Marine Fouling: The Hidden Drag on Global Shipping

Introduction

Marine fouling – also known as biofouling – is the accumulation of aquatic organisms on submerged surfaces, such as the hulls and propellers of ships. It is a natural process: within hours of a clean surface being immersed in seawater, microorganisms begin to cling on and form slimy films, followed by the growth of larger plants and animals. For ships, this “coat” of marine life significantly increases drag (resistance) as the vessel moves through water, forcing engines to work harder. The result is higher fuel consumption, increased operating costs, and more greenhouse gas emissions. In fact, even a thin layer of biofilm or “slime” on a ship’s hull can lead to surprisingly large inefficiencies.

One recent International Maritime Organization (IMO) study found that a minor slime film just 0.5 mm thick covering half of a hull’s surface can increase a ship’s greenhouse gas emissions by 20–25% (maritime-executive.com). If small barnacles or tubeworms manage to attach, the impact is even worse – an “average” ship with light calcareous fouling could see fuel use and emissions jump by as much as 55% (maritime-executive.com). In global terms, researchers estimate that roughly 10% of the shipping industry’s fuel is wasted overcoming the extra drag from biofouling, equating to about 90 million tons of excess CO₂ emissions every year (IMO,2020).

Classes of Marine Fouling Organisms

Scientists classify marine fouling organisms into broad categories based on size and the nature of the growth. The two primary divisions are microfouling and macrofouling, whereas macrofouling is further split into hard and soft fouling. Each class has distinct biological origins and characteristics:

	Microfouling (Biofilms or “Slime”) refers to the microscopic life that initially coats a surface including bacteria, and single-celled algae. The organisms in a biofilm produce glue-like polymers (extracellular polymeric substances) that help them stick to the hull and to each other. A biofilm can build up to a thickness of around 0.5 millimeters or more, and it provides a base.
	Macrofouling – Soft Fouling Organisms: After the biofilm stage, larger organisms can anchor themselves to the hull – this is called macrofouling. One subset of macrofouling is soft fouling, which includes organisms that are fleshy, flexible, or lack hard shells. These organisms often have a plant-like or gelatinous appearance.
	Macrofouling – Hard Fouling Organisms: Includes organisms that build calcareous or rigid shells and attach firmly. Common hard foulers are barnacles, mussels, clams, and tube worms. These organisms create strong, cement-like bonds to the surface and can be extremely difficult to remove once established.

It is worth noting that fouling isn't just a problem for ships; it affects any submerged structure – from ocean buoys to offshore wind turbines – requiring constant maintenance to remove growth and keep things operational. In the context of shipping, however, the stakes are especially high because of the fuel and emissions penalties incurred. We will now examine exactly how marine fouling degrades a ship’s hydrodynamic performance and why that causes such a spike in fuel usage.

How Fouling Affects Ship Hydrodynamics

When a ship moves through water, the majority of the resistance it encounters  comes from hull friction – the drag caused by water flowing along the hull’s surface. Time is a key factor: a hull that’s fresh out of drydock (with new antifouling paint) might stay relatively clean for a few weeks or months, but over longer periods the fouling builds up progressively. In cases of biofouling attached to the hull, the ship’s engines then must generate more power to maintain the same speed through the water. To visualize this, an IMO-sponsored analysis (IMO, 2021) plotted the increase in GHG emissions (as a proxy for fuel consumption) against different biofouling conditions.

Figure: Relationship between biofouling severity and increase in GHG emissions for ships, based on preliminary IMO study data. The red curve in the figure below shows how emissions rise sharply with even small amounts of fouling, based on data from real ship observations and models.

Roughness near the forward part of the hull or near the waterline tends to have a bigger impact on drag than the same roughness in sheltered areas like in recesses or near the stern, because the forward areas influence how the flow develops along the entire hull. However, even fouling in “niche areas” like sea chests, thruster tunnels, or propellers can be detrimental – for instance, a fouled propeller or thruster loses efficiency and can cause vibration. A layer of slime or shells on propeller blades reduces the propeller’s thrust output and can cut propulsion efficiency substantially, on top of the hull resistance increase.

Propellar Fouling Examples

Seaches Fouling Examples

Near Waterline Fouling Examples

Fuel Consumption and Emissions Impact

The global shipping industry is the backbone of international trade, but it also accounts for a significant share of the world’s energy use and emissions. Shipping contributes roughly 3% of global CO₂ emissions (IMO, 2020) with 1.056 million tonnes of CO2 emissions back in 2018. Over all, analyses suggest roughly 10% of shipping’s emissions can be attributed to the drag caused by biofouling,  equating to ~90 million tonnes of CO₂ emissions avoided annually, which likely adds around $6 billion to the annual fuel (bunker) costs of the commercial fleet (IMO, 2020).

