How Does A Hydrostatic Release Unit (HRU) Work?

A Hydrostatic Release Unit (HUR) is a device used to automatically release safety equipment when a vessel sinks. The most common places to find HRUs is on liferafts or EPIRBs, but you could use them on anything that needs to be able to float free on its own.

A HRU uses water pressure to activate a spring-loaded blade, cutting through liferaft securing straps. As the raft floats upwards, it automatically inflates when the painter pulls tight. Additional buoyancy from the inflated raft then breaks the “weak link”, releasing the raft from the vessel.

How a Liferaft HRU works step-by-step

The most common place to find a HRU is on the securing mechanism from a liferaft. Most of the time it sits there, forming part of the securing mechanism which stops liferafts falling off in bad weather. It is important to secure liferafts because you need them to be in place in an emergency.

If all goes to plan, you should have time to launch your liferaft. If not, the HRU takes over and automatically releases the raft as your vessel goes down. Even if you managed to launch a liferaft yourself, HRUs on your other rafts will launch the rest, increasing your survival equipment and your chances of survival.

Step 1: The Vessel Sinks

As your vessel sinks, the securing straps keep your liferaft in place. It stays attached to its cradle. There is some inherent buoyancy within the raft itself, so it does try to float off on its own. The straps that were securing it in place during bad weather now stop it from launching.

On one part of the strap, you should have a hydrostatic release unit fitted. Inside the HRU, water starts to enter, exerting pressure onto a diaphragm. The deeper the vessel sinks, the deeper the HRU sinks, and the more pressure is exerted.

Eventually, the HRU reaches its activation depth. The activation depth is the point where the water pressure is sufficient to activate the HRU.

Step 2: The HRU Activates At 1.5m – 4.0m

Somewhere between 1.5m and 4.0m, the water pressure on the diaphragm is great enough to release a spring-loaded blade. The precise depth is not too important as long as it complies with regulations. It needs to…

automatically release the liferaft at a depth of not more than 4m

LSA Code

The blade fires through the rope within the HRU, severing the part that is attached to the securing straps holding the liferaft. This frees one end of the securing straps, allowing the liferaft to float upwards under its residual buoyancy.

Step 3: The Raft Floats Free

As the liferaft floats upwards, ideally it will have a clear path towards the surface. If anything is overhanging the raft storage location, consideration needs to be placed on constructing guides so that the raft will float clear.

A good example of this is on rafts that are stored on the promenade deck of cruise ships. If you look above them, you will see a metal structure designed to guide the rafts clear of the ship if they ever need to float free.

As it is floating upwards, the only part of the raft now attached to the vessel will be the painter. The raft will continue upwards, paying out the painter as it goes. Once it reaches the end of the painter, tension starts to come on the line.

Step 4: The Tension In The Painter Inflates The Raft

The painter is attached to an integrated canister of compressed gas inside the liferaft. When force is applied to the painter, it activates the valve on the gas canister which inflates the raft. In normal circumstances, you can give the painter a sharp tug to inflate the raft. In the automatic launching scenario, the raft itself applies the force.

As the raft floats upwards, there is enough inherent buoyancy to apply enough force on the painter to fire the gas canister. 

The other end of the painter is attached to a section of the HRU which is strong enough to apply the force required to fire the canister. The “weak link” is designed to give sufficient force to activate the canister, but not so much force that it will then drag the raft down with the vessel. Instead, as the vessel sinks further, the weak link breaks.

Step 5: The Weak Link Breaks

The weak link is designed to break under the equivalent force of a standard size liferaft’s buoyancy. It needs to…

…break under a strain of 2.2 +- 0.4 kN

LSA Code

As the raft pulls on the painter, the weak link breaks and the raft is free to float to the surface.

Pro Tip: You can get different size HRUs to match the size of your liferaft. Small rafts will not provide sufficient force to break a commercial standard HRU’s weak link.

I made this video to illustrate how a HRU works when used for securing a liferaft.

How an EPIRB HRU works

The hydrostatic release unit on an EPIRB works in the same way as the HRU on a liferaft. As it becomes submerged, the water exerts pressure on the diaphragm inside the HRU. Once it reaches its designed depth, there is sufficient pressure to release the spring-loaded blade to sever the EPIRB’s securing mechanism.

The main difference with the HRU on an EPIRB is that it usually severs a link that holds the cover onto the EPIRB’s storage container. When the cover is released, the EPIRB floats clear and heads for the surface.

As there isn’t a gas cylinder that needs activating, there is no weak link on EPIRB HRUs. Once the EPIRB is released, no further action is required.

EPIRBs will activate themselves in different ways. Some will use contacts that detect the presence of water. Others will use a simple cord that pulls out a pin as the EPRIB floats away. You should consult your own EPIRB’s instruction manual to determine the exact arrangement that yours uses.

What depth does a HRU activate?

A Hydrostatic Release Unit will activate at a depth not exceeding 4m. This depth is defined in international law through the International Convention for the Safety of Life at Sea (SOLAS). SOLAS refers to the Lifesaving Appliances (LSA) Code which states that a HRU should…

automatically release the liferaft at a depth of not more than 4m

LSA Code

Given that the only requirement is that the depth is shallower than 4m, there will be a range of depths that the HRU could activate at.

