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Buoyency tank question?

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  • Buoyency tank question?

    I don't know if this is appropriate for this forum but I thought someone on this board might have an opinion on this idea.This is kind of a DUMB question,but I was trying to come up with a plan for a jet ski lift. I wanted to use something like a 55 gallon drum(mounted under the water below the lift) that could be pumped up with air to make the lift float up under a frame for the jet ski.You could bleed air out of the drum to lower the lift into the water,then pump it up to raise the lift with the jet ski on it.The queston I have is, will increasing the pressure in the drum make the drum more buoyant, or is there a limit to what will make the drum rise higher in the water?Will the drum be buoyant enough, no matter what, if no water is in the drum? Will the weight of the jet ski make the drum sink even if it's not filled with water?Would the bottom of the drum have to be open to let water in so that the weight of the jet ski would cause it to lower as the water was displaced?I also wondered how much pressure a 55 gallon drum might be able to stand?(20psi or 50psi or what) I don't want it to be a bomb waiting to explode. I have an older friend that used refridgeration tanks to make an air tank. One day he picked one up(was not filling it ) and it blew up on him ,shot shrapnel into both legs and he had several operations to fix one of his knees.I f anyone has an opinion on this I would appreciate it! Thanks ,David

  • #2
    I am not going in to all that, but,, increasing air pressure will not make it lift more and a drum is good for about 5 lbs of air before it bulges the ends way out.


    • #3
      IF I'm understanding this, you are trying to make a mineature floating dry dock.
      Going with that presumption, the amount of air pressure in the drum won't have any positive effect on buoyancy, you are working strictly on displacement.
      The main thing you need to determine is how much water you will need to displace to float the object you are lifting. Cubic feet of water displaced by the flotation device must equal or surpass the weight of the item being lifted + the weight of the displacement system.

      As to the subject of safety in such a device, using compressed air can get extremely dangerous because you could easily get a clog in the outlet line that would prevent ballast escape, an the natural tendency would be for the operator to increase pressure. The next step in such operation would usually be devistating.
      Pumps are normally used to dewater ballast tanks to preclude the possibility of deforming the tank by pressurization, unless the ballast tanks are pressure vessels, such as submarines.


      • #4
        Thanks Franz & S berry

        Franz you have it right. I am going to build an aluminum frame with bunks like the jet ski trailer or aluminum boat trailer without wheels. I am going to hinge one end to my boat dock. I wanted to attach either a 55 gallon drum or some plastic blue drums that I can get from some of my local dairy farmer friends under the opposite end of the hinge.I wanted them to lift the jet skis or aluminum boat out of the water.Jet ski weighs maybe 500lbs,aluminum boat maybe 1000lbs. I knew this was kind of a physics question(battleship water displacement).I didn't know if there was a way to firgure out how much displacement I might need. I also thought I might have to flood the barrels with water to make the frame sink.Then pump them back up till the water displaces the water to make it float again.I've got an old small compressor I could use to fill the tanks and I have plenty of power at the dock.Thanks for the replys!!!


        • #5
          floating tank


          Franz is correct (as usual) about the air pressure but you could cut a large hole to let the air out. For example you could stand the drum(s) on end and completely remove the bottom end. Unlikely that it could plug up. Also set your compressor regulator to only as much pressure as needed, which is probably a good rule for any time. I would suspect a single drum would have enough lifting power but you will need more than one to keep the whole thing from flipping over. I'm picturing 4 drums, one at each corner with the jet ski in the middle. Maybe somone can think of a way to make it stable with fewer drums. (One at the front and on one each side?)

          Don't put the air vent at the top. You want enough air always trapped to keep the empty "floating dry dock" floating without having to shut off the air release just before it sinks.


          • #6
            The big problem with an open bottom system is you'll have to maintain pressure to keep it afloat.

            I'll agree the best option is a pump out system..
            no pressure.. nothing to blow up.
            Scott Schering


            • #7

              Well our replies crossed. If you have one end secured then you can let it sink. The lifting force is equal to the weight of the water displaced so ignoring the weight of the container and the weight of the air in the container, if a container holds a pound of water it will lift a pound of weight when filled with air. Note that this is only true when the container is completely under water. If the container is half out of the water then it will only lift 1/2 pound.

              As for how much water weighs there is a little ditty that goes "a pints a pound the world around" . Maybe someone has a more accurate number handy.

              Also if the weight is centered between the dock and the drum then half the weight is on the dock and only half is on the drum. Adjust depending on the where the weight is on your boat. With the engine there is probably more weight at that end so the drum will be lifting more than half.



