Most HRUs operate by water pressure alone depressing an internal diaphragm to operate the release mechanism or water ingress into the unit activates a sharp blade which cuts the strong rope that secures the raft to its cradle or mounting position. With either type, upon immersion and at a depth of 1.5 to 4 metres, the raft will float free and inflate as the painter line is fully extracted from its container. A weak linkage finally frees the life raft’s painter line from the vessel as it continues to sink and the life raft floats free.