What is a Hydrostatic Release Unit & How Does it Work?

by | Last updated May 9, 2024 | Lifesaving, Maritime Safety | 0 comments

If you watched many ship-sinking movies before, you might have already indirectly seen a hydrostatic release unit in action.

Movies or news about a sinking vessel sometimes mention liferafts, fully inflated but without anyone inside. 

You may already hear of rescuers finding a locating beacon, in this case, an EPIRB, but with nobody nearby.

Sounds grim, isn’t it?

Liferafts and EPIRBs can operate on their own in case the ship sinks.  A hydrostatic release unit or HRU enables these devices to detach from the sinking vessel.

What is a hydrostatic release unit or HRU?

A hydrostatic release unit or HRU is a device installed together with our EPIRB and liferaft securing arrangements.

Since all types of vessels going on international trade require them to have an HRU, expect one on board installed as part of the vessel’s safety equipment.

To find its meaning, let’s define each word one by one.

It is Hydrostatic because it operates using the pressure exerted by fluids, in our case, the sea.  Release means its function is to cut, severe, or free something up. Unit refers to the device.

In simple terms, it is a quick-release mechanism that operates automatically when submerged underwater.

In the case of a liferaft, this unit has a built-in knife that cuts the raft’s lashings. Additionally, the same raft connects to a weak link that disconnects from the vessel.

The hydrostatic release unit of an EPIRB operates differently but works under the same principle. 

Since an EPIRB is installed in an enclosure or casing, a knife cuts through its securing bolt when the HRU activates. With the lock broken, the EPIRB releases itself and floats to the water’s surface.

All of these mechanisms operate when the ship is sinking underwater.


Nobody knows how this simple yet very important device originates. The closest we can find is a patent that dates back to 1976 from Birgitt I. Wildhagen.

He named his invention “Release Device for emergency sea rescue apparatus”.

Here is the abstract of his invention:

A release device for an emergency sea rescue apparatus includes a water pressure release mechanism whereby a lifecraft can be automatically released if its parent ship sinks, the device being designed to accommodate forces at any angle between the normal horizontal and vertical direction whereby the release will operate without jamming regardless of the angle of the ship’s deck when the release is triggered.

Early hydrostatic release unit design patented in 1976
Early hydrostatic release unit design patented in 1976.

His invention mounts the emergency rescue craft (liferaft) into a ship. It is then attached to an eyehook with a belt strap that fastens the craft to the vessel.

Additionally, a painter line connects the raft to the ship.

The said release device frees the eye hook on a pre-determined water pressure level. This releases the raft to its securing arrangement.

If you look more closely at the image above, the setup looks similar.

After that initial contraption, many inventors followed patenting similar release mechanisms for liferafts until what it is today.

How does it work?

You can deploy liferafts using two methods. One and the most common is manned and the second one is unmanned.

Manned deployment involves the ship’s crew.

The unmanned release is automatic. The liferaft automatically frees itself from the sinking ship. Furthermore, it can break free from its cartridge and inflate all on its own.

This is possible using the hydrostatic release unit installed on its lashing arrangement.

An HRU works not necessarily when the ship is submerged but when the unit itself is immersed underwater. 

Most of them operate when submerged between 2 to 4 meters but not more than 4 meters according to SOLAS regulation.

This is the reason why liferafts installed on the forward part of the vessel are not fitted with this device. Rough seas during heavy weather will trigger the HRU thereby losing your raft.

Liferaft arrangement

The float-free arrangement for liferafts has three main parts.

First is the painter which connects the raft to the weak link.

Second is the weak link which connects to the painter and the ship’s strong point. The painter has a breaking strength of 2.0 kN. Weak links are weaker than that.

Lastly is the hydrostatic release unit.

An HRU connects the liferaft’s lashings and a strong point of the vessel.

It has a spring-loaded knife that activates when it is submerged less than 4 meters underwater. This knife cuts the lashing thereby freeing the liferaft.

As the ship continues to sink, the liferaft creeps its way to the water surface while it is still attached to the painter.

Remember that the painter is still attached to the sinking ship.

Once all of the painter is pulled out, it activates the non-toxic CO2 bottle and starts inflating the liferaft.

As the buoyancy of the raft becomes stronger, the weak link breaks! The liferaft is now free from the sinking vessel.

Hydrostatic release unit of a liferaft
Parts of a liferaft lashing arrangements.

Hydrostatic release unit for EPIRBs

EPIRBs or Emergency Position-Indicating Radio Beacon is a part of the vessel’s GMDSS.

This device is outside the bridge and transmits homing signals when the ship sinks.

EPIRB is an electronic object. Since this is placed outside, it is fixed in an enclosure to protect it from sun, rain, snow, seawater, etc.

EPIRB HRU is simpler than that of the liferaft.

The front of its cover is removable. That is how you replace or inspect the EPIRB or its hydrostatic unit.  A plastic rod or bolt from the HRU locks that same cover using a small pin.

When a ship sinks, the HRU triggers cutting off the plastic rod. The cover then removes itself followed by the EPIRB.

Inside of an EPIRB casing showing the hydrostatic release unit. Second picture is its enclosure.
Inside of an EPIRB casing showing the hydrostatic release unit. Second picture is its enclosure.

Important Specs

There are two types of hydrostatic release units.

One is the disposable type which has a lifespan between 12 to 30 months. Once it reaches that service period, you need to replace it with a new one.

The other type is serviceable. A qualified service technician must inspect these HRUs at intervals not more than 12 months.

Whatever the type, they must operate at depths of less than 4 meters. 

Moreover, they must not be released prematurely when seawater washes over the unit. Hammar HRU products have a release setting of 1.4 meters to 4 meters.

Additionally, it must have a draining mechanism so the water inside the HRU does not accumulate.

Hydrostatic units, especially for liferafts have different sizes. Larger ones installed on smaller rafts may not release the weak link as it should. Be sure to check the right type to get the right measurement for the right raft.

Liferaft HRUs are exposed to different meteorological conditions 24/7. The Third Officer or the safety officer must inspect them and ensure they are in good order.

May the winds be in your favor.



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