Please Sanity Check My Catamaran Design

Discussion in 'Multihulls' started by Nautically Obsessed, Mar 4, 2018.

  1. TANSL
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    TANSL Senior Member

    I was not referring to you, I think it is obvious, as you, apparently, have experience in catamaran design. But my idea is that a "good design" could be, in some cases, impossible to build by the client that has commissioned it. A "good" design can be defined in several ways but a design that does not take into account the technical, mechanical and human means of the builder can never be a "good" design. Therefore, the design that the OP has made can be a super good design for him.
    In summary, a "good design" could be "harder to build than a poor one".
    At your disposal for anything you may need.
     
    Last edited: Mar 9, 2018
  2. Nautically Obsessed
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    Nautically Obsessed Junior Member

    Thanks again. I'm re-evaluating now. As Gonzo implied, it may not be too much of an adjustment to add rocker. I see that most beach cats have banana-shaped hulls. I would also like to go 12 feet if I can. I found some plans for the "hobby kat", from an old Mechanic's Illustrated article that was posted, which I will study.
     
  3. gonzo
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    gonzo Senior Member

    There are several other fundamental errors mentioned here. For example, the hulls if completely submerged will displace about 730 lb. The boat with motor and battery, anchor etc. will weigh 200 lb plus two people 340 lb plus some minimal gear 40 lb equals 590 lb. The reserve flotation is about 140 lb. The deck will be about 3 inches above the waterline. In a short length like you need, a punt or jonboat is the usual and best solution.
     
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  4. TANSL
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    TANSL Senior Member

    The OP says that each hull displaces 629 lb, so since both things can not be, someone should redo their calculations and correct an error that seems quite large.
    This is what some would call a "fundamental error" that can make a "good" design appear very bad, or vice versa.

    Best solution, .... for what?
     
    Last edited: Mar 8, 2018
  5. Nautically Obsessed
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    Nautically Obsessed Junior Member

    I calculated about 10 cu ft per hull using Volume = 1/2h X b x l. That is about 620 lbs per flotation per hull, using the fresh water weight of 62.4 lbs per cu ft. Length average 7.5 ft (subtracting about 1/2 foot for the bow point). Base is 18". Height is an average of 21.5".
     
  6. tmark
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    tmark Junior Member

    Start with two hulls something like this.
    They are 15' x 20". Center height is 12" with 3" rocker at either end.
    They are 4 panel construction (top bottom and two sides) and built with a simple chine log.
    Total displacement is 550lbs at 4", 700lbs at 5" and 850lbs at 6"

    [​IMG] [​IMG] [​IMG]
     
  7. gonzo
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    gonzo Senior Member

    I must have gotten the numbers wrong from the drawing. In that case, the deck will be about 5.5 inches above water.
     
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  8. Nautically Obsessed
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    Nautically Obsessed Junior Member

    I will look into it. Thanks for working that up.
     
  9. Manfred.pech
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    Manfred.pech Senior Member

    The best I have seen here are the well tested and over the years developed and easy to build small Catamarans of SAILHAND and Bob Oram.
    [​IMG]

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    Last edited: Mar 9, 2018
  10. Nautically Obsessed
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    Nautically Obsessed Junior Member

    That is inspiring !. I need the hulls to bolt together though. It gives me some ideas to work on....
     
  11. upchurchmr
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    upchurchmr Senior Member

    8' is too short, especially for sail.
    When sailing the sail can tip the hull sideways and forward. You need to take both into account
    You need displacement with a reasonable amount of freeboard - 10" to 12" would be a minimum and that wont take much in the way of waves.
    The "design weight" needs to be estimated and the immersion of the hulls (and freeboard) needs to be determined.
    As was said the triangular hull cross section is the worst possible for load carrying and drag.

    5' width is very narrow, especially for 2 people - those people look really small.

    Don't make each hull in two peices - adds weight, complexity - for no apparent reason. You don't need any internal bulkheads for 8', and they certainly don't need glass reinforcement.
    Perhaps you should look at kayak designs. Guillemot designs use foam bulkheads to keep water out of the ends - no strength required.
    You might also look at "human powered designs" to see what can be easily driven thru the water.

    Look on Duckworks for the designer with two models, there is a commercial rotamolded model, and there have been several threads on the same subject. Here or on woodenboat forum.
    Last there is another person with the same idea posting today.
     
  12. Nautically Obsessed
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    Nautically Obsessed Junior Member

    I researched the kayak designs. However, it seems to me they are necessarily made wide enough to fit and balance a person. In fact, expert-level kayaks are narrow. From what I gather, a narrow hull is more efficient / faster / lighter.

    It seems like I would be well-served to build or modify hulls of The Glider offered at duckworksbbs.com:
    upload_2018-3-10_12-16-45.png


    I would have a wider "bridgedeck", and it would need to support two people, an outboard, and the weight of the boat. So I might have to adapt the design to fit my weights.

    It would be more fun if the boat would go fast, like 12 knots or so. I haven't researched yet planing vs displacement. I do like the idea of Skoota at Sailing Catamarans - Home http://www.sailingcatamarans.com/
     
    Last edited: Mar 10, 2018
  13. upchurchmr
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    upchurchmr Senior Member

    Why don't you send Gumprect a question?
    Might be better to talk to someone who designed the boat.
    Don't get your hopes up about planing - lots have gone down that rabbit hole in a short narrow hulled catamaran.

    If you widen the bridgedeck will you be able to sit with a leg on each side like the designers pictures?

    Please, please don't use the Hobby cat as a good model for anything. The narrow, almost triangular hulls will support very little weight, the banana shaped hulls are not very typical of the majority of catamarans.
     
  14. Clarkey
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    Clarkey Junior Member


  15. Manfred.pech
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    Manfred.pech Senior Member

    Catamaran purists will not like these hulls. But the Atlanic was crossed with a similar boat ( The crossing of the Atlantic without compass by the twin brothers Emmanuel and Maximilien BERQUE on their outrigger canoe MICROMEGAS 3 / Adventure/Sail/Surf/Film/Video/Photo/Book http://www.creartisto.com/sansboussole/index_en.html ) . This one here is a design from Jerome Delaunay (Katta 395 : Katta 395 http://www.duckworksbbs.com/product-p/jd-katta395-id.htm )
    [​IMG]
    [​IMG]
     
    Last edited: Mar 11, 2018
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