Designing a solo 16' Cruising Cat(amaran)

Discussion in 'Boat Design' started by LP, Sep 11, 2015.

  1. LP
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    LP Flying Boatman

    I've hit a design inspiration and have been following it down several paths. Downsized cruisers(coastal) for the solo sailor or maybe a couple that can handle close quarters, but not to the extreme of Paradox. I'm working three different designs, but this one is holding my attention the most and is the only one that is a cat design. I have never played with any catamaran designs so this is very preliminary and inherently flawed, but I want to put it out for the my fellow forum members to chew on and I'm hoping get some valid feedback for improving the design.

    Basic SORs include 16' LOA and 8' Beam (7' for lower ama sections to fit low on trailer between fenders), trailerablility (road legal), lightly constructed in plywood, accommodations for one (possibly two), 1-2 week capability, open water within acceptable weather windows, spartan accommodations (camp style), sloop rig with maybe a mizzen for self steer capability (yawl),

    I've roughed a model in Freeship with about 1200 pound displacement and about 800 lbs of hull. 400 lbs of useful load might be a bit of a tight budget, but I'm concerned about trying to build too much displacement into the model.

    The design originally had more rocker. but I flattened the run (dampen pitching moments?). I need to track down some good cat design reading media if anyone has any suggestions. I have to admit that I am in a bit of a quandary about finishing off the trailing edge of the amas. Especially regarding amount of emersion and width. I'm torn between deep and narrow or wide and shallow and currently have neither. I know I will need to changes things up and would like positive criticisms about my design so far.

    BoxCat16-01b_Linesplan.jpg
     
  2. gonzo
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    gonzo Senior Member

    A Hobie Cat 16 is 1/4 of the weight. They are not lightweight by modern standards. The weight you are calculating seems really high.
     
  3. rasorinc
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    rasorinc Senior Member

  4. LP
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    LP Flying Boatman

    Hey, Gonzo, Thanks.

    It seemed high to me and I've probably been conservative in my weight estimates (on the high side). It's still very early in the design process. I'll need to go back re-scant my materials list. I may need to start looking ay this as a skin on frame type of build where the skin is ply and the internal structure is load based.

    Rasorinc, thanks for the link.

    Powercat. Saw they had their hull weight at 700 with some very similar scantlings to what I have. They don't mention a top speed, but I think I will be considerably less so I feel that reinforces the need to reduce my scantlings a bit in order to lighten it up some.

    Here is a link to Miss Cindy. A catamaran in the same genre as I am suggesting. Looks like he built in mostly 1/4" ply and came in at 500 lbs. That would certainly be a nice number to beat.

    http://turtleislands.net/tmc/specequip.html
     
  5. Manfred.pech
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    Manfred.pech Senior Member

  6. LP
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    LP Flying Boatman

    Wouldn't you?

    Just back it down the launch ramp.

    No problem. :p

    image.jpg
     
  7. peterAustralia
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    peterAustralia Senior Member

    ahem
    a 16 x 8ft cruising catamaran at around 500 pounds (why the extra 300 pounds, I dont know)

    Why the chine on the outside of the hulls, why not just make it simple and flat


    Gee, kinda sounds like,,, Slider by Ray Aldridge
    plus u can buy plans
    http://www.duckworksmagazine.com/09/designs/slider/index.htm
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    The mode of transportation is in pair with the design with its rocking chair hulls...Poor car, I have pain for it. The engineer who designed the car's roof is crying, he made all his calculations for 200 pounds of luggage. The suspensions are destroyed. God is sometimes good with the fools, the guy survived and was not arrested by the police.

    "Gee, kinda sounds like,,, Slider by Ray Aldridge
    plus u can buy plans
    http://www.duckworksmagazine.com/09/...ider/index.htm"
    That's very antiquated...But at least rational except the sail plan.

    I do not understand the wish, first of making a 16 feet when a 18 feet will cost just a bit more, second to put dwarf cramped cabins on a small cat, it's simply useless and even dangerous. A good tent is far better.
    I hate to be critic but the design, besides other flaws, is far too complicated; I imagine all the pieces to draw, cut, adjust, glue for just a 16 feet cat. Hours, hours, hours with all these details. I shiver of the idea of sanding, puttying, sanding again all these corners. A full deck on a 16 feet is useless, except as hydrodynamic brake.
    Wheel fenders of the trailer are not a good reason for losing a substantial part of stability.

    I have some experience on coastal "cruising" on small monohull and cats. If you can get some spartan amenities for two on a 18 to 20 feet monohull, it's simply impossible to have it on a cat of same size without sacrificing totally the performances. That gives an ugly, desperately slow and dangerous cat, the windage of the cabin kills any possibility of going decently upwind. Do not forget that on small cat, the weight of the skipper and eventual crew are primordial for keeping the boat upright. Daggerboards, or a very good design of the hull like the Dart 18 or Keltic 17, are essential to be able to tack fast and sure. Your life may depend on in a tricky situation.

