Plywood sectional barge

Discussion in 'Boat Design' started by GhostGeek, Nov 7, 2019.

  1. GhostGeek
    Joined: Nov 2019
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    GhostGeek New Member

    I'm interested in floating a standard 20' shipping container with rectangular plywood pontoons.

    The pontoons would be 8 feet long, by four feet wide by four feet high, in order to utilize standard dimensional lumber with a minimum of cutting.

    The 4x8x4 pontoons will utilize galvanized fasteners, and all joints secured and sealed with Titebond II.

    My thought is 3/4" pressure treated plywood, and 4x4 cross members and rails. Overkill?

    a "log" of this size, shape and dimension, would two of them be capable of floating a standard 20' container?

    Thanks for your ideas and insight.
     
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    My first impressions;
    How heavy is the stuff in the container?
    4x4 framing is very stout. 2x4 more appropriate.
    The log would probably roll over and hang from those floats. The raft should be much wider.
    Will it just need to float in a stationary position or be towed around as a barge?
     
  3. bajansailor
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    bajansailor Marine Surveyor

    In theory there should be enough buoyancy in the two plywood pontoons - if we assume that the draft is 2', then the volume of displacement is 8' x 4' x 2' (ie 64 cubic feet) for each pontoon.
    In salt water this is approx 4,100 lbs.
    And for two pontoons it would be 8,200 lbs.
    What is the weight of the empty container - maybe 4,000 lbs?

    All fine in theory re the buoyancy aspect (but you can't put an awful lot of cargo in the container) - however in practice I think you would probably find that this contraption will be very tippy and not very safe at all.
    If each pontoon was 16' long, 4' wide and 4' high, where the 16' is across the width of the container (forming a broad 'I' shape in plan view) then the stability would be much improved.
    For manoeuvering you would probably have to regard it as a catamaran 16' long and 20' overall beam, with 4' wide hulls.

    Edit - some more thoughts.
    What do you want to do with this container / pontoons?
    Will the container be permanently attached to the pontoons?
     
  4. fallguy
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    fallguy Senior Member

    It sounds to me like an instant capsize.

    I very highly doubt that you are stable.

    The length of the pontoons needs to be the length of the container.
     
  5. messabout
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    messabout Senior Member

    What Fallguy said. ^
     
  6. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Need to know what you expect to do with this floating container...
    And why?
     
  7. GhostGeek
    Joined: Nov 2019
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    GhostGeek New Member

    Thanks for all the insight guys.

    fallguy - absolutely correct. Capsize city! :eek:

    Let me elaborate more on my idea...

    My plan is to "tiny house" a shipping container as a live aboard house barge. Build several of these logs, build a large deck on top, and secure the container on the deck.

    Essentially I'd build a raft floating on 4x8x4 sectional logs. The attraction to this design is that (A) they would be inexpensive to build, and (B) would be easy to build quickly, pretty much anywhere. The logs being separate means that if one floods, it can be replaced easily and inexpensively.

    bajansailor - so these logs will each displace roughly 4,000 lbs in salt water. Good to know! I can affordably over-engineer my creation to play it safe. If we need more buoyancy, build another log to displace another two tons!:cool:
     
  8. bajansailor
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    bajansailor Marine Surveyor

    Good morning GG - where are you planning on mooring this houseboat?
    If you are using 8' x 4' x 4' plywood block rafts, buoyancy is the least of your concerns - you will have this in abundance if you have four of these blocks.
    However stability will be a concern - I think you would need to have the hulls in a catamaran configuration in order to get the stability required.
    As a variation of what I proposed earlier, each hull could be 24' long x 4' wide x 4' deep (ie 3 blocks per hull, 6 blocks in total); they could be joined together by 2' deep truss frames giving an overall beam of 16'.
    So you have a cat with deck area 24' x 16'. And a displacement of 24,600 lbs at 2' draft - this should give you lots of cargo capacity for fresh water tanks, stores etc.
    You could have the bow and stern of each hull raked at 45 degrees to reduce the resistance for the times when you want to tow the barge to another location.
    You can then position the container longitudinally in the middle of the deck, and you still have 4' on each side, and either 2' at each end, or 4' at one end and 0' at the other.
    However this still does not give you much free deck space, eg for a table and chairs outside.
    Do you want to build a top deck as well?
     
  9. fallguy
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    fallguy Senior Member

    The logs need to be engineered for the needed size; not the ease of build or ease of replacement, although the latter can factor in.

    And sorry, but that lumber is going to fail. It will open on the endgrains.

    Even green foundations are sealed on emdgrain exposures.

    The deck is structural element as well.

    The shipping container is a horrible idea. They are basically tin cans and transfer heat instantly which means the sun will bake you and the cold shake you. And they are overbuilt on the bottom.

    Sorry, but the idea is a non-starter.
     
  10. bajansailor
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    bajansailor Marine Surveyor

    Fallguy makes a good point above re the container would be an overkill - once you build your raft it might be easier to just build a simple 'Tiki House' of timber on it.
    And if you want to build the hulls and deck 'properly' using glass cloth and epoxy to sheath everything to keep the water out, well, you will be amazed by how much epoxy you will need. Fallguy mentioned elsewhere on this forum about how much epoxy he has used on a 32' catamaran he is building - it is a lot.
    Edit - here is his post mentioning 250 gallons. Although you shouldn't need as much as this, the cost of your epoxy would still be a significant part of the build cost.
    Quality of wood https://www.boatdesign.net/threads/quality-of-wood.63040/#post-863722
     
  11. fallguy
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    fallguy Senior Member

    Also, the wolmanizing would need to be the highest available and that is not done on marine grade plywood.

    Marine grade plywood still requires sealing and end grain exposure is still a problem.

    That said, I have a cold molded mahogany runabout with exposed endgrain. The endgrains have been well sealed throughout the boats life of 60 years. But not subject to constant immersion either; nor salt water.
     
  12. rnlock
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    rnlock Junior Member

    Maybe weld the lower part of the container shut so it can float itself? To keep it from cooking you, cover it with something else that's insulating and white.
    Seems to me that if you built, say, six of those floats, you'd be close to the houseboat already. I've found that even 3/4 oz. glass and epoxy holds up to weather quite well. Not sure if it does,when submerged.
    ------------
    The size of the floats makes me think of the infinite modular sharpie. I think,at one point they had 12 4X8 foot sections together, end to end. Probably not too good in swells!
     
  13. fallguy
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    fallguy Senior Member

    if you are 'not sure' what 3/4 oz glass does submerged; why promote it?

    I have never seen anyone claim 3/4 oz glass was impervious to ingress. If it were; non structural tape would not be 1208 (17 oz glass) and we'd use the 3/4.

    And 3/4 glass and epoxy chalks in weather unpainted.

    Others may see your post and take is as black and white when in fact it os rather chalky.

    I realize I have fallen to polemics, but too many wrongs not to mention.

    His floats are not sized correctly. We would never encourage building a boat for nominal size sheets of ply. It is a ridiculous concept. You further the idea suggesting he use the container as part of the cockamamie notion.

    Have you ever opened the doors on a ship container? They'd get a bit rusty submerged. Then think, well, you'd take the doors off and fab. And by the time you fab new doors and cut in windows and insulate agonst the tin; you'd realize the ship can was a horrible idea!
     

  14. gonzo
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    gonzo Senior Member

    Firstly, Titebond will fail in this application. Secondly, a bunch of small floats will be heavier and way more expensive than a single section barge. Lastly, what is the reason to use a container? They are heavy, which is a really bad quality for something that must float. Build a plywood barge and lightly frame a house on top instead.
     
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