"reverse" trimaran

Discussion in 'Multihulls' started by nimblemotors, Mar 19, 2013.

  1. nimblemotors
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    nimblemotors Senior Member

    This is more a super wide catamaran, the center hull is reducing the needed strength of the crossbeams, and gives more length for a taller mast, and thus more sail area.

    I've attached a bad block diagram picture to give the basic idea.

    Certainly there is a reason not to build it this way, probably first on the list is just make the crossbeams stronger, it will go faster without the center hull right? But just how WIDE can you make a catamaran, square? 30ft wide with 30ft long hulls?

    JackB
     

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  2. catsketcher
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    catsketcher Senior Member

    If you go too wide you will increase the weight a fair bit. The stress on a beam is proportional to its length squared. So the weight goes up fast.

    More importantly is the fact that you don't want a boat that is more stable sideways than fore and aft. Otherwise you get a funny assymetry of stability so the boat can handle its rig if it is on a reach but it goes over its bows on a run.

    It has been done before and the boats were tricky to sail. So cat beam tends to stay at around 60% of LOA centreline to centreline.

    Try it if you want or go for a sail on a Twiggy. Lovely boat but they need a careful hand.

    cheers

    Phil
     
  3. Corley
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    Corley epoxy coated

    It's been tried quite a bit and the results are distinctly underwhelming.
     
  4. champ0815
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    champ0815 Senior Member

    Just musing about the same problem... .
    What's the difference in terms of engineering about an oversquare cat and a tri of the same dimensions, both build to fly one or two hulls, respectively?
    In "flying mode", the beam is the same, the wetted area is the same (or less), the rig loads on the cat can be accommodated in a center pod the same way as in the tri without the weight of a complete third hull.
     
  5. nimblemotors
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    nimblemotors Senior Member

    Can you point to any examples? I've never seen one this way.

    This type of configuration is used in powerboats, particularly the hydroplanes
    with quite overwhelming effect. :D

    [​IMG]

    Which has me thinking I'll be fine with a big enough motor to put this 50ft boat on plane.
     
  6. Doug Lord
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    Doug Lord Flight Ready

  7. champ0815
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    champ0815 Senior Member

    Ok, I have read the linked thread and still in my opinion the question is not sufficiently answered:
    Why should a (pod) cat of the same length and width as a tri be inferior or more complicated to design?
    If the design allows for flying the other hull(s) the loads on the beam could be less on a pod cat, where the pod has not to be a complete hull and so the total weight would be lower than the equivalent tri.
     
  8. Corley
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    Corley epoxy coated

    In pitch stability, the trimaran wins hands down, The Decision 35 has a vestigal centre hull structure to help with this problem the centre vestigial hull helps to reduce the chance of a pitchpole and also stiffens the boat so when they wind on the mainsheet they tighten the mainsail rather than bending the boat. This approach is better than nothing but still is inferior since the vestigal hull will only come into play at a greater angle of pitch (still better than nothing though). So even at relatively L/B ratios the catamaran structural problems start to appear.

    A trimarans actual sailing beam is lower than a catamarans so you fly a hull sooner due to the windward float being hoisted out of the water immediately by dihedral. This means as you transistion from leaning on the float and unloading the main hull you reduce WSA to the point where you are finally sailing on one hull. On an square catamaran you have excessive wetted surface area until the windward hull is hoisted clear but at that point you are already approaching a point where righting moment is reduced due to the differences between trimaran and catamaran righting curves and maximum stability being approximately 25 degrees of heel versus 35 degrees on a trimaran. This means you have to overpower the square catamaran to get decent performance and reduce the wetted surface area penalty but you end up with a very tender boat that's hard to keep on it's feet and is quite vicious and unforgiving in the way it sails.

