Catamaran Cross-deck Design

Discussion in 'Multihulls' started by ktimg, Aug 28, 2016.

  1. ktimg
    Joined: Apr 2014
    Posts: 28
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Myanmar

    ktimg Junior Member

    I have found that so many catamarans are designed with continuous transverse frames, as shown in the picture. But, what I have known so far is that it would also be possible to build one with two separate demi-hulls attached with the deck plate and a number of girders running between the two hulls. What are the pros and cons of the two designs?
     

    Attached Files:

  2. TANSL
    Joined: Sep 2011
    Posts: 5,811
    Likes: 205, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    A catamaran needs to have several cross beams to absorb the stresses occurring due to the relative movements between the hulls. Once this is done, you can put as many longitudinal elements as you like, but they will not help you to solve the problem mentioned.
     
  3. ktimg
    Joined: Apr 2014
    Posts: 28
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Myanmar

    ktimg Junior Member

    My question is_ I think there're two configurations of catamaran structures.
    1. continuous transverse frames existing in both two hulls and also between them. (as seen in the photo)
    2. transverse frames are not continuous from hull to hull. A number of cross beams connect the two hulls and a deck plate cover over them.
    What is the difference between them? Thank you, for your attention.
     
  4. SailorTom
    Joined: Sep 2013
    Posts: 4
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: NE USA

    SailorTom New Member

    Mr James Wharram has said that his designs have ropes attaching the cross beams to allow some flex in the structure. I suppose the that the ropes dissipate some of the energy vice the hulls or cross beams.
     
  5. Richard Woods
    Joined: Jun 2006
    Posts: 2,179
    Likes: 145, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

    As asked I'm afraid the question is too vague to get a sensible answer. Are you talking about a commercial power catamaran, as in the photo. Or an open deck sailing catamaran as eg, Wharram designs?

    Obviously the two concepts are very different and need completely different structural designs to hold them together. You could never use a lashing system to hold the hulls that you show together

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  6. ktimg
    Joined: Apr 2014
    Posts: 28
    Likes: 1, Points: 3, Legacy Rep: 10
    Location: Myanmar

    ktimg Junior Member

    I'm sorry, if my question was too rough. I was talking about a commercial power catamaran here, not a sailing catamaran. I'm afraid if I might annoy you with this question, but, could you explain a little more detail about these two concepts? or, guide me some resources where I can learn about them?
     
  7. TANSL
    Joined: Sep 2011
    Posts: 5,811
    Likes: 205, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    You need to understand that, simplifying, there are three types of effects that should be supported by the structure of decks between the two hulls:
    - Relative movements of the two hulls, absorbed mainly by the transverse beams, which must be continuous.
    - Loads on the deck, which can be supported by the plate deck panels, with beams and girders.
    - Beats of sea, waves, slamming, on the wet deck.
    Each requires a specific solution.
     
  8. Ad Hoc
    Joined: Oct 2008
    Posts: 6,177
    Likes: 400, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    There are two basic differences

    1) Labour. The is a difference in the amount of labour and hence cost of construction. One is better/quicker than the other.

    2) Structural. So long as the main transverse frames of the hull and raft line up between the hull and centre raft structure and the deck plate is continuous between the hulls, it should not pose any issues.
     
  9. Zulu40
    Joined: May 2015
    Posts: 36
    Likes: 3, Points: 8, Legacy Rep: 18
    Location: Australia

    Zulu40 Junior Member

    kinda like this? the deck and house are an integral unit, excepting that the crossbeams are uniform. (although without the central submerged structure obviously)

    [​IMG]
     
  10. TANSL
    Joined: Sep 2011
    Posts: 5,811
    Likes: 205, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    This is a nice boat but the problems of its deck are only similar, not identical, to the deck of a catamaran. The central body introduces different stresses to a catamaran. It has little to do way of working for both structures.
    From some point of view deck and deckhouse are always an intergral unit on all ships. From the standpoint of longitudinal strength they do not form an integral unit, in this boat or any other.
     
  11. Zulu40
    Joined: May 2015
    Posts: 36
    Likes: 3, Points: 8, Legacy Rep: 18
    Location: Australia

    Zulu40 Junior Member

    No it is not identical, but it is the theory of the process that counts, and the reasons why those differences exist.

    You could liken most cats to monocoque and this SWATH Klein to a chassis derivative. The pity is we dont make true monocoque cats as they have many frames within that somewhat defeats the principle. Frames that have to be frequent, self supporting to endure deformation free assembly and construction, and are largely and likely redundant in the finished boat.

