Ship

A ship is a large, usually decked watercraft. A ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it doesn't always apply) goes: "a boat can fit on a ship, but a ship can't fit on a boat." Often local law and regulation will define the exact size (or the number of masts) which a boat requires to become a ship. (Note that one refers to submarines as "boats"). Compare vessel.

One alternate, perhaps more modern, definition of a 'ship' is that of any floating craft which carries and transports cargo in an enclosed space between its hulls for the purpose of earning revenue. For example, passenger ships move 'supercargo' (another name for passengers or persons not working on board), and passengers occupy the enclosed space between those ships' hulls. But fishing boats are never considered 'ships' even though fishing boats carry lifeboats and cargo (the catch of the day). However, the space between hulls on fishing boats is unenclosed, and those hulls are usually raised to double as barriers preventing passengers from falling overboard.

Ferries are generally not referred to as 'ships' either. Though ferries float on an enclosed space (a barge usually), that space is left vacant to any sort of cargo. Exceptions are large car ferries and long-distance ferries run by shipping companies, for example, those that connect China and Japan.

During the age of sail, ship signified a ship-rigged vessel, that is, one with three square-rigged masts and a bowsprit.

Nautical means related to ships, particularly customs and practices at sea.

Measuring ships

One can measure ships in terms of overall length, length along the waterline, beam (breadth) and tonnage.

A number of different tonnage definitions exist, the majority of which measures volume rather than displacement. Displacement most frequently applies to naval vessels and equals the actual weight of a ship under specific conditions. "Light ship" tonnage measures the actual weight of the ship with no fuel, no persons, no cargo, no water on board, just as it first entered the water. The term "displacement" occurs due to the basic physical law, discovered by Archimedes, that the weight of a floating object equates exactly to that of the weight of the water that would otherwise occupy the "hole in the water" displaced by the ship.

In Britain, up until the end of the 19th century, shipowners could load their vessels until their decks were almost awash, resulting in a dangerously unstable condition. Additionally, anyone who signed onto such a ship for a voyage and, upon realizing the danger, chose to leave the ship, could end up in jail.

Samuel Plimsoll, a member of Parliament, realized the problem and engaged some engineers to derive a fairly simple formula to determine the position of a line on the side of any specific ship's hull which, when it reached the surface of the water during loading of cargo, meant the ship had reached its maximum safe loading level. To this day, that mark, called the "Plimsoll Mark", exists on ships' sides, and consists of a circle with a horizontal line through the center. Because different types of water, (summer, fresh, tropical fresh, winter north Atlantic) have different densities, subsequent regulations required painting a group of lines forward of the Plimsoll Mark to indicate the safe depth (or freeboard above the surface) to which a specific ship could load in water of various densities. Hence the "ladder" of lines seen forward of the Plimsoll Mark to this day.

Propulsion

Until the application of the steam engine to ships in the early 19th century, oars propelled galleys or the wind propelled sailing ships.

Before mechanisation, merchant ships always used sail, but as long as naval warfare depended on ships closing to ram or to fight hand-to-hand, galleys dominated in marine conflicts because of their maneuverability and speed. The Greek navies that fought in the Peloponnesian War used triremes, as did the Romans contesting the Battle of Actium. The use of large numbers of cannon from the 16th century meant that maneuverability took second place to broadside weight; this led to the dominance of the sail-powered warship.

The development of the steamship became a complex process, the first commercial success accruing to Robert Fulton's North River Steamboat (often called the "Clermont") in the USA in 1807, followed in Europe by the 45-foot PS Comet of 1812. Steam propulsion progressed considerably over the rest of the 19th century. Notable developments included the condenser, which reduced the requirement for fresh water, and the multiple expansion engine, which improved efficiency. The paddle wheel gave way to the more powerful screw propeller. Further efficiencies resulted from the development of the marine steam turbine by Sir Charles Parsons, who demonstrated it on the 100-foot Turbinia at the Spithead Naval Review in 1897. This facilitated a generation of high-speed liners in the first half of the 20th century.

The marine diesel engine first came into use around 1912: either the Vulcanus or the Selandia (depending upon who you talk to) first deployed it. It soon offered even greater efficiency than the steam turbine but for many years had an inferior power-to-space ratio. About this period too, oil came into more general use and began to replace coal as the fuel of choice.

Some historical types of ships and boats