Cat Design

Discussion in 'Multihulls' started by bayrider, Oct 29, 2006.

  1. bayrider
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    bayrider Junior Member

    Hi

    I'm in the middle of a catamaran design for power (conceptual), energy efficiency is the driving force of this design. I haven't much experience with cats but I am eager to experiment with this hull type. I am trying to minimize resistance as much as possible in order to keep propulsive power down. The design is for smooth water with a max speed of about 6 knots. The preliminary length of the design is 26 feet and the beam is 10. I have been having some difficulty locating info on optimum hull spacing and beams of each the individual hulls. Also which if any is better, asymetrical or symetrical wrt resistance.

    Thanks
     
  2. marshmat
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    marshmat Senior Member

    It's a huge field, bayrider, and there's a lot of variety. http://www.cyberiad.net/multihulls.htm has a few good theoretical papers... Leo, the author of a lot of that stuff, is sometimes found on here. His Michlet program (same site) is absolutely wonderful for this, if you're computer savvy (it's not the most user friendly thing in the world).
     
  3. bayrider
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    bayrider Junior Member

    Hi Matt

    Thanks for the info, I'll be sure to sheck it out.

    bayrider
     
  4. ron17571
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    ron17571 Junior Member

    Easy,thin hulls with little resistance,small four stroke outboard,i hope to one day see what kind of milage such a combo can deliver.keep it simple and light, lately ive been looking at woods design cats.
     
  5. yipster
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    yipster designer

    6 knots is about a normal speed for a 26 feet wl displacement mono hull
    so without going into cat nitty gritty's i dont see great efficiency wins here

    edit: i.o.w. cat has even little more skin friction and at speed mentioned there is little need to minimise wave drag
     
  6. ron17571
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    ron17571 Junior Member

    Yipster, thought the cat would be more slippery,less wetted surface,but i get the feeling you arent as big a cat fan as me.i remember the rolling and heeling of our mono and how much faster our cats were,i once let the entire mono yacht fleet take off at a regatta and while eating lunch and kicking back went way around them all(as not to disturb them)and passed them all by the finish line.im not sure exactly what bayrider is trying to accomplish?either design at slow speeds and a small four stroke i think the fuel usage wouldnt be much.I read post and added to them about buying a sailboat and using a small outboard to convert boat into cheap motor boat also,a twenty some foot sailboat with say a 6hp outboard would cruise along preety cheaply.
     
  7. yipster
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    yipster designer

    me not liking cats? au contraire mon ami, look at the latest drawings in my gallery!
    read some more on hull drag and see that for the given speedrange there is little to win for a cat
    as shown in quikly done drag calculation for a 26 ft hull, note that at 6 knt wavedrag is minimal
     

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  8. bayrider
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    bayrider Junior Member

    Hi Guys

    Read the posts and find them very interesting. The reason I chose a catamaran is because I want narrow slender hulls to minimize resistance. Also, having enough room for six people to lounge about and enough initial stability to keep the comfort level high is of utmost importance. While I am sure a mono-hull of 26 feet can be designed efficiently for a speed of six knots, a long slender mono-hull would probably lack the necessary space and comfort requirements.

    Thanks for the comments

    Bayrider
     
  9. rayaldridge
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    rayaldridge Senior Member

    One thing you might keep in mind is that the cat will be less able to carry the weight of any accomodations than the the monohull would.

    For ideas on long slender monohulls, take a look at Thomas Firth Jones outboard launch, Puxe.
     
  10. yipster
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    yipster designer

  11. bayrider
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    bayrider Junior Member

    Not sure I follow you rayaldridge. Are you suggesting that a monohull can carry more weight in accomodations than a cat of equal length?
     
  12. bayrider
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    bayrider Junior Member

    Oh, bye the way, interesting website Yipster.
     
  13. rayaldridge
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    rayaldridge Senior Member


    In general, this is the case. Weight is the issue. A bridgedeck cabin cat may have greater interior volume than a monohull, but if you fill that volume with stores and heavy boat systems, the cat's performance will be adversely affected.

    This may not be as comparatively true in the case of the sort of long narrow monohull that would be most efficient as a power boat, but monohull sailboats can usually be loaded down with a lot more weight than a multihull of equivalent length, if the multihull is performance oriented. Sailing multihulls are primarily faster than monohulls because they're lighter, or to put it another way, because they have a more favorable power-to-weight ratio. Their great beam gives them the ability to carry a lot of sail without the weight penalty associated with the ballast that monohulls require to carry large amounts of sail.

    There are several useful ways to think about it, from the point of view of a pure power vessel, in my opinion. The monohull will have less wetted surface than a cat of identical displacement (or weight) because the long slender hulls have by their nature more surface area. (The most efficient shape for enclosing volume is a sphere, and the monohull is closer to that shape than the cat's hulls.) On the other hand, the cat will have less wave-making resistance because of the slenderness of the hulls. So, like every design decision, it's a tradeoff. The crucial issue is: how fast do you want to go? If you're content with displacement speeds, the monohull is probably the better choice. If you want to go faster, maybe the cat would serve you better, as long as you can live with the weight limitations.

    An interesting example of a long narrow powerboat is T.F. Jones' Puxe. She's 22' LOA with a beam slightly less than 5 ft. She has a 7.5 hp outboard and cruises at 12 knots, at half-throttle. Pretty efficient.
     
  14. bayrider
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    bayrider Junior Member

    Hey rayaldridge, thanks for the comments. Where can i find some info on T. F. Jones' Boat?
     

  15. rayaldridge
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    rayaldridge Senior Member

    He has a website. Google "Thomas Firth Jones." Unfortunately, he doesn't list plans for Puxe on the site, so I don't know if he sells them. He details the design in his book, New Plywood Boats. He first built the boat when he was a novice boatbuilder, and after quite a few years, the boat began to work its seams and leak, so he built a second version, incorporating what he'd learned about design and construction. My thought on reading that was that it must be a pretty nice boat, if he was willing to build it twice.

    By the way, he's primarily known as a designer of small sailing multihulls.
     
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