The complete guide to ship hull cleaning: methods, biofouling, fuel efficiency and maintenance

What is in-water hull cleaning, and when is it used?

In-water hull cleaning is the removal of marine growth from a ship's underwater hull while she is afloat, at anchor or alongside, without going into dry dock. It is used between dockings, when fouling has built up enough to cost fuel or speed, or before arriving somewhere that requires a clean hull.

The work is usually done by commercial divers using brush carts on the flat bottom and vertical sides, and hand tools on the areas a cart cannot reach. Remotely operated systems are also used on some hulls. At Cleanship our underwater hull cleaning is diver-operated brush cart and hand cleaning, with debris capture where the port requires it, and before-and-after video for the vessel's file. You can see the full scope on our underwater hull cleaning service page.

The job normally covers:

• Vertical sides and flat bottom, the large areas where most drag comes from
• Niche areas: bilge keels, sea chests and gratings, thruster tunnels, rudder, stern frame and anodes
• The propeller, usually as a separate polishing scope
• A report: what was found, what was removed, coating condition and photographs

How does fouling develop, and how is it rated?

Fouling grows in a broadly predictable sequence:

1. Slime (microfouling). A biofilm of bacteria and diatoms forms within days on any submerged surface, including fresh antifouling.
2. Soft macrofouling. Weed and soft-bodied organisms settle, especially near the waterline where there is light.
3. Hard macrofouling. Barnacles, tubeworms and mussels attach and calcify. This stage happens fastest when a ship sits idle in warm water.

The IMO's 2023 Biofouling Guidelines (resolution MEPC.378(80)) use a fouling rating scale running from no fouling, through microfouling, to increasing degrees of macrofouling coverage. The rating matters in practice: it is the number that guidance and some ports use to decide whether a hull may be cleaned in the water, and whether the debris must be captured.

The single biggest accelerator is idle time. A ship that trades steadily at service speed sheds some soft growth; the same ship at anchor for weeks in the Gulf, off West Africa or in South Asian waters can arrive at hard fouling surprisingly quickly.

What does fouling do to speed and fuel?

Fouling increases hull roughness, roughness increases frictional resistance, and the engine has to burn more fuel to hold the same speed. The effect starts earlier than most people expect. Even slime adds measurable resistance.

How much? Published research gives a sense of scale. The IMO GloFouling Partnerships' 2022 study on biofouling and energy efficiency estimated that even a thin layer of slime can raise a ship's fuel use and emissions substantially, with heavier fouling costing far more. Those figures are modelled for particular ships and conditions. Your vessel's result depends on hull form, speed, coating, where the fouling is and how well the cleaning is done.

So we never promise a percentage saving. What we recommend is measuring it on your own ship: compare speed and consumption before and after a clean, corrected for weather and draught. The data in your noon reports is usually enough to see whether a clean paid for itself.

Choosing a cleaning method for your coating

The cleaning method has to suit the coating, because the coating is what keeps the hull clean between cleanings.

The rule is simple: the least aggressive method that removes the fouling. A pre-clean dive to rate the fouling and check the coating is how that decision gets made properly, rather than on the day.

Capture, local rules and the regulatory direction

In-water cleaning releases what it removes. With slime that matters little; with macrofouling it can mean releasing living organisms and biocide-laden paint particles into a port's waters. That is why rules are tightening:

• The IMO's 2023 Biofouling Guidelines and its later guidance on in-water cleaning (MEPC.1/Circ.918) link the cleaning method to the fouling rating, and point towards capturing debris when macrofouling is removed.
• BIMCO and ICS publish an industry standard on in-water cleaning with capture, with an approval procedure for cleaning companies.
• Ports set their own rules. Some allow in-water cleaning freely, some only with capture, some only above the waterline or only for propellers, and some not at all.
• Arrival rules in places such as Australia, New Zealand and Brazil require ships to show they manage biofouling, which drives cleaning before departure.

The direction of travel is clear: IMO has been working towards a mandatory framework on biofouling management. Expect more ports to ask who is cleaning, how, and whether the debris is captured.

Propellers and niche areas

The propeller is a small area with a large effect on efficiency, and it is usually cleaned and polished as its own scope. Roughness on the blades is graded on the Rubert scale, and polishing restores a smooth finish. See our propeller polishing service.

Niche areas, meaning sea chests, gratings, thruster tunnels, bilge keels and rudder recesses, carry a disproportionate share of fouling and of biosecurity risk. They need hand work and are often left out of cheap quotes. If a quote does not name them, ask.

In-water cleaning or dry dock?

In-water cleaning keeps the ship trading and costs a fraction of a docking. It cannot recoat a hull, repair a failed coating or deal with problems that need the ship out of the water. When the coating has broken down badly, cleaning in the water may simply expose more steel. The decision is about coating condition, docking schedule and the fouling you are dealing with. Where a docking survey can be replaced by an underwater inspection, see our UWILD service.

Planning frequency and budget

There is no universal interval. The right frequency depends on:

• Trading pattern: steady service speed or frequent long waits
• Water temperature: warm water fouls faster
• Coating: type and age
• Performance data: the trend in speed and consumption

Many operators move from cleaning when a problem is obvious to cleaning against a trigger, such as a set loss of speed, a fouling rating after an inspection, or a long idle period. Writing those triggers into the ship's energy efficiency plan makes the decision routine instead of reactive.

Where can cleaning be done?

Cleaning is only possible where the port allows it, the water and weather allow divers to work, and a qualified contractor can mobilise. Cleanship works from bases in the UAE (including Fujairah and Khor Fakkan) and India (including Kandla and Mumbai). See hull cleaning in the UAE and hull cleaning in India for the ports covered.

What a good report contains

A hull cleaning report should let a superintendent who was not there understand exactly what was done:

• Vessel, date, location, water conditions and team
• Fouling found before cleaning, by area, ideally with a rating
• Areas cleaned and areas not cleaned, with reasons
• Coating condition and any damage seen
• Anode condition
• Before-and-after photographs and video, labelled by location

If a report cannot answer "what did the hull look like before, and what does it look like now?", it is not a report.

Getting a quote

Send us the vessel name or IMO number, the port and expected dates, the time since the last clean or docking, and any recent photos or performance data. We will come back with a scope and a realistic duration. Contact us here.