Hydrostatic Release Units are designed to automatically free inflatable liferafts from their stowage after they have been submerged. There may be occasions when manual release is impossible (e.g. if the vessel were to sink very rapidly). The use of HRU is therefore, a very desirable safety measure.
When the ship is submerged about 3 metres deep, the lashing strap of the liferaft is released. The painter is held to the vessel by the weak link. As the liferaft floats free of the vessel, the painter will be pulled out and will inflate the liferaft. The weak link breaks free from the cradle and releasing the liferaft at a certain depth. Otherwise, the liferaft will still be attached with the sinking vessel.
Hammar is committed to safety and there have been imitation of their HRU lately which prompt them to enhance a few security features on their products. Hence, as of February this year all H2O has a unique Holospot security marking with serial number and production date next to the Holospot marking. The Holospot shimmers in rainbow colours when put under direct light. The Holospot label has a six number alphanumerical code which is different on each product.
This red part is called the weak link. The liferaft painter must be secured here to ensure the HRU is capable of releasing the liferaft in the event the ship submerged. Most of the common mistake by the ship crew is the failure to locate the weak link and install the painter correctly. This might cause the liferaft to be dragged down along with the sinking ship and unable to be released for the use of the survivors at the surface of the ocean.
After installation, the HRU will have a lifespan of 2 years and should be marked appropriately from the month it was installed. There is actually a blade inside the unit which will be activated by the hydrostatic pressure (as water pressure will rise as it gets deeper in the ocean) in the event the ship submerged. The blade will then cut loose the rope which holds the weak link and release the liferaft.