    Too few frames and the surfaces by this process wont be fair, too many and the weight becomes a burden. Whether or not that balance is achieved is in my view questionable.

    On the other hand the chassis deck of the Klein is simply the hub to which all else is attached too, rather like most cars of the bygone era. But because the deck is a necessary component of the finished boat, it quite probably has less redundancy that the framing of the almost monocoque cats. Especially when the design is a one off, so this should be of interest to home builders.

    There aren't many cats that demonstrate the same utility, although Wharrams are one, and off the beach cats another, the need will possibly become greater with displacement power cat designs presently becoming popular infiltrate the self build market.

    This because the narrow 1:16 to 1:20 B/L ratio of the hulls do not allow so much adequacy for lateral framing, and have a potential weakness at the deck to hull join that complicates the structural thickness's required. The hulls are so narrow they are best afforded by other construction methods like infused flat panel foam.

    The solution is a self supporting strongback deck with hulls buttoned up under it, and fewer bulkhead frames within those hulls. The house then simply sits on top in a reversal of how the hulls attach.

    Forgetting the propulsion module in the illustration above, an examination of Klein's prototype reveals the skinny hulls at the periphery and the house situated on top
    So the drivers become more advanced hull construction methods, skinny displacement hulls, and sufficing the self build market or in the case of the Klein a one off.
     
  12. TANSL
    Joined: Sep 2011
    Posts: 5,811
    Likes: 205, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Frankly, I do not know what does all this discussion of a type of boat that has nothing to do with OP's question.
     
  13. Barry
    Joined: Mar 2002
    Posts: 1,152
    Likes: 87, Points: 48, Legacy Rep: 158

    Barry Senior Member

    I assume that when you say continuous that you are referring to the frames being part of the same piece as the transverse beam. As compared to two separately built hulls and the transverse beams being bolted/attached to the hulls

    The paragraphs below refer to two hulls, and to relatively thin transverse beams to resist loading stresses. Similar to a smaller cat like a Hobiecat

    The continuous transverse beams attached and part of the frames, aluminum in this case, enable the stresses that are created to be transmitted from the hulls to the beams without running through a joint that may create stress concentrations

    While this joint can be designed, it will be require more than running a few bolts through ONE frame and one transverse beam per side, as there will be stress concentrations near the bolts.


    The following refers to the upper deck, cabin structure providing bending resistance.

    If the upper deck and cabin structure are designed in such as way as to provide bending/torsion resistance as one contributor suggested, then stress concentrations can be significantly lowered between the deck and hulls.
     
  14. Zulu40
    Joined: May 2015
    Posts: 36
    Likes: 3, Points: 8, Legacy Rep: 18
    Location: Australia

    Zulu40 Junior Member

    The op asks "I have found that so many catamarans are designed with continuous transverse frames, as shown in the picture. But, what I have known so far is that it would also be possible to build one with two separate demi-hulls attached with the deck plate and a number of girders running between the two hulls. What are the pros and cons of the two designs?"

    I answered in the affirmative.
    But ok if you do not accept the Klein as an example, (where tris commonly exhibit these asked for characteristics), take a look at a PDQ 32.
    By eyeball measuring the hulls look to be about 4ft wide aft.

    It demonstrates the same things. The long narrow hulls wont accommodate double berths as is traditional and popular with Lagoons and that ilk, theyre probably just fat enough to squeeze a pair of diesels in there. We can go narrower still if we use outboards but the consequence of doing this makes the hulls too narrow for much crew use, then almost all the people friendly climate is in the deck house.

    You may ask what is this boat good for? Displacement cats with very narrow hulls will give very economic performance. Its possible a 30+ft boat will be so easily driven as to operate with 10hp on one engine for extended range, or both engines WOT (wide open throttle) for speeds in the area of 20 knots.

    Mostly these boats will be similar in that the house is inboard of the very skinny hulls which doesn't allow for enough mechanical jointing between the two for the lateral frame construction method to properly support, the alternative is to use a self supporting deck as previously described.


    [​IMG]
     

  15. TANSL
    Joined: Sep 2011
    Posts: 5,811
    Likes: 205, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Zulu40, maybe I'm wrong but I'd swear ktimg is talking about catamarans, and no other type of vessel.
    The boat that you insist on talking is very interesting. Maybe you should open another thread to talk about it.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.