How does buoyancy work?

The bulk modulus for sea water is apx 2.38E9 Pa. You can expect a 1% change in volume for every 238 bars.

HDPE has a specific gravity a bit less than water, so it should float (especially if it has a few bubbles attached).

There isn't really that much difference in density between HDPE and LDPE (maybe 5%).

I don't count on my kayak floating if it fills with water. They sink unless they have air compartments.

Thing is, it's not linear, thus the bulk modulus is not a constant- it increases with pressure.

Water has about 11% volume change at 4000 bar:

"Spehro Pefhany"

Water is extraordinarily incompressible, less so than steel.

Diamond and few modern materials are less compressible than water.

.... Phil

ds

I'd call that the bulk modulus

George H.

yOn 2013-02-19, snipped-for-privacy@bid.nes wrote: On Feb 19, 8:41 am, Jim Thompson > Ignoring gravity gradient effects, isn't the simple buoyant rule that

The bit that's above the water isn't diplacing any water. so ignoring boyancy in the atmosphere the volumne of wter displaced has the mass of the whole floating object.

See "Archimedes's principle"

Again, buoyancy is about density. All other things being equal, a submarine-shaped volume of water does not rise or sink because it has the same density as the water surrounding it (it displaces a volume of water that has its exact weight).

For a submarine to occupy that space without rising or sinking in must have the exact same density as the sub-shaped volume of water; it will therefore by definition *weigh* the same.

Submarines *do not* need active "propulsion" to change depth; water's density conveniently increases with depth so they fill or flood their ballast tanks which changes the sub's overall *density*, forcing it to rise or sink to a depth where the water's density matches its own.

Mark L. Fergerson

" snipped-for-privacy@bid.nes"

Submarines *do not* need active "propulsion" to change depth; water's density conveniently increases with depth so they fill or flood their ballast tanks which changes the sub's overall *density*, forcing it to rise or sink to a depth where the water's density matches its own.

Water increases in density by only 4 ppm per metre of depth - temperature and salinity variations have much bigger effects.

As the pressure on the hull of a sub increases the hull is compressed, causing a loss of buoyancy that is greater than the density increase of the surrounding water.

So, once buoyancy becomes even slightly negative, a sub will sink to the bottom unless corrective action is taken.

Maintaining a set depth is a continuous process of adjusting the subs buoyancy with trim tanks to keep the outside pressure nearly constant.

... Phil

I believe that's wrong. At least modern (atomic) submarines are slightly buoyant such that if they lose propulsion they don't sink. They use the propulsion (and planes) for fine control.

Yup. Plus the use of trim planes, which are the analogues of pectoral fins.

Cheers

Phil Hobbs

Phil is correct, hull compressibility is the over-riding factor, all other things being equal. Unfortunately, things never stay equal. Sea water is brought in to be made into fresh water; brine, bilges, sanitary tanks, and garbage go out. Trim is always changing but you really want to aim for a neutral trim. It's a point of pride to be able to slow from a long, high speed run (where trim compensation is done based on numbers reported from engineering) and to keep depth control nearly "hands off" as the boat slows to a hover.

I think this is an *excellent* answer, very well thought out with great images to illustrate the concepts.

Yes, they basically fly through the water although they don't depend on wings for support, they do use the trim planes to "steer" up and down. It would be way too difficult to try to balance the buoyancy to maintain a depth.

Really? They dump garbage when submerged? Wouldn't that tend to show where they were? I suppose they weight it so it sinks.

I remember years ago they were talking about tracking subs by satellite images. You don't see the sub wake directly, but it has a calming effect on the surface waves and this is visible or at least detectable in theory. Don't know if they ever turned it into practice.

Have you been a submariner?

Have to. The sound of an entire crew gagging on the smell of rotting garbage would be a give-away, otherwise. Think of a small torpedo tube that is aimed down instead of forward.

They're too shallow and too fast. Guessing that this was a "familiarity" exercise with a P-3. "This is what a periscope looks like, noobs" kind of thing. Also, too:

Yes. They have a special little airlock to blow the garbage overboard. The fish eat it.

In look for the section titled "Malfunction of trash disposal unit".

From what I've read in defense publications, it does work, but not if the sub is some classified combination of too deep and too slow.

Joe Gwinn

rickman schrieb:

Hello,

they even store ballast stones to weight the dumped garbage.

Bye

When I say "wake", I'm not talking about a wake made when breaking the surface. I'm talking about the pressure wake made when moving far below the surface. It spreads like the wave front from an airplane and reaches the surface, calming the surface waves a bit.