outboard conversion

Discussion in 'Boat Design' started by ChrisUK, Nov 29, 2014.

  1. ChrisUK
    Joined: Nov 2014
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    Location: south shields, uk

    ChrisUK Junior Member

    Good day group,

    I am looking to convert my matelot 20 hull from an inboard to outboard configuration. The hull is in excellent condition. My concerns are:

    will I have to strengthen the transom in any way?
    How would be best to mount a 20-25hp engine?
    If I had to cut away part of the upper transom, would this be a problem in regards to the strength of the boat?

    Thank you so much in advance for your help.
     
  2. Easy Rider
    Joined: Oct 2009
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    Easy Rider Senior Member

    Yes .. Cut down the transom and beef it up some perhaps w an intrigal self bailing well ... small IMO.

    But consideration should be given to the change in the boat's gentler of gravity. Many inboard engines are quite heavy so changing to OB power may ruin the boat balance wise. Don't know the boat or it's original engine so ???
     
  3. ChrisUK
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    ChrisUK Junior Member

    Hi, thanks very much for your reply! I have posted pics of the boat and the transom for reference. It had an inboard in it which weighed around 250KG, as the bilges are now clear, I was planning on ballasting down once I have completed the conversion. My main concern was transom strength as outboard conversion is not something I have any experience of.

    Regards, Chris.
     

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  4. philSweet
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    philSweet Senior Member

    Yours looks like a pretty good candidate for a conversion. I'd build a well just forward of the transom and use a long shaft motor. The leg would be protected by the keel. It can be designed to kick right up through a keyhole slot in the transom to keep the motor clear when not in use and to access the prop. If you plan on keeping the rudder, you don't need the kickup keyhole, but lifting a Bigfoot fourstroke that size is a pain. A bracket mount in the well can work though it makes the box a lot bigger. You will want to detail the trailing edge of the keel to a high degree to work properly with the smaller prop. Meaning a nice, long keel taper down to nearly nothing.
     
  5. rasorinc
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    rasorinc Senior Member

    I would not touch the outside of the transom or the height. It is to nice looking. Beef it up from the inside with say 2 layers of 1/2" ply laminated together
    and bolted threw recessed heads on the outside. Then mount a metal bracket to the transom for the engine. Or go with making a box mount forward of the transom as suggest above (the least work) The transom really adds to the looks of the boat . I would go with Phil's box plan if it was MY BOAT. NO NEED TO TOUCH THE TRANSOM...........
    Here is a sample of the box mount http://www.glen-l.com/designs/workboat/sweetcaroline-notes.html
     
  6. ChrisUK
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    ChrisUK Junior Member

    Thank you guys, that is excellent advice! So just to be clear, the transom would remain exactly the same. The ply reinforcement would be simply bolted to the transom then a box built in forward of the transom with a well in the centre in which the outboard would sit when lifted out of the water? And the outboard mounting and clamps would be bolted directly to the transom?

    Thanks again for your help guys!
     
  7. rasorinc
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    rasorinc Senior Member

  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    An outboard could be put in a well, but if it's very close to the transom, it'll require a cutout of some sort, so the engine can be tilted up. I think this boat deserves to keep her transom, so maybe a slide up type of engine install is in order or maybe positioning the outboard further forward so she can tilt without the cutout in the transom, similar to the link Stan has posted. The engine box you currently have is a lot bigger then necessary for this arrangement, though some fiddling will be necessary to see what size works.
     
  9. whitepointer23

    whitepointer23 Previous Member

    definitely a well for aesthetics and to keep the prop in the water. a 9.9 four stroke would be ideal , 25 hp is overkill for a small disp hull.
     
  10. viking north
    Joined: Dec 2010
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    viking north VINLAND

    This old style lapstrake wooden hull reminds me of our make and break powered skiffs we used in the fishery pre out board motor days. If so you possibly have a stern knee fastened to the keelson situated above and extending roughly from the just before the end of the keel running aft and up the transom (stern) This structural member plus the transom (stern) itself provide necessary framing strength and attachment points for the individual lapstrake planks. Any cutting or alterations here could weaken this area to the point where you might never have the framing integrity to properly maintain the hulls aft section strength and water tightness. As previously mentioned the ideal conversion would be a nice sail drive outboard situated on rails in a well, however I would recommend having a traditional wooden boat builder have a look before any alterations are made. On such a beautiful traditional looking inboard set up it almost seems a sacrilege to clamp on an outboard . Having said that outboards located in wells do hide that clash in cultures. :D

    A yacht is not defined by the vessel but by the care and love of her owner---
     
  11. ChrisUK
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    ChrisUK Junior Member

    Hi Viking, thank you very much for your input. The hull is actually constructed of GRP, she does have a great traditional and genuine wood look about her though!
     
  12. ChrisUK
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    ChrisUK Junior Member

    I believe if I went for Stan's design, I would have to cut out the bottom of the boat aft and put kind of an inboard outboard system in place? like the small pebble cobbles here in the UK.

    On engine power, I use the boat in coastal waters, for pleasure fishing and working a few creels for crab and lobsters. Would a 9.9hp be sufficient power? the boat is 20.4ft LOA and 18ft LWL.

    Thank you guys, the advice is top notch!
     
  13. philSweet
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    philSweet Senior Member

    So we all agree it is a very handsome craft and it is worth a little extra effort to keep the looks of the thing. I'd really not want to mess with the transom either, if it could avoided.

    1. Where do you run this boat- Inland, coastal, or off shore?
    2. What do you use it for and can you accept a box big enough to tilt up a motor like in the Sweet Caroline plans?
    3. Are you keeping the rudder or not?
    4. What is the hull made of? I was assuming 'glass.
     
  14. ChrisUK
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    ChrisUK Junior Member

    Hi Phil, to answer your questions:

    1. The boat is used in coastal waters
    2. I use the boat for angling and fishing with a few lobster pots, space isn't too much of an issue, but id not like to cut a well through the bottom of the boat.
    3. The rudder is something I could keep or remove, im not quite sure what would be the best option?
    4. The hull is made of fibreglass.

    Thank you guys!
     

  15. ChrisUK
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    ChrisUK Junior Member

    This is the kind of design I was looking to install initially, but hopefully with not so deep a cut into the transom.
     

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