Deck mounted crane

Discussion in 'Stability' started by Chris MacDonnell, Jan 16, 2020.

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  1. Chris MacDonnell
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    Chris MacDonnell New Member

    I am looking to purchase a lobster style boat and add a crane to do "light" such as setting moorings or installing docks.

    My question is about sizing a boat to accept the moment loads a crane would impose on the boat.

    I have specs for a knuckle boom crane that will suit my needs. Max lift moment is 13.6kNm (10,020 ftlbs).

    What are the main consideration for sizing the boat? Is mounting at the stern out of the question?

    Thanks Chris
     
  2. TANSL
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    TANSL Senior Member

    It would be necessary to know the body lines plan of the ship, current weight and the loading conditions of the ship, crane weight and weights that must be lifted with it as well as to what height, among other things, to give a reliable response. You should also check that the deck, or the area that will support the crane, is sufficiently reinforced.
     
  3. Chris MacDonnell
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    Chris MacDonnell New Member

    Let me ask this another way. I am looking at a 31ft BHM lobster boat. Weight is ~15k lb and has a beam of 11'-6". I don't have any lines of the boat and am not looking for a 2 decimal point analysis, I just want to know an order of magnitude of what a boat this size can handle . Considering a crane with max moment of (13.6kNm/10,020 ftlbs) mounted midship on the keel.
     
  4. gonzo
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    gonzo Senior Member

    Without lines and weights, it is not really possible to do a stability calculation. However, if you are doing mooring work, the crane boom will need to be at least 7'. For the maximum moment you propose, that will be a maximum load of 1431 lbs. That is really low, unless you are planning only working on very small moorings.
     
  5. bajansailor
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    bajansailor Marine Surveyor

  6. Ad Hoc
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    Ad Hoc Naval Architect

    Safety!

    Then there must be a stability booklet for the vessel....yes?...if not.
    Don't buy it...because you'll need to get one, and that will cost you lots if you have, as you note, no data.
     
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  7. Chris MacDonnell
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    Chris MacDonnell New Member

    Yes in fact we may look at that particular one if we can get up to MDI.
     
  8. Chris MacDonnell
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    Chris MacDonnell New Member

    Is a stability booklet something typical for a 30ft fishing boat?

    I have not selected a boat yet. I want to make sure that size the boat correctly for the type of crane I want to install. I am a civil engineer but have no experience in nautical design so I just want to get some order of magnitude idea of moment a 30 ft LOA 11.5 beam 15k lb boat can take.
     
  9. bajansailor
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    bajansailor Marine Surveyor

    Here is an excellent introduction to fishing vessel stability.
    https://www.dco.uscg.mil/Portals/9/DCO Documents/5p/CG-5PC/CG-CVC/CVC3/references/Stability_Reference_Guide.pdf
    On page 39 there is a brief mention re lifting weights over the side.
    I don't know if 30' fishing vessels are required to have stability booklets in the USA - Gonzo, do you know?

    Chris, will you be using your boat for fishing as well, or just as a general purpose workboat for laying moorings and such?
    If the latter, then a catamaran might be more suitable for the task (much better stability generally than a monohull)?
    You could even have a 'moon pool' in the deck between the hulls with a gantry crane to avoid having to lift over the side?
     
  10. gonzo
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    gonzo Senior Member

    For mooring work, a stern mounted crane is usually used in smaller boats.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    As noted by bajansailor:

    However, even if it is not required by USCG, you need peace of mind. They only way is to have a stability book for that vessel.
     
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  12. TANSL
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    TANSL Senior Member

    Although there was a stability booklet, it is very likely that the leight weight has changed as well as the position of its CoG (occurs on all ships) and, therefore, it would be necessary to estimate the current center of gravity of the boat.
     
  13. gonzo
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    gonzo Senior Member

    The USCG does not require a stability book. However, to lift and move moorings from the side, I can't see a boat that size being able to do it. There are two usual solutions. One is to have the crane at the stern because the longitudinal stability is greater. The other is a barge with a hole in the center so there is minimal moment.
     
  14. TANSL
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    TANSL Senior Member

    The moment produced by the crane does not depend on its position on the ship but on the amount and position of the weight to be lifted, as well as the height of the end of the boom
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Any crane has a maximum moment, that is very obvious, whether on a ship or lorry or on jetty or anywhere...the moment is the same.
    Where a crane is placed on a ship however, does greatly influence the response of the ship from that applied moment.
    It is easy for those unfamiliar to be confused by the two very separate issues.
     
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