When does a monohull become a multihull??

Discussion in 'Boat Design' started by Abby cat, Jan 1, 2017.

  1. Abby cat
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    Abby cat Junior Member

    I have noticed the trend in wide monohulls that are meant to gain hull form stability while decreasing the reliance on lead ballast thus lighter and faster, but they are slowly evolving into multihulls are they not.

    So where does the line cross between a monohull and a multihull, ie how much like a multihull can a monohull be before it is classified as a multihull.

    Example an imoca 60 has a length/beam ratio of about 3/1 and the 100 footer Comanche has a ratio of about 4/1. and most offshore cats have a ratio of about 2/1 so the beam is getting there.

    But also in hull shape can a monohull have "hulls" that are only below the water but the boat only has one piercing of the surface of the water, but when sailing the windward "hull" comes out of the water as a catamaran does.
     
  2. TANSL
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    TANSL Senior Member

    I think I understand what you mean but the first answer I can think of is that a monohull can never become a multihull. And forgive you for the simplicity.
    To answer properly, if there is an answer, we should first clarify what each one understands by a hull and how it differs from a float / ama. Clarifying that concept maybe the answer is easier.
     
  3. Angélique
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    Angélique aka Angel (only by name)

    -
    For power boats the Cathedral Hull goes a bit towards a Tri, here's a subject thread . . .
    Cathedral Hulls ? ? - - - - Wikipedia: Cathedral Hull
     
    Last edited: Jan 3, 2017
  4. Doug Lord
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    Doug Lord Flight Ready

    Some said the boat in the first picture was a trimaran but it was a mono with molded in leeboards. In 1898 the boat in the third & 4th picture-Dominion- won a series of races that got her type banned. And the fifth picture shows a classic relatively"modern" tunnel hulled scow shape similar to the M20.
    The last picture shows the worlds first foiling keelboat that has some features of a catamaran in its forward sections but is definitely a monohull. http://www.quant-boats.com/documents/BoatdescriptionQ23_July_2016.pdf
    --
    I think if the boat has more than one distinct hull in the water when it is level it is not a monohull. Lots of beam, in and of itself , doesn't make it a multihull.
     

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  5. Angélique
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    Angélique aka Angel (only by name)

  6. Abby cat
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    Abby cat Junior Member

    Thanks for the replys
    Angélique thanks, but I am more speaking in sailing terms but is still interesting.
    So TANSL what do you define as a hull, I have attached a grey area hull whether it is a mono or a multi, I would be interested at what your opinion is.

    thanks

    Nick
     

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  7. TANSL
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    TANSL Senior Member

    I rather tried to get you to say what a monohull is to you because, interpreting literally the words, if a thing you define as monohull it is impossible for it to become multihull.
    For me a hull could be defined as the part (or tarts)s of the boat that generates buoyancy and helps to support the weight of the boat. From this point of view a float is only a buoyancy reserve for certain cases, damage, large heelings, ... (This is my way of trying to answer Abby cat. Others can think what they want and it will be interesting to know other opinions)
    The boat in the picture of your last post, for me is a monohull with sponsons, or with a tunnel ???. I do not see it well. In any case, hydrodynamically speaking, in my opinion, it is very deficient, so whatever it is, I would not call it a boat.
    Your welcome, Nick.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Abby cat

    Your question is an interesting one but a bit misleading; especially since it covers several different disciplines (often at variance with each other) at once. However you may find the following HERE an interesting read that provides some insight to why
     
  9. Abby cat
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    Abby cat Junior Member

    Hi

    yea the image was of a concept and would require much more to make it efficient but what I believe the understanding is that a hull is the bottom, sides, and deck but not the masts, superstructure, rigging, engines, and other fittings.
    But the difficult part is when does the bottom of the hull become the sides as to whether a boat is a monohull or a multihull.
    The reason that i had these thoughts is because the sydney to hobart yacht race has not allowed multihulls to race, so I have been thinking about how you could get a multihull to be, by definition a monohull so it could do the race, this is because an equivalent length racing multihull is faster than a racing monohull.

    thanks
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    It is very simple.

    At the design waterline, if you cut the hull(s)...what do you see...one, two or three hulls?
     
  11. Ike
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    Ike Senior Member

    Both the USCG and ABYC use a simple definition: If the waterline forms a continuous line all around the hull it is a monohull. This is with the boat sitting level on the boats waterline with normal load. If it makes two or more footprints under the same conditions it is a multihull.

    H-5 Boat Load Capacity:
    5.4.1.6 Monohull Boat - a boat on which the line of intersection of the water surface and the boat at any operating draft forms a single closed curve.
     
  12. Squidly-Diddly
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    Squidly-Diddly Senior Member


    http://www.wildernesssystems.com/us/products/ride-135

    Kayak has tunnel hull for stability. Still a mono IMO but blurring the line.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    There'd be a few planing cats and tris that would qualify as monos based on that continuous waterline rule.
     
  14. TANSL
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    TANSL Senior Member

    The reasoning that I want to expose, although with little success, is that if a floating object is a "monohull", it can never be "transformed" into a multihull. It is what it is. I mean, the question is badly posed. You can ask if a floating object is mono or multihull but one thing we have already described as mono, can not be multi.
    On the other hand, I think explanations of Ad Hoc and Ike are very clear although, in occasions, in case of hulls of very different sizes, it would be necessary to qualify them.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Dynamic lift can do it, in some cases.
     
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