Barnacles

What Are Barnacles?

Barnacles are small marine crustaceans that permanently cement themselves to hard surfaces underwater, including the hulls, propellers, and sea chests of ships. Once attached, they form rough, calcified colonies that disrupt the smooth flow of water along a vessel’s hull, increasing hydrodynamic drag and forcing the engine to burn more fuel to maintain the same speed. In shipping, barnacles are the most operationally consequential component of biofouling, the broader category of marine organisms (algae, slime, mussels, and tubeworms) that colonize submerged surfaces.

Key Takeaways

• Barnacles are marine crustaceans that attach to ship hulls and are the most consequential component of biofouling for vessel performance.
• Heavy calcareous fouling can require up to 86% more shaft power at cruising speed, with corresponding penalties to fuel consumption and CO₂ emissions.
• The Arabian Gulf’s warm, shallow, saline water has accelerated biofouling on roughly 2,000 vessels stranded since the Strait of Hormuz situation began in early 2026.
• Biofouling carries regulatory exposure under the IMO’s Carbon Intensity Indicator (CII) and 2023 Biofouling Guidelines, plus contractual exposure under hull-performance clauses in time charters.
• Dark fleet vessels are disproportionately affected because sanctions limit their access to dry dock and in-water cleaning, which can itself become a behavioral signal in vessel tracking analytics.

How Do Barnacles Affect Maritime Operations?

Barnacles affect maritime operations through three connected consequences: hull drag, fuel consumption, and regulatory exposure. When a vessel sits stationary in warm, biologically productive water, as roughly 2,000 tankers and cargo ships have done in the Arabian Gulf since the Strait of Hormuz situation began in early 2026, biofouling accelerates rapidly. The Gulf is shallow, averaging around 50 meters, highly saline, and warm year-round, with surface temperatures often above 30°C. Those are textbook conditions for the striped acorn barnacle (Amphibalanus amphitrite, formerly Balanus amphitrite) and similar species to colonize a hull within weeks.

For a tanker chartering desk or technical manager, the practical impact is measured in fuel burn and speed loss. Even a thin biofilm layer can cost a vessel meaningful efficiency. Once hard fouling like barnacles takes hold, the penalty escalates sharply:

Beyond the fuel bill, barnacles drive regulatory and contractual consequences. Under the IMO’s Carbon Intensity Indicator (CII) regime, sustained efficiency losses push a vessel’s annual rating downward, which has direct implications for chartering attractiveness. Time-charter parties increasingly include hull performance clauses that allow charterers to claim against owners for excess fuel consumption attributable to hull condition. BIMCO’s Hull Fouling Clause for Time Charter Parties 2019 sets a default 15-day idling threshold in Tropical and Seasonal Tropical zones, after which the contractual speed and consumption warranties may be suspended pending hull inspection. The framework applies directly to vessels stranded inside the Arabian Gulf. In addition, under the IMO’s updated 2023 Biofouling Guidelines, flag states are tightening expectations around biofouling management plans and in-water cleaning records.

The 2026 Gulf situation also has a secondary, less obvious effect. When a vessel finally moves after weeks of stationary exposure, sea chests — the grated openings that draw seawater for engine cooling — can be partially blocked by barnacle colonies and, depending on the season, by jellyfish drawn into the same warm water. That moves the problem from “fuel inefficient” to “operationally unsafe to sail.”

Anti-fouling coatings compound the problem for stationary vessels. Most modern coatings are engineered around regular operating profiles and rely on water flow across the hull to shed settled organisms. When a vessel sits at anchor for weeks, that flow disappears, and the coating no longer performs as designed, allowing fouling to accumulate faster than it would on a sailing ship.

How Much Do Barnacles Slow a Ship Down?

Barnacles can slow a vessel significantly. Even a hull with just 10% barnacle coverage requires roughly 36% more shaft power to maintain its design speed, and heavily fouled vessels often cannot maintain laden speed at all. The exact penalty depends on hull form, design speed, and fouling distribution, but the relationship is well established. A rougher hull means more drag, less speed at the same fuel burn, and higher emissions per nautical mile. For a Suezmax operator running on a long voyage, that translates into measurable extra days at sea and additional bunker fuel costs.

Propeller fouling compounds the effect. While hull fouling drives the larger penalty by surface area alone, fouling on the propeller separately degrades thrust, torque, and open-water efficiency, meaning a vessel turning its shaft at the same RPM no longer delivers the same speed or acceleration. Combined hull-and-propeller fouling produces a meaningfully larger power penalty than either alone.

Why Are Barnacles a Problem for the Dark Fleet?

Barnacles are a particular problem for the dark fleet because sanctioned and shadow fleet tankers often cannot access reputable shipyards or commercial hull-cleaning services. A vessel under U.S., U.K., or EU sanctions may be turned away from dry dock in most jurisdictions, and even underwater hull cleaning in many ports requires environmental permits and provider relationships that sanctioned operators struggle to maintain. The result is a fleet of vessels carrying progressively worse biofouling, with the operational and behavioral signatures that come with it, which include slower transit speeds, longer voyage times, and unusual fuel signatures that a chartering desk or sanctions analyst can detect through pattern-of-life analysis. 

The Gulf stranding situation during the Strait of Hormuz conflict has amplified this. Many of the vessels stuck in the Arabian Gulf since early 2026 are dark fleet or sanctions-implicated tankers that were already overdue for cleaning before they became stationary, compounding the hull condition problem when traffic eventually resumes.

How Are Ships Cleaned of Barnacle Fouling?

Ships are cleaned of barnacle fouling through a combination of dry-docking, in-water hull cleaning (IWC), and the use of anti-fouling coatings. Dry-docking remains the most thorough approach but is expensive and disruptive, so most operators rely on periodic in-water cleaning by ROV-equipped diver teams between dry-dock intervals. Modern anti-fouling coatings (biocidal, foul-release silicone, and hybrid systems) slow attachment but do not eliminate it, especially on vessels that sit stationary for extended periods, which is why prolonged anchorage is the worst-case operational scenario for hull condition.

Biofouling is also treated as a major pathway for the transfer of invasive aquatic species, which is why the 2023 IMO Biofouling Guidelines tightened expectations around inspection, cleaning, capture, and waste disposal. Some destination jurisdictions, including Australia, New Zealand, and California, may require inspection, controlled cleaning offshore, or additional biosecurity evidence before allowing normal port operations on heavily fouled vessels. For vessels leaving the Gulf with weeks of accumulated growth, that adds a second gating layer beyond physical hull cleaning capacity.