Hydrostatic Release Unit: A Complete Guide

Hydrostatic Release Unit

A Hydrostatic Release Unit (HRU) is a pressure-activated mechanism. It is designed to automatically deploy a life raft under certain pressure. In case a vessel sinks, the HRU will activate and release the life raft at a depth between 1,5 and 4 meters. The pressure of the water against a diaphragm within the sealed casing causes a knife to cut through a rope. Then, it will release the life raft.

A hydrostatic release unit or HRU is an automatic releasing pressure-activated mechanism installed in the life rafts of a ship. The unit is designed in such a way that when a ship sinks or is in danger of sinking, the HRU will activate and automatically release the life raft i.e cut-off the rope and inflate the raft.

How Hydrostatic Release Unit Works

The Hydrostatic Release Unit (HRU) is a critical equipment in the safe operation of life rafts and EPIRBs (Emergency Position Indicating Radio Beacons) on board ships and vessels.

SOLAS (Safety of Life at Sea) regulations, specifically SOLAS 1974, provide clear requirements for the construction and positioning of HRUs to ensure the proper functioning and reliability of these life-saving devices.

  • The Hydrostatic Release Unit acts as a connecting media between life raft container and ship deck, where it is stored.
  • The HRU comes in action under the pressure of water exerted on HRU when a ship sinks below 4 meter of water level.
  • The HRU consists of a sharp knife or chisel which is used to cut the strap. This strap is lashed over the container carrying life raft. Still, it holds the painter line at the weak link.
  • The HRU is connected to the container through a lashing arrangement. This can be disengaged quickly by means of slip hook when launching the raft manually.
  • The HRU is connected to a strong point on deck through a weak link.
  • When a ship sinks, the HRU cuts the rope and the container floats to the surface of water.
  • As the ship sinks further, the tension on the painter line causes the life raft to inflate out of the container.
  • The tension acting on the weak link will cause it to break which frees the life raft from the ships.

Also read: Automatic Identification System

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