Hammar, who manufacture the popular H20 HRU state on their website that their HRU has a release depth of 1.5m – 4.0m.

Similarly, Seamate, who also manufacture HRUs for liferafts and EPIRBs also state that their release depth is 1.5m – 4.0m. 

Do you need to service a HRU?

Most HRUs are disposable so they do not need to be serviced. You should still regularly check that the HRU has not reached its expiry date and that it is fitted correctly.

If you have a hydrostatic release unit that is serviceable, SOLAS is clear that it needs to be serviced annually at a competent servicing station.

“Hydrostatic release units, other than disposable hydrostatic release units, shall be serviced:

…at intervals not exceeding 12 months, provided where in any case this is impracticable, the Administration may extend this period to 17 months;

…at a servicing station which is competent to service them, maintains proper servicing facilities and uses only properly trained personnel.”


Given the expense of annual servicing and maintenance, disposable HRUs are a popular choice. Every ship that I have worked on has used disposable HRUs, usually the Hammar H20. The H20 has a service life of 2 years, so it is often more cost-effective than the equivalent serviceable HRU.

What HRU do I need for my liferaft?

The HRU that you select for your liferaft depends on the size of the liferaft you use. This is because the weak link of a SOLAS approved HRU is designed to break under a load of 2.2 +- 0.4kN. There is a chance that smaller rafts may not exert sufficient force to activate the HRU.

If you are on a small boat and have a 4-person liferaft, you can use a different HRU. Taking Hammar as an example, they offer a “small raft” version which is designed to break under a load of 1.2 +-0.4kN.

The reason that you cannot use the smaller HRU on a large liferaft is that the breaking load of the weak link may be too low to inflate the raft. If the weak link breaks before the raft inflates, it will be harder for survivors to use the raft. The container might still reach the surface, but someone will then need to pull the painter hard enough to activate the inflation mechanism.

Do all liferafts need a HRU?

On a small boat, fitting a HRU to your liferaft is advised, but it is still only optional. On a SOLAS regulated ship, all liferafts do need to have a means of automatically launching themselves. The HRU is the easiest way of complying with the requirements.

The only exception to this is the extra liferaft that you need on larger cargo ships. On these ships, if it is more than 100m to the lifeboats, you need to have an additional liferaft available. As cargo ships usually have their main lifeboats at the stern, they often store another liferaft on the forecastle. This liferaft does not need to have a HRU fitted. The regulations state that it…

…may be securely fastened so as to permit manual release


You often find in heavy weather that ships have waves breaking over the bow. If you had a liferaft with a HRU, there is a chance that it will automatically deploy if you take a large wave over the bow.

Can I secure multiple liferafts with one HRU?

HRUs are only usually approved to secure a single liferaft. When you secure multiple rafts, there is more residual buoyancy, providing more force on the securing straps in the initial stages of sinking.

The additional tension in the securing lines may change the characteristics of the rope that needs to be cut with the spring-loaded blade. If the tension is too great, there is a chance the blade may not rut the rafts free.

In addition to that, when a single HRU is used you will need multiple weak links for the painters. If you attached more than one to a single weak link, the link will probably break before it can apply sufficient force to inflate the liferaft.

If you do decide to use a single HRU to secure multiple rafts, you need to ensure that the HRU has been specifically designed for that purpose

Do you need a HRU if you only operate in shallow water?

If there is no chance of your boat sinking below 4m, a HRU is unlikely to activate. You need to decide as to the best way of securing your liferafts in those conditions. You’ll have to consider how best to stop the rafts from falling overboard when your boat rolls, but at the same time being able to float free if they need to.

The other consideration is the weak link. You should still have the painter attached to the boat by a weak link so that it will inflate the raft if it does launch itself.

The UK MCA has published some guidance about HRUs in shallow water. Read their M-Notice here: MGN 343.

Do all EPIRBs need a HRU?

All Category 1 EPIRBs need to be able to float free in an emergency. Most of them will be fitted with a hydrostatic release unit. Most manufacturers integrate the HRU within the housing when they sell you a Category 1 EPIRB.

Category 2 EPIRBs do not need to be able to automatically float free, so do not need to have a HRU fitted. Category 2 EPIRBs are ones that only require manual activation. The EPIRB itself is the same, it’s just that you need to manually take it out of its housing and activate it yourself.

Local regulations will dictate whether you should have a Category 1 or Category 2 EPIRB on your boat. For example, ships need to have a Category 1 EPIRB, capable of floating free and automatically activating in an emergency.

Can you still launch a liferaft manually if a HRU is fitted?

You must always be able to launch a liferaft manually, even if there is a HRU fitted.

This usually involves attaching the securing straps to the HRU with a senhouse slip. The senhouse slip means that you just need to move a single metal bar and the liferaft is free. Not only is it easy to release manually, but the senhouse slip does also have a second function. Its smooth metal construction means that the HRU will slide off smoothly. If you attach the HRU to a rope, the rope could snag and reduce the chance of the HRU operating correctly.

You hope that even in an emergency, the HRU isn’t necessary because you will have time to manually launch your liferafts. Particularly with davit launched liferafts, it is much better to be in control of the process.