Catamaran vs Trimaran

Discussion in 'Multihulls' started by Maciek188, Jul 17, 2005.

  1. frosh
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    frosh Senior Member

    Chris, I would like to point out that almost all tris nowadays are designed so that when one float is slightly immersed the other is well clear of the water.
    This means that the tri will sail at a slight angle of heel in light winds and allows a slightly lower wetted surface than a similar size cat.
    Interesting that Ian Farrier in an interview in "multihull magazine" explained that accomodations were superior in a tri over a cat in sizes up to 40 feet and this was largely why all his designs up to about 12 months ago were tris. (Note his speciality has been folding ama tris between 22 and 36 feet). His biggest production boat design was being featured which was 41 feet, and interestingly a cat, and he explained how this had a relatively huge amount of interior space. This argument seems the opposite to yours.
     
  2. Chris Ostlind

    Chris Ostlind Previous Member

    Chris, I would like to point out that almost all tris nowadays are designed so that when one float is slightly immersed the other is well clear of the water. This means that the tri will sail at a slight angle of heel in light winds and allows a slightly lower wetted surface than a similar size cat.


    Did I say something different from that?


    Interesting that Ian Farrier in an interview in "multihull magazine" explained that accommodations were superior in a tri over a cat in sizes up to 40 feet and this was largely why all his designs up to about 12 months ago were tris


    I believe that the F-41 cruising cat was designed in 1998.


    ... and he explained how this had a relatively huge amount of interior space. This argument seems the opposite to yours.

    What I said was this, "That kind of room is just not up to luxury cruising standards on a tri until you get to the 40'+ LOA designs" The key here is cruising standards and I take that line of thinking from the huge numbers of boats in the marketplace. While Ian has had a nice run of success with his F-boat tris since their inception, the total sales of under 40' cruising multihulls are easily dominated by cats. There's a very good reason for that. They have more livable area for the dollar invested.

    Virtually all cruising appointed multihulls of 40'+ LOA have serious room available. It is my take that the vaka hull of trimarans generally does not have the same amount of interior volume, as does a cat in the smaller sizes as discussed. Without knowing how Ian is arriving at his comparison stats, it is impossible to argue the point. Perhaps Ian is counting the tramp surface of his boats as part of the space?

    I have designed a 6.5-meter folding tri with a cabin, as well as a 21' cruising cat with a trailer legal beam. These boats have nominal dimensions for their type. The tri has a vaka hull volume of 90 cu ft. and the cat’s volume is 100 cu. ft. Please see the attached renderings.

    This isn’t the end of the discussion, as you can probably find examples of designs that reverse the relationship. It’s my experience that the references I have provided are fairly representative.
     
  3. Chris Ostlind

    Chris Ostlind Previous Member

    Faulty attachment routine apparently
     

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  4. rayaldridge
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    rayaldridge Senior Member

    "Gosh, Ray, doesn't all that substantiation of the cruising cat-over-tri argument, really depend upon the design details of the tri?"

    No, I don't think so, if we're talking about modern designs.

    "For example, if the tri amas are designed to just kiss the water surface at design load, there is very little heeling going on when the sails are engaged and the ama is pressed to equilibrium. Any gusts beyond the norm would cause both boats to experience a degree of enhanced heel and that comfy notion that a martini will sit still in a seaway goes right out the door."


    The tri ama does have to sink a little to reach equilibrium, which translates into heeling. Maybe I don't understand what you're saying, but to me, heeling is deviation from level. A cruising cat will rarely heel as much as 5 degrees even in a big puff. A cruising tri will often heel 10-15 degrees. Makes a fairly big difference in the MSQ (martini spillage quotient.) Of course, the other side of that is tris give more warning when hard-pressed than cats, but I'm talking about cruising boats, where flying a hull is pretty unlikely, for the prudent sailor. It's just physics, it seems to me. A tri ama will always have less buoyancy than a cat hull for boats of similar displacement. So the same forces will depress the tri ama more than the cat hull-- and that disregards the fact that the lever arm will usually be longer on a tri, because of its greater overall beam.

