Hydrostatically Released Units (HRU) – Essential Safety Device for Liferafts and EPIRBs
When a vessel faces sudden emergencies such as capsizing, sinking, or fire, life-saving appliances must be deployed instantly and reliably. One such critical device is the Hydrostatically Released Unit (HRU). This small but vital mechanism ensures that liferafts and Emergency Position Indicating Radio Beacons (EPIRBs) are automatically released and float free when a ship sinks.
The HRU is a mandatory component under IMO and SOLAS regulations, providing automatic release at a certain depth. At Seanav Marine, we specialize in supplying, servicing, repairing, and certifying HRUs for vessels worldwide. With decades of expertise, a globally connected partner network, and a highly skilled technical team, we ensure full compliance with international safety standards.
What is a Hydrostatically Released Unit (HRU)?
A Hydrostatically Released Unit is a mechanical device fitted to a liferaft or EPIRB. It operates based on water pressure (hydrostatic force). When a vessel sinks to a depth of around 1.5 to 4 meters, the HRU automatically cuts the securing strap, allowing the liferaft or beacon to float free and inflate or activate.
This ensures survival equipment is available even if the crew cannot manually deploy it.
Technical Specifications of HRU
HRUs are simple yet highly reliable devices designed according to SOLAS and IMO guidelines. Typical specifications include:
Activation Depth: Between 1.5 m and 4 m below the sea surface.
Construction Material:
Corrosion-resistant stainless steel components.
High-strength plastic or composite housing.
Breaking Strength:
Minimum 2.2 kN for liferafts.
Designed to withstand shipboard conditions until activation.
Release Mechanism:
A sharp blade or pin cuts the securing strap under water pressure.
Weak Link: Ensures liferaft or EPIRB floats free without being trapped by sinking vessel.
Lifespan: Normally 2 years (pyrotechnic-type HRUs) or 3–4 years (mechanical-composite HRUs).
Weight: Lightweight, typically between 1.5 to 3 kg.
Certification: Must comply with IMO Resolution A.689(17) and MSC.81(70) testing standards.
Types of HRU
There are two main categories of HRUs in maritime use:
HRU for Liferafts
Connects liferaft to deck securing arrangement.
Releases automatically at specified depth.
Weak link ensures separation from sinking vessel.
HRU for EPIRBs (Emergency Beacons)
Releases EPIRB automatically to surface when vessel sinks.
EPIRB then transmits distress signals to satellites.
Both types are mandatory under SOLAS for most vessels.
IMO and SOLAS Regulations
The International Maritime Organization (IMO) and the Safety of Life at Sea (SOLAS) Convention mandate the use of HRUs under SOLAS Chapter III:
SOLAS Regulation 4.1.6: All liferafts secured on deck must be fitted with a hydrostatic release device.
SOLAS Regulation 6.2.2: EPIRBs must be capable of floating free via a hydrostatic release arrangement.
Release Depth Requirement: HRU must activate at a depth between 1.5 m and 4 m.
Servicing: HRUs must be replaced at intervals not exceeding manufacturer’s expiry date (usually 2 years).
Approval: Only HRUs tested and certified according to IMO Res. MSC.81(70) are allowed on SOLAS vessels.
Importance of HRU in Maritime Safety
The HRU is crucial because:
It ensures automatic deployment of survival equipment even if crew cannot manually launch.
Provides fail-safe protection during capsizing or sudden sinking.
Increases chances of survival by ensuring liferafts and EPIRBs are not trapped.
Complies with international safety requirements, preventing port state control detentions.
Common Global Brands of HRUs
Several globally recognized manufacturers produce IMO-SOLAS-approved HRUs, such as:
Hammar (Sweden) – H20 series, widely used across merchant fleets.
Dongtai Jianghai Lifesaving Equipment (China)
Lalizas
Viking Life-Saving Equipment
Survitec
At Seanav Marine, we work with leading global partners, offering our clients a wide range of SOLAS-approved HRUs.
Maintenance, Inspection, and Certification
Hydrostatically Released Units are precision devices that require proper handling and timely replacement.
Visual Inspection: Regularly check for cracks, corrosion, or damage.
Expiry Date: Each HRU is marked with a clear expiry date (typically 2 years). It must be replaced before this date.
Replacement Procedure: Simple to install; however, must be fitted correctly with weak link and securing strap as per manufacturer instructions.
Annual Servicing: Check as part of annual safety equipment inspection.
Certification: Carried out by trained technicians to confirm compliance.
At Seanav Marine, we provide annual service, five-yearly certification, replacement, and repair of HRUs. Our team ensures that every device fitted onboard meets international safety standards.
Applications of HRUs
Hydrostatically Released Units are mandatory on many types of vessels and installations, including:
Cargo Ships
Passenger Vessels (Cruise ships, Ferries)
Offshore Oil Platforms
Fishing Vessels
Naval Ships and Coast Guard Vessels
Frequently Asked Questions (FAQ)
Q1. How often should an HRU be replaced?
Most HRUs must be replaced every 2 years (or as per manufacturer’s instructions). Expired HRUs are non-compliant and unsafe.
Q2. Does an HRU inflate the liferaft?
No. The HRU only releases the liferaft from the vessel. Inflation occurs automatically once the liferaft reaches the water surface and the painter line is pulled.
Q3. What is the difference between an HRU for liferaft and for EPIRB?
Liferaft HRU: Releases liferaft from deck cradles.
EPIRB HRU: Releases beacon to float free and activate distress signal.
Q4. Can HRUs be reused after expiry?
No. HRUs are designed for single service life and must be replaced with certified new units after expiry.
Conclusion
The Hydrostatically Released Unit (HRU) is a small but essential device that ensures liferafts and EPIRBs are released automatically during vessel emergencies. Compliant with IMO and SOLAS standards, HRUs save lives by guaranteeing that survival equipment is not trapped on a sinking ship.
At Seanav Marine, we supply, service, and certify HRUs for liferafts and EPIRBs. With our decades of expertise, global partnerships, and skilled technical team, we provide reliable solutions that keep vessels safe and compliant worldwide. From annual inspections to five-yearly certifications, we ensure that your safety equipment is always ready to perform when it matters most.