My current Project-Pontoon Junk Raft.

Discussion in 'Boatbuilding' started by djwkd, Jan 27, 2009.

  1. djwkd
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    djwkd Senior Member

    Hi everyone-
    Just to let you all know about my most recent project-it's a 3' long junk Pontoon Raft made of plywood for the hulls and foam for the floatation.The deck will be made from scrap wood.

    My Question-
    The Draft is only 8".Will this be enough, considering that the foam will give extra buoyancy to it?


    Thanks,Dominic.
     
  2. Petros
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    Petros Senior Member

    We need more information to give you an answer:

    What is your intended use? Is is a model, toy or human carrying, at 3 ft long it will not carry much, or is that the correct length?

    What is the total weight, or payload weight?

    what is your beam?

    How is it powered, sail, paddle or motor?

    What is your hull/deck configuration, monohull or catamaran?
     
  3. djwkd
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    djwkd Senior Member

    1-For use in still waters carrying humans.3' is the correct length(i'm not very tall lol)
    2-weight shouldn't be more than about 160 Ibs (73 KG)
    3-Beam is 3' also, (Its because of the limit of the plywood i have) but only 2' of that is of the hull.
    4-It is powered with a paddle.
    5-It is catamaran-there are two 1' Wide pontoons.
     
  4. robherc
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    robherc Designer/Hobbyist

    Well, if your pontoons are perfectly rectangular, at 1' square all the way through, you're only talking about a TOTAL of 372lbs displacement. You're also talking about only 186lbs buoyancy per pontoon, which means that you'll get your feet wet if you get too close to either side, and that's assuming you are on mirror-flat water!

    I'd highly recommend going with slightly larger pontoons; or wearing a wet suit.
     
  5. djwkd
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    djwkd Senior Member

    Do the Bottles and foam not add extra buyoancy, or have you considered that in your calculations?

    Thanks,Dominic.
     
  6. djwkd
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    djwkd Senior Member

    Oh-Just noticed-the name of the thread-The boat isnt a sailing boat, its not a Junk raft as in the sail rig, its junk as in scrap.
    Hope this clarify's things.
     
  7. robherc
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    robherc Designer/Hobbyist

    The bottles and foam keep you from losing buoyancy when everything goes to hell, and the boat fills with water (I.E. hole in a pontoon).

    In everyday use, though, they actually have weight to them, and thus slightly reduce buoyancy. The only way you could improve your buoyancy to .62lbs/cu.ft. would be to fill your pontoons with helium, or draw a vacuum in them...neither one is a very realistic approach, however.

    My recommendation would be to go with (at MINIMUM) 4' long x 18" wide x 18" high pontoons (assuming you make them square-profiled). This would give you up to 558lbs (9 cu ft) buoyancy in each pontoon...that's probably still gonna make for a wet ride in any kind of waves, but at least you'll be staying ABOVE the water.

    Myself, I don't plan on going out on anything smaller than my 9'4" x 4'10" cat. The "pontoons" on that one slope from 8' x 12" at the bottom, to 9'4" x 18" at the top. They give me about 15cu.ft./930lbs. of displacement/pontoon before I end up submerged, and yield about 11" of freeboard with only me on-board, so I can stay dry(er) on most protected/semi-protected inland lakes. If I decide to take that one out in the Gulf, I'll be expecting to come home wet! ;)
     
  8. robherc
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    robherc Designer/Hobbyist

    Just re-read your 1st post...if you're wanting to make the pontoons 3' long by 12" wide, and only 8" draft, you're only going to have 2 cubic feet, or 124lbs per pontoon...that's less buoyancy than some inflatable pool floats!
     
  9. djwkd
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    djwkd Senior Member

    Hmm...What i may do to save my back (from kneeling down marking ply wood again, including erasing current markings) is build another 2 pontoons of the same dimensions (although maybe just 2' long, i dont know) and attach them to the stern or bow of the raft.
    It will also help me complete the project as i will delve into the building of the first two pontoons fairly quickly.
    What do you think of this idea?
     
  10. robherc
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    robherc Designer/Hobbyist

    Add the Length x width x height (all measurements in inches) of all four pontoons together and divide the total by 1728. This will give you four total raft displacement in cuft; multiply that number by 62 and you have your displacement in pounds.
    If you're happy getting wet while on your raft, you MIGHT be ok with as little as 400lbs (6.5cuft) displacement (though that'd be REALLY pushing it and the raft would want to flip over every time you got near the edge). If you want to stay relatively dry (i.e. don't get your feet wet every time you dip the paddle in the water, shift your weight, or encounter a wave that's more than 2" high) I'd recommend staying on the high side of 600lbs (9.7cuft) displacement.

    In the end, it's your raft, your life, and your decision...I'm just telling you my opinion. Any way you go about it, I wouldn't dare take a new/experimental (like this raft) craft anywhere I wouldn't be able to easily swim to shore from, until I'd put it through some fairly extensive testing (including leaving it on the water for a few days to see if anything leaks).

    Hope that was helpful,
    Rob
     
  11. djwkd
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    djwkd Senior Member

    Thanks rob.I have decided that i will make the raft with 4 pontoons, as this will give 744lbs of displacement.
     
  12. robherc
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    robherc Designer/Hobbyist

    744lbs sounds pretty safe to me...show us some pictures once it's complete :)
     
  13. djwkd
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    djwkd Senior Member

    Ah...Due to my masterful Plywood Marking, the width of each pontoon will be 8" also.
    I cant however, seem to get the calculations right on the new amount of buoyancy, as i keep getting 320lbs Per pontoon, which cannot be right.
    Could anyone help me out?

    Thanks,Dominic.
     
  14. robherc
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    robherc Designer/Hobbyist

    What are your overall dimensions for each pontoon now?
     

  15. djwkd
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    djwkd Senior Member

    3' long 8" deep and 8" wide.
     
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