What air pressure must be supplied to remove water from the ballast tanks of a submarine 100m below the surface of the water in order for the submarine to surface?

Answer 1

This question isn't so good, and I'll explain in a second.

First, the answer your teacher wants is "more than 165psi." At 100 meters, the water pressure is 11 atmospheres, or 165psi. If you want to blow the water out of the ballast tanks, you need to apply enough air pressure to force the water out of the boat--or more than 165psi.


There are a couple of reasons why the question is bad.


When a submarine is submerged, they put enough water in the ballast tank to give the submarine neutral buoyancy. If the boat is large enough to displace 10,000 gallons of water and it weighs 60,000 pounds, it will float because 10,000 gallons of water weighs 80,000 pounds. If you add 20,000 pounds of water to it, the boat will weigh 80,000 pounds and it will neither float nor sink--and once it weighs this much, they make it rise or fall in the water with control surfaces like an airplane has. So you don't really have to blow the tanks to get the boat up from 100 meters.


Second, blowing the water out with air is only done in a real emergency. It's noisy, and doing anything noisy in a submarine gets you killed. They even have special shoes with soft soles so the enemy can't hear sailors walking around, and the beaters on the mixer in the galley are vinyl covered so you can't hear them scraping the sides of the bowl. They use pumps to remove the ballast when they need the sub to float.

Good point. That makes it do-able. In neutral buoyancy it would have to be more than 9.9*10^5N/m^2, in order to push the air out of the ballast tanks.