Need motor mounting advice with low transom.

Discussion in 'Boat Design' started by suprathepeg, Oct 12, 2015.

  1. suprathepeg
    Joined: Jul 2015
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    Location: Winnipeg, Canada

    suprathepeg Junior Member

    I'm rebuilding an older 3 point boat and it has a really low transom 8-3/4" [​IMG]

    Looking online standard short shaft transom height is 15". I'm currently hunting for a short shaft 50 merc 2 stroke to power this thing but they aren't really common, I may need to use a long shaft.

    I'm thinking that I should add a jack-plate to lift the motor up enough to get the motor to sit at proper height or raise the transom? Whatever it do will need to be pretty light. I'm assuming that I will need to brace to top of the transom/jack-plate back to the boat somehow since it will be so high off the boat.

    I could easily build something like this:

    [​IMG]

    But how beafy will it need to be to handle 50hp? If I use aluminum box tube how thick and what dimension tube should I use?

    [​IMG]

    I could build something like this:

    but would mounting the motor an additional

    Can anyone shine some light on this or give some advice?
     
  2. boatenthusiast
    Joined: Jul 2015
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    Location: canada

    boatenthusiast Junior Member

    looks like your motor got a hole in it
     
  3. suprathepeg
    Joined: Jul 2015
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    Location: Winnipeg, Canada

    suprathepeg Junior Member

    Thats not my motor, its just a pic from google showing a jackplate.
     
  4. boatenthusiast
    Joined: Jul 2015
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    Location: canada

    boatenthusiast Junior Member

    sorry should have read the thread better
     
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Typically, these types of designs were intended to run outboards of 120 pounds or less, generally in class racing. You're looking to install a 250 pound outboard, so how much buoyancy do you think that little hull can support? These boats have a point of no return, in terms of longitudinal stability. Meaning once at these speeds, there's no recovery when the hull is even slightly upset, which is extremely easily done, in anything other than glass smooth water. All you have to do is watch some boat drag racing on TV and you'll quickly see what happens, when these points are reached. The results aren't pleasant and usually quite destructive, both to boat and it's skipper. The biggest you should consider is a 20 HP engine and this will push you into holy grail speed ranges anyway, without sinking the transom top, below the LWL.
     

  6. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Full on race engines have very short shafts maybe 8.75 from clamp base to cavitation plate. There are numerous reasons for having so short a shaft. Back in the day some engines, most notably Mercury, had ultra short shaft accessory kits. Those mods may still be available for newer model motors. The short shaft layout also included a nicely streamlined gear housing. (Quicksilver goodies cost a plenty however)

    Short shaft stock engines are about 15.5 +/- inches from base to plate.

    Heed Pars admonition. Weight on the transom is a critical matter for such a boat.
     
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