Surfacing and Submerging The principle of bouyancy explains why a submarine stays above the surface or submerges. The force that water exerts on any immersed object is called a buoyant force. This upward force counteracts the downward gravitational force that acts on the object. This downward gravitational force can also be called weight and is equal to (“mass” x “acceleration of gravity”)or “mg”. When these forces are equal, the object will float in static equilibrium. So when a ship is floating in static equilibrium, the following equation holds true: B = Fg = Mg Where B is the buoyant force, Fg is the force of gravity, and Mg is the weight of the object. More importantly, the total weight of the water displaced is equal to the total weight of the object displacing the water (Archimedes's Principle). Therefore, “displacement” is what we call the volume of water that is moved aside by an object. From the equation above we see that if B = Mg and M =pV, then B = pVg. Where V is volume, p is density, M is mass, and g is the acceleration of gravity. So the buoyant force is a function of density if volume remains constant. From this we can conclude that the density of an object of constant volume determines whether or not it will float. The illustration above shows a surfaced submarine. When the sub is on the surface its ballast tanks are full of air. The air inside the hull ( air is 1000 times less dense than water) brings the average density of the entire ship below the density of saltwater(64 pounds per cubic foot). Because the sub is less dense, it floats. A submarine that is on the surface or is rising towards the surface is said to have positive bouyancy. So if a submarine has an average density greater than saltwater, it will not remain on the surface. The average density of a submarine is increased by flooding the ballast tanks with water through flood ports on the bottom of the tanks by releasing air out of the top of the tanks. Once the density of the ship exceeds that of the water, the submarine has acheived negative bouyancy and it submerges as illustrated above. The ballast tanks in a sub are filled with air in order to surface the ship. In order to do this, high pressure air is blown into the ballast tanks to push the water out. This air must be blown in at a pressure greater than the sea pressure at the flood ports. All subs have high pressure air banks that are always filled with 3000 psi air. If the sub crew has an emergency, they can pull the "chicken switches" and this will rapidly expel the 3000 pd. air into the ballast tanks. This rapid water removal causes the sub to begin to surface very rapidly. This effect is compounded by the further expansion of air due to the rapid reduction in sea pressure. This procedure is called an "emergency blow" and when done from great depths, the large positive bouyant force created causes the sub to launch out of the water. Picture from (www.lycos.com) Home/ Basic Anatomy of a Nuclear Submarine/ Propulsion /Controlled Surfaces: Planes and Rudder/Bibliography