Unit system of machinery

[1] The unit system was developed during World War I 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 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 torpedo hit would typically blow a 35-by-15-foot (10.7 m × 4.6 m) hole in the hull and would compromise the integrity of adjacent watertight bulkheads over twice that length and further if the ship was of riveted construction rather than welded.

[2] 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 funnels which reduced the fields of fire of the ship's anti-aircraft guns and added topweight.

For example, the second batch of the British Leander-class light cruisers of the 1930s (which were all eventually sold to Australia) were modified to use the unit system.

The extra weight of the armor required the beam to be increased by 1 foot 8 inches (0.51 m) to preserve stability.