Increased resistance, invasive species and growing regulatory attention.
Hydrodynamics, energy consumption and port entry implications.
Biofouling management plans, inspection logic and clear cleaning criteria.
Treat it as a cross-cutting topic: technology, consumption and compliance.
Biofouling is not a uniform process. It occurs in phases: within hours, a biofilm of bacteria and microalgae forms (microfouling or “slime layer”). This provides the foundation for macrofouling – the colonisation by barnacles (Balanidae), mussels, tubeworms and algae. The speed of this process depends on water temperature, salinity, light and nutrient availability. In tropical waters, heavy macrofouling can develop within two to four weeks of idle time.
The hydrodynamic consequences are substantial and well documented. Even a light slime layer can increase frictional resistance by 10-15 per cent. Heavy macrofouling (barnacles, mussels) can raise resistance by over 40 per cent, translating directly into increased fuel consumption. For a Panamax bulker consuming 30 tonnes of heavy fuel oil per day, a 20 per cent resistance increase equates to additional fuel costs of approximately USD 3,000-4,000 per day at current bunker prices.
Anti-fouling coatings have evolved considerably over the past two decades. Following the ban on TBT (tributyltin) under the AFS Convention in 2008, copper-based self-polishing copolymer (SPC) systems initially dominated. Today, silicone-based foul-release coatings are increasingly deployed, which do not kill the growth but detach it through a smooth, non-adhesive surface at sufficient sailing speed. These systems perform well on faster vessels (from approximately 15 knots) but less reliably on slower units or during extended idle periods.
Niche areas – sea chests, rudders, propeller shafts, bow thruster gratings – are particularly problematic because they often lie outside the coating system or are difficult to access. At the same time, these are precisely the areas that play the greatest role in the transfer of invasive species. The IMO Biofouling Guidelines (MEPC.207(62)) explicitly call for management of these niche areas, but implementation varies considerably.
Regulatory pressure on biofouling is increasing. Australia and New Zealand already have stringent requirements: vessels operating in Australian waters must be able to demonstrate a biofouling management plan. Since 2018, New Zealand can refuse entry or order cleaning if fouling levels exceed defined thresholds. California has implemented similar regulations.
For operators this means: biofouling management is no longer a voluntary best practice but a compliance requirement with operational consequences. A vessel refused entry incurs costs not only for cleaning but also for schedule disruption, alternative routing and potential contractual penalties.
The CII regulation (Carbon Intensity Indicator) adds further economic pressure. A fouled hull increases fuel consumption and thereby worsens a vessel’s CII rating. From a rating of “D” in three consecutive years or “E” in any single year, a corrective action plan becomes mandatory. Biofouling management thus becomes a direct factor in the regulatory rating of a vessel.
In practical terms, this requires a combination of preventive coating management, regular underwater inspection (via ROV or divers) and a clear cleaning strategy with defined trigger thresholds. Those who only react when growth becomes visible are typically already too late.
Vessels with irregular operating profiles are particularly affected: offshore units stationed for weeks, tankers at anchor awaiting access to congested terminals, and cruise ships in seasonal lay-up. During the Covid-19 pandemic, hundreds of vessels were laid up for months – many in tropical waters. The result was massive biofouling problems that led to significant cleaning costs and delays upon reactivation.
A typical scenario: a product tanker waits four weeks off a terminal in Southeast Asia. Water temperature is 29°C. Without countermeasures, moderate to heavy macrofouling develops during this period around the waterline and at niche areas. Upon resuming the voyage, fuel consumption is markedly increased, manoeuvrability may be reduced, and the next underwater inspection reveals findings that require either cleaning or early docking.
Proactive operators deploy planned in-water cleaning during the idle period or immediately afterwards. An important consideration: cleaning must take place in a port that permits it and has capture systems to prevent releasing detached organisms into the local ecosystem.
Effective biofouling management rests on four pillars:
1. Coating strategy: Does the current coating system match the operating profile? Slow vessels or units with frequent idle periods require different systems from fast container ships.
2. Inspection cycle: Underwater inspections should be conducted at least every 12 months – more frequently for at-risk vessels. ROV-based inspections offer cost-effective interim solutions.
3. Cleaning thresholds: Define clear criteria for when cleaning is triggered – not only upon obvious macrofouling.
4. Documentation: The biofouling record book must be kept current. It forms the basis for regulatory inspections and internal trend analysis.
Once a biofouling management plan identifies that cleaning is needed, the choice of method matters as much as the timing. Proactive grooming – light, frequent brushing of a foul-release coating before macrofouling establishes itself – preserves the coating and can often be done without removing the vessel from service, but it only works on coatings designed for grooming and requires a reliable schedule. Reactive in-water cleaning of established macrofouling is more aggressive, risks damaging the coating system, and depending on the fouling type and the port's environmental rules may require certified capture equipment to collect the debris rather than releasing it into the harbour.
Diver-based cleaning remains common but is slower and harder to document consistently across a fleet. ROV-based systems with recording capability produce a verifiable record of what was removed and where, which matters increasingly for port authorities that want evidence a vessel was properly managed before entry. The decision between grooming, reactive cleaning, and dry-docking should be driven by the coating type installed, the vessel's idle pattern, and whether the calling ports accept in-water cleaning at all – some jurisdictions restrict or prohibit it outright regardless of the capture technology used.
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