A study, conducted by I-Tech and Safinah Group (2020), surveyed 249 ships between 2015–2019 and noted over 40% had at least 10% of the hull covered in hard fouling. Using published drag data (from Schultz 2011), they calculated that this extent of barnacle coverage across a large portion of the fleet would translate to roughly 110 million tons of excess CO₂ emissions per year (I-Tech and Safinah Group, 2020).

 On a global scale, if international shipping were a country, it ranks 7th place on the global CO² emission sources in 2023. Despite having emitted more CO² than the airline industry in 2023, shipping still remains the most efficient and economical friendly mode of transportation based on a CO² per tonne moved comparison.  

Source: ATAG.org (2024), Statista 2024), SEAISI (2024)

Furthermore, if a ship’s performance degrades too much, it might fail to meet charter party speed requirements or need unscheduled drydock, leading to lost revenue and maintenance costs. In an era when the shipping industry is also being pressed to reduce its carbon intensity, the extra emissions from fouling can even have regulatory costs such as higher carbon levy or may not meet EEXI/CII targets.

Global Regulations and Initiatives

Marine fouling is unique as an issue that straddles both engineering efficiency and environmental protection because of invasive species. Consequently, international bodies like the IMO, as well as national governments, have developed guidelines and regulations to address different aspects of biofouling management. Here we discuss the key regulatory measures and global initiatives:

	IMO Anti-Fouling Systems (AFS) Convention 2001 (into force 2008): This treaty dealt with the pollution aspect of antifouling paints. It prohibits harmful antifouling systems, specifically banning organotin compounds like TBT from ships’ hulls 26 evidenced by an International Anti-Fouling System Certificate that ships carry. The AFS Convention doesn’t directly regulate biofouling performance, but by banning the most toxic paints, it spurred the development of alternative antifouling technologies and ensures that environmental harm from coatings is limited.
	IMO Biofouling Guidelines (MEPC.207(62). In 2011, the IMO issued a set of voluntary guidelines for best practice being the first international instrument to directly address how ships should manage biofouling on their hulls. The primary aim was to reduce invasive species spread, but an obvious co-benefit is improved efficiency. In 2023, after a decade of experience, the IMO adopted an updated Biofouling Guideline which revokes and replaces the 2011 version. The influence is significant as several jurisdictions have made parts of the guidelines effectively mandatory.
	In-Water Cleaning Guidelines: Organizations like BIMCO (Baltic and International Maritime Council) and the International Organization for Standardization (ISO) have been working on standards for in-water hull cleaning to ensure it’s done safely and with minimal environmental impact 42. BIMCO released guidelines on in-water cleaning with capture technology in 2022, which align with the idea that proactive grooming is preferred and any cleaning that removes macrofouling should capture debris.
	While not explicitly about marine fouling, the IMO’s regulations on reducing GHG emissions from ships like the Carbon Intensity Indicator (CII) ratings and the Energy Efficiency Existing Ship Index (EEXI) (effective from 2023) provide penalties for ineffective vessels. These regulations indirectly incentivize good fouling management. A fouled ship has worse fuel efficiency, which could jeopardize its CII rating (which measures grams CO₂ per cargo capacity-mile) and ban the ship from operations if the rating is too low. With CII, ship owners have an added motivation to keep hulls clean to meet environmental performance targets.

Regulations and Initiatives

	New Zealand was one of the first to enforce strict biofouling requirements for incoming vessels. Since 2018, ships are required to arrive with a clean hull. As of October 2023, New Zealand updated its rules to align with the new IMO guidelines requiring a Biofouling Management Plan and evidence of proper cleaning before entering its waters.
Australia also has guidelines and some mandatory requirements on biofouling management for international arrivals, essentially requiring a biofouling management plan and documentation similar to the IMO guideline (both Australia and New Zealand were driving forces behind the guideline at IMO).
	California (USA) has its own biofouling management regulations for vessels arriving at its ports, including requirements for regular hull inspections and cleaning and reporting of such activities.
Other countries like Chile and Brazil have begun implementing biofouling rules for ships, and there’s movement in the EU (mostly driven by Scandinavian countries) as well to consider tighter biofouling measures.

What can be done about marine fouling?

Dealing with marine fouling on your ships first requires a comprehensive knowledge about the status-quo of underwater growth and your anti fouling paint condition. Generally, vessels are inspected underwater every 2,5 years as part of their class renewal. This means, that 900 days, ship owner have no knowledge about their hull condition and even when hiring conventional divers, the video quality is often too bad to draw managerial conclusion from IWS (in water survey) reports.