    At anchor, crew out on the side decks of a tri will depress the ama and the deck will develop a definite and less-than-comfortable slant, unless it's an enormous tri. The cat won't. Also, modern tris at anchor have a terribly annoying habit of walking back and forth, first dipping one ama and then the other. Cats don't.

    As the former owner of a 27' cruising cat, I can tell you that the no-spillage thing is not hyperbole, or even a "comfy notion." It's entirely true. (Though in my case it was a lot likelier to be beer than martinis.) Anyway, the extraordinary stability of cats in a seaway is definitely not just a hopeful marketing ploy. As an extreme demonstration, I came in Destin's East pass one day in high winds and a strong following sea, in my old cat, with wind against tide. The seas were breaking heavily in the pass, and like an idiot I still had the main up. I broached to on a 10' breaker, surfed at a high rate of speed toward the jetty, and recovered just in time. I had guests with me who'd never been sailing before, and when I screeched "Hang on!" they set their beers down on the deck to cling to something. After it was over and we were inside, they just picked their beers up, and they didn't know enough about sailing to understand how remarkable that was.

    We're having a pretty old argument, but in my opinion tris are the better go-fast platforms (due to lower wetted surface and greater sail-carrying stability) and cats are the better cruising platforms. The existence of monsters like Playstation makes for some ambiguity on the first assertion, but I regard the second as pretty much settled.

    Ray
     
  5. Chris Ostlind

    Chris Ostlind Previous Member

    5 degrees...that's all, really?

    I guess that would mean I've been pushing too hard with my boats in the past.

    All silliness aside, the following seas story so totally explains your perception.

    Beer has a much lower center of gravity than does the typical martini. That is especially true if the beers in question were dark and German.

    Here's the pre-requisite 5 degree heel I prefer...
     

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  6. rayaldridge
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    rayaldridge Senior Member


    Yep.

    It's easy to work out with graph paper and protractor. Assume a 35' cat with 15' beam and 18" draft. By the time the windward hull is clear of the water (NOT an excellent idea in a cruising cat) and the leeward hull is depressed 6" you're only at 6 or 7 degrees of heel. Most folks will never see that much heel on a cruising cat, unless they're beam-on to a big steep sea, and then it would be worse aboard a tri or monohull.

    I'll quote Thomas Firth Jones on this: "It may seem niggling then, to say that Carol and I prefer the motion of a cat. It goes to windward at only 3 to 4 degrees of heel, where a tri may heel 10 to 15 degrees in gusts." ...from Multihull Voyaging.

    This is not to claim an unvarnished benefit for cats-- the downside of all that stability is that the mast and rigging of a cat must sustain much higher shock loads... because why? Because, of course, a cat does not heel in gusts.

    But I like your theory about the CoG of consumable liquids.

    Ray
     
  7. Chris Ostlind

    Chris Ostlind Previous Member

    Ray, you're getting way too serious for the topic. (for me anyway) You already made those points, all in attendance got it and the last letter from me was to put the whole thing in perspective by taking the humor from the subject and making it the focal point.

    Essentially, it's an endless argument, though I suppose that if you'd like to press-on, we can always find some obscure stuff to toss on the fire. There's always quotes from one guru or another to fan the flames if you want, so whad'ya say it just goes away?

    Besides, it's really hot here, I'm kinda tired from being in the boat shop all day fashioning a carbon cockpit rim for my new canoe/trimaran and my wife wants to go to the movies. Now there's a loaded list for which you'll never have the better answer.
     
  8. rayaldridge
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    rayaldridge Senior Member

    Sorry, Chris. When you said, above, that:

    "For example, if the tri amas are designed to just kiss the water surface at design load, there is very little heeling going on when the sails are engaged and the ama is pressed to equilibrium. Any gusts beyond the norm would cause both boats to experience a degree of enhanced heel and that comfy notion that a martini will sit still in a seaway goes right out the door."

