There has a variety of connection methods between Life Raft Hydrostatic Release Unit and the hull. Here will explain the correctness of various connection methods from the principle of release.
When we test the inflatable life raft for new shipbuilding or It’s annual service, we could always find the connections of HRU, painter line and weak link is wrong. The following is a discussion of hydrostatic release unit connection.
Common HRU like picture A.
The Painter line of each life raft(except the liferafts that are stored at bow or stern in accordance with SOLAS Chapter III, paragraph 31.1.4) should be connect with hull through HRU and weak link. Common connection method of painter line like picture B and C, the main purpose is, if life raft need to be thrown overboard by man,then the painter line is firmly connected with hull through HRU; in case of ship sinking, the painter line is connected with hull through weak link.
The HRU should automatically release life raft when at a depth of not more than 4 meters under water, and it’s designed that the structure should not be disengaged when subjected to waves. When the ship sank, the life raft sank with the ship, reaching a certain depth (no more than 4 meters), the HRU automatically release, at this time the life raft is connected depends on the painter line and weak link with the hull, life raft will float up by it’s own buoyancy,the painter line that connected with hull will release the life raft subjected to the tension, finally the weak link broken under the tension at 1.8KN ~ 2.6KN and disengage the life raft from the hull.so when inspection, we should check the connection of HRU carefully, otherwise the life raft could not release at an emergency by wrong connection of HRU.
The picture D and E are right connection of HRU, while F and G are wrong.
In picture D and E, the painter line(green one) is connected with HRU through a shackle,and HRU is connected with hull, but the weak like(red one) is connected with shackle on painter line and hull; when HRU released by pressure of water,The shackle on painter line also disengaged from the HRU, at this time the life raft is connected depends on the painter line and weak link with the hull, when the buoyancy is big enough, the life raft will be automatically inflated, and the weak link also broken by tension, then complete the whole procedure of life raft.
In Picture F and G, when HRU automatically released, weak link broken by buoyancy of life raft, but caused by directly connection between the shackle on painter line and the shackle tied on life raft(blue one)(The shackle one side is tied on life raft and another side is connected with cradle, while cradle is welded with hull), not only connected with weak link, so when weak link broken, the painter line still connected with the shackle that tied on life raft, finally the life raft will sink with ship and not released.
Article from: http://www.prosmarine.com/connect-life-raft-hru/