weight calculations for pontoons

Discussion in 'Boat Design' started by pjmar10, Nov 2, 2015.

  1. pjmar10
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    pjmar10 Junior Member

    I am planning on building a stationary boat/ house on aluminum pontoons specifically I plan on using four pre-manufactured 26' x 25" aluminum pontoons. Can the optimum carrying weight of the pontoons be calculated?
     
  2. TANSL
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    TANSL Senior Member

    Welcome to the forum.
    The first thing you should know, or calculate,are the hydrostatic values of the pontoons. Once this is done, calculate the weight of the pontoon with all their equipment, know the rules you must follow for security and stability, for example the minimum freeboard required, and with all that can you deduct the maximum (not optimal) loading the barge can carry.
    I will gladly help you if you give me enough data from the pontoons and the set that you want to build.
     
  3. pjmar10
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    pjmar10 Junior Member

  4. TANSL
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    TANSL Senior Member

    Frankly, I do not see enough information to make any serious estimation.
    It Think twice before you buy a product they do not offer any guarantees. There may be hidden defects that take months to appear.
     
  5. pjmar10
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    pjmar10 Junior Member

    logs

    what information do you need, I am not concerned about warranties as I will hydro-static test the logs as well as ultrasonic test all circumferential and lateral welds.
     
  6. TANSL
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    TANSL Senior Member

    Body lines and weights.
    I feel very good that you inspection welds because some, circunferentials, not look too good.
     
  7. lance linked
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    lance linked Junior Member

    25" circular logs 26 feet in length are calculated at 490 square inch section (3.4 feet). 3.4 x 26 = 88 cubic feet. a cubic foot of space holds about 7.4 gallons water, so displaces about 62 pounds. 88 X 62 = 5456 pounds fully immersed displacement per log, assuming no log weight and fill dimensional fill.

    if we assume that the aluminum sheet these are made from is .125" thick, we see a weight of about 1.75 pounds square foot. this works out to about 200 pounds per log, plus 10% for bracketing and hardware.

    you don't want more than 40% immersion in fully loaded condition, so your total live and dead load is just over a ton per log. you are better off with less than 40% immersion, more like 30%, so we figure closer to 1500 pounds per log.

    as for UT inspection of .125" aluminum joints, hogwash. UT is one of the fields I work in, and what you pre proposing is un-validatable, as well as prohibitively expensive. you can do a visual inspection, looking for wear points or obvious flaws, then a static pressure test at about 15-20 pounds pressure, and know everything you need to know. leaks can be detected with a spray bottle and watered down dish detergent, and marked for GTAW repair.
     
  8. pjmar10
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    pjmar10 Junior Member

    Thanks very much for the info

    John
     
  9. TANSL
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    TANSL Senior Member

    Be careful because pontoons have sharp ends and therefore the displacement is smaller.
    On the other hand, in my opinion, the inner structure and hardware can be a lot more than 10% by weight. But I'm just stating my opinion, since we know almost nothing of the pontoons.
    Lance linked, when you talk about the 40% or 30% immersion, you are talking about % of volume or depth, because the result can be very different.
     
  10. gonzo
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    gonzo Senior Member

    On pontoons the % of immersion refers to the diameter, not the volume.
     
  11. TANSL
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    TANSL Senior Member

    That will be true, perhaps (let me hesitate, but I do not know), on cylindrical, circular section, pontoons. In all other pontoons that can't be right.;)
     
  12. lance linked
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    lance linked Junior Member

    I was referring to % of displacement. 30-40% of displacement is startlingly close to midpoint of diameter when using round section logs, as opposed to boxes. this is actually desireable, so the differential between dead load and actual live is not a significant distance on the freeboard. this helps if your platform is attached to other structures or platforms, and when adding entry features like swim platforms and docking piers to your structure. it is also a good safety factor if you have a catastrophic or gradual failure in one of your floatation chambers.
     
