Converting an Outboard to Diesel.

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

  1. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Diagram of bevel gearbox arrangement:
    http://workshop-manuals.com/volkswa...tiondiff./dismantling_and_assembling_gearbox/

    Any gearbox experts here? What cogs could be dumped while leaving drive to the bevel gearbox.....
    Diff planetary gears
    Drive shaft flanges
    Output shaft for 5 & 6 the gear and reverse plus reverse shaft

    ...could the cogs off 1-4 gears be dumped while keeping the cogs to transfer power to diff at a 1/1 ratio?



    [​IMG]
     
  2. Joris
    Joined: Mar 2013
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    Joris Junior Member

    I didn't read the whole thread so my apologies if the following is useless information;
    To determine which parts you can take out you're better off with an exploded view of the gearbox.( Or better, take one apart)
    4th gear i believe will be around 0.85 ratio and the diff on VW synchro's is around 4:1 (or more)
    I would make my own bell housing and mount a Jaguar IRS diff. with 3:1 ratio.
    It will save weight, space and money and you can mount it straight to the flywheel or with a belt to change the ratio.

    If you only need 70hp, the 1.4hdi that is used in Peugeot/Citroen might be something?

    Joris
     
  3. rustybarge
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    rustybarge Cheetah 25' Powercat.


    Do they make a 4 wheel drive g'box, as we need to feed power vertically into the outboard shaft at 1/1 ratio with the engine sitting on top of the OB leg, just like in the 557 v8 monster OB.

    I believe in lazy engineering, if somebodies already done it use that system!:D

    ...although a chain or belt drive would be simple to rig up and cut out the gear box completely.


    Just looking at the diagram in post 61, the main diff(7) must be reducing the ratio down to 3/1 or whatever to drive the front wheels, and then the 90 deg bevel box crown wheel (10) is INCREASING the ratio by 3 times back up to 1/1 to power the rear diff.....

    Is this correct or am I crazy?
     
  4. Joris
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    Joris Junior Member

    If you have a 90° drive (a diff for example) and you can adjust the end-ratio, i don't see why you need a complex unit like a 4-drive gearbox?
     
  5. rustybarge
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    rustybarge Cheetah 25' Powercat.

    The OB leg will have a 2.5/1 ratio in the bottom g'box already, so if you feed the engine power through another 90 Deg diff the overall gearing will be 6.5/1.

    So I was hoping to find a 4 WD G'box that has a 1/1 output into the prop shaft/rear diff; just fabricate a frame on top of the leg to hold the engine g'box combination......

    0f course an agricultural 90 deg box used in rotorvators/hay turners/mowers would work, but they tend to be very heavy.

    .....or the top gear case off a Merc outdrive. Is this a 1/1 drive ratio?
     
  6. rustybarge
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    rustybarge Cheetah 25' Powercat.

    The crown wheel on the diff (red)seems to be the same size as the bevel gear (11. red) and the input gear cog at (14 blue) is the same size as the bevel gear (blue) = 1/1 ratio, I think.

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

    I will visit a scrap yard to view likely candidates; can't find any definite info on the i'net........

    Please keep on making suggestions : if 7 marine can do it with 550 hp, we can with 70hp:D
     
  8. Joris
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    Joris Junior Member

    The crown wheel is driven by gear no6 and you only coloured half of the crown wheel, it's diameter is about double that. On VW's it's 4:0 to 4,4:1 reduction.
    If you block the diff the input (blue) bevel gear will be turning at the same rpm as the diff so little gain in the bevel i'm afraid.
     
  9. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Ok, I think the diff unit crown wheel must be directly connected to the bevel gear 10(blue), otherwise when the diff was slipping no drive would go to the 90 deg gearbox to drive the rear wheels....???
     
  10. AndySGray
    Joined: Jun 2014
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    AndySGray Senior Member

    Correct - The differential (7) is purely for the front wheel outputs 9 and 13 allowing one wheel to make more rotations on the outside of the corner while the other slows on the inside of the corner. The rear axle has its own diff to do the same job. In this system there is always a 50:50 power distribution front to rear - fancier systems designed for more off road work feature a central diff (and limited slip).

    The implication is that the box may not have been designed to take more than 50% of the power to the rear output shaft - it may not be strong enough for 100% if the front outputs are not being used? Of course it may be the box was used across a model range and 100% of a lower output motor maybe less than 50% of the largest?

    But yes if you wish to remove 7,9 and 13 and blank off - they are redundant as you propose to use it. If you do this, angle grinder on (9) and cut out the center part of the shaft and weld back in its previous location to effectively turn the hollow shaft supporting gear (10) into a solid one.
    :idea:
     
  11. Joris
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    Joris Junior Member

    Rustybarge,
    when all of this is done, are you planning to put the engine on it's side to get the output pointing downwards? Or are you mounting the gearbox 90° to the original position?
    btw, to block the diff i would recommend welding inside the crown on the side gears and spider pinions to avoid distortion.
     
  12. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    Thanks Andy,
    Never thought of that, the whole internals of the diff unit can go but obviously the crown wheel will need to stay to bring drive down to the 90 deg bevel g' box, and as you say also the left drive shafts, and maybe the RH drive shaft that runs inside the tube: this is beginning to sound more and more do-able.:)

    Ill check what the top of the range VW turbo diesel output, but I guessing it's more than 150hp so hopefully it can cope with 70hp .

    I took a quick look at the specs for a upper Merc outdrive unit; seems like they give lots of trouble with the teeth sheering off, bit as far as I could figure out its a 1/1 ratio or very close to it. I never owned one, or indeed know anyone with one; nearly everyone here in Ireland has shaft drive or OB's.
     
  13. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Yes I was hoping to keep the engine vertical, and just swivel the 90 deg g'box into the vertical position to connect directly to the outboard vertical shaft. It would be easy to fabricate an adaptor plate if the mounting isn't exactly square.

    Unfortunately as you probably realise the engine will be slightly offset to one side by a few inches, this should pose no problem in reality; but it should be balanced nearly perfectly 'fore' and 'aft'.......

    I was just thinking, the engine could be mounted on the leg longitudinally or transversely, the latter would be in perfect balance in every axis.....

    [​IMG]



    PS: tilting the engine slightly over to one side is a good idea, as this would bring the balance back over the centre point of the bevel g'box if it was a longitudinal setup.
     
  14. rustybarge
    Joined: Oct 2013
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    Location: Ireland

    rustybarge Cheetah 25' Powercat.

    For a proof of concept test can anyone recommend the biggest strongest outboard leg which could be bought very cheaply; I know that the big monster OB 's are only a recent innovation, but what would be suitable from say 10 years ago that has cheap parts to recondition the tilt and steer rams, and cheap props that are readily available.

    I idea is to buy an old engine/ gearbox that still runs, but may not be in rude health, and an old OB leg, weld up a mounting frame and see if this concept will work before spending mega bucks on a marinised version! The engine could run on the standard radiator for test purposes.

    So the most import thing is availability of cheap parts.......
     

  15. gonzo
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    gonzo Senior Member

    You would be better off getting the upper unit of any type of sterndrive rather than getting automotive parts and trying to modify them. They are already made for the purpose.
     
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