Catamaran vs Trimaran beam question

Discussion in 'Multihulls' started by Red Dwarf, Jun 28, 2012.

  1. Red Dwarf
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    Red Dwarf Senior Member

    Why are most catamarans, length divided by beam, near 50% while trimarans appear to be closer to 66%?

    Is there any reason you couldn't make a catamaran just as wide as a trimaran of the same length?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You obviously mean beam overall, rather than demihull.

    The width apart of the hulls dictates the stability (for driving force v sail area) but also structural strength. Thus if you make the hulls further apart, the biggest influence will be the strength of the beams and being aluminium or mostly composite, the deflection is the driver. To minimise the deflection becomes harder with hulls which are further and further apart. Not impossible, but the compromises required can lead to a design that is not satisfactory.

    A tri-has hull spacing much closer, thus the structural loading is different and easier to control deflections.

    In a nut shell.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Beam

    A post by me from another thread-actual beam of a number of racing catamarans. Trimarans vary from less than square to over square. For instance Hydroptere is 1.36 times as wide as it is long. The Rave, Osprey and Hobie trifoiler are over square as well.
    There is a conviction among some that nearly square or over square cats don't tack well. Thats not my experience in RC models. Oversquare trimarans can tack very well.

     
  4. ChrisSR
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    ChrisSR Junior Member

    Hi Red Dwarf,

    Having built multihulls since the mid sixties, I remember the problems cat designers had in overcoming the stresses trying to tear the hulls apart. It was overcome much later in cats than in tris.
    So we have the stresses over a wider span in cats, and the compression load from the mast in the middle of the longer crossbeam, and the tension involved in trying to keep the forestay tight and straight,(absorbed by the main hull of the tri), and the underwing impacts with the waves.
    All easier to overcome in tris than cats by the limited knowledge of the time and most important, by trial and error by amateurs such as myself who were prepared to build ocean going boats that nobody had built before.
    Hey, the joy of youth!

    Cheers,
    ChrisSR
     
  5. TANSL
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    TANSL Senior Member

    Probably there are some parameters that I do not know because I have only projected 5 or 6 cats, passenger, cargo and fishing. I have always used the rules of Lloy's Register for "Special Service Craft." In them, torques supported by deck beams depend on several factors, displacement, waterline length and greatest breadth of the hulls at water line. I have seen nothing to suggest that these depend on the ratio LWL / Total Breath of the ship.
    I would be interested if someone knows, tell me where I'm wrong, for I am sure I have some error.
     
  6. ChrisSR
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    ChrisSR Junior Member

    Hi Tansl,
    I would see is as abeam with mast load in the middle(compression) and get it engineered by a Nav. Arch. familiar with multihulls,(not all Nav. Archs are)to handle the boat being droped off waves.
    Cheers
    ChrisSR
     
  7. Silver Raven
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    Silver Raven Senior Member

    Gooday Chris. I'm sure we'd all be interested in your boat - I know that I am - so please some pic's & some design homework please - so we can all learn from your extensive homework & design knowledge. From what I gather it's over 40' but I don't know much else. What does it weigh, sail area, width, etc etc. All information is good.

    What trouble did the designers of multihulls have to overcome - back in the 60's & 70's - ??? could you please explain, thanks

    Also you might care to expand on how 'amatures such as yourself' managed to help these designers overcome the 'barms' problems ??? Thanks, james
     
  8. TANSL
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    TANSL Senior Member

    Thanks for your comment ChrisSR. I understand what you say but, I think, does not answer my question about the need to set the ratio LH / BH.
    The beam that supports the mast, in my oppinion, is a beam under a punctual load at its midpoint, other continuous charges and various torques. All these charges do work to the beam under bending and torsion. The effect of compression I can not see it. If you have time to spend with me, please, I would like an explanation.
    regards
    Ignacio López
     
  9. teamvmg
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    teamvmg Senior Member

    most trimarans only use 2 hulls at a time
     
  10. Richard Woods
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    Richard Woods Woods Designs

    Ad Hoc has it pretty much in a nutshell, as he says.

    But also: Boat designers are conservative. If you draw a wide beam catamaran it looks very strange on paper. Looks a bit weird being built, but very "right" on the water. So early designs were narrow because it looked right on paper

    Then many early boatyards were building in sheds that usually launched monohulls. So doorways and slipways were narrow. One major reason why the Iroquois, for example, was the beam it was was because the Sailcraft door was 14ft wide. When I worked with Derek Kelsall his shed door was 20ft wide so we tried to keep boats narrower than that, if they were wider (like the 38ft wide GB4) they had to be assembled outside.

    The Gemini 105 was designed to suit a standard 14ft wide slip. Many European boats are less than 5m wide so they can fit in the French canals and so get to the Med without going out to sea.

    Wide bridgedeck cabinned catamarans are very heavy as there is so much extra deck area. And heavy means more expensive to build as all the weight has to go through the builders hands. On a monohull at least 30% of the weight is in a bolt on keel.

    So generally it is open deck boats that are wide. And then you have the beam strength problem. More width adds weight. Many racing boats are deliberately narrow so they can hull fly in lower winds. A trimaran has shorter cantilever beams than a catamaran

    The widest catamaran I have designed was 17ft wide on a 22ft waterline. It sailed really well. But as I said at the beginning, it looked very strange on paper, especially when I first drew it 25 years ago and compared it to its competitors. It was wider than the 26ft Telstar trimaran for example.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  11. Red Dwarf
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    Red Dwarf Senior Member

    Thank you Richard that is very interesting. It also gives me more incentive to go ahead with my ideas for a catamaran. It is very much in the pipe dream phase but all ideas start somewhere.
     
  12. hump101
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    hump101 Senior Member

    My Irens F40 cat is 11.7m long and 7.75m wide, open bridgedeck, all kevlar/carbon/epoxy. It was designed for offshore use originally, so not to fly a hull routinely, though with a 19m wing mast it will do so. It spins around the daggerboards easily, very maneuverable compared to a typical beach cat, but of course has a lot more inertia which helps.

    Width/length should be a balance of transverse/longitudinal stability requirements, as well as other considerations. On a fast multihull the longitudinal stability is only required when accelerating, as the wind is always forward, but if you only have transverse you are risking a pitchpole when accelerating/decelerating. So a race boat can be wider than a cruising boat, for example, but a race boat for use in light winds would want to be narrower to limit stability and fly a hull earlier.

    As such, decide how you intend to use the boat, and adjust the width accordingly.
     

  13. Silver Raven
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    Silver Raven Senior Member

    Gooday 'hump' Very nice post - well balanced - very well put forward & extremely rational. I - personally - want a rather - forward - speed cruising multihull - however I can appreciate others points of view - - to each therir owm - however very well put . Thanks fo your wisdom . Ciao, james
     
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