How does a submarine submerge and then float on water?

Answer 1

There are ballast tanks in a submarine. If the submarine weighs 150,000 pounds, it will push aside, or displace, 18,750 gallons of water. (We're using an even 8 pounds per gallon here.) If the submarine's volume is actually 25,000 gallons, part of the submarine will be out of the water--what we call floating.

To make the submarine submerge, you must increase its weight to 200,000 pounds--the same as 25,000 gallons of water. This is done by pumping water into tanks in the front and back of the submarine. Once they do this, the whole submarine will be underwater and they can control its depth with planes. They can also "trim" the sub by putting more water in one tank than in the other. Submarines change their weight distribution all the time. If you put a case of canned peaches in the front of the boat when you were provisioning it for the mission, and you fed the peaches to the crew, you've got to compensate for the loss of that case of fruit by adding a little more water to the forward ballast tank. That's how sensitive these things are--you have to compensate for the food you eat to make them ride right. (And if you actually have to shoot at anyone you have to compensate for the torpedos or Tomahawk Missiles you fired.)

Increasing the weight of a submarine beyond its displacement will make it sink to the bottom. You might want to do that if you were going to park the boat at the entrance to a harbor and wait for a ship to come by so you could block the channel by sinking it, but if you did it all the time you'd have to waste lots of power keeping the boat off the bottom.