Start hereRules checked 20 October 2026
A dry-docking takes a ship out of service for days or weeks, plus the passage to the yard. For the intermediate bottom inspection, the one that falls in the middle of the five-year cycle, many owners ask whether a team of divers or a remotely operated vehicle (ROV) can look at the hull while the ship stays afloat. Often the answer is yes. The international rules have allowed the “alternate” bottom inspection to be done afloat for decades, provided the ship, the water, the equipment and the people are right.
Getting it wrong costs more than a dock. An in-water survey done in murky water by an unapproved team is not credited, the 36-month limit keeps running, and the statutory certificate can lapse. This lesson shows when an in-water survey (IWS, the IACS term) is allowed, how it is run, and how the extended dry-docking schemes work. In US practice the same idea is called underwater inspection in lieu of dry-docking (UWILD). Work through the case first.
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In this lesson
- An in-water survey for a mid-life container ship
- In plain words
- In detail
- What the rules say
- Flag and class notes
- Key numbers
- What inspectors find
- Quiz: 6 questions
Try it
An in-water survey for a mid-life container ship
In plain words
Every cargo ship must have the outside of its bottom inspected twice in each five-year period, never more than 36 months apart. One of those inspections, the one at the five-yearly renewal, is in dry dock. The other one, in the middle, is called the alternate inspection, and it may be done with the ship afloat.
An in-water survey is a dry-dock survey done under water. Divers or an ROV swim along the hull with a colour video camera, lights and a two-way radio to the surface. The class surveyor watches the live picture on deck, talks to the diver, and asks for close-ups, cleaning, thickness readings or measurements where needed. The aim is to get the same information the surveyor would get standing under the ship in dock: the condition of the shell plating and welds, sea chests and gratings, rudder, propeller, stern seals and bilge keels.
Some things cannot be seen from outside, such as the clearance in the rudder bearings and the stern tube bearing. For those, the surveyor relies on operating history, tests on board and, for oil-lubricated stern tubes, oil sample results.
Not every ship may do it. Bulk carriers and oil tankers of 15 years and over (under class rules, all ships in the Enhanced Survey Programme) must go to dry dock for every bottom inspection. Other ships over 15 need special consideration. And conditions must be right: sheltered water, weak current, clear enough water and a hull clean enough to see.
In detail
Which ships, and when
| Ship | Alternate bottom inspection afloat? | Source |
|---|---|---|
| Cargo ship under 15 years | Yes, if conditions, equipment and staff are suitable | UR Z3.1.3; HSSC 4.6.2.2 |
| Cargo ship 15 years and over (not bulk carrier or oil tanker) | Only with special consideration | UR Z3.1.3; HSSC 4.6.2.2 |
| ESP bulk carriers and oil tankers 15 years and over | No, dry dock | UR Z3.1.3; ESP Code 2.2.2 |
| ESP bulk carrier 10 to 15 years, intermediate survey | Underwater survey may be considered in lieu of the dry-dock part | UR Z10.2 4.2.3.3 |
| ESP bulk carrier over 15 years, intermediate survey | Dry dock survey is part of the intermediate survey | UR Z10.2 4.2.4.3 |
| Passenger ship | Two dry-dock inspections per five years; for non-ro-ro passenger ships the flag may accept one (max 60 months apart) under MSC.1/Circ.1348 | HSSC 5.10 |
The 36-month maximum between bottom inspections applies whichever method is used.
Planning the survey (UR Z3.3 and Rec 133)
- Proposal to class, in advance. Location and date, the approved diving or ROV firm, how the hull below the waterline will be cleaned, access to sea chests, sea valves and box coolers, and how rudder bearing and stern bush clearances will be assessed (operating history, on-board tests, stern tube oil analysis).
- Orientation. The surveyor must be satisfied with how divers or the ROV locate themselves on the hull, using permanent markings on the plating at selected points where necessary.
- Equipment check. Time is allowed for the firm to test all equipment before the survey.
- Planning meeting. Surveyor, owner’s representative, diving firm and master agree that everything in the plan is in place.
- The survey. In sheltered water, preferably with weak tidal streams and currents, under the surveyor’s surveillance, with a good two-way link between surveyor and diver and an agreed pictorial record.
- Findings. If damage or deterioration needs early attention, the surveyor may require dry-docking. Temporary repairs must be made permanent by the date the surveyor sets.
- Report. A comprehensive report of findings, gaugings, clearances and work done, with representative video images, signed by the surveyor for each job.
What is examined
The scope is the same as a docking survey under UR Z3.2: shell plating for corrosion, chafing, grounding damage, unfairness or buckling (with particular attention to bilge strakes and bilge keel connections); sea chests, gratings and overboard valves; visible parts of the rudder, pintles, stock couplings and stern frame; the propeller, stern bush and oil gland; side thrusters and other propulsion units. Where thickness is gauged under water, the diving firm must also hold certification as a thickness measurement firm.
