3 equal hulls vs 2 wider hulls for a WIDE cat

Discussion in 'Boat Design' started by black_sails, May 10, 2016.

  1. black_sails
    Joined: May 2016
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    black_sails Junior Member

    Is there ever a situation where a 3 hulled multihull (technically a trimaran but all 3 hulls the same instead of the center being weight bearing and the outer being for stability) with narrow hulls would be better than a 2 hulled catamaran where the hulls were just 50% wider?


    I'm trying to analyze a design I saw of exactly that trying to figure out why they thought it was better... maybe global strength issues with the desired construction material like wood? (at certain large sizes, the desire of the cats to sail off in opposing directions/breakaway becomes a bigger problem) How would stability in waves be? Or safety under high seas (waves slamming into the broad deckhouse of the cat vs broken up by the third hull in the tri) or drag... there's more wetted area on the trimaran clearly but is the actual drag likely to be that much worse vs some potential benefits?
     
  2. gonzo
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    gonzo Senior Member

    Tritoons are made of modular tubes. If they need more carrying capacity, the easy and cheap way is to add a center pontoon.
     
  3. messabout
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    messabout Senior Member

    The two hull configuration will have more initial stability if the hulls are the right size for the design displacement. Two hulls of appropriate size will have less wetted surface than three hulls that are sized to carry the same weight as the two hulls. There is more..... When moving, the hulls will create bow waves. It is more practical to space two hulls such that the converging bow waves between the hulls do not cause problems by building up a tall peak at the convergence. If a single outboard is used it will be running directly in the convergence disturbance and there will be loss of efficiency.
    Three hulls would double the problem, but relieve the possible difficulties for the single outboard.

    Nonetheless three hulls are common for pontoon boats. Much power is wasted and sometimes there are mini geysers somewhere under the deck, but when the boat attains planing speed (if it ever does) then there is a whole different set of circumstances.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    It is called marketing....and it is working too :eek:
     
  5. Rurudyne
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    Rurudyne Senior Member

    "Spaceballs the Party Barge!"
     

  6. black_sails
    Joined: May 2016
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    black_sails Junior Member

    Oh no, it wasn't really for sale, actually it was someone who at least used to post here and reading through his other reasoning i'm a bit of a fan of his designs:

    http://www.magiccarpetcatamarans.com/What-the-hell.html

    But he added a third hull (in his design, which is mostly seat of the pants experience from building things like houses more than CAD engineering, but his opinion is 45-50 feet is too wide for two hulls out of plywood and wooden beams, but 40 feet is about the limit of okay) and i'm trying to figure out all the consequences positive and negative of such an arrangement. I'm not sure why wave dynamics would be dramatically worse than a ship of that size just being 20 feet wide between hulls though... just times two because theyre side by side.

    My guess is still that 'global strength' issues could be better/as the box of the deckhouse on top now only has 20ish feet between each hull instead of 40 feet so the stresses and tendency for the cat hulls to go sailing off in different directions is reduced. If I built a simple house to live in for comparison i'd feel better having some foundation pier support every 20 feet instead of 50 feet so it feels like that is what he's doing - however i'd love to have an engineer verify or disprove that instinct with applicable math as to how much effect it is. My guess is that hydrodynamic drag from more wetted area wouldn't be too substantially worse at the slower speeds vs the two wider hulls and thats the tradeoff for not needing superstrong materials for holding together such a wide beam. Though as i've said i've got no clue about the wave dynamics going on under there. I definately like the possible option of simply sticking one big propulsion diesel dead center instead of two matched diesels at the cat ends so that's another plus. (rather use two small ones for slow speed maneuvering at that size and which together can 'limp it home', and only buy one big expensive diesel as primary power)


    Okay that part makes sense, but does it make much difference if the hulls are wave piercing and as long and narrow as you can possibly make them?
     
    Last edited: May 10, 2016
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