Davit SWL (Safe working Load)

Discussion in 'Class Societies' started by athvas, Aug 1, 2015.

  1. athvas
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    athvas Senior Member

    Hi

    i have a issue in specifying the SWL of the Davit to give in purchase specification to vendors. The Boat weight is 2.5T and the Pay load is 1.2T. What Min SWL value to be specified in the specification on purchase of Davit.

    Reply with explanation will be good

    Thanks
    athvas
     
  2. jehardiman
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    jehardiman Senior Member

    125% of the boat+payload+entrained water * G-load calculated by the class society at the location of the davit (or use MIL-STD 1399 Section 301a and at least SS5).

    Really, you can't make them too stout.
     
    Last edited: Aug 1, 2015
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  3. AndySGray
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    AndySGray Senior Member

    Absolutely - another way to calculate would be to look at impulsive tensions - the momentary shock they would experience if a wave lifted the tender and then dropped it until the cables became taut and it's fall would stop suddenly with a jolt - that is hard on the davits, the cables, tender mountings and the CREW.

    :!:
     
  4. gonzo
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    gonzo Senior Member

    Another consideration is whether the boat can get full of water.
     
  5. TANSL
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    TANSL Senior Member

    Who calculates a davit to support a boat full of water ?. No I had never seen. Although, of course, it is a possibility.
     
  6. gonzo
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    gonzo Senior Member

    Somebody that has fished the North Atlantic
     
  7. athvas
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    athvas Senior Member

    Is 3.7 T SWL is right value to specify (Boat weight + payload). or any value higher than 3.7 T to be specified ?
     
  8. TANSL
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    TANSL Senior Member

    In addition to the total weight, which can include water filling the boat (although it seems to me nonsense), you must take into account the acceleration produced by the movement of the boat, which can make 4 or 5 times the total weight. The CS give instructions about it and maybe someone who fish in the North Atlantic can give you more information.
     
  9. daiquiri
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    daiquiri Engineering and Design

    Davit for what purpose? Is it a lifeboat or a tender?
     
  10. hoytedow
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    hoytedow Wood Butcher

    Would there be boats full of water on davits? Absolutely! Saw it here in Hudson just a few days ago, with boat owner in boat bailing with a bucket. Same day another boat with failed automatic bilge pump sank due to the extremely heavy rains we have been experiencing. Build them strong.
     
  11. TANSL
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    TANSL Senior Member

    Can a ship be completely filled with water ?. Absolutely. The structure of a ship, are calculated for when fully filled with water ?. Of course not
     
  12. daiquiri
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    daiquiri Engineering and Design

    Does this mean that the boat will be hoisted with the payload on board?
    Does the payload comprise persons too?
     
  13. TANSL
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    TANSL Senior Member

    It depends of course on whether the boat is hauled down with people inside or not. You also have to consider under what conditions the lifting of the rescue boat should be done, if this is the case.
    Only athvas knows for what purpose he should design his davit.
     
  14. daiquiri
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    daiquiri Engineering and Design

    In fact my question was, quite obviously, for Athvas.
     

  15. Barry
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    Barry Senior Member

    As so frequently happens, a thread is started with insufficient information to begin to help the OP

    We do not know if the davit is attached to a boat, a wharf, carries people ( which would affect the factor of safety), specifications of the lifting boat to determine rate of roll characteristics to determine loading due to acceleration, and Tansl, the davits would have to include the possibility of the extra weight due to water ingress unless the lifted boat has self bailing scuppers or is covered, rain fills boats ( of course perhaps the boat owner might rely on bilge pumps, which can fail, to mitigate this aspect.


    So we can agree that there are quite a few calculations required before one can DETERMINE tension the 3.7 ton loaded boat can put on the lines considering the above issues.

    BUT I think that we are putting the cart before the horse.

    The Safe Working Load is 3.7 tons. The specification to the vendor should be 3.7 tons, on a ship with certain characteristics that will or will not carry people.

    Ie the factor of safety is determined BEFORE the safe working load is determined.

    And you want the designer/engineer of the davit to do the calculations to arrive at a factor of safety.

    If they are a company that builds davits for boats, they will have the people employed to calculate the FS, factor of safety

    Your really want them to take the liability for the calculations.

    Alternatively, if you are trying to determine a factor of safety to lift the 3.7 tons by asking the contributors of the forum, you have not supplied enough information in the post to get an accurate result.

    If you are a purchasing agent providing max working loads due to all of the issues the contributors have recognized , this is indeed a tenuous position.

    My recommendation is this
    1) hire a competent naval architect firm that can do the calculations ; OR
    2) word the vendor specification " supply a davit, that can support a 3.7 ton safe working load in
    expected sea states" ( and let them do the calculations)

    Really what you are wanting is a Safe Working Load of 3.7 tons for the conditions that the lifted boat will experience. The design firm will supply the appropriate factor of safety above this load


    You will almost have to go with twin davits as with the possibility of uneven distribution of loading, twin davits will be easier to position the boat properly
     
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