    A small cat has to be kept light if you want better hydrodynamics than a barn with closed doors. I made a few miles raiding along the coast of Brittany with very different boats from the 18 feet Muscadet to the F40 catamaran and trimaran.
    In small cats there were 2 clear winners;
    1- a 20 feet cat based on the Tornado. Wings and double trapeze. It had even a gennaker. Hardware for righting the cat in sea. A true beast only for experienced people. 20+ knots. Owned by a couple, the wife was 6 foot, 200 pounds and stronger than a Russian woodcutter.
    2- a 18 feet Hobie Cat with wings, I love wings on a coastal cat; sit without butt aches, legs in a natural position, out of reach of the spray, clear vision. The Hobie 18 is strong, stable, comfortable, it's not stopped by a chop and it's not afraid by a breeze. Good performances without effort. Modifications: hardware for righting the cat in sea, special hatches for keeping the material in the hulls, 2 waterproof bags tied to the front beam, a chinese scull as engine, 2 small anchors, lines for the anchors, a nice DIY tent, some kitchen items and you can go. Simple, sure, funny and far more comfortable you can imagine.
    There are a lot waiting a new owner, as for a "regatter" the boat is not competitive and no more built.
     
  9. LP
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    LP Flying Boatman

    Thanks for the input Peter.

    I'm not sure I follow about the extra 300 lbs. Over a Hobie 16? Even the Slider is promoted at 500 lbs. That seems like a lot for an open cat. I think a cruiser needs a cabin and cabins and accommodations are going to have a weight penalty. I would be eager to see a cruising cat that comes in at 200 lbs and offers the weather protection of a cabin. :cool:

    I'm trying to fit the hulls between the fenders on an 8'6" wide trailer. The step would also add strength to an otherwise large flat panel. I am also looking at going the opposite direction where the hulls ride outside of the wheels "a la pontoon" trailer. It may be easier to find a short narrow trailer for the task rather than a wide short trailer. Pontoon style would let me remove the step.


    Thanks for the link, though it doesn't fit my SORs.
     
  10. LP
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    LP Flying Boatman

    Ilan, Thanks for the critiques.

    Why not 20 instead of 18......why not 22 instead of 20........ It's a viscous circle and I'm too early in the design to start pushing outside my intended design envelope. I'm building a 24' mono right now where a 20-21'er might have been enough. As you know boats cube with length, but maybe not quite so if it is only the amas that are growing. 16 is my chosen length....for now. 18' is not unreasonable and even 20 if I were only extending the amas, it would do wonderful things with regard to accommodations and rig. It would move the CLR forward and give me better placement for the rig. Still very early in the process.

    I would like to hear why it is useless and dangerous; emphasis on dangerous. I just can't agree with the tent notion. Apologies.

    Don't worry about being a critic. That is why I put up the thread. I am in no way an expert and I value constructive criticism from those with experience where I am lacking. Shiver, shiver, shiver. I agree!:D I guess I'm not expecting to bury the bows. I'm not expecting phenomenal performance, but if I can get double the speed of a mono of the same length, then the design will be a success. Note taken though.:idea:


    :eek: Part of the process. I am looking at an alternative. See my previous post.

    Interesting information. Thanks again for sharing.
     
  11. Stumble
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    Stumble Senior Member

    For the trailer, just do what we do on the A-Cats and rest the boat on the cross beams. No need to let the hulls rest on pads. The cross beams should be more than strong enough, and it eliminates the need for worrying about the trailer at all. Alternatively just raise the pads for the hulls high enough to clear the fender. No boat should be designed around a trailers fender, the trailer needs modification instead.
     
  12. LP
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    LP Flying Boatman

    Hi, Stumble.

    I agree that the trailer shouldn't be the sole driving force in my design. I think that it needs to be a consideration to optimize launch characteristics. The initial stability calcs of this design give it close to four times the initial stability of a 6' beamed mono design I'm playing with. I didn't feel that the "notched" hulls were necessarily a bad thing. Admittedly, I was giving up speed potential by giving up potential righting moment.

    I plan to look some alternative configurations. I can't say that I have cross beams in the conventional sense. There will be internal bracing that accomplishes the same task and should present hard points for supporting the craft on the trailer from under the bridgedeck, if that is the method of support I use.

    Thanks for contributing.
     
  13. peterAustralia
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    peterAustralia Senior Member

    what about a Jarcat 5
    5m catamaran with a cabin
     
  14. LP
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    LP Flying Boatman

    Looks very similar to what I have been working with. I noticed in the specifications that it weighs in at 640 lbs. My own initial weigh estimate is definitely on the high side, but it is looking like anything less than 500 lbs is not a reality. At least with any kind of a cabin.
     

  15. Steve W
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    Steve W Senior Member

    Fun project, I do understand the desire to design your own although there are plenty of existing designs to choose from. The Jarcats would be good ones to look at for scantlings. Why only 8 ft beam when the legal road width is 8ft 6" the extra 6"would be huge in a boat this size. You could use a narrower trailer with the wheels inside the hulls supporting the boat under the bridgedeck instead of the hulls. These trailers are readily available as they are used for pontoon boats. I believe the Gougeons used such a trailer for their 32ft cat years ago. This opens up a lot of design options for the actual boat.

    Steve.
     
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