Surfacing and Diving
In this case, it is probably not hard for you to imagine
how a Los Angeles class submarine weighing 6,900 tons can dive. It is
probably very easy to make it dive, but how do you make it float and how can
you control its depth? First, a basic understanding of Archimedes's
principle is needed. This principle states that the magnitude of the
buoyant force always equals the weight of the fluid displaced by the object.
The buoyant force is the force exerted by water on any immersed object,
like a submarine. What this means is that in order for the submarine
to float, the buoyant force has to be equal to the weight of the sub. In
other words, the density of the sub has to be equal to the density of the
water around it for it to float and more than the density of water to dive.
You also have to be able to control the density of the sub so that you
can surface and dive at will.
With a basic understanding of the buoyant force, we can discuss how a submarine controls its density. This is done with tanks on the outside of the submarine called ballast tanks. When the sub is on the surface, the ballast tanks are filled with air, making the overall density less than that of the surrounding water. In a dive situation, the ballast tanks are flooded with water and the air is vented out making the overall density more than water and causing the submarine to sink. The ballast tanks are open to the outside water at all times, but when the vents are closed no water can get in because no air can get out. This is similar to holding a glass upside down under water. As long as no air bubbles get out, no water gets in. When a dive is called for, the vents are simply opened up.
Image courtesy of: Diving and Surfacing
Image courtesy of: How Submarines Work
With a basic understanding of the buoyant force, we can discuss how a submarine controls its density. This is done with tanks on the outside of the submarine called ballast tanks. When the sub is on the surface, the ballast tanks are filled with air, making the overall density less than that of the surrounding water. In a dive situation, the ballast tanks are flooded with water and the air is vented out making the overall density more than water and causing the submarine to sink. The ballast tanks are open to the outside water at all times, but when the vents are closed no water can get in because no air can get out. This is similar to holding a glass upside down under water. As long as no air bubbles get out, no water gets in. When a dive is called for, the vents are simply opened up.
Image courtesy of: Diving and Surfacing
We have now discussed the easy part of this topic,
diving. Now that the sub is underwater, however, how do you make it
come back to the surface? This is actually not as difficult as it sounds.
When a submarine needs to surface, it needs to lessen its density so
that the buoyant force pushing up is more than the weight pulling down. This
is down by closing the vents and pumping compressed air into the ballast
tanks in order to force the water out.
Image courtesy of: Diving and Surfacing
There is also another aid that submarines have in surfacing
and diving; hydroplanes. Hydroplanes are very similar to wings on airplanes
in their use. Hydroplanes can be angled up or down to control the fore-aft
tilt of the sub. They are located on the back of the sub and therefore
can be used to push the back down and the nose up or to push the back up
and the nose down. These are very valuble in steering the submarine.
The hydroplanes are the horizontal "wings" on the sub:
After the dive, the next problem is controlling depth. This is
not very difficult at all however. The biggest thing to worry about
is keeping a balance of air and water in the ballast tanks so that the density
of the sub is equal to the density of the water. This keeps the sub
from either rising toward the surface or sinking toward the bottom. The
hydroplanes are also straightened out to keep the sub traveling in a level
path.
