capacities? The small capacity version would make a perfect choice.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Not sure whether there are any 4 strokes that answer that description, some two strokes were different bore sizes in the same block.
     
  7. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Just looking at the yanmar marine engine site, their standard 2ltr diesel block comes in several hp outputs from 45hp to 150hp.....!

    http://www.yanmarmarine.eu/Products/Leisure-Engines/
     
  8. rustybarge
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    rustybarge Cheetah 25' Powercat.

    A 2ltr diesel engine block capacity looks good, going on the basis that the yanmar 2ltr can produce from 45hp to 150hp.......

    .....so maybe a 2.5ltr petrol engine would have the cylinder walk thickness to sleave down to 2ltr?
    I will need to find out the wall thickness on the Yanmar engines to find a similar petrol engine.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Don't forget the diesel will have an iron block, the outboard alloy.
     
  10. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Forgot about that, you're quite right.

    The Chinese Ronsun OB that Hastings Harry has bought has an alloy block?

    Just guessing from the light weight of 135kg........
     
  11. rustybarge
    Joined: Oct 2013
    Posts: 533
    Likes: 3, Points: 18, Legacy Rep: 37
    Location: Ireland

    rustybarge Cheetah 25' Powercat.

  12. rustybarge
    Joined: Oct 2013
    Posts: 533
    Likes: 3, Points: 18, Legacy Rep: 37
    Location: Ireland

    rustybarge Cheetah 25' Powercat.

  13. rustybarge
    Joined: Oct 2013
    Posts: 533
    Likes: 3, Points: 18, Legacy Rep: 37
    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Question about cylinder pressures:

    Diesels run at twice the compression ratios of petrol/gas engines. Does a Diesel engine running at half the revs only produce half the cylinder pressure, reducing the probability that it might crack the block?

    In other words would a 4ltr petrol/gas engine be safe running at say 2000 revs on diesel at 16/1 compression, instead of the 4000revs on petrol at 8/1 compression?

    It might be possible to buy 'off the shelf' cast iron cylinder liners for a Diesel engine , and machine the petrol outboard cylinders to take the liners; would this increase the strength of the alloy block?
     
  14. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    A alloy engine is not going to work; it might be possible to insert steel liners to strengthen the cylinders, but the alloy head is going to warp with the high compression ratios.

    Just wondering if early outboards fron the 70's and 80's were made from cast iron?

    ....saildrive looks like a neat unit.

    [​IMG]
     

  15. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Looking at the saildrive concept, the problem is that the engine is not centrally located over the outboards leg.

    The Vw Diesel engine used by Mercruiser is from the VW Golf front wheel drive 2ltr diesel car: a front wheel drive engine transversely located like this photo.
    https://mercurymarine.com/en/us/engines/diesel/mercury-diesel/20l-tier-3/


    [​IMG]

    You will notice that the output shafts at the back of the engine/gearbox are nearly centrally located to connect to the drive shafts.

    Take a big OB leg, put a saildrive 90 degree gearbox on top, connect to the Vw gearbox/engine located in line with the boat: the engine will then be balanced nearly centrally on top of the OB leg because of the output shaft on the VW gearbox.

    The mercruiser is a marinised Vw 2ltr Diesel engine, so all you have to do is buy a VW gearbox and a 90 degree saildrive gearbox and weld up a frame to locate them on top of the OB leg.
     
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