Start hereRules checked 20 October 2026
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.
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In this lesson
- 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
- Quiz: 6 questions
Try it
When is the HRU (or liferaft) due, and how far can an extension go?
In plain words
There are two kinds of HRU. A disposable HRU has a fixed life, set by its maker and marked on it. Most last two years from the date they are fitted. When the date passes, you throw it away and fit a new one. It is never serviced.
A serviceable HRU is a heavier unit that is opened, checked and tested by a service station at least once a year, just like a liferaft. If that is impossible, the flag can allow up to 17 months.
Whatever the type, the unit only works if it is rigged as the maker shows: the raft’s straps go through it, the raft’s painter is tied to its weak link, and the HRU itself is tied to a strong point on the ship. Nothing else should hold the raft down.
In detail
What the LSA Code requires of an HRU
Paragraph 4.1.6.3 of the International Life-Saving Appliance (LSA) Code sets the design rules for an HRU used in a float-free arrangement. It must:2
- be made of compatible materials so that it does not malfunction; galvanizing or other metallic coatings on its parts are not accepted;
- release the liferaft automatically at a depth of not more than 4 m;
- have drains so that water does not collect in the hydrostatic chamber in its normal position;
- not release when seas wash over it;
- be permanently marked on the outside with its type and serial number;
- be marked, on the unit or a securely attached plate, with date of manufacture, type, serial number and whether it is suitable for a raft of more than 25 persons;
- have every part connected to the painter system at least as strong as the painter;
- if disposable, be marked with a means of determining its date of expiry in place of item 6.
The type test in the IMO Revised recommendation on testing of life-saving appliances (MSC.81(70)) puts two samples through 160 hours of salt spray, stowage at -30 °C and +65 °C followed by operation in seawater at -1 °C and +30 °C, a submerged release with its rated buoyant load at no more than 4 m, and a 30-minute tensile test of at least 10 kN, or 15 kN for units meant for rafts of more than 25 persons.3
Disposable HRUs: life, expiry and replacement
The LSA Code gives the Administration the job of setting the period of acceptability of any appliance that deteriorates with age, and prefers a permanent expiry date on the item.9 For HRUs, the service life is set by the manufacturer. AMSA notes that most disposable HRUs have a service life of two years after installation, and that the installation or expiry date is marked on the unit.4
When you replace a disposable HRU:
- use the model the liferaft maker specifies, rated for the raft’s capacity (more than or not more than 25 persons);
- follow the HRU maker’s fitting instructions, including how the weak link and the painter are attached;
- write the expiry date on the unit if the maker asks you to, and record the replacement in the maintenance records;
- check that the old unit is removed from the ship so it cannot be refitted by mistake.
Serviceable HRUs
SOLAS regulation III/20.9 requires HRUs other than disposable ones to be serviced at intervals not exceeding 12 months, extendable by the Administration to 17 months where 12 months is impracticable, at a servicing station that is competent to service them, has proper facilities and uses only properly trained personnel.1 What is checked and renewed at that service is set by the HRU maker’s service instructions, so ask the station for a service report that shows it.
Rigging: the four checks that matter
AMSA lists the points its inspectors find most often wrong:4
- Right unit for the equipment. Liferaft HRUs and EPIRB HRUs are different products; small rafts may use a different model.
- Correctly installed. The securing straps must release when the HRU fires. Straps should be “taut but not over tight”: over-tensioning and too little load can both stop the unit working.
- Painter on the weak link. The raft’s painter must be tied to the HRU weak link, not to the cradle or the deck.
- No extra lashings. On a float-free EPIRB, additional lashings on the housing can stop the cover releasing, so the beacon never floats free.
HRUs on EPIRBs
A float-free EPIRB must release itself and float free before reaching a depth of 4 m.10 At the annual radio survey the surveyor checks the hydrostatic release and its expiry date as part of the EPIRB test, following IMO circular MSC.1/Circ.1040/Rev.2.67 Lesson 8 covers the rest of the EPIRB check.
