A Little History
Engineers toyed with the idea of a sea-going vessel that could submerge itself completely for quite some time before the first true submarine was ever successfully tested. Today, the submarine forms an important cog in the control of the seas for any nation and has since taken the mantle of "King of the Seas" from the equally-important aircraft carriers. Aircraft carriers came into their own during World War 2, rendering the previously impervious battleship utterly useless in the wake of torpedo aircraft and dive bombers. Likewise, the undetectable nature of the submarine soon rendered the aircraft carrier a juicy target where a submarine could make its way through fleet defenses and ultimately sink thousands of sailors and airmen in one swoop. The submarine certainly proved its worth in World War Two, particularly in the hands of the German Navy, where her U-Boats nearly destroyed any outside support for the United Kingdom.
The modern battleship takes along a few known principles that most of us learn in science class. Whereas surface ships need to content with their displacement (weight) on the surface of the water and immediately below it, the submarine must contend with the colder operating temperatures and increasing pressures of the deep blue. As such, submarines must be constructed internally and externally as a vessel capable of keeping her crew alive and her systems intact, lest the vessel sink from a leak or be completely smashed to the size of a water pail from the pressures below.
The External Anatomy of a Modern Submarine
Taking a page from nature's own, the submarine has evolved from Jules Verne-like shapes in more "ocean-friendly" designs along the same lines as a shark or porpoise. A typical modern submarine's profile is dominated by this elongated shape which in turn is broken up by the "sail" (or "conning tower") and various planes about the design. The sail is the tower that contains various communications equipment and available periscopes and may or may not contain the sail planes - planes that operate in the same way that an aircraft's elevators might, allowing for movement upwards and downwards. The stern (or aft or rear) section of the submarine is usually made up of the propeller, the rudder (allowing for side-to-side movement similar to the tail of a shark) and the stern planes.
Armament of the submarine at one time primarily revolved around its torpedo tubes, usually several mounted as forward-facing and several as rear-facing tubes. This eventually gave way to strictly forward-facing ones as it is in today's designs, opening up space for further systems and equipment to be carried. Additionally, deck guns were installed to combat lighter vessels on the surface of the water. Modern submarines have no deck guns afforded to their armament as that need in battle has all but past. Today's submarine vessels still take on the need to operate torpedo tubes but, more importantly, many are undergoing or have undergone conversions to become ballistic missile submarines. This has provided them with long range strike capabilities unheard of in World War 1 or World War 2 by allowing the vessel to be near surface or surfaced entirely and launch its payload of vertical-loaded cruise missiles into the air, only to be guided furthermore by satellite and GPS systems.
The Internal Anatomy of a Modern Submarine
Submarines are designed with two hulls, one inside of the other. These are aptly known as the inner hull (or "pressure hull") and the outer hull. The inner hull is vital to the crew in both protection against exterior pressures of the deep and the freezing temperatures inherent in roaming the depths. Between these two hulls is a pocket that contains the ballast tanks. Ballast tanks are the real key to submerging and raising a submarine. Ballast tanks operate with both air and water to maintain the desired level of buoyancy. There are three types of buoyancy and are positive buoyancy, neutral buoyancy and negative buoyancy. Positive buoyancy merely means that the vessel is lighter - or less dense - than the density of the water around it, hence it floats. Neutral buoyancy is a state where the vessel is neither lighter or heavier than the water around it so the submarine maintains a constant depth. Negative buoyancy is maintained by taking in seawater and jettisoning out the compressed air. This causes the submarine to sink.
How Surface Ships Work: The Archimedes' Principle
The Archimedes' Principle is nothing more than a law describing what we know as floating or buoyancy. Two basic things happen when an object is dropped into the water - either it will float or it will sink - as simple as that. The principle explains that the weight of any object in water naturally works against the water, wanting to sink. The displaced fluid naturally works against the object and pushes back up. If these two forces pushing against one another are at any time equal, the object will float. If the density of the object ever exceeds the density of the water pushing on the object, the object will sink. So in essence, in order to float, a ships weight (or displacement) has to be less than or equal to the water underneath it, the water being pushed aside and down in order to make room for the ship. This helps explain why something as large as an aircraft carrier can stay afloat.