Catamaran Bridgedeck structural design

Discussion in 'Multihulls' started by APP, Oct 3, 2011.

  1. APP
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    APP Junior Member

    Catamaran Bridgedeck structural design

    The Elements of Boat Strength (Dave Gerr) gives a variety of elements for the scantlings of an aluminum hull. However I cannot find anything about the Aluminum Catamaran Bridgedeck structure as dimensions of transverse and longitudinal frames or stiffeners (spacing, plate thickness, etc.) on the basis of Bridge deck dimensions (length and width, Center Hulls distance, clearance ), weight supported and other input data. Further, it seems there are two main methods. One is the integral one frame 2Hulls+Bridgedeck and second the independently built Bridgedeck.

    How do you calculate the ALU BridgeDeck for small Cats up to 14 m? If you know, you may give an example of an existing ALU Catamaran, if possible.

    Thanks and Regards
    APP
     
  2. Alik
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    Alik Senior Member

    I would throw away the Elements and take HSC Rules of German Lloyd... :)
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Agree.

    I would thow into the mix DNV HSLC rules ( very easy) or the sligthly more complex LR rules.
     
  4. Alik
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    Alik Senior Member

    What I don't like about DNV HSLC and LR SSC - they define min thickness/laminate for sandwich bottoms that is not reasonable for smaller boats, say below 24m. In this terms, GL HSC is more flexible.

    But it does not matter for aluminium.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    I can't recall for LR (been a while since i used their composite rules) but for DNV there is no minimum thickness. Just a miniumum weight of reinforcement.

    The values are not excessive when compared to older existings vessels.
     
  6. Alik
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    Alik Senior Member

    Yes, correct, in DNV there is weight of reinforcement but it is directly related to thickness. Meanwhile, for carbon it is about half of requirement for glass.

    For boats below 24m this lamination in glass seems excessive to me, GL gives much smaller values.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Only in the sense that it is related to the material properties of your layup. Which defines the allowable stress limits.

    Thus you can select a low end to a high-end material, choice is yours.
     
  8. Alik
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    Alik Senior Member


    In DNV Rules, Table A2 specifies minimum requirements for amount of reinforcement; for glass there is no dependence on specs of material and requirement is 2400g/m2 for outside skin of bottom, for craft L<20m. With ISO12215, one can get compliance with much lighter laminates. Say, for 18m cat designed for 27kts 1700g/m2 of glass would be sufficient.
     
  9. APP
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    APP Junior Member

    Thanks for the suggestions.

    If someone has design data/drawings of an aluminum Cat up to 14 m, I would like to know (as example) the ship general data and for the Bridgedeck a) The spacing or span of the transverse frames and logitudinal girders, b) the Plate Height and Thickness of the transverse deck frames and dimensions of the longitudinal Deck girders, if possible.
    Here is a pictures with the frames of an errected Cat.

    Best Regards
    APP
     

    Attached Files:

  10. Ad Hoc
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    Ad Hoc Naval Architect

    If you use any of the rules cited above, then the answers shall "pop" out from the page. :p

    Assuming you know how to "design it" and then the number crunching from the rules to arrive at the scantlings.
     
  11. APP
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    APP Junior Member

    Right, I think the rules are for professionals, I just wanted to have an amateur's feeling on the bridgedeck dimensioning for small Alu Cats (e.g. 10m). I am hoping someone cam post an example, if he has dealt with aluminum Cats. That would be good, I think, for discussions among professionals as well or not?

    Thanks again
    APP

    PS: Is there any software doing this Job? (Bridgedeck structure calculations)
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Care to define what you mean by “professional”?

    The rules can be used by anyone with a basic grasp of maths. From the basics of your boats, displacement, speed shape etc etc which provides the loads for your boat you select your frame spacing and stiffener spacing, you plug in the numbers into the formulae given and out pops the answer. If it isn’t what you want, you change your variables. It isn’t rocket science.

    Since you’re requesting software for this job, doing it by hand, as noted above, is exactly the same, just takes longer that’s all. You still need to do exactly the same.

    Being an amateur or professional is irrelevant. There are not 2 sets of rules, one for professionals and one for amateurs!
     
  13. APP
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    APP Junior Member


    OK. Thanks. Encouraging.
    From GL Rules I have isolated the Structures Part for High Speed Craft {under: GL: I Ship Technology, 3 Special Craft Edition 2002 I - Part 3 Section 3.(see the attached PDF). Then, I have to find the corresponding Aluminum Bridgedeck calculations inside these GL Rules, is that OK? Thereafter, I shall also check with DNV corresponding rules. Maybe someone has already collected the right formulas, etc. from such Rules and has written an Excel Sheet.

    In the attached picture I am showing the dimensions I search to calculate, which is an independent Catamaran Aluminum Bridgedeck (to weld eventually to the Hulls, if this practice can be acceptable).

    Regards
    APP
     

    Attached Files:

  14. Ad Hoc
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    Ad Hoc Naval Architect

    APP

    When working out the scantlings, you need to address only several issues.

    1) Global strength.
    So this relates to the thickness of the main deck and the under side of raft (USR) plate thickness and of course the distance of the two plates being separated. Since that provides the stiffness.

    So that is C.3.4.2.3 and C3.4.2.4

    Once you have worked out the stress from the given loads and your selected plate thicknesses and distance apart of the 2 plates, you need to do a buckling check, to ensure the plates don’t buckle. But you can’t do this until you have finalised the frames spacing and long.t spacing.

    2. USR plate.
    C.3.5.4…this gives you the pressure to establish what load to apply for the min plate thickness. Which is based upon your frame spacing and long.t spacing. If you have a frame spacing of 200mm and a long.t spacing of 50mm, this will yield VERY different results from a frame spacing of 2.00m and long.t spacing of 500mm. Since the area of support is greater thus the thickness increases!

    3. Main deck plate.
    This is in C.3.5.8.1 & C.3.5.8.2. Again as above once you work out the pressure and apply the pressure deepening upon your frame and stiffener spacing will yield different results. Of plate thickness and of course the frame & long.t stiffener modulii.

    Once you have done #2 and #3, go back and workout the buckling strength of that panel size with the given global loads worked out in #1.


    I don’t use GL rules, I use DNV 80% of the time and LR 20%. But the procedure is exactly the same.
     

  15. APP
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    APP Junior Member

    Hi,

    Extremely useful indeed! Can you give us please the equivalent articles of DNV to use as you did above with GL?

    Thanks again
    APP
     
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