Using the calculator
The calculator works out three dates from the last service (or fitting) date:
- Due date = last service date + interval. For a serviceable HRU or a liferaft the interval is 12 months.1
- Latest date with an extension = last service date + interval + allowed extension. Under SOLAS III/20.9 and III/20.8.1.1 the most a flag can allow is 5 more months (17 in total), and only on application where servicing is impracticable.1
- Disposable HRU: enter the life the maker gives (often 24 months) and an extension of 0. There is no extension for an expired disposable unit: it is replaced.4
Some flags accept servicing in the survey window, 12 months ± 3 months up to 18 months (the Bahamas, under MSC/Circ.955). That is a flag concession, not a SOLAS right: check your flag’s notice before relying on it.5
What the rules say
- SOLAS regulation III/20.9 requires hydrostatic release units, other than disposable units, to be serviced at intervals not exceeding 12 months, extendable to 17 months by the Administration where this is impracticable.1
- SOLAS regulation III/20.9 requires that servicing to be done at a station that is competent to service HRUs, maintains proper facilities and uses only properly trained personnel.1
- LSA Code paragraph 4.1.6.3.2 requires an HRU to release the liferaft automatically at a depth of not more than 4 m.2
- LSA Code paragraph 4.1.6.3.1 forbids galvanizing or other metallic coatings on HRU parts.2
- LSA Code paragraph 4.1.6.3.8 requires a disposable HRU to be marked with a means of determining its date of expiry.2
- LSA Code paragraph 1.2.3 leaves the period of acceptability of appliances that deteriorate with age to the Administration and prefers a permanent date of expiry.9
- MSC.81(70), Part 1, section 11, requires HRU prototypes to release under load at no more than 4 m and to withstand a tensile load of 10 kN, or 15 kN for rafts of more than 25 persons.3
- MSC.1/Circ.1040/Rev.2, paragraph 4.9, includes checking the EPIRB’s hydrostatic release and its expiry date in the annual EPIRB test.6
Flag and class notes
- Bahamas. Marine Notice 06 applies the same servicing and extension rules to serviceable HRUs as to liferafts: 12 months, survey-window servicing up to 18 months, and no extension of more than 5 months otherwise.5
- Australia. AMSA’s Safety Alert 02/2021 is the clearest public guide to HRU rigging, with photographs of correct and incorrect installations; the Indian Register of Shipping recirculated it to its fleet.4
- Class at the annual survey. Under the IMO survey guidelines the surveyor examines the HRU and float-free arrangement of every inflatable liferaft at each annual survey, and the HRU of the EPIRB at the radio survey.7
Key numbers
| What | Value |
|---|---|
| Release depth | not more than 4 m2 |
| Serviceable HRU service interval | 12 months, flag may extend to 171 |
| Typical disposable HRU life | 2 years from installation (maker’s figure)4 |
| Prototype tensile test | 10 kN for 30 min; 15 kN for rafts over 25 persons3 |
| Salt spray test at type approval | 160 h at 35 ± 3 °C3 |
| Weak link breaking strain | 2.2 ± 0.4 kN2 |
| AMSA HRU deficiencies in 2021 | over 1004 |
What inspectors find
- Painter not connected, or not attached to the HRU weak link as required, so the raft may not inflate or may not leave the ship.48
- HRU incorrectly fitted, or securing straps over-tightened (or too slack), which can stop the unit releasing.4
- Expired disposable HRUs, and expired service certificates for serviceable ones; the Indian Register of Shipping’s 2024 port state control (PSC) report lists an expired HRU survey report among detainable deficiencies.8
- Extra lashings or padlocks on EPIRB housings that stop them floating free.4
FAQ
Can the crew fit a disposable HRU?
SOLAS sets no servicing for disposable units, so replacing one is normally a crew task done to the maker’s fitting instructions. Record the date and keep the instructions on board. Check your company’s procedures and flag notices, which may say more.
Our serviceable HRU went ashore with the liferaft. Do we need a spare?
The raft must not be left on board without a working float-free arrangement while it is in service. Plan the swap so that the HRU comes back with the raft or a replacement is fitted, and note it in the log.
Is the expiry date on a disposable HRU a manufacturing date or a fitting date?
It depends on the maker. Many label the unit with an expiry date when it is fitted. Read the label and the instructions; if they disagree with your records, replace the unit.
Does an HRU need to be serviced together with the liferaft?
No. They have separate cycles. A disposable HRU follows its own expiry date, and a serviceable one its own 12-month service.
Quiz6 questions
Sources
- 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
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
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
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
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
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
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
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
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
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
Rules checked 20 October 2026. VSL Safety is not an approved training provider and doesn't issue certificates. These lessons don't replace STCW training, or the manufacturer's training and certification that service technicians need.
