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Thread: Catamarans, beam, and stability question

  1. #1
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    Default Catamarans, beam, and stability question

    I've always been a bit smitten by this crab claw cat from Shellboats. The beam is only 8'4" (by 21 feet long) and so is legally trailerable just about anywhere without any need for folding or detatchable parts, as well as being light enough to tow with a smaller vehicle. Also, there's plenty of sheltered cabin space with an aft cabin and center 'cockpit' and so is not too different from the monohull ideas I've been pondering, less 8 or 900 pounds ballast. http://www.shellboats.com/sbcrabclawcat.html :










    My first question is: What sort of sea conditions could one expect such a narrow lightweight cat to remain reasonably safe in? I'm dreaming about our future BC Gulf Islands weekend cruiser again so I'm not considering off shore but am including the Straight of Georgia, which can be a formidable body of water at times. I realize catamaran questions such as this have come up before but I'd rather not spend hours doing the searchy thing if anyone out there with catamaran experience would like to help a guy out. We'd be equipped with an outboard motor as well.

    My other question is: Any one know of diy plans for a similar craft? Shellboats apparently sells only kits and finished boats for this design.
    Last edited by JimD; 04-16-2008 at 12:30 PM.

  2. #2
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    Default Re: Catamarans, beam, and stability question

    An important thing to look at when choosing a catamaran design is the clearance between the underwing and the water. Many designers have lowered the wingdecks to increase accommodation and the result is severe pounding and possible damage.
    I can't tell from your photos what the clearance is, but in a cat this small, I'd want it no lower than the gunwales.

    Gary

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    Default Re: Catamarans, beam, and stability question

    In the same vein, I'd worry about waves hitting the cabin sides (vertical) and breaking out the windows, which don't look particularly stout. If the boat swamped, the freeboard would be further reduced. No fun.

    The design looks best-suited to waters where waves are small, i.e. lakes without a long fetch and inland waterways.

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Gary Dierking View Post
    An important thing to look at when choosing a catamaran design is the clearance between the underwing and the water. Many designers have lowered the wingdecks to increase accommodation and the result is severe pounding and possible damage.
    I can't tell from your photos what the clearance is, but in a cat this small, I'd want it no lower than the gunwales.

    Gary
    Gary, thanks for stopping by my thread. What do you think of this sort of footwell idea?


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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Chip-skiff View Post
    In the same vein, I'd worry about waves hitting the cabin sides (vertical) and breaking out the windows, which don't look particularly stout. If the boat swamped, the freeboard would be further reduced. No fun.

    The design looks best-suited to waters where waves are small, i.e. lakes without a long fetch and inland waterways.
    Toughening her up with smaller windows and similar details can be dealt with rather easily. I'm primarily concerned with how well a cat without a lot of space separating the hulls can stay on her feet.

  6. #6
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    Default Re: Catamarans, beam, and stability question

    I think that has been used with some success, but with a round trough rather than a flat-bottomed one.

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by johnw View Post
    I think that has been used with some success, but with a round trough rather than a flat-bottomed one.
    Round or vee would be easy to do in sheet plywood. Whatever design I settled on would be for sheet plywood. Either dory or vee bottom hulls.

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    Default Re: Catamarans, beam, and stability question

    I like the idea of that boat, and it is handsome enough. But with a crabclaw rig with (what seems to be) an extreme back-cant optimized for reaching (at least that is the consensus in the proa community), and all that cabin windage, will it at all get much to windward without a motor? Even as good a close reach?

    As for hull stability, I hear tell that a catamaran with roundish hulls and lateral resistance appendages drawn up fares pretty well in big seas -- the boat slides down the waves without rolling (conforms to the slope of the wave) whereas a monohull especially with a keel is going to roll further than perpendicular to wave slope. There is a good on-line article available called something like "Multi-Hull Design Considerations....etc." written by a well known multi-hull guy. Should be findable.

    Can the tack-end of that crabclaw sail be winched up so that sail stands higher (higher aspect ratio) for windward work? That might be interesting, if the mizzen sail is powerful enough to maintain balance. --Wade

  9. #9
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    Default Re: Catamarans, beam, and stability question

    Sounds like a Jim Brown article......

    NOPE....wuz

    Multihull Design Considerations for Seaworthiness.
    By John Shuttleworth.
    John Shuttleworth's Biography.

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    Abstract

    This Paper evaluates how a multihull performs in waves with respect to rolling and pitching. Stability is evaluated both in relation to wind and wave action. In particular reference will be made to Prof. Marchaj's recent work - ' Seaworthiness the Forgotten Factor.' Multihulls are studied under the same criteria as monohulls are evaluated in the book, giving a clear
    Wakan Tanka Kici Un
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    Default Re: Catamarans, beam, and stability question

    Chuck, you expect me to be able to understand this stuff???







