Ships Burn 25-30% More Fuel Because Ports Won't Let Them Clean

A slime layer of just 0.5 millimetres covering up to half a hull can raise greenhouse-gas emissions by roughly 25 to 30%, according to IMO-linked research — yet many ports still ban the in-water cleaning that would remove it. The result, Alex Byelyavtsev argues, is a self-defeating cycle: ships sail dirtier, burn more fuel and emit more, because the places where cleaning is permitted are disappearing.

The paradox is structural. Each individual restriction makes sense. Hull cleaning releases organisms, coating particles, biocides and microplastics into the water, so ports limit it. But once cleaning is restricted, the ship often keeps sailing with a fouled hull — more resistance, more fuel, more emissions.

What does fouling actually cost?

The numbers are concrete. DNV put the penalty in April 2025 at up to 20% extra fuel consumption for light slime and up to 85% for heavy shell fouling. Those figures are not a promise for every ship, but they price what operators pay to keep a fouled hull in service while waiting somewhere cleaning is allowed.

The cost is no longer purely a bunker line item. Extra shaft power is extra fuel; under the Carbon Intensity Indicator and, where it applies, the EU Emissions Trading System, the same fouling can also move a vessel's rating and inflate its allowance bill.

When cleaning fails to count as clean

Two cases recorded by Gard show how the rules bite. A vessel on a short time charter, on its first voyage from Malaysia toward New Plymouth, was rejected by New Zealand authorities for biofouling non-compliance. Owners diverted to Bundaberg, Australia, for hull cleaning. Bad weather meant the ship reached New Zealand roughly three weeks late, missed its laycan, and forced charterers to find other business.

The second case is sharper. A vessel had already undergone underwater inspection and cleaning in West Africa before a New Zealand call. New Zealand's Ministry for Primary Industries still found the evidence on the sea chest insufficient and issued a Notice of Direction, requiring extra work before arrival. Cleaning, in other words, is not always the same as a regulator accepting the ship is clean.

What do the contracts and rules say?

The market has already priced the geography gap into charter parties. Under BIMCO's 2019 Hull Fouling Clause for time charters, where the clause applies and cleaning is called for after an inspection, speed and consumption warranties can remain suspended if cleaning at the place of inspection is not permitted or possible. The clause assumes the fouling. It does not assume a berth.

Regulators have responded, but only partially. The IMO revised its biofouling guidelines in 2023 and in April 2025 approved guidance on in-water cleaning — while itself acknowledging that cleaning can damage coatings, shorten coating life and release harmful substances and invasive species. If the answer were simply "clean the hull," that circular would not be needed.

In March 2026, ISO 6319 set requirements for planning, conducting and documenting in-water cleaning, including sea chests and other hull openings. It tells ports and operators how a clean can be done. It does not require a port to allow one.

GreenVoyage2050 states the port barrier directly: many ports do not allow hull and propeller cleaning during a stay. The obstacles include biomass discharge, missing reception facilities, conflicts with cargo and bunkering operations, diver safety, and permits that cleaning companies often cannot obtain. If cleaning costs time, the ship may speed up afterward to recover its schedule — eating part of the efficiency gain.

Is a network forming?

Singapore shows capability, not a finished system. In 2026 the Maritime and Port Authority selected six partners to trial robotic in-water inspection and cleaning under real port conditions from the second half of the year. The MPA's point is that commercial systems already exist; the hard part is running them alongside busy operations, currents, hull geometry and debris-capture rules.

Better coatings help, but they do not answer the berth question. Idle time, warm water, route changes, sea chests and thrusters still produce fouling.

From the bridge, the sequence is ordinary: this port forbids in-water cleaning, the next has no approved contractor or lacks capture equipment, the call is short — and the ship departs with a dirty hull. Until ports permit and equip cleaning at scale, expect fouling penalties, diversions and suspended warranties to keep landing on operators' accounts.