    ...I thought you were seriously putting forward the idea that there was little substantive difference between the heeling characteristics of tris and cats. Needless to say, I found this inexplicable, considering how knowledgeable I find you to be on the subject of multihulls in general.

    I like this discussion. In laying out the differences between two hulls and three, a lot of light is shed on the differences between one hull and more than one hull. Interesting stuff, to me.

    Hot here, too, and I'm putting a new roof on the old homestead. I'd rather be boatbuilding.


    Ray
     
  9. RMG
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    RMG New Member

    Alinghi Catamaran

    After reading through the many posts I was surprised to see that nobody has brought up the Lake Leman multihulls used in the Bol d'Or regatta and other various lake events in Switzerland. Following the 2003 Bol d'Or, the Lake Class box rule, which limits overall length to 19.6 meters, waterline length to 12.5 meters, and height to 24.5 meters, was scrapped due to escalating costs and was replaced with the Decision 35 class, which was based off of the Alinghi Catamaran that dominated the scene from its launch in 2000 through 2003.

    Throughout the 1990s, trimarans won a majority of the lake regattas in Switzerland. Prior to the commission of the Alinghi Catamaran, Ernesto Bertarelli was sailing a yellow Gino Morrelli designed trimaran also named Alinghi. In 1999 Ernesto Bertarelli asked Sebastien Schmidt & Jo Richards, Gino Morrelli, and Van Peteghem & Lauriot Prevost to propose a new multihull design under the Lake Class box rule. The commission ended up going to Schmidt and Richards.

    What I found so interesting was Schmidt and Richard’s design process. They first started out by designing the most optimal sailplan that best suited the tricky winds of Lake Leman often ranging 360 degrees from 0-30 knots, without taking into consideration the rig, beams, or hulls. Computer tests were then run using the already designed sailplan on optimum tri and cat platforms. They found that in light air, the difference in wetted surface area favors the tri only if sailing on the central hull, which is not possible. The tri goes through an increase in wetted surface area until flying on one hull. To fly two hulls, the tri will be heeling at the very minimum of10 degrees. The wetted surface area of a cat only decreases with heel. The cat has a weight advantage of 400kg in this case, 1200kg compared to 1600kg. The cat only needs 3.5 degrees of heel to fly on one hull. What about righting moment? Obviously a tri is going to have much more righting moment than a cat. This was solved with 320 liters of water ballast and six crewmembers on racks. This extra weight is movable unlike on a tri.

    On this 1.2 ton boat, 23.7 tons of mast compression was calculated, similar to that of AC boats. An innovative carbon truss/tie rod structure was used to solve the compression and torsional deformation problems prone to cats. Two carbon trusses spaced 4 meters apart on the rear beam meet at the mast base and continue forward as a single truss to form the bowsprit. The structure looks like an upside down Y. A complex system of PBO rigging was used to support the trusses attached by tie rods.

    This catamaran was untouchable. None of the trimarans under the same rule came close to beating the Alinghi Catamaran. I think that this is a perfect example of how a catamaran platform is more optimal than a trimaran platform. It would be interesting to see this concept applied to larger racing catamarans. (Attached is an article from seahorse written by Sebastien Schmidt on the Alinghi Catamaran that I got most of my information from.)
     

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  10. jam007
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    jam007 Junior Member

    Some thought on tris and cats.
    Background:
    Maximum traverse righting moment determines the maximum sideforce from the sails and depends on weight and beam. Maximum longitudal righting moment depends on length, weight and shape of the hulls. Maximum speed possible depends on longitudal stability since it is the driving force that has to be maximized.
    Sideforce is greater than driving force for relative wind angles up to ca 50 degrees, for larger angles the sideforce rapidly gets smaller.

