Turn 2 monohulls into 1 catamaran. Is that viable solution?

Discussion in 'Projects & Proposals' started by TruckoftheSea, Feb 29, 2020.

  1. TruckoftheSea
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    TruckoftheSea Junior Member

    The true catamaran is the Ferrari of the sea. Monohull is so fat and heavy. So turning 2 monohulls into one catamaran is double the problem. But if we are not looking for the speed is it still a viable solution for DIY project?
    mono to cata 1.png
     
  2. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Truck.
    What size / type of sailboat hulls do you currently have in mind for this project?
    And what size outboard engines do you intend to use?
    If you can acquire two identical sailboat hulls for next to nothing (or nothing even), then it might possibly 'work' - however to achieve your objective of owning a power catamaran it would probably still be much less cost in the long run if you simply buy a second hand power cat.
    And this SH power cat would most probably be far more suitable for any Statement of Requirements that you might come up, when compared to a cat built from two sailboat hulls.

    While saying this, I might soon be involved in a project here (in Barbados) to build a new fibreglass power cat using two 24' motor boat monohulls - however we will be 'stretching' the hulls to about 32', adding some rocker and reducing the deadrise towards the stern, and raising the topsides.
    And the mould being used is very different to a typical sailboat hull.
     
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  3. kapnD
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    kapnD Senior Member

    Do you have two identical powerboat hulls, or some specs of what you’d propose to use?
    If so, let’s see them.
    It can definitely be done, BUT
    Keep in mind that your original complaint about the monohulls is going to be doubled.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Then the matter of drag from interference between the two hulls, which will be substantial. I guess the virtue is you have a lot of internal room, and in the event of a tiff, two separate areas to repair to, without the annoyance of being in a confined space with someone you are fuming at !
     
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  5. upchurchmr
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    upchurchmr Senior Member

    Wave inteference drag will be huge, unless you separate the hulls a LOT.
    Show us a sketch of the connecting structure.
    I don't know what you said or meant.

    Better to connect the two hulls with two metal tubes, then build a deck as you wish.
    Reinforce the attach point of the tubes to the hull - heavily.

    I'd never do it.
     
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  6. TruckoftheSea
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    TruckoftheSea Junior Member

    Thank you so much for your replies. Here is my sketch idea.
    cata 3.png
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Go to a toy shop and buy a couple of plastic sailboats, test the resistance of one alone using a fishing line and a fish weight scale, then bridge them together, and test the resistance of the coupled boats at the same speed. It will be a lot more than 2 x one alone, one suspects.
     
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  8. upchurchmr
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    upchurchmr Senior Member

    Thanks for the sketch.

    You definitely don't have enough gap between those hulls.
    Each hull will push water aside. In the center gap, the "wave" from each side will add and make a double ( or more) sized wave.
    That wave will do two things. It will cause a lots more water resistance, and the peak of the wave in the center will hit the engine mount platform and possibly the flatbed.
    Once it hits, there will be even more water resistance, and it might damage that structure. That is if you are just motoring on a flat sea. The height of the center wave depends in part upon how fast you are going.
    Even worse, when you hit a wave as you are moving, the peak in the center will get that much more high.
    More resistance and more potential damage.

    Power catamarans in Australia suffered damage to the connecting structure before they raised it and also added a structure to break up the wave, before it hit the boat.

    You would be much better off to keep the motors behind the "old hulls" where they originally were and eliminate the "engine mount platform".

    Are you planning on having the boat plane?
     
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  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Presumably ex-sailboats and certainly any expectation of planing will be dashed.
     
  10. upchurchmr
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    upchurchmr Senior Member

    I guess we should ask. The sketch looks like planning power boats to me.

    Truck - what are you planning - sailboat hulls or powerboat hulls?
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    He talks about removing keels and ballast, so that would mean sailboats
     
  12. TruckoftheSea
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    TruckoftheSea Junior Member

    1- The mentioned NPL report showed that raising the cross-structure on a catamaran model reduced the frequency of slams of a given severity but did not reduce the intensity when they did occur.
    2- Double bottom longitudinal girders to design against local collapse that help the cross-structure survive the slamming.
    3- Bending moment and shear force against the widening of the cross-structure.
    4- Thank for idea of building outboard engine platform to the transom of the main hulls but will that increase the longitudinal in-plane force and vertical shear force?
    5- Should we add the bulbous bow to the main hulls?
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Nope, generally it is around 20-30% depending upon final configuration.

    Nope...one doesn't need to suspect (intuition doesn't work well in hydrostatics nor hydrodynamics!!)...just look at facts.
    One can "generalise" though... a monohull and a catamaran designed for the same objective/SOR, the catamaran's will have either i) 50% saving in installed power,
    or
    ii) a speed increase with engines at 50%,
    The caveat being providing the Fn are in the 0.8- 1.20 range.

    Just check any work by Molland et al.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Let's not worry about "generally", take a look at his diagram, and give an estimate for that, and given he is only likely to be travelling at 6 knots or thereabouts. 20-30% ??
     
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  15. TruckoftheSea
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    TruckoftheSea Junior Member

    1- Question 1: Why not an affordable second hand true cat but a cat from monuhulls? I can't put tons of fuel, water, big refrigerator, big gen-set, air conditioning on medium size cat. I need the vacation AV on the sea. I don't need the speed, fuel efficiency....
    2- Question 2: Why not big old cheap trawler? That's so ugly and hard to access to water. Cat from monohulls with large flatbed, tons of fuel, water, big refrigerator, big gen-set, air conditioning is truly an coastal going vacation platform for the lazy city guys.
     
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