A submarine is a vessel that goes under water. Submarines are almost always war vessels, but some are used for scientific or business purposes. A submarine is always called a boat, not a ship.
Early, submarines were often powered by hand, as boat engines had not yet been invented. They were almost always designed for war purposes, and would attempt to sink enemy ships by crude methods like driving screws into their wooden hulls. Some attempted to blow ships up, but this would often blow up the submarine as well.
Later, during the first and second world wars, more practical submarines were developed, mostly by Germany. They were powered by a diesel-electric system. A diesel motor would be used to turn a generator which would charge a large number of lead-acid batteries while the submarine was above the water surface. This power was then used to power the submarine when it went underwater. These submarines were dangerous for enemies and hard to see while under water but it was easy to attack them when they were above the water suface and recharging. Their were only used to attack ships.
Most modern military submarines are powered by nuclear reactors. These submarines often have a system which can get oxygen from the surrounding seawater as well, this combination allows them to stay above the water surface until there is no food anymore. Their most important functions are attacking ships or launching rockets like cruise missiles, or nuclear missiles (SLBMS (Submarine Launched Ballistic Missiles)) The main difference is the difference in range. A military submarine will usually be designed for only one of these two functions, because a submarine carrying a missile is larger, which is not useful when attacking other ships. Some small submarines are still used to transport commando forces such as the U.S. Navy SEALS.
The first real submarine was made by John Philip Holland, in 1881. It had a petrol engine. He was from Ireland. It was called the Fenian Ram.
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