    Doug will probably disagree but I consider Alinghi V to be a failure and it is the closest to a square cat that's not a foiler listed in his outline of L/B ratios. Alinghi V basically scaled up the concepts of Le Black (which was successful by the way but was only a 41' boat intended for lake sailing) but the concept didn't scale effectively. You can see that with the exaggerated twisting of the platform under mainsheet load that kept dumping the windward hull back into the water when Alinghi V was powered up. When the BOR90 trimaran was fitted with it's soft rig it never showed any sign of the exaggerated and uncontrollable platform twist that Alinghi V did in a very moderate sea and wind state off Valencia.

    You might like to read John Shuttleworth's article on considerations for seaworthiness and comparisions between the catamaran and trimaran formats it's a good reference.

    http://www.john-shuttleworth.com/Articles/NESTalk.html
     

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    Last edited: Mar 19, 2013
  9. nimblemotors
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    nimblemotors Senior Member

    Cory, did you mean to say wide cats have been tried? While I did ask that, the design in question is a trimaran with fat amas forward and a narrow center hull that extends to the rear, something I have not seen before. I've read that shuttleworth text, not sure how it relates to this tri layout. The boat being contemplated is a cruiser, I don't think any hulls will be lifted on purpose.

    JackB
     
  10. Corley
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    Corley epoxy coated

    Tony Grainger did some interesting work on a powered high efficiency tri hull concept not sure it really translates to what you want to do but interesting none the less. Quite narrow beam due to the powered format.

    http://youtu.be/8zjPQ_t09Nw



    Just as an observation I don't think having the hull towards the back as illustrated in your diagram is a good idea. If you wanted to use a vestigial or immersed centre hull it would be better to put It forward where more of the forces require compensation. On a cruiser you get away with more as the speeds and stresses are not as great as on a race boat.
     
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  11. Doug Lord
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    Doug Lord Flight Ready

    Theres a theory that the "amas" should be aft on a powered tri because there is less chance of the ama not being supported as in a wave formation that supports the bow and stern leaving the boat to roll if the ama's were midships. I think it was Craig Loomis that mentioned that. http://www.lomocean.com/
    And Nigel Irens puts the amas fairly close to midship. http://www.nigelirens.com/FRAMEpower.htm scroll down
    And John Shuttleworths Adastra-sorta in between: http://www.john-shuttleworth.com/
     
  12. Corley
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    Corley epoxy coated

    Square and extremely wide cats have been tried without much success but I'm scratching my head to think back and put a name to any of them. Maybe one of the more senior members of the forum can give some examples I'm sure I recall reading some articles in an old AYRS about it.

    Note I'm talking about non foiling boats of which some wide catamarans like Hydroptere.ch have been successful different kettle of fish once you add foils really.
     
  13. Doug Lord
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    Doug Lord Flight Ready

    There's a table listing some of them referenced in post #6 above. Hydroptere.ch is an almost square cat designed to sail well in "normal" mode, in stepped hull mode and as a foiler. It has set some records in Europe. All almost square cats are not failures. Square or over square tri's seem to offer more advantages, though.
     
  14. Gary Baigent
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    Gary Baigent Senior Member

    Parlier's stepped ORMA cat is very wide, around 70% at a guess - but carries a double rig on/near the two hulls; Alinghi's monster was around 75%, then there is Jo Richards' lake racing cat Ghamsin (sp), also very wide with an arched beam and diagonal connections to behave somewhat like a central hull, then as Doug says, Hydroptere ch. There are not many catamarans with this configuration. The Supercat 20 and 22 had wide beam (around 12-13 feet?).
     
    Last edited: Mar 25, 2013

  15. champ0815
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    champ0815 Senior Member

    Just a thought experiment: take a nice and proven tri design like the one in http://www.youtube.com/watch?feature=endscreen&v=YQfEmZ8HM9U&NR=1 , large amas with, don't know, 200 % volume or so, remove everything from the center hull not needed for rig stability including rudder and centreboard (they are only just skimming the surface...) and the bottom of the hull
    Why should this thing have other stability characteristics than the unmolested tri, especially in the flying mode, where the centre hull is nothing more than ballast in the middle of the large overall beam?
     
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