The Hidden Cost Below the Waterline

Editor’s Note: Jennifer Merck is vice president of maritime and defense at the Association for Materials Protection and Performance (AMPP), where she has held leadership roles during her 26 years with the organization. She has worked extensively with the Department of Defense, the military services, and the Department of Transportation to advance corrosion mitigation, materials protection and workforce competency.

A ship doesn’t need a major mechanical problem to become less efficient. Sometimes, all it takes is a thin layer of slime.

Research supported by the International Maritime Organization (IMO) found that a biofilm just 0.5 millimeters (about 0.02 in) thick, covering up to half a ship’s hull, could increase greenhouse gas emissions by 25–30%, depending on vessel characteristics, speed and operating conditions.

For fleet operators, selecting an underwater hull coating is about more than the initial application. The system will spend years exposed to seawater and marine growth, and its condition can affect maintenance requirements and the surface roughness.

Those factors come with costs.

When Hull Condition Becomes an Operating Cost

Biofouling begins within hours of immersion. Microorganisms attach to submerged surfaces, creating a biofilm that can develop into growth such as algae and barnacles.

As fouling accumulates, hull roughness and hydrodynamic resistance increase. More power is then required to maintain speed, increasing fuel consumption and associated emissions.

The IMO’s 25–30% figure is a modeled estimate, not a fuel penalty that applies to every ship. Actual effects vary with vessel characteristics, speed, operating conditions and the amount and type of fouling present.

It shows the potential relationship between hull condition and energy efficiency.

This is where coating selection enters the economic equation.

The lowest initial coating cost does not necessarily translate into the lowest cost over a vessel’s operating life. Owners must also consider how the system will perform under actual operating conditions.

Speed, activity level and expected idle periods all shape a vessel’s operating profile. In most cases, a ship underway faces different fouling conditions than one sitting idle for extended periods, and antifouling selection should match those expected service factors. Trading routes expose hulls to different water temperatures, salinity and fouling organisms. Maintenance schedules, drydocking intervals and cleaning practices add still more variables that must be accounted for.

The IMO identifies operating profile, trading routes, maintenance history and the type, age and condition of antifouling coatings as factors that influence biofouling.

No single coating choice addresses every condition. Selection has to begin with how and where the vessel will operate.

Looking Beyond the Price of the Coating

The lowest initial cost may not be the lowest lifecycle cost. Compare it against expected service interval, maintenance or cleaning requirements and demonstrated in-service performance.

An antifouling system is intended to limit the accumulation of marine organisms. However, if fouling builds between maintenance periods, added resistance can affect fuel demand, while cleaning or other intervention adds expense.

Protective coatings shield the underlying structure from the marine environment. If that protection deteriorates prematurely, exposed steel can become vulnerable to corrosion and require repair.

For an owner, the relevant question is not simply, “What does this coating cost?”

It is: “How is this system expected to perform on this vessel until we plan to maintain it again?”

That puts surface preparation, application requirements, expected service conditions, antifouling performance, inspection and maintenance into the same lifecycle calculation.

Fuel efficiency also cannot be separated from coating performance. A hull may be structurally sound while marine growth adds resistance. An effective antifouling surface may control growth, while deterioration elsewhere leaves steel susceptible to corrosion. Both influence the cost of maintaining the underwater hull.

Selection Is Only the Beginning

Even a coating system well matched to a vessel’s operating profile must be applied correctly.

Surface preparation is particularly important. Contamination or inadequate preparation can interfere with adhesion and compromise performance. Coating performance also depends on achieving the specified surface preparation, environmental controls, film thickness, recoat intervals and curing requirements, which application and inspection must verify.

Work quality is therefore part of the coating’s economics. A system selected for long-term performance cannot deliver that value if preparation or application problems shorten its useful life. Premature deterioration can lead to unplanned work or additional time in drydock.

That lifecycle thinking should continue after the vessel returns to service.

What Is the Hull Telling You?

Underwater inspections allow owners to see whether coating and antifouling systems are performing as expected. But simply recording that fouling exists has limited value. Consistent records of fouling extent, severity, and location; coating condition; and comparable photo or video evidence let operators evaluate performance trends over time.

AMPP SP21487-2021, Inspecting and Reporting Biofouling and Antifouling Systems’ Condition during Underwater Inspections on Ships, was developed to provide a consistent methodology for planning, conducting and reporting inspections. It applies to inspections performed by divers and remotely operated vehicles.

AMPP Guide 21517-2025, Digital Fouling Report Format for Underwater Hull Cleanings and Inspection, defines a standardized digital report format that lets hull roughness and coating condition data be compared over the vessel’s lifecycle, regardless of the service provider.  Comparable reports can help owners identify increasing fouling, changes in coating condition and whether maintenance may be needed before the next scheduled drydocking.

Actual performance from vessels in service can shape future coating specifications.

People Still Determine Performance

Coating technology alone cannot solve every part of this equation.

An applicator who misses the specified film thickness or recoat window can shorten the coating’s service life, and the problem may not become apparent until the vessel is back in drydock.

Someone has to prepare the surface, apply the system and verify the finished work. Once the vessel is operating, an experienced professional must recognize deterioration, document inspection findings and determine whether further evaluation is necessary.

Training, certification and experience in turn are essential because coating economics depend in part on those decisions.

New coating technologies and advanced inspection tools can improve hull performance, but they don’t eliminate the need for technical judgment.

A Different Way to Calculate Coating Value

Fuel will remain a major operating consideration for fleets, while pressure to reduce maritime emissions will persist in shaping vessel design and operations. But alongside emerging fuels and propulsion technologies, hull condition will remain as a lever fleets already control.
                                                                                                                                    

Instead of considering acquisition and application cost alone, owners can evaluate how a system fits the vessel’s operating profile and expected fouling conditions, along with maintenance and hull-cleaning requirements, drydocking costs, expected service life and potential fuel impact. Inspection data can then show how that choice performs in service.

Hull condition affects the energy required to move a ship, though coatings are only one part of reducing maritime emissions. For fleet operators, protecting that surface is not only a maintenance decision. It is part of managing operating costs.