Biofouling · · 6 min read
Most fleets still clean hulls the way they change the office water filter: on a calendar. Every nine months, every drydock, every charter handover. But biofouling doesn't grow on a calendar - it grows with exposure. Two sister vessels on the same service can carry very different fouling loads depending on where they sailed, how warm the water was, and how long they sat at anchor. A fixed interval is guaranteed to be wrong for at least one of them.
The cost of a dirty hull
Fouling begins within hours of a hull touching seawater: a biofilm first, then slime, then - given time and the right conditions - hard growth like barnacles and tubeworms. Each stage adds frictional resistance, and the engine pays for it. Studies collected by the IMO's GloFouling Partnerships put the added fuel burn from even a light slime layer in the double digits, with heavy calcareous fouling far beyond that.
Since fuel is the largest controllable line in most voyage budgets and burns straight into a vessel's CII rating, hull condition is no longer just a maintenance topic. A hull cleaned too late quietly degrades margins and carbon intensity for months; one cleaned too early spends money on divers, robots, and coating wear that the fouling load never justified.
Why the calendar gets it wrong
Fixed intervals are calibrated to an average vessel on an average trade - and almost no vessel is average. The drivers of growth are physical: water temperature, salinity, nutrient load, and above all time spent idle. A hull moving at service speed through cold North Atlantic water accumulates growth slowly; the same hull anchored for two weeks in a warm, nutrient-rich roadstead is a colonization experiment.
This is why the interval that protects your worst-exposed vessel over-services the rest of the fleet, and the interval tuned to the fleet average lets your tropical traders sail dirty. There is no single number that gets it right.
Exposure-based modelling: growth as a function of the voyage
The alternative is to model what actually happened to each hull. A vessel's AIS track says where it sailed and how long it lingered; satellite observations supply the sea conditions along that track, with chlorophyll concentration as the standard proxy for the nutrient load that feeds growth. Weight the hours by those conditions - idle hours most heavily - and you get a per-hull fouling index that accrues voyage by voyage.
From there, cleaning becomes an optimization rather than a ritual: trigger the scrub when the accumulated fuel penalty overtakes the cost of cleaning, and not before. The interval stretches for vessels running cold, fast rotations and tightens for vessels that idled through an algal bloom. SeaWiz builds exactly this model for every tracked vessel, and sets a best-fit cleaning window per hull instead of a fleet-wide calendar.
The takeaway
The calendar was a reasonable tool when nobody could see where a hull had been. With AIS tracks and satellite ocean data, that excuse is gone: fouling exposure is measurable per vessel, and the cleaning schedule should follow the measurement.