    I would then guess that a tri can use a large beam more effective than a cat since the longitudal stability of the large central hull and one side hull is higher than for one cat hull. In other words: the risk of pitchpoling when coming off the wind is higher for a catamaran than a timaran of the same length to beam ratio.

    If the above is correct it would lead to the following:
    For very high performance boats that seldom sails with a relative wind angle above ca 50 degrees a beamy cat should be the fastest in all but the lightest winds as the central hull of a tri will fly anyway.
    With lowering performance the tri will start to be advantageous due to it´s higher maximum speed off the wind (smaller wetted area and high longitudal stability) and in light winds.

    Comments anyone?

    Anders
     
  11. frosh
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    frosh Senior Member

    Hi Jam007, No argument with your conclusions from me. In a way the discussion applies more to racing craft as in cruising multihulls it is more about ease of handling, liveability and so on. Speed is a much lower priority, however inevitably it will be considerably more than a similar size monohull.

    The Volvo Extreme 40 catamaran is relative to almost all other multihulls overpowered to an extent that longitudinal stability is as fragile as tendency to heel in even light winds. See this link with some good photos:
    http://www.catsailor.com/Stories_Temp/VolvoExt_UK-Day2.htm
     
  12. jam007
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    jam007 Junior Member

  13. jam007
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    jam007 Junior Member

    Found this post from Tom Speer about multihull stability:
    http://boatdesign.net/forums/showpost.php?p=15588&postcount=15

    Some other thought about cats and tris:
    Is there not a built in conflic for catamarans. To lower the wetted area you want to fly one hull in as low windspeed as possible. But that must mean that you have a low maximum sideforce. If you make a wide catamaran you get a high max righting moment but also a high "flying" windspeed and will have a large wetted area in light winds. A trimaran can be as wide as you want and still have a small wetted area in light winds. Again the difference is small if the boats have high enough performance to be limited by sideforce and minimum relative wind angle in all but the very lowest windspeeds.

    A note on longitudal stability. When iceboating you can actually lift the front runner when releasing the sheet when traveling at high speed as the drag is almost only from the rigg and hull. There is never any high net forward momentum even when traveling at maximum speed for the same reason. A similar effect should be possible for boats with very low hydrodynamical drag reducing the need for longitudal stability. (And giving us spectacular pitchpole capzises when something goes wrong:)

    There is also a scaling effect. With increasing size the righting moment and longitudal stability grows much faster (forth power) and wheight (third power) than the wetted surface (squared) this would make cats superior when it comes to very large boats as a cat can be made lighter than a tri of the same size.
    In a way tris are occupying a (shrinking?) part of the high performance size spectrum between skiffs and cats.

    Anders
     
  14. Doug Lord

    Doug Lord Guest

    Tris/ cats

    I don't think you can look at longitdinal stability(pitch resistance) or roll stability on high performance multi's these days without considering the effect of the use of foils whether it's rudder t-foils to aid pitch stability or a combination of rudder and lifting foils to not only aid pitch stability but reduce wetted surface at high speed.
    Orma 60's have proved the viability of lifting foils on the ama's as well -on some of them- as the combination of lifting foils and rudder t-foils. Parlier's planing cat relies on rudder t-foils to keep the stepped planing hulls at the right attitude.Steve Clark has experimented with the combination on a C class cat and at least one production beach cat comes with rudder t-foils. The Catri trimaran uses foils advantageously as well.
    I think the careful application of foils in high performance multihull design can allow shapes that would not be possible w/o foils and lead to better handling, higher speed and greater safety.
    ------------------
    edit: I'm referring to the use of "foil assist" hydrofoils not full flying hydrofoil systems.
     
    Last edited by a moderator: Aug 13, 2006

  15. jam007
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    jam007 Junior Member

    oh no Doug Lord...
    I will not take up that glove but stop foiling every thread or someone will stick one in you...

    Anders
     
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