  13. messabout
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    messabout Senior Member

    Forty percent of the 25 inch tube is 10 inches of draft. Beware: the displacement is not 40 percent of the total displacement of the tube. The 25 inch tube has an area of 490.87 square inches. The displaced area of the tube at 40 percent immersion is about 183 square inches , not 196 which is 40 percent of the 490.87. At lesser draft ....say 30% the difference is dramatically different. from the mere six percent area differential at 40%.

    Some of us can amuse ourselves with the math. At 10 inch draft we need to know how wide the chord is. Call draft B and chord C. But we don't know how wide the chord is which is actually the waterline beam until we fiddle with the numbers. Here we go...........C ^2 = 8BR - 4B^2 In this case of 25 inch diameter, 12.5 inch radius, the waterline beam, or chord, is 24.49 inches. .....turns out to be about 141 degrees of wetted wrap around or something like 31 inches.

    With a bit of basic trig we can determine the displaced area. The tubes are to be 25 feet long and let us presume that the front end will be five foot cone shaped. The cones would displace maybe 120 pounds of water at ten inches immersion and the main sections of the tubes at 20 feet long would displace about 1590 pounds. At 30%, displacement will be about 1100 pounds including the small part of the forward cone. WLBM will be 22.9 inches.

    If you calculate this stuff in terms of cubic inches of displacement, you can simply multiply by 0.03611 to get the pounds of displacement for the number of displaced cubic inches of fresh water. This works dandy for small boats. Good for designing boats in a cocktail lounge on a martini napkin.

    For sure the boat will go better, with less power if the total weight is minimized. There is also a considerable safety factor if draft is 30% as opposed to 40%.
     
  14. pjmar10
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    pjmar10 Junior Member

    Thanks for all the cogitation guys, answer me this, do the calculations and estimates change if in fact this boat (actually more of a barge) will never go anywhere, will not leave its slip, will more than likely have a permanent attachment to the shore. I just want it to float and need to know how heavy of a structure to design assuming I have 4- 26'x 25" pontoons. I have a 14' x 24" cabin design that when completly furnished and occupied will weigh about 8000#'s. some of your calculations say it will float nicely and others have it sunk very quickly.
     

  15. lance linked
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    lance linked Junior Member

    your design is very similar to the 16X20 I am planning to build beginning summer 2017, so I have considered the question thoroughly. Messabout assumes you will be planing in rough water, while I assume you will be in the eddies of a slow river. The width of your intended will give you much more stability than a traditional boat. You can go with the 30% buoyancy immersion figure safely, so total dead and live loads (including pontoons weight) over 7000 pounds. If you are at 40% in still water, 9000 pounds is not unreasonable.

    I should note the difference between barges and powered craft may interest you. In my home state of PA a powered craft needs a registration, and, if made at home, may need inspection for seaworthiness and capacity. An unpowered barge requires neither. In some areas it may be advantageous to be registered, as this keeps you soundly within maritime law, and free from civil law and statutes regarding housing. This is often problematic closer to cities, and can involve taxes and codes inspection. Check with your local Natural Resources Office for the top "boat cop" in your area, and have a conversation with him about your ideas.

    So, my plan is 3 barges. Two of them will be "half houses", and the third will be an open deck. The deck will be used with a jon boat to set private bouys in a cove of the local river, to which it will be anchored. The cabin sections will be launched and towed to the deck barge separately, and attached to it, and joined mechanically. On board solar electric, lp gas fired refrig and stove, rainwater recovery and pumped water, kitchen sink, shower and head with a smaller 8 foot long waste punt carrying 80 gallon plastic tank that can be towed to local marina pump out station stored under the platform (PA requires 100% wastewater containment).

    My loaded weight calcs are similar to yours, in proportion. Costs of a build it yourself will be about $2 a pound. Comfort level would be high. Performance of your footprint, underway, wouldn't be impressive. Maybe 6 knots with a 50 Hp outboard centrally. Twin 50's mounted front and rear on axial centerline, then you got something, and talk about parallel parking!
     
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