Connecting the demi hulls

Discussion in 'Multihulls' started by pavel915, Mar 22, 2023.

  1. pavel915
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    pavel915 Senior Member

    I usually design and build steel ships and new in wooden boats.
    I work with monohulls but now I am very much interested in multihulls.
    I need to know the structural details for connecting the demi hulls. How the connecting beams should be joined with the deck and frame of the demi hull? Are there any rule for the scantlings?
    Any structural drawing and good pictures will be helpful.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Hi Pavel,

    You should read "Essentials of Catamaran Structure" in Proboat magazine, dec/jan 2020, issue 182.
    That'll give you pretty much what you seek. The rest.. is just basic structural design.
     
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  3. pavel915
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    pavel915 Senior Member

    @Ad Hoc Thanks a lot for the nice reply.
    I will try to collect that paper.
    I think I have lot more to know from you on this regard.
     
  4. oldmulti
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    oldmulti Senior Member

  5. pavel915
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    pavel915 Senior Member

    @oldmulti thank you very much for the link.
    Are there any document which is particularly on wooden catamarans?
     
  6. oldmulti
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    oldmulti Senior Member

    Pavel915. Catamarans in wood range from older 21 foot designs with solid timber 50 x 100 mm beams to 80 foot long with deep I beams with timber flanges top and bottom and supporting infrastructure in between. It really depends on the function of the catamaran and designers preferred style. Adhoc is a good designer and has his approach, Wharram catamarans have there style of cross beams and NG designers have wooden box beams etc. There are no "default" rules of thumb style of beams as a full bridgedeck catamaran has an entirely different approach to handling the x beam and torque loads versus Wharram type 2 hulls with a series of interconnecting beams.
    Below is some jpegs of a Wharram 63 foot cross beams structure to smaller Wharram beams. A full bridgedeck cat has from I beam structures to box beam structures. If you have time look through Multihull Structure Thoughts thread for a lot more information on structures and crossbeam.
     

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  7. pavel915
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    pavel915 Senior Member

    @oldmulti Thanks a lot for the wonderful set of information.
    One question. In one picture I see that the cross beams are tired by ropes. Can we make another means of connections like bolting with the demi hull frames/deck or any other method?
     
  8. oldmulti
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    oldmulti Senior Member

    Pavel915. As I said each designer has there own style. Wharram was famous for his "flexible" connection to the hulls allow some independent movement of hulls. Reality is very few designers have flexible connections. Most designers have solid bolt or glued cross beam to hull bulkheads to form a rigid platform. Even Wharram does not publicize that in some designs (Tiki 46) he has 20 mm stainless steel bolts going through the cross beams connecting to bulkheads. The above is just one approach. There are many more approaches. I have attached some views of an aluminim cat about 64 foot long that has rigid connections between the hulls of box beams with attached bulkheads. Timber cats are similar.
     

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  9. pavel915
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    pavel915 Senior Member

    @oldmulti Thanks a lot for your wonderful information.
    I tried to model the process of bolting the cross beam with the bulkhead in Rhino and attached the picture here. Please let me know your opinion if this can be option for joining the cross beam with the bulkhead.
     

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  10. Ad Hoc
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    Ad Hoc Naval Architect

    You can make the connection whatever way you like.
    So long as the method selected satisfies the requirement and can transfer the loads from the hull to the beam and to the other hull within its design allowable stress limits and with an eye on fatigue.
    But usually with low modulus materials, like wood, the main driver is deflection.

    We recently did some box beams for a passenger catamaran built out of plywood.
    The beams were ally, and designed to satisfy Class rules....as were the connections, - these being bolted - to the hulls.
     
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  11. pavel915
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    pavel915 Senior Member

    @Ad Hoc thanks a lot for explaining the methodology of the design. I hope to follow those in my design.
    I will ask you specific questions when I proceed.
     
  12. oldmulti
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    oldmulti Senior Member

    pavel915. Sorry, my internet has been down. Fully support what Ad Hoc has said. Look at the total design and concept. If you do not need to build and transport the cat in parts, build the cat as one solid integrated unit. The loads are more easily handled when spread over a wider area. Bolts tend to have point loads, where as the edge of a full bulkhead with an integrated crossbeam attached, spreads the loads over a greater area when attached to a hull.
     
    Last edited: Mar 27, 2023
  13. pavel915
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    pavel915 Senior Member

    @oldmulti thanks for your kind opinion.
    I think the bulkhead and crossbeam can be made as an integrated object by welding if it is made of steel or aluminum. But is it possible when it is made of wood?
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Yes of course, wood is just another material, that's all.
    And like all materials, you just need to know the limits.

    Here is one we did, 2 off steel beam bolted to a catamaran hull made entirely from plywood -she is 18m in length.

    upload_2023-3-28_8-50-22.png
     
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  15. oldmulti
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    oldmulti Senior Member

    pavel915. To give you an idea look at the following PDF plans' of a Spindrift 37 catamaran crossbeams. This is an old design but gives the idea. In effect the plywood bulkheads go from one side of the boat to the other and timber top and bottom flanges are glued on with some intermediary supporting trusses. All glue virtually no bolts. Loads spread across a large surface area of the hulls. As Adhoc said ply is just another material, if you can calculate it in steel or aluminum then you can do it in ply.
     

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