Afloat repair lets you inspect and fix a ship's underwater hull while it stays in the water, usually at anchor or alongside. It covers propeller polishing, hull cleaning, anode replacement, sea chests, stern tube seals and temporary patching. Classification societies accept some of this work in place of dry-docking, under strict conditions.
A ship that isn't moving isn't earning. That line, heard in any shipowner's office, explains why a whole specialty exists within commercial diving commercial diving, dedicated to doing underwater what used to require pulling the ship out of the water. This is afloat repair work, and it's one of the most accessible ways into the trade.
The logic is simple: putting a ship into dry dock costs money for the docking itself and costs far more in lost charter days. If the work can be done while the ship is already alongside the quay where it was going to load, or at anchor in a roadstead while it waits its turn, the economics change completely.
What work gets done afloat
Hull cleaning and propeller polishing
This is the most common and the most repeated job. Biofouling — the layer of organisms that colonises the underwater hull — increases drag and, with it, fuel consumption. Cleaning is done with rotary brushes handled by divers or with self-propelled units, following a pattern that covers the whole surface.
Propeller polishing is a separate, more delicate task: blade roughness affects propulsive performance directly, and the work uses abrasives of decreasing grit to restore the finish. The technical and environmental sides of these operations are covered in the article on hull cleaning and biofouling, including the restrictions many ports now place on cleaning within their own waters.
Anodes and cathodic protection
Sacrificial anodes get consumed and need replacing. Replacing them afloat avoids waiting for the next dry-docking and keeps the protection system working, which means less structural corrosion in the long run. The diver removes the spent anodes, preps the surface, fits the new ones and checks electrical continuity; on many contracts they also take potential readings with a reference electrode to document the system's condition, as explained in cathodic protection and sacrificial anodes.
Sea chests and gratings
Sea chests feed cooling, ballast and firefighting systems. They get blocked by marine growth and debris, and their gratings get damaged. Cleaning, repairing or replacing them afloat is routine work, and so is fitting blanking plugs that isolate a sea chest so the ship's crew can work on it from the inside without going into dry dock.
This is probably the job with the highest suction risk in the whole catalogue, and the one that demands the strictest permit discipline.
Rudder, stern tube and steering gear
Inspecting the rudder stock, bearing clearance, the condition of the stern tube seal, and any build-up of rope or netting wrapped around the shaft are regular checks. Clearing a rope fouled around the propeller or shaft is a classic emergency job, often with the ship stopped mid-port-call.
Hull repairs and patching
When there's damage — a crack, a hole, an impact — an afloat repair is usually temporary: a bolted patch or a cofferdam is fitted, allowing the area to be pumped dry and worked on from the inside, and the permanent repair is deferred to the next dry-docking. The underwater welding comes into play here, with the limitations detailed below.
The classification society framework
This is the part that sets this work apart from a simple cleaning job. A ship in service is subject to a regime of periodic surveys overseen by its classification society — DNV, Lloyd's Register, ABS, Bureau Veritas and the others — under the international regulatory framework.
That regime requires the ship's bottom to be surveyed twice in every five-year period, with a maximum interval between surveys. The key for diving is that one of those two surveys can be carried out with the ship afloat, through what's known as in-water survey, if specific conditions are met.
There are usually four conditions:
- The ship must hold the class notation that enables an in-water survey, which brings design requirements such as identification marks on the hull that let the surveyor establish position, and features that make it easier to examine the critical parts.
- The diving company must be approved by the classification society. Not just any company will do: the approval covers procedures, video equipment, staff competence and the quality system.
- A surveyor from the society must be present, following the survey live by video with direct communication to the diver, and able to ask for any examination to be repeated or extended.
- Conditions must allow it: sufficient visibility, clean water, and suitable draught and sea state. With turbid water, the survey isn't valid.
The result is that this work has a strong documentary side. The diver doesn't just look: they record, measure, reference every view against the hull marks, and produce a report that becomes part of the ship's class file. The techniques are the same ones used for underwater visual inspection , applied to a very specific object.
The limits of welding afloat
It's the question everyone starting out asks: can you weld the hull with the ship in the water? The short answer is yes, but with restrictions worth understanding.
Wet welding — with the arc struck directly in the water — produces joints that cool very fast and absorb hydrogen, which gives hard microstructures prone to cracking. Its mechanical properties are inferior to an equivalent weld made dry, which is why classification societies restrict its use on load-bearing structural members and accept it mainly for temporary repairs, secondary elements and reinforcement work under conditions approved case by case.
