Biofouling on Antifouling Coatings under Static and Dynamic Conditions

Rapport, 2025

Dynamic conditions are known to have large impact on the formation of marine growth, biofouling, on both natural surfaces and antifouling coatings but are however still understudied. While testing of antifouling coatings in static conditions are widespread and well documented, data from testing under dynamic conditions is more scare and only described in a limited literature (around 30 publications) using various devices, methods and assays. 

In this report we analyse the scientific literature on dynamic testing of antifouling coatings to describe how testing in static versus dynamic conditions can influence the results, i.e., measurement of biofouling (type and cover) on panels coated with antifouling. 

The report includes a description of the biofouling characteristics (size, attachment strength and motility) of importance for the marine organisms like bacteria, plants and animals when hydrodynamic forces are present in natural environments (ie the ocean). The forces along the hull of a ship moving through the water are schematically illustrated with reasoning on how flow conditions impact the biofouling process ie both during attachment stage and for removal of the biofouling. 

Results from the compiled literature showed that testing under dynamic condition primarily impact the lower forms of biofouling often referred to as the ‘slime’ layer. The slime layer consists of microorganisms (bacteria), microalgae (diatoms) and juveniles (larvae) of larger organisms. The organisms within the slime layer are surrounded by a matrix of ExtraPolymeric Substances (EPS) which is important for their attachment, and the composition of substances within this matrix shift under dynamic conditions compared to static conditions. The buildup of EPS under dynamic conditions creates a more tight and low form of growth and will enhance the ability for smaller biofouling to stay attached. 

The attachment very close to a surface is further facilitated by the prevailing reduced water movement in the close vicinity to a surface like the ship hull. To achieve the height of this ‘low velocity layer’ both hydrodynamic calculations and knowledge about the present biofouling organisms are required. For larger hard type of biofouling (like blue mussels, barnacles and calcareous tubeworms) the structures/features used for attachment differs, where mussels use threads of glue which gives several attachment points while for barnacles and tubeworms the entire underside of the animal adheres to the surface (giving a large attachment area). To fully resolve the forces (flow velocities) needed to hinder attachment and also to remove fouling organisms, a combination of several scientific disciplines (hydrodynamics, surface chemistry and biology) will be needed. 

Through the French research platform GDR Biofouling & Environment, established 2025, we are discussing previous knowledge on dynamic testing and the recently developed testing methods and facilities. In ongoing discussions between the antifouling paint production industry and academia, there is consensus that standardisation of dynamic testing is needed for sound interpretation and comparison between results. 

An overview of the dynamic devices used today are presented with their advantages and limitations. As a summary we compare static versus dynamic testing and discuss around their use for different types of tests and applications. Finally, we reflect on future development and suggest a multidisciplinary approach to advance work within this area.