Hydrostatic Release Unit Guidance

During the last 11 years as both a seafarer and a surveyor, it's amazing how many incorrectly rigged hydrostatic release units I have seen. These are a simple but critical piece of safety equipment that can save valuable time and even life in an emergency situation – provided they are rigged correctly of course.

Please take the time to read this bulletin and check that your own devices are suitable and rigged correctly. It would be even better if all boat owners had this information to hand onboard, so if you would like a PDF copy of the safety bulletin to keep onboard your own motorboat, yacht or working vessel, please email us.

Safe Sailing!

Marine Surveyor and Safety Consultant


Hydrostatic release units, or HRU'S for short are designed to automatically release a liferaft or EPIRB in the event of a sinking, saving valuable time. The HRU will operate at between 1.5 and 4 metres depth, at which time a spring-loaded blade is released inside the unit, cutting the rope and releasing the container from its cradle.

On a liferaft, the painter is paid out as the container floats to the surface. Upon reaching its maximum length, the gas cannister will operate and begin to the inflate the liferaft. The additional buoyancy will be sufficient to break the weak link that the painter is attached to, preventing the raft from being dragged down by the vessel. For MED approved rafts the weak link should break at a force of between 1.8 and 2.6kN.

HRU's may be either disposable or re-usable. Re-usable units must be serviced annually and disposal units must not be used past their date of expiry - usually 2 years. Which type do you have onboard?

Key points to consider when fitting Hydrostatic release units:

Correctly rigged HRU

MGN 343 – Hydrostatic Release Units. 04/2007. Figure 1 A correctly rigged HRU

It is worth noting that whilst these (Hammer H20) are arguably the most common type of hydrostatic release unit, there are other designs on the market so always take the time to familiarise yourself with the ones you carry onboard.

For more information on the legislation surrounding hydrostatic release units and how they should be fitted, please see or MGN 343 which is produced by the Maritime and Coastguard Agency. Alternatively, please give us a call for a chat or to organise a safety equipment inspection.

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