    I like pictures better:













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

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    Default Re: Catamarans, beam, and stability question

    Jim ya just gotta remember certain maxims about boats....
    1) Stone Boats Sink
    2) Any boat whose entire stabilitay program consistes of 5 tons of lead pulling you toward the center of the earth is inherently dangerous.
    3)Catamarans are very stable boats....upside down or rightside up, sometimes not your choice.
    4) trimarans can still fly with the main hull and one outrigger submerged, and let loose the sail pressure and it will right itself
    5)Murphy was an optimist....
    Wakan Tanka Kici Un
    ..a bad day sailing is a heckuva lot better than the best day at work.....
    Fighting Illegal immigration since 1492....
    Live your life so that whenever you lose, you're ahead."
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    Default Re: Catamarans, beam, and stability question

    I thought you wanted to build a Chelaydra.......
    Wakan Tanka Kici Un
    ..a bad day sailing is a heckuva lot better than the best day at work.....
    Fighting Illegal immigration since 1492....
    Live your life so that whenever you lose, you're ahead."
    "If you live life right, death is a joke as far as fear is concerned."

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by paladin View Post
    I thought you wanted to build a Chelaydra.......
    Just looking at my options. Did you see in Building/Repair I'm making a mini Chelaydra?



    Gonna look kinda like this when its done, only not blue:

    Last edited by JimD; 04-16-2008 at 09:55 PM.

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    Default Re: Catamarans, beam, and stability question

    4) trimarans can still fly with the main hull and one outrigger submerged, and let loose the sail pressure and it will right itself
    Maybe I should take a gander at a few tri's...

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    Default Re: Catamarans, beam, and stability question

    I've admired that design too. I think it works specifically because the sail area is kept low. A tall rig would probably be risky. I think it's telling that there are very few catamarans with those proportions, except for the very light beach cats. And the more experienced designers prefer more beam.

    I have a 35-foot cat, btw, and I can tell you that no matter how much clearance you have under the bridgedeck, you'll find yourself in conditions were it will pound alarmingly, and copious amounts of water will wash right over the cabin. If you build one, assume you'll have days like that.

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Woxbox View Post
    ...no matter how much clearance you have under the bridgedeck, you'll find yourself in conditions were it will pound alarmingly, and copious amounts of water will wash right over the cabin. If you build one, assume you'll have days like that.
    With this Shell design there's probably only about four feet of open water between the hulls. I wonder how much wave can actually get in there?

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    Default Re: Catamarans, beam, and stability question

    The width of the space doesn't matter. The hulls forward on almost all catamarans are quite fine. So when they dive into a wave, they don't lift immediately. Anything that's between them smacks the top of the wave at least, sending water in all directions.
    Here's an extreme example: http://www.youtube.com/watch?v=HZ53C...eature=related

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Woxbox View Post
    The width of the space doesn't matter. The hulls forward on almost all catamarans are quite fine. So when they dive into a wave, they don't lift immediately. Anything that's between them smacks the top of the wave at least, sending water in all directions.
    Here's an extreme example: http://www.youtube.com/watch?v=HZ53C...eature=related
    I see what you mean. Probably a cat is not for us as a dry ride makes all the difference around here where the summers are short and cool and the water is always cold. The Crab Claw's cabin looks quite snug though and she is sailed from inside. And I bet a ten hp outboard would really zip her along, too.

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    Default Re: Catamarans, beam, and stability question

    Jim -- I'm not trying to make a case that the cat's unsuitable. Just be aware that, as with any boat, things aren't always like the publicity shots.
    90 percent of the time the decks on our boat are free of spray (that's East Coast sailing in the summer for you), and folks can lounge on the forward tramp. And the cabin does serve as an excellent dodger. But you want to know that your boat can handle a bit of weather, too.

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    Default Re: Catamarans, beam, and stability question

    The Strait of Georgia ain't exactly a mill pond.

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Chip-skiff View Post
    The Strait of Georgia ain't exactly a mill pond.
    Vancouver Island and the mainland form a long wind tunnel and the winds can be very strong. Even the large car ferries have their crossings cancelled occassionally, although usually the winters are much much worse than the summers. Many small boaters have had white knuckle crossings by failing to keep an eye on the weather.

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    Default Re: Catamarans, beam, and stability question

    I sailed in that area ONCE in the 70's.....solo in my last boat....Seattle to Vancouver and around.......lot more comfortable in the open sea....
    Wakan Tanka Kici Un
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    Fighting Illegal immigration since 1492....
    Live your life so that whenever you lose, you're ahead."
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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Woxbox View Post
    ... you want to know that your boat can handle a bit of weather, too.
    Actually, being a bit of a fraidy cat on the water I would like to know that my boat can handle rather a lot of weather. A well ballasted monohull still sounds like a better boat for us. For me the fear factor is directly related to my confidence in the boat's ability to handle the conditions in spite of my poor seamanship.
    Last edited by JimD; 04-17-2008 at 02:04 PM.

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    Default Re: Catamarans, beam, and stability question

    Jim,

    If you like the design of the Shellboat’s crabclaw and are worried that the bridgedeck is too low for your waters, you can always raise the sheer line some without seriously manipulating the stability issues to the negative. A boat of 21' with a trailerable beam has quite a ways to go before it begins to suffer from other design considerations. This design has quite a low COE with that crabclaw rig, so the risks are small.

