Biofouling

Biofouling is the build-up of marine life on the submerged parts of a ship — the hull, propeller, sea chests, rudder and other underwater surfaces. It begins with a slime layer of bacteria and microscopic organisms and progresses to visible growth of algae and seaweed and then to hard fouling by barnacles, mussels and tube worms. Any surface left in the sea will foul over time, and a ship provides a large, slow-moving habitat, so keeping the hull clean is a continual concern in ship operation.

The most immediate consequence of biofouling is added resistance. A fouled hull is rougher and disturbs the flow of water past it, so the ship needs more power, and therefore more fuel, to maintain a given speed. Even a modest slime layer measurably increases fuel consumption, and heavy hard fouling can raise it dramatically, so biofouling has a direct effect on operating cost and, because more fuel means more emissions, on a ship's carbon intensity. Keeping the hull and propeller smooth is one of the simplest and most cost-effective ways to save fuel and reduce emissions.

Biofouling also poses an environmental risk quite separate from fuel: it is a pathway for spreading invasive aquatic species. Organisms that attach to a hull in one part of the world can survive the voyage and establish themselves in another, damaging native ecosystems in the same way that species carried in ballast water can. For this reason the transfer of species by biofouling is increasingly regulated through guidelines and, in some regions, requirements for hull cleaning and management. Ships combat fouling with anti-fouling coatings applied in dry dock, which release or present surfaces that discourage growth, and with in-water hull cleaning and propeller polishing between dockings, all managed under a biofouling management plan that balances fuel efficiency, cost and the protection of the marine environment.