Transom scantlings

Discussion in 'Boat Design' started by meren, Sep 13, 2014.

  1. meren
    Joined: Sep 2005
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    Location: Northern shore of The Baltic Sea

    meren Junior Member

    What are forces to be considered when highpowered outboard powerboat in question? Is it just to find out maximal trust and vertical acceleration and are there some fine rules for transom constructions?
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The thrust the engine is capable of producing is the maximum thrust the transom will have. However, remember that the leg is a lever between the bracket and the propeller shaft. Vertical acceleration will depend on the type of hull and maximum speed.
     
  3. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    Transom is formally calculated as bottom (lower part) and side of the boat. Given thickness required for engine installation, it will be strong enough to hold the engine's weight (with acceleration) and thrust. ISO12215-6 specified thickness of plywood core and skins for transoms depending on installed power, for engines <100kW.

    Usually we do not calculate transom for engine loads - this might be challenging and will require FEA including modelling of all small details and fillets, and simply not worth the efforts. We base transom design on ISO standard requirements or experience. Load transfer is important - we usually fill bottom/transom and side/transom joints with foam blocks of triangle section, for better joints and loads.
     
  4. meren
    Joined: Sep 2005
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    Location: Northern shore of The Baltic Sea

    meren Junior Member

    So You have some 3000 N outboard hanging from transom and speed up until airborned. Then slamming effect and your back and kidneys says...Autch! What does transom feel at this moment? 10 g's or something plus trust. In addition to possible deceleration effect of hogging wave... To me it's vertical accelerations which are The transom killer as well as in case of injuries during high speed boating.
     
  5. Alik
    Joined: Jul 2003
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    Location: Thailand

    Alik Senior Member

    10g is very high load; I would day 6-8g of vertical acceleration is upper limit of structural design for RIBs.

    If You want to design transom using load approach, take collision load of 12g forward direction, as well. This is design case under HSC Code and acts perpendicular to the transom.

    And again, due to the fact that transom is relatively thick to its size, it would not be easy to reliably model it using FEA approaches for composite. There are many geometry factors involved which are not easily modeled.
     

  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    I figure if I hit something at speed, I want to wring the powerhead off the bracket, not tear the transom out of the boat. Imagine collecting a crab pot warp in the prop about five feet from the crab pot. Suddenly, an 80# crab pot explodes from the water travelling straight up and pulls the motor up behind it. All of the energy of the boat has been transferred to the crab pot. Once the motor kicks up, the crab pot becomes a wrecking ball and swings forward into the boat. Design for that. The crab pot isn't going to be the point of failure. You can choose between the boat and the motor.
     
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