Converting an Outboard to Diesel.

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

  1. Westfield 11
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    Westfield 11 Senior Member

    I would think that the heavier power impulses from the diesel (if you could do the conversion) would destroy the lower unit in no time at all unless you upgraded the whole thing internally. Given that there have been a number of attempts at making a diesel OB and that there is an obvious need, I have to think that if it was possible to convert a gasoline OB block to diesel it would have been done by now. The only way I see this happening is with a longtail hung over the transom. But, anything is possible if you throw enough money at it. Are you a machinist, have access to a fully equipped machine shop? If not, this is going to get very expensive, very fast........
     
  2. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    I was hoping that a 300hp OB leg would be strong enough to take a 70hp diesel.

    Do long tails work in rough conditions?

    Maybe a more elegant solution would be an extra long catamaran outdrive leg, the 90 deg gearbox is built in already, so all you would have to do is fabricate a frame to hold the VW engine, but limited to only 37hp input.

    http://www.sillette.co.uk/sterndrives_and_saildrives_list.html


    [​IMG][​IMG]
     
  3. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Missed this 'hydro drive 80 unit' that will take 90hp diesel input.
    http://www.sillette.co.uk/documents/Hydrodrives_004.pdf

    Longest leg is 61 cm /24" long: perfect height to make a standard 20" outboard leg height. From memory you lose about 20% power with hydaulic drive.:(

    Easy to mate this straight onto an engine without the need for a gearbox.......

    Ps: is this a hydraulic drive or not...?
     
  4. Westfield 11
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    Westfield 11 Senior Member

    It's not the HP, it's the torque. And mostly it is the shock loads from the harder hit from fewer cylinders and fewer power pulses. Most 300hp OB lower units are designed for the smooth power flow of a 6 cylinder block derived from an automotive design. Replace that power head with one with fewer cylinders, perhaps as few as two and you have a very different situation. There are ways around this: you can add a heavy flywheel to absorb and smooth the power delivery which means an even heavier engine or you can design and engineer some other solution out of cubic dollars.

    You also will need a much bigger prop to put that torque into the water efficiently. Is there room in the lower unit? I am assuming that you will want more of a prop than the dinky folders shown above in your link. Finally, will the internals of the gearbox be up to the torque and the shock loading in the long term, because when you really NEED this engine it better be able to run for hours at a time........
     
  5. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    .......:D:D:D

    Just noticed that hydro drive 80 costs over £22,000 !!!!
     
  6. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

  7. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    This site explains the power train:http://www.boattest.com/engine-review/Seven-Marine/17000068_557_2013

    A very similar arrangement to the VW transverse engine/gearbox drivetrain, except the gear off the crankshaft on the 557 comes out directly under the v8 engine to feed to the 90 deg gearbox, whereas the VW comes out at the back centre.

    Horizontal V8 sitting on top of 90 deg gearbox
    [​IMG]

    This might just work out on a much smaller scale........
     
  8. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Some more thoughts:

    The 557 V8 is a proven concept that works.....

    VW 2ltr diesel Mercruiser marinised engine, with standard VW gearbox with all gears removed except 4th 1/1 gear, sitting on top of Mercruiser alpha 1 upper gearbox as 90deg g'box located centrally under the engine.
    The Merc input drive UV joint will enable the drive line to be connected from the rear 'VW gearbox output shaft' to the 'Merc input upper box' located centrally under the Diesel engine.

    ....a big OB leg to suit diesel power input.:)
     
  9. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Why not convert the engine to run vertically instead of fooling with all this aligning of gears,chain drives,building cases,machining,adding complexity etc?

    Random thoughts:
    -change the oil pickup location to the bottom, and enlarge the pan (build a new one) so oil won't seep into the lower cylinders or perhaps just dry sump it.
    -obviously ditch the factory oil pump/drive and install dual electric oil pumps.
    -run a rail up with spray holes to give the cylinders an oil mist.
    -run a rail to mist lube the OHC on the VW. On a push rod engine this may be the sticking point.
    -be sure the oil can drain out of the cam tower/head. Use an external line if necessary to drain.
    -the injection pump shouldn't care which way it is oriented.
    -you'd need a thrust bearing under the crank to keep the downward force of the crank from doing harm?
    -etc

    I don't think it would be too much of a problem,would be kinda fun in fact.I thought of this years ago as a business, however in the largest market-USA- the EPA stands in the way.

    Of course pull the injectors with electric motor drive testing,inspections,adjustments etc and then fire it. Assume it'll blow up,but then you figure the problem and try again.

    All sorts of engines have been converted to be used in airplanes and stunt planes,which fly upside down for periods of time.


    CDK is the local VW expert....any thoughts?
     
  10. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Dry sumping the engine did occur to me as the parts are easy to obtain from a racing engine specialist, but I wonder how the big ends/crank bearings etc would like being run vertically, and pistons could wear on one side of the cylinder surface. The camshaft bearings would also need support.

    Could be a minefield........

    Just another idea for a 'built in' 90deg gearbox: front wheel drive cars with a transverse engine that have 4 wheel drive have gearbox that has a output shaft that goes to the rear differential.
    It might be possible to rotate that spur gear output through 90deg to point vertically downwards to engage with the OB shaft.

    [​IMG][/URL
     
  11. WestVanHan
    Joined: Aug 2009
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    Location: Vancouver

    WestVanHan Not a Senior Member

    Thats what the thrust bearing is for

    Subaru and Porsche's pistons run horizontally with no problems...and 30 million VW engines did as well.

    Yes I agree..support the cam.

    However,why not just keep the engine normal and have a simple forward/reverse transmission:do away with all this stuff and run a long chain or industrial belt drive right down to the prop shaft?
     
  12. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    More detail of VW 2ltr 4 wheel drive system.

    [​IMG]

    Looks promising: the bevel gear drive to the rear diff looks like it could be rotated by 90deg to point downwards.

    [​IMG]

    [​IMG]
     
  13. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Good point about Subaru and Porsche engines.:)

    Great idea, use one of those big belt drives they use on Harley Davidson bikes instead of a chain. Put the big cam belt pulley onto the engine flywheel, bolt the input shaft of the 90 deg gearbox (Merc alpha ) to the bottom of the sump with a couple of bearings, fit a suitable pulley to the end to get correct gearing from the flywheel.

    ....fantastic idea, simple, light, easy to fabricate, can easily take the power.:)
     
  14. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    Something like this from the Diesel engine flywheel to the Merc input shaft:

    [​IMG]

    Merc input shaft could be bolted under the sump with a simple bearing:

    [​IMG]

    Merc upper gearbox output connects to outboard leg drive shaft......
     

  15. whitepointer23

    whitepointer23 Previous Member

    Probably cheap compared to your diesel outboard.
     
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