Hydrostatic Release Units for Liferafts and EPIRBs

Engineering guide to Hydrostatic Release Units (HRU): Hammar H20 float-free mechanics (1.5m to 4m depth), liferaft weak-link rigging rules, and 2-year service life mandates under SOLAS.

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A Hydrostatic Release Unit (HRU) is a compact, autonomous mechanical safety device engineered to perform a single, critical task: automatically releasing a liferaft or EPIRB from a sinking ship without requiring human intervention. In catastrophic maritime casualties—such as rapid vessel capsizing, severe hull breach, or sudden structural failure—crew members may have zero time to reach muster stations or manually unlash survival craft. Under **SOLAS Chapter III** and the **IMO LSA Code**, HRUs must deploy reliably under water pressure at depths between **1.5 and 4.0 meters**. Despite their compact size, HRUs are precision-engineered pressure instruments. Installing an HRU incorrectly is one of the most common and dangerous errors found during maritime safety audits. ## Mechanical Working Principle of the HRU The most widely deployed HRU technology globally is the **Hammar H20** hydrostatic release: - Inside the sealed composite casing, a flexible stainless steel diaphragm separates an air chamber from the external marine environment. - As the vessel sinks, water enters the outer chamber. At a depth between 1.5 and 4.0 meters, water pressure forces the internal diaphragm inward against a pre-calibrated spring. - The movement releases a spring-loaded stainless steel knife blade that severs the synthetic retaining lashing holding the liferaft canister in its cradle. - The liferaft floats free toward the surface while its painter line remains connected to the sinking vessel v...