new boater in need of assistance

Discussion in 'Boatbuilding' started by bigfboater, Aug 26, 2013.

  1. bigfboater
    Joined: Aug 2013
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    bigfboater Junior Member

    hey just wondering about changing from inboard to out board..... I have fiber glassed the hole as well as filled in the hole with wood.... I just purchased a 60 hp Johnson but needs a bit of work like plugs, starter and so but what I would like to know is with the transom hold up or should I also put a plate across the inside or outside or both, all which has been recommended to me but I figured to join and gets the real facts as I am unable to find this info online except here....thanks for any help... :D
     
  2. FMS
    Joined: Jul 2011
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    FMS Senior Member

    Inboard or sterndrive? An inboard has a shaft which exits from the bottom of the hull. A strut holds the shaft in front of the propeller. A sterndrive has a leg that fits on a transom assembly over a large hole in the transom though which a power shaft, incoming water, and exhaust pass. An inboard needs the least transom strength. A sterndrive requires more, and an outboard the most.
    What are the dimensions of your transom?
    Did you epoxy in the infill? And did you angle the edges of the hole?
    For additional strength, transom knees to transmit loads from up on the transom to the stringers are commonly added on outboard boats.
     
  3. bigfboater
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    bigfboater Junior Member

    thanks for the info I did angle the edges before glass but didn't use fill when you say knees what are these referring to? the dimensions are 17" from bottom of boat to the top of the transom .... had to cut it down the thickness is approx. 2.5".... and will the plate work or would you suggest the knees?
     
  4. FMS
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    FMS Senior Member

    What type and weight of fiberglass did you apply?

    Transom knees are angled braces that form a triangle from up on the transom down to the stringers. The load the outboard places in the center and up high on the transom is partially carried by the transom knees and through these the stringers.
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    60hp sounds underpowered if it is replacing an inboard. Unless it is a displacement hull.
     
  6. bigfboater
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    bigfboater Junior Member

    just used the fiber glass mat and resin with hardener from NAPA uncertain of weight but am working on getting pictures to help a little better...
     
  7. bigfboater
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    bigfboater Junior Member

    mt friend and I have almost identical boats and he has a 70hp and I figured 60 hp would be enough ....... guess I should be looking for a bigger motor?
     
  8. FMS
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    FMS Senior Member

    Post details and photos of the boat.
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A 60 HP outboard will need a minimum of a 1.5" thick transom. This is usually done with two pieces of 3/4" plywood, glued together and bonded to the inside of the transom 'glass, as well as tabbed to the hull shell all around it's perimeter.
     
  10. RodUK
    Joined: Aug 2013
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    RodUK New Member

    Plate it.

    Hi
    as a new member myself I reckon your question needs answering by as many opinions as you can get.
    I have a flecher 16 speedboat project which i bought for about a grand 2 years ago.. It is fitted with a 115 mercury which is quite heavy .. we used a forklift to remove it.. The boat has an alloy U shaped plate over most of the transome which is obviously to strengthen it for the motor.. the 115 is the maximum reccommended for the boat. I am not sure of the size of your boat but, if theres any question in your mind plate it.... you dont want to be in the middle of the sea or a lake with a boat full and have your motor dissapear into 30 feet of water.

    The plate wont be the end of it as I can see on my project that it has cracks in the corners outside the plate but I cant see the motor falling off with the plate on.

    Good luck
    RB
     

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

    Using angle stock or plates creates "point loading" which causes localized stresses. These will eventually cause failures at contact points and around contact points, which the cracks you see likely are. This is why we use a cored assembly in a transom, that is tabbed all around it's perimeter to the hull shell. You have core choices, but with the exception of metal boats, where similar materials are desirable, 'glass and wooden boats use plywood or a structural foam or honeycomb core.

    It's not a matter of opinions, so much as understanding the engineering behind the usual choices. This isn't to say you could develop your own structure, but is to say, this is a subject that's well understood and addressed a few different ways. Lastly, if you pulled into my yard with a hunk of angle stock on your transom as a reinforcement, I'd likely not want to touch the project, as this is a band aid approach (suggesting other band aid approach) and doesn't address the issues associated with load transmission, from the engine bracket to the hull shell and bottom, which is where they need to end up ultimately.
     
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