How Does a Hydrostatic Release Work?

When submerged in water, a Hydrostatic Release Unit (HRU) is activated by water pressure at a specific depth, usually around 4 meters. 

This pressure causes the unit’s release mechanism to function, allowing the liferaft or EPIRB to separate from the sinking vessel and float to the surface. 

There are different types of HRUs, some of which are disposable and must be replaced every 2-4 years, while others can be serviced and reused.

In my yachting career, I thankfully never had to use one in an emergency, but I saw a fair few deployed in my extensive training and certifications. Here’s how they work.

Principles of Operation and Functionality

Role of Water Pressure

The HRU functions based on the water pressure it senses when submerged. As your vessel sinks (hopefully, it doesn’t), the pressure increases, and this change in pressure plays a crucial role in activating the HRU. 

The activation depth typically ranges between 1.5 and 4 meters, ensuring that the liferaft’s deployment occurs promptly as the vessel submerges.

Automatic and Manual Release Mechanisms

Hydrostatic Releases are designed with both automatic and manual release mechanisms for optimal functionality in different situations.

The automatic release mechanism relies on the water pressure to activate the deployment process. When the pressure reaches a pre-defined level, usually between 1.5 and 4 meters below the surface, the diaphragm inside the sealed casing is forced to cut a weak link, allowing the liferaft to float free.

Conversely, the manual release mechanism enables you to detach the liferaft from the vessel during normal operation or in emergencies. By disengaging this mechanism and activating the inflation system, the liferaft can be deployed and float clear without needing to submerge to the activation depth.

The Weak Link and Knife

The weak link and knife system are integral to the HRU’s release mechanism. In an automatic release scenario, the water pressure causes the diaphragm to activate a knife that cuts the weak link. 

Upon severing this connection, the liferaft or EPIRB automatically detaches from the vessel and floats to the surface.

This design ensures easy release and buoyancy for the liferaft, allowing it to float clear and provide a safety apparatus if your vessel sinks. 

The combination of weak link and knife components enables the HRU to function reliably and efficiently, providing essential assistance in emergencies.

Deployment in Case of an Emergency

When deployed, the mechanism releases the liferaft, which automatically inflates and floats to the surface. 

Simultaneously, the EPIRB is released, and its transmitter activates. The EPIRB then starts transmitting a continuous 406 MHz distress signal, which search and rescue teams use to locate the emergency and provide assistance.

To ensure the effectiveness of the HRU system, it’s essential to maintain and replace the device when necessary. 

HRUs must be approved by regulatory authorities and should be serviced or replaced according to manufacturer guidelines. Regular inspections and timely servicing will ensure performance when you need it the most.

The Role of HRUs in Liferafts

Remote Release Systems

Employing remote release systems such as HRUs significantly increases the chances of successful deployment and retrieval of liferafts by the crew. This is because manual release may prove difficult or impossible in panic-stricken situations.

Key Components and Assembly

A few key components work together to ensure proper release and deployment of the liferaft:

  • Painter: A rope or line connecting the liferaft with the vessel. This painter will help in pulling the liferaft towards the vessel for easy boarding by the crew.
  • Securing Straps: These straps hold the liferaft container in place, ensuring it is securely attached to the vessel.
  • Release Mechanism: This is the actual hydrostatic release unit that detaches the securing straps when the vessel sinks to a certain depth.

When securing a liferaft, it is vital to ensure the HRU is correctly installed with appropriate lashing arrangements. This includes proper positioning of the painter and securing straps to keep the liferaft container attached to the vessel.

To set up your HRU, follow these general steps:

1️⃣ Attach the painter to both the vessel and the liferaft.
2️⃣ Place the liferaft container on its cradle or in a designated location on the vessel.
3️⃣ Connect the securing straps from the HRU to the liferaft container.
4️⃣ Properly lash and tension the securing straps according to the HRU and liferaft manufacturer guidelines.

Be mindful of regular maintenance and inspections to ensure the HRU functions correctly when needed. Proper installation, maintenance, and understanding of your hydrostatic release unit can save precious time during an emergency and potentially save lives.

I’m the founder and chief editor here at Sailing Savvy. I spent a decade working as a professional mariner and currently, I mix those experiences with digital publishing. Welcome, and I hope that we can be the hub you need for safe passage.