Al Safwan Marine
Al Safwan Marine
Get a quote Call +971 55 659 9860

Liferafts and personal LSALesson 3 of 8

Hydrostatic release units: disposable, serviceable and rigged right

You will know how an HRU works, how to tell when it is due, how to fit one correctly, and what the surveyor and PSC officer check.

13 minutes · For signed-up learners, free · Rules checked 20 October 2026

Start here

A hydrostatic release unit (HRU) is the cheapest part of a liferaft installation and the one that decides whether the raft ever leaves the ship. It is a small sealed chamber with a membrane and a blade. When the ship sinks, water pressure pushes the membrane in at a few metres’ depth, the blade cuts the rope holding the raft’s straps, and the canister floats up. The same idea frees a float-free EPIRB from its bracket.

Because the HRU sits outside in sun, salt and frost for years, the rules give it its own servicing cycle, separate from the raft. Many HRUs on ships today are disposable: they are not serviced but replaced when the date on their label runs out. Get the date or the rigging wrong and the raft may be held to the ship. This lesson includes a calculator to work out when an HRU or any annually serviced item is due, and the latest date a flag extension could allow.

Continue this lesson free

The rest of this lesson, the interactive When is the HRU (or liferaft) due, and how far can an extension go? and the quiz are open to signed-up learners. It's free, takes a minute, and opens every lesson in all 14 courses.

We use your details to save your progress and to understand who uses the Academy. Already signed up? Get a sign-in link.

In this lesson

  • Try it: When is the HRU (or liferaft) due, and how far can an extension go?
  • In plain words
  • In detail
  • What the rules say
  • Flag and class notes
  • Key numbers
  • What inspectors find
  • Questions people ask
  • Quiz: 6 questions

Sources

  1. 1 SOLAS 1974, chapter III, regulation 20.9 (Periodic servicing of hydrostatic release units) and 20.8.1.1, as adopted by IMO Resolution MSC.47(66) (in force 1 July 1998). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.47(66).pdf
  2. 2 LSA Code, IMO Resolution MSC.48(66), section 4.1.6 (Float-free arrangements for liferafts), paragraphs 4.1.6.2 and 4.1.6.3. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.48(66).pdf
  3. 3 IMO Resolution MSC.81(70), Revised recommendation on testing of life-saving appliances (adopted 11 December 1998), Part 1, section 11 (Hydrostatic release units). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.81(70).pdf
  4. 4 Australian Maritime Safety Authority, Safety Alert 02/2021, Correct use of hydrostatic release units, as enclosed in Indian Register of Shipping Technical Circular 060/2021 (24 December 2021). https://www.irclass.org/media/5801/technical-circular-no60.pdf
  5. 5 Bahamas Maritime Authority, Marine Notice 06, Servicing of inflatable lifesaving appliances, version 1.3 (13 January 2026), section 3. https://www.bahamasmaritime.com/wp-content/uploads/2026/01/MN006-Servicing-of-inflatable-lifesaving-appliances.pdf
  6. 6 IMO MSC.1/Circ.1040/Rev.2, Guidelines on annual testing of emergency position-indicating radio beacons (EPIRBs) (18 October 2021), Annex paragraph 4.9. https://wwwcdn.imo.org/localresources/en/OurWork/Safety/Documents/Documents%20relevant%20to%20GMDSS/MSC.1-Circ.1040-Rev.2.pdf
  7. 7 IMO Resolution A.1186(33), Survey Guidelines under the HSSC, 2023, Annex items (EA) 1.2.2.20 and (RI) 4.1.2.16.8. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/AssemblyDocuments/A.1186(33).pdf
  8. 8 Indian Register of Shipping, PSC Annual Report 2024, detainable and other life-saving appliance deficiencies. https://www.irclass.org/media/7954/psc-annual-report-2024.pdf
  9. 9 LSA Code, IMO Resolution MSC.48(66), paragraph 1.2.3 (period of acceptability of appliances subject to deterioration with age). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.48(66).pdf
  10. 10 IMO Resolution MSC.471(101), Performance standards for float-free EPIRBs operating on 406 MHz (adopted 14 June 2019), Annex paragraph 2.6.3. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.471(101).pdf

Free awareness training. Al Safwan Marine is not an approved training provider, so no certificate is given, and these courses are not a qualification. They don't replace STCW training, manufacturer training or the certification technicians need for this work. The manufacturer's manual and your flag's rules always come first.

Sign in to save your progress.

This lesson is also published by our sister site vslsafety.com.