Hydrostatic release unit (HRU)
A part of the liferaft lashing used for automatic liferaft release. At up to 4m water pressure activates release mechanism and the liferaft is free to float clear to the surface. HRUs may be either of the disposable type, in which case they are replaced every 2-4 years, or they can be of the type that has an unlimited life provided they are serviced and tested (normally every year).
Note: A liferaft stowed forward at a distance of over 100m from other survival craft does not need be fitted with an HRU. This is because the unit may release the liferaft after becoming submerged in heavy bow seas.
Further reading: MGN 343 Hydrostatic Release Units (HRU) - Stowage and Float Free Arrangements for Inflatable Liferafts