Dive to Survive


An HRU unit.


  • How are liferafts deployed?
  • What is HRU?
  • Why is the HRU so important on a lifeboat?
  • Next week in Invisible History: First female US transcontinental pilot.
  • Discuss the article here.

Imagine that you’re on a cruise. Everything is good until you hit an iceberg. Now, I know it sounds like Titanic, but there is one twist. This time there are many lifeboats. The reason behind this is a small device that makes concise packaging possible for a huge boat.

Back in 2014, I saw a youtube video about the HRU - hydrostatic release unit, and my mind just exploded!. I didn't know the working principle behind the unit, but I instantly knew that this would save many lives if it worked like clockwork. So without further ado, let's get into it.

The Principle

Your cruise is in the middle of the ocean, sinking as the vessel slips beneath the waves. Your only hope is that the life raft will do its job and appear at the surface. Which in other words, means you're utterly reliant on its HRU. The HRU is a hydrostatic release unit, and its correct rigging and operation can be the difference between life and death.

HRU securing the liferaft.

Life rafts are installed at an open deck so that access to it is relatively simple. They need to be secured at the same time they need to be free to float, should the vessel sink. These units automatically cut the life raft free and trigger its inflation if you're unable to launch it yourself. Even if you've already successfully abandoned ship into other rafts, the remaining release units will release all your remaining life support, dramatically increasing your resources available to survive.

The mechanism of working

Let's consider the image you see below. It is a CAD model of the lifeboat with HRU attached to it.

There is the main strap that runs over the top. At the end of the belt is a senhouse slip. These are provided for quick manual release if you're launching it yourself. If the senhouse slip is directly secured to the deck, the raft will never be able to break free. So instead of this, we fit the hydrostatic release in between. On this unit, the attachment is on the yellow section (as seen in the picture above). Now this yellow section is critical as it has a low coefficient of friction. The other end of the components is fixed straight to the deck.

The lifeboat assembly.
The lifeboat assembly.

The final part of the unit is the red section. It is the weak link in all the connections here. We must attach the rope (also called the painter) to this part. The link is designed in such a way that it is strong enough to pull the painter tight to inflate the raft inside the casing but weak enough to break before the vessel pulls it down while sinking.

So what happens in the HRU when the boat is sinking? Inside the HRU we have a flexible diaphragm in a sealed chamber. When this unit is submerged in water, it will be pressurized from all directions. This pressure pushes the diaphragm and frees the securing blade. Finally, with the magic of some spring force, the line is cut with the blade, and the raft is free.

weak link
The release unit from the ship with the weak link.

The raft after this is still attached to the weak link with the painter. Due to buoyancy, the painter is pulled from the weak link. As mentioned before, the painter will trigger the inflation of the raft inside and then break the weak link. And by this, the life raft is free to be occupied by the survivors.

The release of the blade due to the hydrostatic pressure on the diaphragm.
The release of the blade due to the hydrostatic pressure on the diaphragm.

Characteristics of the HRU

Some common characteristics of HRUs are:

  • Water pressure-sensitive at depths not to exceed four meters or less than two meters
  • Single-use only, require replacement if activated
  • Cannot be serviced; only replaced
  • Waterproof; sealed against moisture and tampering
  • Must be labelled with an expiration date
  • The expiration date is two years from month of installation applies to unit and rod

This week in Invisible history.

The clue for this week in invisible history was a difficult one as no one could give the correct answer. Siddharth Singh came very close but was off due to incorrect reference. The clue, "Locomotion Number 1" referred to the first locomotive to haul a passenger train. It was operated by George Stephenson's Stockton & Darlington line in England on 27th September 1825.

The engine "Locomotion No. 1" pulled 34 wagons and one solitary coach on its journey of 21 miles from Shildon, via Darlington to Stockton in County Durham. This epic journey was the launchpad for the development of the railways, which was to take the world by storm over the following years. The Stockton to Darlington line was the first railway in the world which carried passengers and freight, both primarily using steam locomotives, and to any meaningful timetable. Earlier, other railways had carried passenger coaches adapted for rails, but only on an occasional basis.

Locomotion number 1 at Darlington.
Locomotion number 1 at Darlington.

The clue for next week is “First female US transcontinental pilot.”

See you next weekend!

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Reference links

  1. Hydrostatic release unit (HRU)
  2. Life Raft Release System and Launching Procedure
  3. Hammar hydrostatic release unit HRU H20 for liferaft | Products | Traconed
  4. Locomotion No 1 photo - Colin Bainbridge photos at pbase.com


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