The Hydrostatic Release Unit serves as the connecting device between the liferaft container and the ship’s deck where it is secured.

Its operation takes place in the following sequence:

1. The HRU secures the liferaft container to the ship while allowing rapid manual release whenever required.
2. When the vessel sinks to a depth of approximately 4 metres (13 feet), increasing water pressure activates the hydrostatic mechanism.
3. The built-in cutting device severs the securing strap holding the liferaft container.
4. The container floats freely toward the surface while the painter line remains attached through the weak link.
5. As the ship continues sinking, tension develops in the painter line.
6. This tension activates the gas cylinder inside the container, inflating the liferaft automatically.
7. Once the inflation force exceeds the strength of the weak link, the weak link breaks.
8. The fully inflated liferaft separates completely from the sinking vessel and floats freely on the sea surface, ready for immediate boarding by survivors.

The Hydrostatic Release Unit therefore provides an automatic, reliable, and internationally approved method of ensuring that liferafts remain available even when there is insufficient time for manual launching during a ship sinking emergency.

Rigid Liferafts

Rigid liferafts are specially designed survival craft that provide a safe and stable platform for survivors after abandoning a ship. Unlike inflatable liferafts, they are constructed using rigid materials that offer strength, durability, and reliable flotation under emergency conditions.

The floor of a rigid liferaft must be designed to prevent the ingress of water, ensuring that the interior remains as dry as possible. It should also provide adequate support to keep the occupants above the waterline while offering effective insulation against cold water, thereby reducing the risk of hypothermia during prolonged survival situations.

The approved carrying capacity of a rigid liferaft is determined according to international safety standards. It must be capable of accommodating the specified number of persons, each assumed to have an average body mass of 82.5 kilograms (181 pounds) while wearing both an immersion suit and a lifejacket. The available seating space and headroom must allow every occupant to remain reasonably comfortable without interfering with the operation of the liferaft or its emergency equipment.