Converting an Outboard to Diesel.

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

  1. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Do outdrives have a 1/1 ratio in the upper gear box?

    It would certainly be lighter, and is designed to take the torque of a diesel.
     
  2. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    yes some top boxes are 1:1 and several have carious ratio's
     
  3. whitepointer23

    whitepointer23 Previous Member

    There is a 30 ft cruiser here on the lakes that ran a home made diesel outboard for years. But now I see he has replaced it with a normal outboard. I think he mated a nissan diesel to a johnson leg. If I see the bloke at his boat I will see if I can get some photos. If he has kept the motor.
     
  4. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Sounds a promising idea; any diesel could be mounted on top with a v belt drive to the crankshaft/Merc upper g'box.

    A couple of pillar bearings bolted to the sump would hold the drive shaft in position and be very easy to set up.
     
  5. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Thanks, sounds very interesting.:)
     
  6. powerabout
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    powerabout Senior Member

    not rocket science you just need to deal with 2 90 degree angles.
    Typical outboard already has done one for you
    As Gonzo says a sterndrive already has another 90 degree in a small self contained housing so no need to reinvent that.
    Problem is the driveshaft of an outboard comes up toward the front/middle of the lower pan. That would then out your diesel way forward in the boat...
    Start with a vertical shaft diesel and most of your troubled engineering has already been done.
     
  7. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Any suggestions.:D
     
  8. AndySGray
    Joined: Jun 2014
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    AndySGray Senior Member

    So you could use a reflex v-drive marine gearbox? The type where the gearbox is at the front of the engine and the output and shaft run below it.
     
    Last edited: Aug 31, 2014
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    You mean any suggestions for a vertical shaft, water-cooled diesel ? Can't think of any myself, but they would exist, surely. But maybe not so easy to find in the HP range you want.
     
  10. rustybarge
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    rustybarge Cheetah 25' Powercat.

    I think Honda make small vertical 20hp diesels for ride on lawn mowers, never heard of anything much bigger...?
     
  11. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Ok a stupid question: I've never had a boat with an outdrive, only shafts and outboards.

    Why are outdrives so unreliable, and what goes wrong with them, and would it be possible to engineer a reliable outdrive?
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Who says they are unreliable ? Expensive to repair maybe, and of course there is added complexity, but I have never heard anyone say it is a fundamentally flawed concept.
     
  13. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    All that machinery under the water line, or half in it; I've heard of lots of problems with seawater getting inside the casings and turning the oil to Mayo which then led to gear failure. looking at the parts diagram there seems to be hundreds of parts, gaskets, spacers, splines, gears, split shafts, UV joints, hydaulic rams, pivot points, castings etc etc etc.......all protected by a thin rubber membrane which could sink the boat if it rips.......:D

    My local dive club has a big rib with twin Yamaha 200hp 2str engines. Am I right in saying 2str are very torquey, and would those OB legs be suitable for this project?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    You would need to talk to a mechanical engineer about what effect a diesel would have on a gearcase designed for a (somewhat higher HP) petrol engine, but since a lot of cars have optional diesel engines on much the same platform as the petrol engined version, I doubt there will be dire consequences, if you are running a modest output diesel on that leg. Mating it all up will be a bigger problem.
     

  15. UNCIVILIZED
    Joined: Jun 2014
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    UNCIVILIZED DIY Junkyard MadScientist

    Hmm, interesting stuff. Tagging it for later study.
     
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