The diving or ROV firm (UR Z17)
| Requirement | UR Z17 Annex 1, section 3 |
|---|---|
| Approval | Required whenever class uses the results (in-water survey in lieu of docking, or internal survey of flooded compartments) |
| Diving supervisor | At least 2 years as a diver carrying out inspection |
| ROV supervisor | At least 2 years conducting inspections with ROVs |
| Inspection diver | At least 1 year as assistant diver on inspections, including at least 10 different assignments |
| ROV operator | At least 1 year inspecting vessels with ROVs |
| Knowledge | Underwater structure and appendages, bearing clearance measurement, video and still photography, underwater communication; NDT only if the firm does NDT |
| Equipment | Colour closed-circuit TV with lighting, two-way communication, video recording, still camera, gauging, NDT and clearance equipment as relevant, hull cleaning equipment; for ROV firms, the vehicle and its positioning |
| Verification | Each job signed by the attending surveyor |
Coming on 1 January 2027: UR Z17 Rev.22 splits the knowledge requirements into external hull survey and internal survey of flooded compartments, and firms approved for remote inspection techniques (section 16) that do in-water close-up surveys of internal compartments by ROV must also meet the section 3 technical requirements, including the internal survey knowledge in 3.2.2.
Extended dry-docking schemes (IACS Rec 133)
Rec 133 Rev.1 (May 2026) describes schemes in which a qualifying ship may do two consecutive in-water surveys, so it dry-docks less often. It is a three-way agreement between owner, flag and class. The minimum of two bottom inspections per five years, 36 months apart, still applies. Main conditions:
- Not eligible: passenger ships, ESP ships, ships under UR Z7.1, ships with propulsion thrusters, ships with a keyed-taper propeller connection, high-speed craft.
- Entry: at any time until the ship is 10 years old. Exit: at 15 years, when the special survey dry-docking must be in dry dock; also on change of owner, manager or flag.
- Ship condition: underwater coating designed for the extended period, cathodic protection that can be maintained afloat, coating in double-bottom and double-side ballast tanks, void spaces and other spaces next to the shell kept GOOD, tailshaft condition monitoring, hull maintenance in the ISM system.
- No extension is granted for the dry-docking at the end of each extended period.
US practice
For US-flag tank vessels, 46 CFR 31.10-21(d) lets vessels under 15 years old (not wooden-hulled) with a 2.5-year drydock interval in salt water, or a 5-year interval in fresh water, apply to the Officer in Charge, Marine Inspection, for an underwater survey instead of alternate drydock examinations, if they have an effective hull protection system. The application covers the procedure, location, how diver position is fixed, through-hull fittings, shaft bearing clearances, the vessel’s condition and draft, and the hull protection system.
What the rules say
- UR Z3.1.3 says bottom examinations are normally done in dry dock, but an alternate examination may be an in-water survey; ships of 15 years and over need special consideration, and ESP ships of 15 years and over must be in dry dock.1
- UR Z3.3.1 requires the in-water survey to provide the information normally obtained from a docking survey, with proposals submitted in advance.1
- UR Z3.3.2 requires sheltered water, preferably weak currents, and visibility and hull cleanliness that permit a meaningful examination of plating, appendages and welds.1
- UR Z3.3.4 requires the in-water survey to be carried out under the surveillance of a surveyor by a firm approved under UR Z17, and Z3.3.6 lets the surveyor require dry-docking if damage needs early attention.1
- The HSSC Survey Guidelines paragraph 4.6.2.2 say inspections afloat should only be done “when the conditions are satisfactory and the proper equipment and suitably trained staff are available”.2
- HSSC paragraph 5.10 requires two dry-dock bottom inspections per five years for passenger ships, which the flag may reduce to one for non-ro-ro passenger ships, with no more than 60 months between dockings.2
- UR Z17 Annex 1, section 3, sets experience, training and equipment requirements for diving and ROV firms, and paragraph 3.8 requires the surveyor’s signature on each job.3
- UR Z17 Rev.22, in force 1 January 2027, separates the knowledge requirements for external hull surveys and internal surveys of flooded compartments.4
- UR Z10.2 paragraph 4.2.3.3 allows an underwater survey in lieu of the dry-dock survey at the intermediate survey of bulk carriers 10 to 15 years old; 4.2.4.3 requires dry dock over 15.6
- IACS Recommendation 133 Rev.1 excludes ESP ships, passenger ships, Z7.1 ships and others from extended dry-docking schemes and ends enrolment at 15 years of age.5
- 46 CFR 31.10-21(d) allows US tank vessels under 15 years with an effective hull protection system to apply for an underwater survey instead of alternate drydock examinations.7
Flag and class notes
