Ship propulsion refers to the method of moving a ship through the water. The three main types are oars, sails, and mechanical means. This article discusses only the mechanical means of ship propulsion, which began in the late eighteenth century. As the field developed over the next 200+ years, various technologies were developed and used, based on different fuels, engines, and ship/water interfaces.
Fuels. The first fuel used to power ships was wood, but the first widely-used fuel was coal. In the early 20th century, coal gave way to a low grade of fuel oil and, in some ships, to diesel fuel. In the mid-20th century, nuclear power was first used to power warships, followed in the late 20th century by high-quality fuels such as kerosene and gas oil. Today, most merchant ships use a low grade of diesel fuel, and most warships use either high-quality fuels or nuclear power.
Engines. Early marine engines were reciprocating steam engines, powered by steam from wood-, coal- or oil-fired boilers. Starting in the 1930s, steam turbine engines were introduced, powered by steam from the same sort of boilers, or with the advent of nuclear power, from nuclear power plants. Most of these engines used reduction gears to reduce the turbine's high rotational speed to a level suitable for driving the propeller. Some, called turbo-electric engines, used the turbine to drive an electrical generator, which in turn drove electric motors that powered the propeller. During the same era, reciprocating diesel engines were also used in many ships. In the late 20th century, gas turbines began to be used in warships. Today, most merchant ships use diesel engines, most surface warships use gas turbines, and most submarines and some surface warships use steam turbines fed by nuclear-powered boilers.
Ship/water interfaces. Early marine engines used paddlewheels to drive the ship through the water. In the mid- to late 19th century, paddlewheels were replaced by screw propellers. In the late 20th century, technical advances provided controllable pitch propellers, thrusters, and propulsion pods.