[Goscience]

Goscience asked the Naked Scientists:
   
Dear N.S.
 
The other day on a walk by San Francisco Bay I was watching boats and thinking about floating and sinking and what keeps a boat afloat.
 
It is easy to understand that something will float if the water it displaces weighs more than the weight of the object.
 
If it weighs less it floats; if it weighs more it sinks.
 
Now here's where my question comes in.
 
Since liquids are only slightly compressible (unless this changes at very high pressures), the object that weighs more than the water it displaces will presumably continue to sink until it reaches the seabed.
 
So how do submarines manage to stay afloat at some specific depth? I understand that they have tanks that are filled with a proportion of water and air and this determines the total weight of the object.
 
Therefore, if an object floats at, say 20 meters below the surface, can we assume that the water it displaces at that depth is equal to its weight at that depth?
 
But doesn't water at a depth of 20 meters have the same weight as water at the sea surface?
 
So something else must come into it.
 
The only thing I can think of is that a gas bubble would rise to the surface while an object that weighs more than the water it displaces would sink. Therefore, floating at some depth would require an equilibrium between these two forces.
 
Can you confirm, clarify, or explain?
 
Cheers,
 
Alan Schein

What do you think?

[JnA]

Neutral Buoyancy.

[chris]

Hi Alan

excellent 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

[JnA]

Hi Alan

excellent 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

well, yeah, if you want to get specific.   

[LeeE]

Just thought I'd mention that the Trieste, when it dived to the bottom of the Challenger Deep in the Marianas Trench, used gasoline for buoyancy instead of 'air' to get around the density issue.

[chris]

I presume, LeeE, that's because a gas under those sorts of pressures might well liquefy, which would make it very difficult to expel much water from a buoyancy tank?!

I think the calculations show that, at the pressures experienced at the Ocean floor, dry ice (solid CO2) would remain in solid form for an extremely long time. People have discussed this as a method of carbon sequestration.

Chris

[LeeE]

I personally don't think that sequestering CO2 at the bottom of oceans is a good idea.  It might be ok for a long time, but then it might not - who knows how long that particular part of the ocean seabed will remain undisturbed?  Should any disturbance occur there would be a risk of the CO2 rising up the water column and turning back into gas, as has happened in a couple of water filled volcanic craters, resulting in the deaths of quite a few people.  The thought of it happening on a much larger scale is frightening.

[lyner]

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.
Fish, of course, have been doing this for millions of years, They (mostly) have a swimbladder inside them. This consists of a bubble of air in a muscular sac which they squeeze or relax, a la Cartesian diver to modify their average density. They have an advantage in that their bodies are more or less neutral density in any case so they don't need to try as hard a submarine, which actually needs to float high in the water at times.

[syhprum]

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.

[JnA]

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.

maybe that's what happened to the Mary Celeste.

[chris]

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.

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

[lyner]

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.

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

It must be a major help in the long run, though. Keeping at near neutral buoyancy would be relatively easy for a rigid hull. A squeezable hull would always be in an unstable situation - go down a bit and the pressure would cause your density to increase and, just like a swimmer, you'd go down even faster. (And vice versa). Having a rigid hull (albeit with some small but finite change due to water pressure) would mean that only a small amount of forward movement would be enough to let you steer up and down.
Apart from when coming into port, there's seldom a reason to be stationary, in any case. You would only be stationary under water during some 'interaction' with the Enemy. Most of your six month tour would involve being on the move. It's a bit like sharks, which have to keep moving to get fresh water through their gills, I believe - they are an 'old design' of fish.

[LeeE]

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.

maybe that's what happened to the Mary Celeste.

Have a read of http://www.maryceleste.net/ concerning the Mary Celeste.

As syhprum says though, large bubbles of gas rising from the ocean floor are a plausible cause for the loss of some ships, along with rogue waves; try googling 'rogue waves' in Google Images: http://images.google.co.uk/images?hl=en&q=rogue waves&um=1&ie=UTF-8&sa=N&tab=wi

[lyner]

The MC didn't sink, tho!

[Bored chemist]

Imagine you have a submarine which is weighted to exactly neutral bouyancy at sy 100 metres. If you put it at more tha 100M the hull will compress (not much) and so it will become more dense. If that happens it will no longer be neutrally bouyant; it will sink.
Similarly, if you put it in just 50M of water it will expand slightly and so become less dense. This too means it will not longer be neutrally bouyant and will rise to the surface.
Neutral bouyancy isn't easy to achieve and it's not a stable equilibrium anyway. You could do it if you could ensure that the compressibillity of the sub was exactly the same as that of the sea water. On the other hand, that compressibility depends on depth and temperature.
Have fun.
The only way to keep a constant depth is to measure the depth and move up or down by adjusting bouyancy accordingly.

[lyner]

No. You 'fly' up and down because it's more efficient.