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Hi Alanexcellent question. Actually it's not just submarines that need to grapple with this problem - scuba divers also need to achieve so-called "neutral buoyancy" to prevent them from ripping their wetsuits on coral or stirring up clouds of sediment by crashing into the seafloor.To do this they use a BCD - buoyancy control device - which is an inflatable vest connected to the air tank. A valve can be used to add or release air from the BCD, altering its volume. Water is pushed out of the way by air in the BCD. This, in turn, alters the density of the diver (when you consider him as a complete unit), and this affects his buoyancy - if something is the same density as the medium in which it is suspended then it will neither rise nor fall. So what happens when the diver swims to a greater depth? He needs to add air to his BCD to stop himself sinking further. This is because, as he sinks, the pressure from the surrounding water increases, which compresses the volume of his BCD and the air-spaces in his wetsuit, increasing his density. Consequently he is now more dense than the surrounding water and will continue to sink, unless he compensates by adding more air to the BCD to increase the volume again.Now consider the submarine. As it dives it too "feels" a steadily increasing pressure from the water surrounding it because the deeper it goes the greater the mass of water above it. This, in turn, squashes the submarine and compresses the gas volume in the floatation tanks. As the volume decreases, so the effective density of the submarine increases (because density (D) = mass (m) / volume (v)). Therefore, to achieve neutral buoyancy with increasing depth you have to progressively expel some water from the buoyancy tanks, reducing the mass of the submarine as a unit. This helps to decrease the density again, so that it matches that of the surrounding water. The density of the surrounding water hasn't changed that much, because water is virtually incompressible.Our experiment on how a Cartesian Diver works, which we also explain in much more detail in our book, Crisp Packet Fireworks, will also shed some light on this for you.Chris
Large bubbles of CO2 or Methane rising from the bottom of the ocean as been put forward as the cause of the mysterious sinking of ships, it would be most unfortunate to be directly above one when it breaks surface.
Submarines, when on the move, use hydroplanes to 'fly' themselves up and down and achieve th depth they want. This is better because it involves no loss of air.
Quote from: sophiecentaur on 31/05/2009 17:03:11Submarines, when on the move, use hydroplanes to 'fly' themselves up and down and achieve th depth they want. This is better because it involves no loss of air.Good point, SC, although this is only helpful when the submarine is actually moving. It is comes to a stop then the neutral buoyancy issue kicks in doesn't it?Chris
Quote from: syhprum on 31/05/2009 18:23:25Large bubbles of CO2 or Methane rising from the bottom of the ocean as been put forward as the cause of the mysterious sinking of ships, it would be most unfortunate to be directly above one when it breaks surface.maybe that's what happened to the Mary Celeste.