Underwater & Diving Services

Underwater hull inspection and diving services

Underwater and diving services allow owners to inspect, clean and repair the ship below the waterline without dry docking. The most common jobs are in-water surveys in lieu of dry docking (often called UWILD or IWS), hull cleaning, propeller polishing, sea chest and grating inspection, and underwater repairs such as blanking sea chests, replacing anodes or fitting temporary patches.

vslSafety arranges commercial diving teams and ROV operators at the vessel’s port, including firms approved by the vessel’s class society for in-water surveys, and coordinates the port permissions, surveyor attendance and reporting format.

Regulatory requirements and standards

IACS Unified Requirement Z3 requires a minimum of two examinations of the outside of the ship’s bottom during each five-year special survey period, one of them in conjunction with the special survey, and the interval between any two examinations is not to exceed 36 months. Examinations are normally done in dry dock, but an in-water survey may be accepted instead. Special consideration applies to ships of 15 years or over, and ESP ships of 15 years and over must have these examinations in dry dock. UR Z3 requires the in-water survey to be carried out under the surveillance of a surveyor by a firm approved as a service supplier under IACS UR Z17, with good two-way communication and pictorial representation for the surveyor. Rudder and stern bush clearances are assessed from operating history, on-board tests and stern tube oil sample reports.

Hull cleaning is shaped by biosecurity rules. IMO’s 2023 Biofouling Guidelines (resolution MEPC.378(80)) recommend a ship-specific Biofouling Management Plan and Biofouling Record Book, and IMO issued Guidance on in-water cleaning of ships’ biofouling (MEPC.1/Circ.918) in 2025. Australia has applied mandatory biofouling management requirements to international arrivals since 15 June 2022, and New Zealand applies its own biofouling standard. Many ports restrict or prohibit in-water cleaning, particularly of heavily fouled hulls or biocidal coatings, or require capture of removed material.

Diving operations themselves are regulated by national diving legislation and port authority rules, and reputable contractors follow recognised industry guidance on team size, supervision, equipment and emergency procedures. The ship must isolate propulsion, thrusters, sea suctions and impressed current systems before divers enter the water.

Inspection intervals

Item	Requirement	Interval	Reference
Bottom examination	Minimum two per five-year period, one at special survey	Max 36 months between examinations	IACS UR Z3
In-water survey firm	Approved service supplier, surveyor in attendance	Each in-water survey	IACS UR Z3, UR Z17
ESP ships 15 years and over	Bottom examinations in dry dock	Each examination	IACS UR Z3
Biofouling inspection and cleaning	Per Biofouling Management Plan and local rules	Ship-specific	MEPC.378(80); national requirements

What the service includes

• Class in-water surveys in lieu of dry docking, with live video and communications for the surveyor
• Pre-purchase, damage and condition inspections of hull, rudder, propeller and appendages
• Hull cleaning by diver-operated brush carts or ROV systems, with capture where required
• Propeller polishing to restore blade surface finish and efficiency
• Sea chest and grating inspection, cleaning and blanking for valve overhauls
• Rudder clearance and stern tube seal checks, and fitting of rope guards or net cutters
• Anode inspection and replacement, and checks of impressed current system anodes and reference cells
• Underwater repairs such as cofferdams, temporary patches, bow thruster work and, where accepted by class, underwater welding
• Recorded video, still photographs and a written report

Ship and equipment types covered

Diving teams routinely work on bulk carriers, tankers, container ships, gas carriers, offshore vessels and passenger ships, covering fixed pitch and controllable pitch propellers, tunnel thrusters, azimuth and pod units, bilge keels, echo sounder and log transducers, sea chests and hull markings. For shafting work beyond an external check, see propeller and shaft services. For documentation and class preparation around the survey, marine surveys and consultancy can support the owner.

Common findings

• Heavy slime and hard fouling reducing speed and increasing fuel consumption
• Coating breakdown, mechanical damage and corrosion on flat bottom and bilge areas
• Cavitation erosion, bent blade tips and rope or net entanglement at the propeller
• Depleted anodes and blocked or damaged sea chest gratings
• Stern tube seal leakage and fishing line damage to seals

Certificates and documents you receive

For an in-water survey, the approved diving firm provides a report with video and photographs referenced to hull markings or frame numbers, measurements such as rudder clearances where taken, and a statement of the equipment used. The attending class surveyor issues the survey record. For hull cleaning, the report should include before and after images, areas cleaned, method used and any capture or waste disposal records, which support the Biofouling Record Book entry. Repair jobs close with a repair report and materials list.

How to prepare the vessel

Check with the agent that diving and in-water cleaning are allowed at the berth or anchorage and obtain the port permit. For in-water surveys, submit the proposal to class in advance and book the surveyor. On the day, lock out propulsion, thrusters, rudder, sea suctions and impressed current systems, hoist the diving flag, and stop overboard discharges. Plan for sheltered water, weak currents and adequate visibility, as UR Z3 requires. Provide the latest docking report, hull drawings and hull marking plan.

Information needed for a quotation

• Vessel name, IMO number, flag, class and ship type
• Port, berth or anchorage, ETA, ETD and draft on arrival
• Scope: in-water survey, hull cleaning, propeller polishing, inspection or repair
• Class requirement and whether the diving firm must be approved by a specific society
• Hull dimensions, wetted surface area, coating type and date of last dry docking
• Propeller type, diameter and number of blades
• Expected fouling level, with any previous inspection images

vslSafety coordinates a suitable diving contractor at the vessel’s port and confirms permit and surveyor arrangements before the team is mobilised.

Frequently asked questions

What is an in-water survey in lieu of dry docking?

It is a class survey of the outside of the ship’s bottom carried out afloat by an approved diving firm, with the class surveyor following live video and communications. It is intended to provide the information normally obtained from a docking survey. It is commonly used for the intermediate bottom survey, but is not permitted for ESP ships of 15 years and over and is at class discretion for other older ships.

How often does a ship’s bottom need to be examined?

Under IACS UR Z3, at least twice in each five-year special survey period, with one examination at the special survey and no more than 36 months between any two examinations. A three-month extension may be granted only in exceptional circumstances such as unavailability of dry-docking or repair facilities. Some flags and ship types have additional requirements.

Can we clean the hull in any port?

No. Many ports restrict in-water cleaning because of invasive species and biocide release, and some permit only light cleaning or require capture of removed material. Record all cleaning in the Biofouling Record Book to support arrival reporting in countries such as Australia and New Zealand.

How much fuel does propeller polishing save?

Savings depend on the initial roughness, fouling, vessel type and operating profile, so no single figure applies. Owners typically plan polishing between dockings as part of hull and propeller performance management, and measure the effect using speed-power data before and after.

Are underwater welding repairs accepted by class?

Underwater welding can be accepted in specific cases, usually as a temporary repair, if the procedure, welders and firm are qualified to a recognised standard such as AWS D3.6M and the class society agrees in advance. Permanent repairs to primary structure are normally required in dry dock. Always agree the method with class before mobilising the team.