    As she sits, I'd go along with Gary on the design tips.

    As to the footwell from Richard Woods to increase the comfort factors while aboard... This is a superb solution for passenger comfort aboard a small cat as long as you retract the collapsible footwell section for sailing/motoring, or you are right back to the slamming thing again.

    I recently did a boat which meets some of the same design brief issues as the original boat shown. I'd love to get your feedback on the Gato Especial design when you have the time.
    http://www.duckworksmagazine.com/08/...gato/index.htm

    Chris
    Last edited by Chris Ostlind; 04-17-2008 at 02:53 PM.

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    Default Re: Catamarans, beam, and stability question

    Chris, wow! Thank you for pointing out your GE. I'm a bit too busy today to give it the attention it deserves but will have a good look later. I do have one question that I hope is not offensive but how would you feel about incorrigable unrepentent tinkerers like me making a few aesthetic changes to the cabin?

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    Default Re: Catamarans, beam, and stability question

    I like the Crab Claw Cat, too, for the imagination that went into it.

    But— Wow! The Gato Especial is a sweet design that lessens the boxy aspect of most small cats with sleeping room. Along with the pleasantly racy lines, it definitely looks safer and saner for open water.

    Rather than a narrow-gauge cat, have you thought of a trimaran (with folding or removable amas)? Seems like I saw an item in a recent Launchings about a Newick Tremolino that had been built of wood and composites. Neat design, with limited living space.

    Another approach is a tri based on a monohull cat ketch– for example the Sea Pearl (which is f-glass). For a look:

    http://www.marine-concepts.com/trimaran.html

    If stability and trailerability (and wood construction) are the criteria, could you build a Core Sound-type hull with mounts for optional amas (recycled Hobie Cat hulls?) and trampoline side decks, to sail both ways?

    Ray Frechette, who builds the Core Sound boats and posts on this forum, might have something to say about the idea. Or delusion— as the case may be.

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    Default Re: Catamarans, beam, and stability question

    I think that this is the best no-fold trailerable cat ever designed: The Gougeon 32. 8 1/2-feet wide, 32-feet long, and with water ballast tanks to keep it on its feet. It's said to be a very fast boat, and goes from trailer to water in minutes. Years ago the brothers themselves were demonstrating mast raising on the boat show circuit. I think it was 5 minutes to raise the mast -- then slide it into the water and go. But as you can see, it's got one small cabin for such a long boat.


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    Default Re: Catamarans, beam, and stability question

    Is there any risk of the heat shield tiles burning up on re-entry?


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    Default Re: Catamarans, beam, and stability question

    Not normally, and the anti-matter isolation is said to be fairly solid, too.

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Woxbox View Post
    Not normally, and the anti-matter isolation is said to be fairly solid, too.
    Its a wild looking boat, alright. Would be neat to see something that long and narrow worked up Polynesian style like some of the Wharrams.

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Woxbox View Post
    Not normally, and the anti-matter isolation is said to be fairly solid, too.
    Glad you didn't interpret my feeble attempt at humour to be ridicule.

  32. #32
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    Default Re: Catamarans, beam, and stability question

    Wharram does very traditional, too. But this is still too wide to trailer.

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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by Woxbox View Post
    Wharram does very traditional, too. But this is still too wide to trailer.
    One of my favourites. Wish I lived some place warm.

  34. #34
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    Default Re: Catamarans, beam, and stability question



    Chris Ostlind, nice looking catamaran! I really like the bevel along the roof line of the cabin. Will study plans be available soon? I would like to see what the cabin is like between the hulls – how much clearance above the waterline? The sugar scoop transoms are a nice touch.


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    Default Re: Catamarans, beam, and stability question

    Quote Originally Posted by kenjamin View Post


    Chris Ostlind, nice looking catamaran! I really like the bevel along the roof line of the cabin. Will study plans be available soon? I would like to see what the cabin is like between the hulls – how much clearance above the waterline? The sugar scoop transoms are a nice touch.


    I'm an unrepentant fan of Chris' work and think for a small family or couple there are few small designs that offer the comfort of a small cat. If you don't intend to trailer through too many states, he has a 10' beam option that makes for a really sweet boat with much more comforting stability figures. However, conservatively rigged, the 8.5' beam boat would be fine as well even in trying conditions. Underdeck slamming is nothing to underestimate though. While sailing in the islands b/w Vancouver and the mainland on G & S (a 36' modified Tennant Tourissimo 10 with a salon between the hulls) we encountered 8' waves that literally lifted and dropped the center salon with alarming force. All multis can slap, but usually reducing sail will mitigate that to a large degree. We were sailing 10-12 knots upwind into solid seas at the time...

    Anyhow, Chris' design is sweet and prettier than the competition in the Jarcat 6 and the Waller 670. For 99% of the cruising I do this would be perfect for me.

    Dan

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