New comer fitting small Diesel engine

Discussion in 'Inboards' started by Michael and A, Mar 8, 2013.

  1. Michael and A
    Joined: Mar 2013
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    Michael and A Junior Member

    Hi I have an old 26 ft Caporn cruiser, it's engines (twin 115 pentas seized years ago, and it's been just a holiday house on the water ever since.
    I am now retired and will be moving the boat closer to home in a couple of months.
    Anyway my idea is to replace the two engines with a much smaller single diesel engine and run it as a displacement vessel.
    My main question is if I sit the engine flat to the bottom of the boat it fits really well and I don't have to change the decks. But of course the prop shaft has to angle down. Mechanically a universal joint should handle this but what sort of experiences have people had with this. It seems most engines are lined up to transfer power in a straight line there must be a good reason for this.
    The engine I plan to fit is 30 hp.
    Thanks in anticipation for your feed back
    Michael
     
  2. Landlubber
    Joined: Jun 2007
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    Landlubber Senior Member

    Michael,
    Yes the old Caporn will do OK as a displacement cruiser, no worries.
    The engines (and gearboxes) on boats are coupled together in a straight line, sometimes there is a downset on the box of 10 degrees, but basically, the box has a thrust bearing, it is what allows the prop thrust to be absorbed in the boat, so you will need a plumbers block type of thrust bearing installed if you wish to drive the shaft with a uni.
    The hull speed of a 26 foot boat is only low, so 30hp will achieve that. (approx 1.3 x the LWL)
     
  3. Michael and A
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    Michael and A Junior Member

    Thanks for your feedback. It is all welcome and somewhat reassuring, I did the calculations on the displacement hull speed and it is fast enough for me.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Such a project should start with taking measurements and making a drawing.
    Going from stern drives to a prop shaft means relocating the engine forward, making an engine bed and deciding which prop shaft angle and how to steer. You may not have space for a rigid setup, in which case you can use a double universal joint and thrust bearing.
    Read my thread "DIY tunnel drives" on this forum. There are construction details for a stern tube with integrated thrust bearings that may be useful.
     
  5. whitepointer23

    whitepointer23 Previous Member

    Are the volvos shaft drive or has it got legs
     
  6. Michael and A
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    Michael and A Junior Member

    The Caporn has twin shafts. But I noticed when I last anti fouled there seems to be provision for a single centre shaft. There is a plug where a shaft would extend from. I need to investigate it further. The boat comes out of the water at the end of the month. I will post pics. Very happy to get advice
     
  7. jonr
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    jonr Senior Member

    I would look into continuing to use the existing drives with your small engine.
     
  8. whitepointer23

    whitepointer23 Previous Member

    I agree with jonr. I would fit a pair of 15 or 20 horse diesels using the existing shafts. That way you don't need to alter steering or engine bay plus the advantage of twin screw handling at the jetty.
     
  9. jonr
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    jonr Senior Member

    Or even one engine coupled to both shafts with a chain drive.
     
  10. Michael and A
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    Michael and A Junior Member

    The idea of twin engines is of course appealing but I lose the large deck I am looking for as the engine positions is where the old ones were. Covered by cowls. But I do like the idea of driving both off chains from a central engine. At least all the bearings and shafts and rudders are in place. Has anyone got pics of this set up? I guess as there are forces now coming from different directions some considerable beefing up needs to be done?
     
  11. whitepointer23

    whitepointer23 Previous Member

    If you run a chain or belt drive you lose your counter rotating props unless there is a gearbox on each shaft. Twin small diesels will work a lot better. The engine boxs don't take up a lot of space and they make good seats or tables.
     
  12. CDK
    Joined: Aug 2007
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    CDK retired engineer

    So why is this conversation taking place in the stern drive section?
     
  13. Michael and A
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    Michael and A Junior Member

    Hi CDK. This is the first time I have been on this forum I didn't realise I had posted on the stern drive section.
     
  14. Michael and A
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    Michael and A Junior Member

    I have had a quick look at th idea of running two prop shafts from one engine. A notched v belt can apparantly be run in a figure eight pattern therefore reversing the direction of rotation. Has anyone tried this?
     

  15. whitepointer23

    whitepointer23 Previous Member

    It will reverse but with a fairly short belt it will wear out quicker i think
     
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