[Edit] Diesel-Electric

Diesel electric propulsion is becoming more common. By having the engines drive alternators, which supply electricity to motors driving the propellers, gearboxes and clutches can be dispensed with and greater flexibility gained in the positioning of the engines, while still providing the step down in speed required for a medium speed engine to efficiently drive a ships propeller.

[Edit] Gas Turbines

Many warships built since the 1960s have used gas turbines for propulsion, as have a few passenger ships, like the jetfoil.

Gas turbines are commonly used in combination with other types of engine. Most recently, the Queen Mary 2 has had gas turbines installed in addition to diesel engines. Because of their poor thermal efficiency, it is common for ships using them to have diesel engines for cruising with gas turbines reserved for when higher speeds are required. Some warships and a few modern cruise ships have also utilised steam turbines to improve the efficiency of gas turbines in a combined cycle. In such a combined cycle, where waste heat from a gas turbine is used to create steam for driving a steam turbine, thermal efficiency can be the same or slightly greater than that of diesel engines. However, the grade of fuel required for gas turbines is much more expensive than that required for diesel engines so running costs are higher.

[Edit] General terminology

Ships may occur collectively as fleets, squadrons or flotillas. Convoys of ships commonly occur.

A collection of ships for military purposes may compose a navy or a task force.

In the past, people counting or grouping disparate types of ship may refer to the individual vessels as bottoms, but this generally refers only to merchant vessels. Groups of sailing ships could constitute, say, a fleet of 40 sail. Groups of submarines (particularly German U-boats in the 1940s) hunt in wolf packs.