Characteristics of narrow-beam catamarans

Discussion in 'Multihulls' started by srimes, Oct 6, 2020.

  1. srimes
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  2. DCockey
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  3. bajansailor
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    bajansailor Marine Surveyor

    What is the point?

    They answer it quite succinctly with the caption for the second paragraph :
    "The catamaran hull gives Magnet more inherent stability, plus a significantly longer range than monohull yachts of the same size".

    Two long skinny hulls should have better range (miles per gallon - or gallons per mile rather) than a 'normal' monohull of similar overall dimensions.
     
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  4. srimes
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    srimes Senior Member

    The "efficient cruise speed of 11.5 mph" is easily attained with a monohull, and with less surface drag would be more efficient. At high speeds I see the point.

    LOA/B is 4:1, plenty lean for a mono. Is it limited to that beam for some reason? Without an external boa I don't see why you'd end up with those proportions. I haven't come across many catamarans that narrow.
     
  5. Ad Hoc
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  6. Alik
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    You are probably looking at sailing catamarans; yes they are wider.
    High-speed catamarans are narrow.

    For boat in question, the beam is limited due to practical reasons - say, for berthing.
     
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  7. bajansailor
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    In addition, if you have a wide beam cat and then put three decks on it (wow, look at all this room!), something has to give - these three decks add up to a lot of extra weight.
    And this extra weight has to be allowed for in the buoyancy afforded by the hulls.
    And the only effective way really to gain extra buoyancy is to make the hulls wider / more tubby.
    Which then pushes your resistance up, so you need more power, and more weight for the extra fuel..... which requires even more buoyancy...... so you need to find a reasonable compromise somewhere along the line.
    And that is what boat design is all about really - you are looking for the best compromise of all the given constraints that you are faced with.
    There is no 'perfect' boat (except in the eyes of a boat owner re their own boat). :)
     
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  8. Alik
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    Well, personally I dont like these 'pagoda on a raft' designs... ;)
    If they have raised fthe bow it would look much better, to my taste. But Customer is the boss!
     
  9. srimes
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    OK I feel dumb. Guess I got too caught up in the long-range hype and ignored the high speed capabilities. Thanks for pointing out what should have been obvious.

    Are there any examples of narrow-beam sailing catamarans? If so how do they perform?
     
  10. bajansailor
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    bajansailor Marine Surveyor

    How narrow is 'narrow'?
    I would say more than than an L/B ratio of approx 2 (?)
    This Endeavour cat has a length of 36', and a beam of 15' -
    1999 Endeavour Catamaran 36 Catamaran for sale - YachtWorld https://www.yachtworld.com/boats/1999/endeavour-catamaran-36-3678688/

    Gemini cats are also relatively narrow - eg the 34' Gemini 105 with a beam of 14' -
    Gemini Catamarans || Gemini 105Mc Design Touch Specifications https://www.geminicatamarans.com/Gemini-Models/Gemini105Mc-Specifications.html

    All else being equal, a wider beam cat should stand up to her sail area / be less prone to capsizing better than a narrower beam cat (but this is a generalization).
     
    Last edited: Oct 8, 2020
  11. Ad Hoc
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    In addition to Bajansailor's reply....not really many.
    Since a sailing cat, to sail fast, needs stability. It gets that from a wider spacing of hulls. The more closely you place the 2 hulls, the less stability and thus less wind (driving force) it can carry. In a nut shell.
     
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  12. Alik
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    Alik Senior Member

    Sailing catamarans are NOT narrow, excerpt very large ones where berthing restrictions are considered. Beam of sailing catamaran provides stability required to carry sails. Moreover, most of sailing catamarans are displacement craft operating below hump of resistance. For such craft, from hydrodynamcis point, wider beam gives less wave interaction and allows to reduce the resistance.

    For high-speed cats above the hump, very often narrow catamaran will have less drag than wide catamaran - directly due to lower tunnel wash, and also becase it is actually lighter.
    If You look at this book, they quote our research on small catamaran beam selection - High Speed Catamarans and Multihulls - Technology, Performance, and Applications | Liang Yun | Springer https://www.springer.com/gp/book/9781493978892
     
  13. Mr Efficiency
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    The motions of a wide fast motor cat have to enter the equation too ? I assume that tends towards going narrower.
     

  14. redreuben
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    redreuben redreuben

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