i need a set of plans for a cruising hull for a 18 foot catamaran

Discussion in 'Multihulls' started by shmuelbrody, May 12, 2013.

  1. shmuelbrody
    Joined: May 2013
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    shmuelbrody Junior Member

    i am building a catamaran 18 by 8 feet

    it will have a full deck and the deck will weigh about 400 lbs
    i want a set of plans for hulls that can support 3000 to 4500 lbs

    i am not really worried about speed as this is going to be
    a pleasure boat no racing
    sailing in the ocean

    the deck will have a small cabin on it that is basically
    a basket covered in building wrap

    it is dismantable

    please let me know what you think
    shmuel

    please se my other post on my 12 foot sharpie
    thanks
     
  2. redreuben
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    redreuben redreuben

  3. Corley
    Joined: Oct 2009
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    Corley epoxy coated

    I'd have a look at Ross Turner's Jarcats for inspiration too. A lot of good design smarts in a small package and a good double berth.

    http://www.jarcatmarine.com/
     
  4. shmuelbrody
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    shmuelbrody Junior Member

    thanks red and corley
    slider looks stunning
    id like to see her plans
    she has just the hulls that i was looking for
    do you think that they are dismountable from the deck
    thanks again
    shmuel
     
  5. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    No can do

    Schmuel - it can't be done

    You can't get an 18ft cat to take 2000kg - well you can if you make it into a punt like a barge but some quick calcs show why it will not work at all well.

    Each hull will have to take 1000kg. That's 1 cubic metre of immersed hull in 18ft of waterline. Jarcats can't come close to this. To carry 2000kg of crew or payload you need a cat around 40ft long.

    Get some graph paper and draw some sections. Then calculate the volume in a reasonable shape. Then remember that a cat hull has to be designed to take the WHOLE weight of the boat when flying a hull.

    Why so heavy?

    Phil
     
  6. rwatson
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    rwatson Senior Member

    He has a family of nine, based on the previous thread.

    You should read it, it has some fascinating insights :rolleyes:
     
  7. shmuelbrody
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    shmuelbrody Junior Member

    thanks rwatson
    and this week my son just had twins

    by the way i just read a great article about
    using wave power to power a boat
    i am going to try th at on my sharpie

    by the way again redreuben
    turned me on to slider a duckworks boat

    hello catsketer
    please do the math
    30 cm x 18 x 30 cm
    equals 1.62 cubic meters
    times 2.2 lbs
    thats 3564 pound of lift
    for one pontoon
    time 2
    equals over 7000 pounds of lift
    thats a pontoon 1 foot by one ft by 18 feet times
    if you use metric its much easier

    if you make the pontoons
    1/2 foot by 1 foot (15cm x 30cm times 18 feet) times 2 hulls
    you get 3500 lbs of lift
    excuse me but i dont know too much about boats
    i have only built one sharpie
    maybe i am wrong

    shmuel
     
  8. shmuelbrody
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    shmuelbrody Junior Member

    thats a pontoon 1 foot by one ft by 18 feet times
    should read
    thats a pontoon 1 foot by one ft by 18 feet
     
  9. bregalad
    Joined: Dec 2010
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    bregalad Senior Member

    @shmuelbrody
    Please redo the math
    "30 cm x 18 x 30 cm equals 1.62 cubic meters"
    You went from an 18 foot craft to one 18 meters long. BIG difference.

    "if you make the pontoons
    1/2 foot by 1 foot (15cm x 30cm times 18 feet) times 2 hulls you get 3500 lbs of lift"

    .5' x 1' x 18' x 2 hulls = 18 cu ft x 64 #.cu. ft (approx. wt. of salt water)= 1152lbs.
     
  10. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    My 22ft Janus design is similar to a larger Slider. Janus has basic dimensions of LWL 6m, BWL 0.45m, depth 0.2m. A rectangular box to those dimensions is 0.54T. However (simplifying) the boat comes to a point, so roughly only 2/3rds of the box is actually boat. Thus each hull supports only 360kgs, a long way short of what you want. However it does sail well with 6 people on board, as the photo on my website shows.

    http://sailingcatamarans.com/index.php/designs/2-catamarans-under-25ft/166-janus

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  11. shmuelbrody
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    shmuelbrody Junior Member

    dear bregalad
    i beg your pardon
    30 cm is one foot
    .3 meters times .3 meters
    equals .09 sq meter
    .3 meters x18 equals 5.4 meters
    equals .486 cubic meters
    5.4 x.09 sq meters
    .486 cubic meters = 486 liters x 2.2 lbs / liter
    = 1068 lbs
    times 2 pontoons = 2138 lbs

    you are correct
    however not absolutely

    because my original calculation NOT MENTIONED HERE
    was for a pontoon 45 cm wide and 60 cm deep
    which would be 6414 lbs
     
  12. shmuelbrody
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    shmuelbrody Junior Member

    the reason for this was to create a pontoon that was high off the water
    like the slider
     
  13. shmuelbrody
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    shmuelbrody Junior Member

    now you will say that given tapering and rounding
    will cut down your stats
    yes given
    so a 2 1/2 foot pontoon will make up for that
    lets chat about that
    shmuel
     
  14. bregalad
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    bregalad Senior Member

    You have changed the parameters. First you said:
    if you make the pontoons 1/2 foot by 1 foot (15cm x 30cm times 18 feet) times 2 hulls"

    Now you calculate the volume for .3m x .3m x 5.4m x 2 hulls.

    All of that aside you are working way outside the parameters for an 18' catamaran with anything resembling decent sailing performance.

    There are many nice boats on Richard's web site. Some in the length you claim you want, but way shy in carrying capacity. Other, larger designs would carry your stated payload.

    What you have proposed would prove disappointing.
     

  15. shmuelbrody
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    shmuelbrody Junior Member

    i am not doing a double shuffle
    i dont know how 15 centimeters got into the conversation
    it was a mistake
    perhaps i made it
    and i dont need 7000 lbs
    that was theory
    i spoke about 7000
    because i want to get up above t he water and be more dry
    the slider has wide hulls that a person can sit in and it is 16 feet
    and they seem to fairly deep
    he says the the boat weighs 500
    and im not sure of carrying capacity
    but near to 700 lbs
    did you read my original post on making my sharpie
    into a v bottom boat
    there is delineated many reasons why i need a certain kind of boat
    please do read it
    it will clarify this conversation
    as doc watson suggested
    i dont want to write it again
     
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