displacemant of a drum

Discussion in 'Boat Design' started by whitepointer23, Feb 9, 2012.

  1. whitepointer23

    whitepointer23 Previous Member

    your houseboat sounds heavy for 25'. i was working on 100 kg per drum and maybe use four rows, i can get as many as i want .
  2. Submarine Tom

    Submarine Tom Previous Member

    It's barge shaped and on the heavier side for sure.

    100 kg per barrel is 50% loading. I believe you are on dangerous ground there.

    As I recommended earlier, 40% loading max.

    You may be able to push to 45%, if your calculations are correct, variable payload minimal but after your opening question, I'd stick with 40%.


    P.S. Four rows, so no longer a cat? Is this a house-boat or a float-home? In retrospect, don't vent them as they may collapse when you need the reserve buoyancy the most!
  3. whitepointer23

    whitepointer23 Previous Member

    okay , so i work on 40%. it is more of a floating cabin than a house boat, i would tow it with my run about. the idea is to moor it and use it for a base when fishing. it would still resemble a cat but 2 drums wide each side. i have been reading a lot of info on the subject and i think something about the size of your houseboat will suit me tom. i just want a basic fit out . portapotti and solar deck shower. nothing to flash to start with. if i like it i can build a bigger version later.
  4. Pluterday
    Joined: Jan 2018
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    Location: Parkland, FL

    Pluterday New Member

    Hi CDK, interested in your comment about effects of water temperature and air pressure on buoyancy....I have a 26 ' boat on a floating lift, wondering if 24 degrees temperature variation could explain a change in buoyancy as shown in the pictures (last thursday 44 F vs today's 68 F minimum temperatures from weather.com) . Pic lower on the water was from thursday when it was colder. I don't have water temperatures but I expect there might be a correlation. Boat is not exactly leveled on the lift so one side rests slightly lower that the other one. Each pontoon has two chambers (aft and forward) independently filled with air by a blower...with valves on the bottom that bubble once filled with air. We have had issues up to very recently that required repairs on the left pontoon, just wondering if the pontoons are up today because of changes in temperature vs someone filling them with air. Appreciate any comments. Thanks!.

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  5. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    A submerged valve doesn't seem like a good idea to me because it only takes a tiny piece of debris to prevent the valve from closing fully. In that case dropping temperature allows water to enter the pontoon.
    For my not fully airtight RIB tubes I bought a Chinese micro air pump and a small 6V solar panel, installed permanently on an air valve. The little 6$ pump from Alibaba.com slowly fills the tube as soon as there is enough sunlight; the pressure is insufficient to cause any damage.
  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

  7. Pluterday
    Joined: Jan 2018
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    Location: Parkland, FL

    Pluterday New Member

    Noted about the valve, now being a commercial lift, any mods will likely void my warranty (system has a controller box with a blower and a system of hoses to let air in and out of the chambers to raise or lower the boat...). My question was more towards picking your brain about buoyancy changes due to temp and air pressure changes...(assume no issues with water intrusion)...do you have any experience on that?....I provided details of the situation in my prior post...any insight appreciated.
  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    This is a complicated problem to solve quantitatively because we are really unable to determine how the volume of the drum or pontoon changes shape due to changes in internal pressure and that in the wide range of ambient air temperatures and less range of temperatures for the water, whose combination will produce an average air temperature within the drum/pontoon.

    We know that only pressure will cause a change in the volume of the pressure vessel. (or do we, more later) and hence bouyancy

    We also know that with a given closed volume ( constant number of molecules) that when we raise the temperature the pressure rises. The Ideal Gas Law being the definitive equation for this PV=nRT
    Pressure times volume equals the number of moles times a constant times temperature
    so with a constant constant (had to ) and a constant number of moles, (number of molecules) then
    Pressure times volume varies with temperature. PV=KT PV~T

    So if we change the temp, either the pressure changes or the volume changes. (or both)

    So to the volume
    If the closed vessel is rigid and will not expand significantly, then pressure varies to temp. Higher temp higher pressure. P~T

    If the closed vessel is extremely soft, like a balloon that has little air in it, the pressure will stay at ambient and only the volume will change V~T

    In our situation we have a semi rigid vessel where the rigidity will resist some of the internal pressure changes but will noticeably change the volume. Hence the canned or alternately
    bloated drum look.

    But there are other parameters in play here is if a person wanted to actually try to get as close as he can to the actual change of buoyancy mathematically wrt to temperature

    1) Assuming a polypropylene shell. Poly has a higher, than stay steel or aluminum, coefficient of expansion. So even if the drum is open to atmospheric pressure, with increasing
    temperature, the diameter of the drum will increase. Ie buoyancy will increase. The corollary is true as well, decreasing temp reduces volume
    2) In a closed vessel, which is what we are talking about here, with increasing temp change and hence increasing pressure, the volume will also increase due to the poly stretching, low modulus of elasticity, and increase the buoyancy.
    3) the modulus of elasticity, roughly, the amount of strain (stretch per unit length) that material exhibits due to a load, changes with temperature. Ie as the temperature rises, the material gets softer and stretches more than at colder temperatures
    So this drum, at -10 degrees C will be more rigid than at 40 Degrees C
    4) At some point with decreasing pressure, the drum might deflect inwards, radically, reducing the volume when it pops in

    So temperature will affect volume of the drum in the following ways, and I will only include increasing volume, same is true for decreasing except with item 4
    1) Higher temperature increases the pressure and the poly, will stretch increasing volume and buoyancy
    2) Higher temperature will increase the materials length and circumference of the skin due to the coefficient of expansion, increased buoyancy
    3) Higher temperature will increase the material length and circumference due to a decreasing modulus of elasticity, increased buoyancy

    So if you are worried about the above impacting your drums, why not fill them with a low density foam? 1 pound per cubic foot as an example

    I think that a person would be able then to mathematically predict with some accuracy the change in volume/buoyancy due to temperature but it would be difficult to predict
    at what pressure and temperature that oil canning could occur, ie major deflection of part of the drums wall as the temperature was lowered
    Last edited: Jan 25, 2018

  9. thought
    Joined: Aug 2018
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    Location: florida

    thought New Member

    Hi, new to this conversation, but has anyone tried banding sealed plastic barrels end to end with plastic strapping then enclosing the structure in Kevlar cloth (most expensive part of the process) and resin for making pontoons?
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