- Each class society publishes its own in-water survey requirements and keeps its own list of approved diving and ROV firms. Rec 133 requires ships in EDD schemes to comply with the society’s in-water survey provisions, and acceptance into a scheme needs the flag’s formal written agreement.5
- Passenger ship operators wanting one dry-docking per five years need their flag’s acceptance under HSSC 5.10.2, applying the assessment guidelines in MSC.1/Circ.1348.2
- US-flag vessels apply to the Coast Guard Officer in Charge, Marine Inspection, for each vessel; vessels 15 years and older are considered case by case, with final approval by Coast Guard headquarters (CG-CVC) after a full set of hull gaugings at the next drydocking.7
Key numbers
| What | Value |
|---|---|
| Bottom inspections per 5 years | at least 21 |
| Maximum interval between bottom inspections | 36 months1 |
| Age from which ESP ships must dry-dock for every bottom inspection | 15 years1 |
| Diving supervisor experience | at least 2 years3 |
| Inspection diver experience | 1 year as assistant, at least 10 assignments3 |
| EDD entry age limit | up to 10 years5 |
| EDD exit age | 15 years5 |
| Passenger ship (non-ro-ro) with one docking per 5 years | max 60 months between dockings2 |
What inspectors find
- In-water surveys credited without the surveyor’s surveillance or by firms not approved under UR Z17, which UR Z3.3.4 does not permit.1
- Poor visibility or heavy fouling, so the video shows little of the plating or welds and the survey cannot be credited.1
- Rudder and stern bearing clearances not addressed in the proposal, leaving the surveyor without the data a dock would give.1
- EDD ships whose ballast tank coatings have fallen below GOOD without being restored, a condition for staying in the scheme.5
FAQ
Does an in-water survey also count for the special survey?
Normally not. Under UR Z3, one of the two bottom examinations must be in conjunction with the special survey, and bottom examinations are normally in dry dock. The exception is an extended dry-docking scheme under Rec 133, which allows two consecutive in-water surveys; even then the special survey and statutory renewal dates do not change, and the docking at 15 years must be in dry dock.
Can thickness be measured under water?
Yes, if the diving firm also holds certification as a thickness measurement firm. UR Z17 requires this when thickness is measured under water.
Is UWILD the same as IWS?
In practice they describe the same thing: an underwater inspection that takes the place of a dry-dock bottom inspection. IWS is the term used by IACS in UR Z3; UWILD is common in US practice. The requirements that apply are those of your class society and flag.
Who pays attention to the rudder bearings if nobody can measure them?
The surveyor, using the operating history, on-board tests and stern tube oil sample reports, as UR Z3.3.1 requires. These must be in the proposal sent to class beforehand.
Quiz6 questions
Sources
- 1
IACS Unified Requirement Z3, Periodical survey of the outside of the ship’s bottom and related items, Rev.8 (April 2019, applied from 1 July 2020), paragraphs Z3.1.2, Z3.1.3, Z3.2 and Z3.3.1 to Z3.3.6. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/18150117/ur-z3rev8.pdf
- 2
IMO Resolution A.1207(34), Survey Guidelines under the Harmonized System of Survey and Certification (HSSC), 2025 (adopted 3 December 2025), Annex, paragraphs 4.6.1, 4.6.2.2 and 5.10 (with footnote reference to MSC.1/Circ.1348). https://wwwcdn.imo.org/localresources/en/OurWork/IIIS/Documents/A%2034-Res.1207%20-%20SURVEY%20GUIDELINES%20UNDER%20THE%20HARMONIZED%20SYSTEMOF%20SURVEY%20AND%20CERTIFICATION%20(HSSC),%202025%20(Secretariat).pdf
- 3
IACS Unified Requirement Z17, Procedural Requirements for Service Suppliers, Rev.21 (January 2025, implemented from 1 July 2026), paragraphs 4.1.2, 4.2 and Annex 1, section 3 (firms carrying out in-water surveys by diver or ROV). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/14135618/UR-Z17-Rev.21-Jan-2025-CLN.pdf
- 4
IACS UR Z17 Rev.22 (in force 1 January 2027), Annex 1, paragraphs 3.2.1 and 3.2.2 and section 16 (survey using Remote Inspection Techniques). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/05122440/UR-Z17-Rev.22-23-24-CLN.pdf
- 5
IACS Recommendation No. 133, Guidelines for Schemes of Extended Interval between Surveys in Dry-Dock: Extended Dry-docking (EDD) Scheme, Rev.1 (May 2026), sections 1 to 9. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2026/05/27081846/Rec-133-Rev.1-May-2026-CLN.pdf
- 6
IACS Unified Requirement Z10.2, Hull Surveys of Bulk Carriers, Rev.37 (February 2023), paragraphs 4.2.3.3 and 4.2.4.3. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/08/10152246/ur-z102rev37.pdf
- 7
US Code of Federal Regulations, 46 CFR 31.10-21, Drydock examination, internal structural examination, cargo tank internal examination, and underwater survey intervals, paragraphs (a), (c), (d) and (e). https://www.ecfr.gov/current/title-46/chapter-I/subchapter-D/part-31/subpart-31.10/section-31.10-21
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.
