The unit system of machinery was a method of arranging a ship's propulsion machinery into separate units that could each operate autonomously in case of damage to the ship. For a

steamship A steamship, often referred to as a steamer, is a type of steam-powered vessel, typically ocean-faring and seaworthy, that is propelled by one or more steam engines that typically move (turn) propellers or paddlewheels. The first steamships ...

, this would be a boiler room supplying steam to an

engine room On a ship, the engine room (ER) is the compartment where the machinery for marine propulsion is located. To increase a vessel's safety and chances of surviving damage, the machinery necessary for the ship's operation may be segregated into vari ...

. There might also be a gearing room that housed the

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission *** ...

that actually turned the

propeller shaft A drive shaft, driveshaft, driving shaft, tailshaft (Australian English), propeller shaft (prop shaft), or Cardan shaft (after Girolamo Cardano) is a component for transmitting mechanical power and torque and rotation, usually used to connect ...

(s). Ideally each "unit" should have an additional compartment between them to further reduce the risk. Many ships were able to provide steam via cross-connections from either boiler room to either engine room. The unit system was developed during

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

to help mitigate damage and flooding from damage inflicted by a weapon and to preserve a ship's mobility by physically separating the engines and boilers into at least two groups so that a single

torpedo A modern torpedo is an underwater ranged weapon launched above or below the water surface, self-propelled towards a target, and with an explosive warhead designed to detonate either on contact with or in proximity to the target. Historically, su ...

hit, for example, could not flood all of the boiler or engine rooms, disabling all of the ship's propulsion machinery. A single

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

torpedo hit would typically blow a hole in the hull and would compromise the integrity of adjacent watertight bulkheads over twice that length and further if the ship was of

rivet A rivet is a permanent mechanical fastener. Before being installed, a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite to the head is called the ''tail''. On installation, the rivet is placed in a punched ...

ed construction rather than welded. This would usually flood two compartments and possibly three. The unit system invariably added length to accommodate the additional piping to cross-connect the engines and boilers and the widely separated boilers required two

funnel A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening. Funnels are usually made of stainless steel, aluminium, glass, or plastic. The material used in its construct ...

s which reduced the fields of fire of the ship's

anti-aircraft gun Anti-aircraft warfare, counter-air or air defence forces is the battlespace response to aerial warfare, defined by NATO as "all measures designed to nullify or reduce the effectiveness of hostile air action".AAP-6 It includes surface based, ...

s and added topweight. There could be significant knock-on costs as well. For example, the second batch of the British ''Leander''-class

light cruiser A light cruiser is a type of small or medium-sized warship. The term is a shortening of the phrase "light armored cruiser", describing a small ship that carried armor in the same way as an armored cruiser: a protective belt and deck. Prior to thi ...

s of the 1930s (which were all eventually sold to

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

) were modified to use the unit system. This increased the length of the machinery spaces by and the

waterline The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...

belt armor Belt armor is a layer of heavy metal armor plated onto or within the outer hulls of warships, typically on battleships, battlecruisers and cruisers, and aircraft carriers. The belt armor is designed to prevent projectiles from penetrating to ...

needed to protect the boilers increased by a length of . The extra weight of the armor required the

beam Beam may refer to: Streams of particles or energy *Light beam, or beam of light, a directional projection of light energy **Laser beam *Particle beam, a stream of charged or neutral particles **Charged particle beam, a spatially localized grou ...

to be increased by to preserve

stability Stability may refer to: Mathematics *Stability theory, the study of the stability of solutions to differential equations and dynamical systems **Asymptotic stability **Linear stability **Lyapunov stability **Orbital stability **Structural stabilit ...

. All of these changes made the ships more expensive than their predecessors.Raven & Roberts, p. 156

Citations

Bibliography

* *{{cite book, last1=Raven, first1=Alan, last2=Roberts, first2=John, title=British Cruisers of World War Two, publisher=Naval Institute Press, location=Annapolis, Maryland, year=1980 , isbn=0-87021-922-7 , name-list-style=amp Naval architecture