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Most watercraft would be described as either a ship or a boat. However, there are numerous craft which many people would consider neither a ship nor a boat, such as: surfboards (when used as a paddle board), underwater robots, seaplanes, jetskis and torpedoes.
Although ships are typically larger than boats, the distinction between those two categories is not one of size per se.
- Ships typically are large ocean-going vessels. Boats are smaller and travel most often on inland or coastal waters.
- A rule of thumb says "a boat can fit on a ship, but a ship can't fit on a boat", and a ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts.
- Local law and regulation may define the exact size (or the number of masts) that distinguishes a ship from a boat.
- Traditionally submarines were called "boats", perhaps reflecting their cramped conditions: small size reduces the need for power, and thus the need to surface or snorkel for a supply of the air that running marine diesel engines requires; in contrast, nuclear-powered submarines' reactors supply abundant power without consuming air, and such craft are large, much roomier, and classed as ships in some navies.
- A merchant ship is any floating craft that transports cargo for the purpose of earning revenue; in that context, a passenger ship's "cargo" is its passengers.
The term "watercraft" (unlike such terms as aircraft or spacecraft) is rarely used to describe any individual object: rather the term serves to unify the category that ranges from jet skis to Aircraft carriers. Such a vessel may be used in saltwater and freshwater; for pleasure, recreation, physical exercise, commerce, transport or military missions.
Aside from small craft used by individuals, maritime tradition identifies a ship as any water vehicle that has, or is capable of carrying three masts within its hull. Three masted ships at one time were called the Great Ships, and marked transition in European seafaring from shallow coastal waters of the "Age of Navigation" confined primarily to the European shores and those of North Africa, via the Age of Discovery of other continents in relative proximity of Western Europe, to the "deep sea" oceanic sailing of the Age of Sail. The term 'a great ship' therefore holds a deeper and far more significant meaning to seafarers than 'landlubbers', or those who do not serve on ships or other vessels, and has remained so through the Age of Steam and into the 21st century.
While rigged masts have been discontinued for the vast majority of ships since the late 19th century, a rule of thumb based on the hull having a wooden construction would determine her as a ship. If the length of the watercraft deck prevents the mounting of three masts, it is a boat, while a single-masted hull can be termed a craft. This is paralleled with the degree of difficulty that was required in the past by different trades constructing the hull based on complexity. A ship was always constructed by a shipwright working with a naval surveyor or a naval architect. A boat could be constructed by a ship's carpenter. A watercraft such as a lifeboat, a dinghy, or a runabout could be constructed by any craftsman familiar with woodworking, such as a builder's carpenter or a joiner, or by a qualified marine trades person or shipwright.
Usually the purposes behind watercraft designs and skills are for seafaring education or leisure activities, fishing and resource extraction, transportation of cargo or passengers, and for conducting combat or salvage operations. In general, the purpose of a water vehicle identifies its utility with a maritime industry sub-sector.
The design from which a water vehicle is created usually seeks to achieve a balance between internal capacity (tonnage), speed and seaworthiness. Tonnage is predominantly a consideration in transport operations, speed is important for warships, and safety is a primary consideration for less experienced or often smaller and less stable training and leisure vehicles. This is due to the great level of regulatory compliance required by the larger watercraft, which ensures very infrequent instances of foundering at sea through application of extensive computer modeling and ship model basin testing before shipyard construction begins.
Propulsion is the primary application of technology in watercraft. Historically water vehicles have been propelled by poles, paddles or oars, through manipulation of sailing rigs that propel by lifting using the wind, and a variety of engineered machinery that creates subsurface thrust through the process or internal combustion. The technological history of watercraft in the European history can be divided into marine propulsion using the simple paddle craft, oared galleys from the 8th century BCE until the 15th century, lateen sail during the Age of Discovery from the early 15th century and into the early 17th century, full rigged ships of the Age of Sail from the 16th to the mid 19th century, the Age of Steam marine steam engine roughly between 1770 and use of the steam turbine until 1914, the internal combustion engines using diesel fuel, petrol and LNG as fuels from the turn of the 20th century, which has been supplemented to some degree with the nuclear marine propulsion since the 1950s in military watercraft. Current technological development seeks to identify cheaper, renewable and less polluting sources of propulsion for watercraft of all shapes and sizes.
Secondary applications of technology in watercraft have been those of used structural materials, navigation aids, and in the case of warships, weapon systems. The purpose of usage and the physical environment define the materials used in construction which had historically included grasses, leather, timbers, metals combined with timber or without, silicate and plastic derivatives, and others.
Navigation aids have varied over time from astronomical observation, to mechanical mechanisms, and more recently analog and digital computer devices that now rely on GPS systems.
Naval weapon systems have closely followed the development in land weapons, developing from:
- aircraft carriers
- breech-loading rifled guns
- direct enemy hull ramming to use of basic mechanical projectiles
- firing shells
- missiles and remotely piloted devices
- naval mine layers
- smooth-bore cannonball firing guns
- torpedo-armed submarines
- warships armed with fire control directed weapons
Until development of steam propulsion was coupled with rapid-firing breech-loading guns, naval combat was often concluded by a boarding combat between the opposing crews. Since the early 20th century, there has been a substantial development in technologies which allow force projection from a naval task force to a land objective using marine infantry.
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