http://express.howstuffworks.com/express-submarine1.htm



How Submarines Sink & Swim
To dive underwater and then resurface, the submarine's crew must
be able to control the ship's weight or density. We can compare
this to an empty can floating in the water. The empty can could
become more "rock-like" and sink by pouring water into the can.
By controlling the amount of water we pour into the can, we can
control the rate and depth of its dive under the water's surface.


Submarines have an inner and an outer steel shell, called a hull.
The area in between the two hulls is called the ballast tank. It
can be filled with either air or water. When the submarine is on
the surface, the ballast tanks are filled with air and the
submarine's overall density is less than that of the surrounding
water.

Ballast tanks are open at the bottom. To dive, the submarine
operator opens up valves at the top and lets air out. It's
something like blowing bubbles when you are swimming underwater.
Seawater rushes in to fill the space that was taken up by air.
This changes the ship's density.

When the density of the submarine is greater than the surrounding
water, it begins to sink. This is called negative buoyancy. A
moveable set of wings, called hydroplanes, helps control the
angle of the dive.

To keep the submarine at any specific depth, the crew adjusts the
mixture of air and water in separate, smaller ballast tanks. The
operator tries to keep the submarine's overall density about the
same as the surrounding water. This is called neutral buoyancy.
When the submarine reaches its cruising depth, the hydroplanes
are straightened so the craft can travel level through the water.
If something changes the submarine's weight and density, such as
firing torpedoes, the operator must make further adjustments of
the water/air mixture in the ballast tanks.


"Dave Doe" wrote in message
. nz...
In article ,
says...

"Dave Doe" wrote in message
. nz...
In article ,
says...

"Dave Doe" wrote in message
. nz...

Can you sink a 'positive buoyancy' boat with water? Can
you
sink it with
lead? You've proven my own point.



What do submarines fill their ballast tanks with?

positive bouyancy boat submarine - EVER.


Bzzzt...Oh, I'm sorry, the correct answer is ''WATER'' .

Thanks for playing.

Water - ROTFL.

No, subs sink cos either they're heavier than water in the
first place,
or they use ballast that is heavier than water such as: Lead,
or for
modern wartime subs I would think they'd use DU (depleted
uranium).

Water! - LOL.

So the correct answer is most assuredly not water. Subs are
made so
that the mass/volume is denser than that of water - and they
sink. They
then have *AIR* in them that provides bouyancy - the cool thing
about
air is it's *compressability*. The "blow the (air) tanks" -
and the
decrease in bouyancy means they sink. To surface, they blow
the water
*out* of the air tanks, filling 'em with the air from the
compressed air
source.

Think of a diver - FFS. They sink because of? Same as a sub -
only as
said, I would think modern war subs use denser material such as
DU -
'cos they can afford it.

So the correct answer is lead or DU.

--
Duncan