A hydrostatic release unit (HRU) is a device used to automatically release safety equipment when a vessel is sinking. HRUs are commonly found on life rafts or EPIRBs (emergency radio beacons), but can be applied to anything that needs to float independently on the water surface.

The HRU uses water pressure to activate a spring-loaded blade that cuts the liferaft's safety straps. The liferaft floats upward and automatically inflates when the painting becomes taut. The additional buoyancy of the inflated liferaft then breaks the "weak link" and the liferaft is released from the ship.

1. The ship is sinking: If your ship sinks, the safety belts stay firmly in place and the liferaft remains attached to its cradle.

2. Activation of HRU at 1.5m - 4.0m: Somewhere between 1.5m and 4.0m, the water pressure on the diaphragm is great enough to release the spring-loaded blade.

3. The liferaft drifts free: As soon as the liferaft is released, it starts to drift upwards.

4. The tension in the painting inflates the liferaft: Painting is connected to an integrated compressed gas cartridge inside the liferaft.

5. The weak link breaks: The weak link is designed to break under the force of the buoyancy of a standard liferaft.

The HRU on an EPIRB works in the same way as the HRU on a liferaft. Once submerged, the water exerts pressure on the diaphragm inside the HRU. Once it reaches the intended depth, the pressure is sufficient to release the spring-loaded blade, which cuts the EPIRB's securing mechanism.

An HRU is activated at a depth of no more than 4m. This depth is defined in international law by the International Convention for the Safety of Life at Sea (SOLAS).

Most HRUs are disposable and therefore do not require maintenance. However, you should regularly check that the HRU has not reached its expiry date and that it is correctly attached.

The HRU you choose for your liferaft depends on the size of the liferaft you are using. On a small boat, installing an HRU on your liferaft is recommended, but it is still optional. On a SOLAS regulated vessel, all liferafts must be able to launch themselves automatically.