When a structural-quality joint is needed without dry-docking, the alternative is dry hyperbaric welding inside a habitat: a sealed chamber that displaces the water and allows welding in a gas atmosphere. It's an expensive operation, run to a qualified procedure with specifically certified welders, reserved for cases where dry-docking isn't viable.
How a job is organised
An afloat repair job follows a fairly stable sequence. It starts with a request from the owner or their technical manager, describing the work and the available window: usually the hours the ship will be alongside or at anchor in a given port. That window is short and doesn't stretch, so all the planning works backwards from it.
The diving team reviews the hull drawings, the survey history and previous reports, and draws up the procedure: which points need examining, which of the ship's systems need isolating, what tools and consumables to bring aboard, and how many people are needed. If the job is a class survey, the presence of the society's surveyor is arranged as well, and the video equipment is checked against what that society requires.
On board the ship itself, before anyone gets wet, there's a meeting with the master or chief engineer to sign off the isolations, agree communications and settle who can authorise the starting of what. From there the work runs in shifts, with the surface team keeping constant contact with both the diver and the bridge.
The job closes out on paper: a report with video, photographs, measurements and notes on anything the owner needs to bear in mind for the next dry-docking. In practice, a good part of what the client is paying for is that report, not just the hours in the water.
Specific risks and how they're controlled
Working under a ship in service concentrates hazards that don't exist on a fixed structure.
The first is suction. A ship has pumps that draw seawater through intakes spread across the hull. A diver near an intake that starts up gets pinned against the grating with a force they can't break free from. The control is formal isolation of the system: physical lockout, tagging and a signed permit to work, not a verbal heads-up.
The second is machinery starting up: main propeller, thrusters, rudder. The same control mechanism applies, with the machinery locked out and the corresponding signage on the bridge and in the engine room. The divers-down flag and constant communication with the bridge are part of the procedure.
The third is the entorno portuarioport environment: vessel traffic, manoeuvring by neighbouring ships, objects falling from the deck during cargo operations, and often near-zero visibility that forces the diver to work by feel.
All of this is managed with the permit-to-work and risk-assessment discipline that structures commercial diving in general, and which in this niche is especially literal: behind every point on the hull where the diver is going to work there's a list of systems someone has had to take out of service.
Why it's a good way into the trade
For a newly certified diver, afloat repair combines advantages that don't often come together. It happens in port, with shifts that let you go home at night. It's steady work, because ships keep sailing regardless of the oil cycle. And it teaches fast: in a year you rack up many short dives with varied tasks, all of them documented.
It also builds skills that transfer straight to offshore work: visual inspection, NDT, cathodic protection, hydraulic tools and permit discipline. A diver with two solid years of afloat repair has a CV that offshore contractors understand.
Pay rates sit below offshore and well below saturation — the comparison is in how much a diver earns— but it's one of the few specialties you can start soon after finishing training. If you're at that stage, the map of schools, the guide to how to train and the equipment catalogue are the starting point, and the jobs and agencies directory, the next step.
Frequently asked questions
What is an in-water survey (IWS)?
It's a survey of the ship's bottom carried out by divers with the ship afloat, accepted by the classification society as an alternative to dry-docking under certain conditions. It requires the ship to hold the relevant class notation and the diving company to be approved by that society.
Can it always replace dry-docking?
No. The international framework requires two bottom surveys in every five-year period, and only one of them can be replaced by an in-water survey when the requirements are met. The ship still has to go into dry dock on the set schedule.
Can the hull be welded with the ship afloat?
Yes, with significant limitations. Wet welding produces joints with inferior properties to dry welding, and its use on load-bearing structural members is restricted. For permanent repairs, dry habitats are used, or the work is deferred to the next dry-docking.
What do you gain by doing the work afloat?
Time and money. A ship sitting in dry dock isn't earning, and dry-docking is expensive. A lot of maintenance work can be done while the ship loads, discharges or waits at anchor, without interrupting its operation.
What specific risks does this job carry?
The main ones are suction from unisolated sea chests, accidental starting of propellers or thrusters, objects falling from the deck, movement of the ship, and port traffic. They're controlled with permits to work, lockout and tagging of systems, and constant communication with the bridge.
Do you need special certification?
Commercial diving certification is the baseline requirement. For inspection work, underwater inspector qualifications are also asked for, and for specific jobs, underwater welding or NDT qualifications. The company also needs to be approved by the classification society to sign off valid surveys.
Published 2 September 2026 · By Arthur More, professional saturation diver
Photo: LouisvilleUSACE · CC BY via Openverse
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.