In recent years, the rise of micro-ROV (Remotely Operated Vehicles) systems or so called “underwater drones” have made ship hull inspections cheap, efficient and generally available. ROV hoover along ship hulls, capturing HD quality images that allow identification of marine fouling species in different states of growth. The high manoeuvrability allows for a view of a large, undisturbed picture instead of narrow area focus of conventional divers. High quality video data of ROV systems opens the possibility for advanced analytics such as image segmentation AI. On the forefront of AI marine fouling detection is Vesselity Maritime Analytics, a German tech start-up founded in 2023.

Vesselity specialises on underwater image recognition of marine fouling in combination with ROV ship inspections and a content management software Hull-Sight to identify and calculate the risk of marine fouling to attach on a ship during operations and the impact on excess fuel consumption.

With advanced knowledge about the status-quo of ship hull fouling and the anti-fouling paint condition, ship owner and fleet manager obtain optimal decision support for in-water cleaning of ship hulls, if economic factors suggest.

In a blog post by Wendy Laursen (2025) from Maritimemagazines.com, the context of robotic in-water cleaning is introduced and summarized based on first applied examples. The role of micro-ROV systems for marine fouling inspection is highlighted.

Another introduction about the rise of ROV systems and its impact on the shipping industry was published by Michael Stein (2023) about how the micro ROV class will change the maritime sector.

Conclusion

Marine fouling has major implications for global shipping’s economics and environmental footprint. A fouled hull creates hidden drag that forces ships to burn more fuel to maintain speed, which in turn means higher costs and more greenhouse gas emissions. The numbers are eye-opening: roughly 79–110 million tons of CO₂ emitted per year can be linked to the extra fuel burn caused by biofouling costing shipping companies billions of dollars. 

At the regulatory level, the IMO and various countries are increasingly recognizing their environmental protection pushing to make biofouling management a standard practice. Shipowners that invest in good hull maintenance  are rewarded with lower fuel bills and compliance with tightening environmental rules. To realize this, however, knowledge about the ship’s hull condition is required where the human eye cannot reach. Innovative approaches by maritime tech companies tackle this requirement for hidden data with state-of-the-art underwater drones. Vesselity Maritime Analytics operates on the forefront of underwater AI to extract marine fouling knowledge from ROV video streams to provide expert decision support on marine fouling condition evaluation.   

In conclusion, marine fouling is a classic example of how interconnected our industrial systems are with the environment. Tiny organisms growing on a ship’s hull can scale up to influence global fuel consumption and climate emissions. By understanding this phenomenon and deploying science-based solutions the maritime industry can significantly cut down on the excess fuel consumption and emissions caused by fouling.

Reference List

ATAG Air Transport Action Group (2024). Aviation and climate change (Fact Sheet No. 2). https://atag.org/media/gw5cgzzh/fact-sheet_2_aviation-and-climate-change.pdf

I-Tech, & Safinah Group (2020). Quantifying the scale of the barnacle fouling problem on the global shipping fleet [White paper]. https://selektope.com/wp-content/uploads/2020/12/ITECH-WHITE-PAPER_June-2020-1.pdf

International Maritime Organization (2020). Fourth IMO greenhouse gas study 2020. https://www.imo.org/en/ourwork/Environment/Pages/Fourth-IMO-Greenhouse-Gas-Study-2020.aspx

Laursen (2025). Send in the robots. https://www.maritimemagazines.com/maritime-reporter/202505/send-in-the-robots/

Maritime Executive (2021). IMO study shows higher-than-expected fuel cost from fouling. https://maritime-executive.com/article/imo-study-shows-higher-than-expected-fuel-cost-from-fouling

Schultz, M. P., Bendick, J. A., Holm, E. R., & Hertel, W. M. (2011). Economic impact of biofouling on a naval surface ship. Biofouling, 27(1), 87–98. 

SEAISI South East Asia Iron and Steel Institute (2024). Global CO₂ emissions from energy production hit record high of 40 gigatonnes in 2023. Reference to BigMint. https://www.seaisi.org/details/25413?type=news-rooms

Statista (2024). International shipping carbon dioxide emissions worldwide from 2012 to 2023. https://www.statista.com/statistics/1291468/international-shipping-emissions-worldwide/

Stein, M. (2023). How the micro ROV class will change the maritime sector: An introductory analysis on ROV, big data and AI. In Autonomous vehicles—Applications and perspectives. IntechOpen. https://doi.org/10.5772/intechopen.110278