              • #8
                Good point on the maintianing pressure issue. I you use a pump to move the water instead of air make sure that there is an air vent so the you don't blow up the tank when filling it or collapse it when emptying it.



                • #9
                  Whoa there younguns, lets not flunk the physics exam here.
                  First of all a pint and a pound are only equal in liquid measure, 16 ounces each.
                  Water is about 8 1/3 pounds to the gallon. Where the **** did I leave that handy conversion chart?
                  Figure around 64 pounds to a cubic foot of water, give or take.
                  For rough calculations, lets say a 55 gallon drum, fully immersed in water displaces 55 gallons of water, giving it a maximum flotation capability of 450 pounds =/-.
                  Just for the **** of it, lets say 2 barrels would float the jetski, presuming the barrels stay below the surface of the water.
                  This estimate would allow for the weight of the barrels and some framing.
                  Plastic barrels, well I hope you have a better way of sealing them than I've ever found, or you're going to have ballast accumulation problems.
                  The diving bell or inverted open ended barrel concept is workable, but I guarantee will get very entertaining if the barrel tips or breaks surface at the open end, and will require constant compressed air makeup.
                  Given my record holding status relative to propane tanks, I'd check into the availability of 100# propane tanks for this project.
                  They already have a tapped valve port, and can easily withstand the pressure of a compressed air system. Another benefit would be DEC probably wouldn't give you greif about a pressure vessel the way they will about 55 gal drums.
                  I hate to turn a project aproject away from welding on this site, but have you considered a rubberized bladder? There are plenty of them available thru government surplus, and those things are fantastic in terms of capability as well as service life. A bladder could operate the system on compressed air alone, and wouldn't require flooding to submerge.


                  • #10
                    what about a dry dock?


                    • #11
                      Franz's numbers about the weight of water look right on the mark to me. So maybe two 55 gal drums, one on each out board corner. Connect them with a common air tube so pressure is equal in both.

                      Another physics number is that underwater pressure is roughly .5 psi per foot of depth. Assuming none of this is more than 10 feet under, you will only need about 5 psi to force enough air into the barrels to force the water out. You might want to use something like a shop vac discharge or a leaf blower (high volume, low pressure) instead of a compressor.

                      With the open ended barrels there will never be any differential pressure inside the barrel. At 4 feet under there will be 2 psi from the water outside, and 2 psi inside holding the water below out.

                      One thing you must do is post some pictures of this unique and interesting project.



                      • #12
                        I think I would use some kind of chain hoist to lift the jet ski. Once I got it to the desired height I would have some kind of system to hold it in place and I could let the tension off of the hoist. I think it would be a lot easier that way.

                        When I first read the post it reminded me of that junkyard wars show where they had to lift a mini cooper from the bottom of the lake. One team tried using barrels to lift it. I didn't think it worked out to well for them.
                        Art is dangerous!


                        • #13
                          my dad has a floating dry dock like you're wanting to build for his two seadoo's .... it has two large cylinders that will fill with water and float the seadoo's. then when you power on the pump which looks and sounds just like a shop vac to me. it pumps air into the cylinders which float the seadoo's up. evidently on the inside of the tanks is a trap like in a sink or something that doesn't allow water to come back in past a certain point. pretty neat device. his is quite large since it will float 2 jetski's and has galvanized steel hardware supporting the rails that the seadoo's sit on.

                          - jack


                          • #14
                            weight of sea water = 64 pd per cubic foot; 8.56 pd per gal; 261.8 gal per ton
                            weight of fresh water = 62.5 pd per cubic foot; 8.34 per gal ; 268.5 gal per ton

                            55 gal drum displaces 470 pounds of sea water but salvage divers are tought to use 450 pounds allows for weight of drum.

                            Blowing ballast is hard to control buoyancy better to pump ballast water to get desired lift. Auto bilge pump takes care of small leaks.


                            • #15
                              There are commercial floating devices just like Morpheus described. A relative has a 21' ski boat with a 454 Chevy in it. His lift works in a scissors action. Seems two rails on the bottom resting on the lake bed. then a big dual scissors contraption with what looks like about 20' (I'm guesing here since I haven't seen it in a year or so) of rubberized tube bladder on each side. They were about the diameter of a 55 gallon drum. Yeah it was some sort of vacuum cleaner looking contraption air pump on the dock with about 2" hose to the bladders. It would lift the boat out of the water in probably 15 minutes or so. Other people had them for skidoos, etc. (just smaller). The warranty/guarantee was supposedly long term on the mechanism and bladders too. I think all the steel was hot dipped galvenized. This was on Lake Norman near Charlotte NC.