Rethinking biofouling risk in an era of shipping disruption

Author: Riyami Mohamed, Managing Director of Alfa Laval’s Sonihull business

The disruption of shipping through the Strait of Hormuz, which has left vessels stranded in the Gulf for extended periods, has brought a wider operational risk into sharp focus: ships can be forced into prolonged periods of unplanned idling for reasons entirely beyond their owners’ control.

The broader issue is that disruption can take many forms – geopolitical, security-related, commercial or operational – and can cause a vessel’s actual operating profile to diverge sharply from what was planned. The 2021 Suez Canal obstruction, security threats around the Bab al-Mandeb Strait and port congestion are very different situations in cause and scale, but each can leave ships idling or operating at low speed for far longer than expected. Rather than weighing the likelihood of another Hormuz scenario in isolation, owners should consider the combined likelihood of disruptive events forcing a vessel outside its expected operating profile. That strengthens the case for giving such changes greater weight in vessel biofouling management.

Biofouling begins as soon as surfaces are immersed, while the rate and extent of accumulation depend on the vessel's operating profile, environment, coating type and condition, maintenance history and other factors. Extended periods at anchor or alongside, as well as prolonged low-speed operation, can all alter that risk. If actual operations diverge materially from the anticipated profile, existing protection may no longer perform as assumed, with potential consequences for vessel performance and operational readiness.

In the following Q&A, Riyami Mohamed, Managing Director of Alfa Laval’s Sonihull business, shares his perspective on what this means for shipowners and how continuous prevention can help make vessels more resilient. Click on the questions to read the answers.

1. When a vessel's operating profile changes unexpectedly, how quickly can biofouling become a problem?
2. Beyond the hull, where can fouling cause the greatest operational problems?
3. If a vessel is already caught in an extended delay, what can its crew and technical team realistically do?
4. If periodic cleaning restores vessel performance, why is continuous prevention necessary?
5. What difference can continuous prevention make when a vessel is unexpectedly forced to remain idle?
6. Does this change how owners should approach coatings, cleaning and biofouling protection?
7. Should the risk of prolonged, unplanned idling (or low-speed operation) now influence how owners specify and manage biofouling protection?

FACT BOX: Australian sister-vessel study – The value of continuous prevention

FACT BOX: How Sonihull works

1. When a vessel's operating profile changes unexpectedly, how quickly can biofouling become a problem?

There's no universal timeframe. Biofouling begins as soon as a wetted surface is immersed, but the rate and extent of accumulation depend on a combination of factors rather than on speed or time alone.

Water temperature, nutrient levels and proximity to the coastline all play a role, as do the coating type and condition, maintenance history, existing fouling and recent operating profile. Extended periods at anchor or alongside can increase exposure, and prolonged low-speed operation can also be relevant, but neither should be treated as an automatic threshold for significant growth. The IMO's 2023 Biofouling Guidelines likewise recognise a vessel's operating profile, including typical speed, as part of biofouling management. Nor is this exclusively a warm-water issue; we see biofouling challenges in Northern European waters too, particularly during the warmer months.

The important question isn't whether 10 or 20 days at anchor, or a particular period of low-speed sailing, represents an acceptable threshold. It's whether an unexpected delay has taken the vessel beyond the operating assumptions underlying its existing biofouling protection, and at what point inspection or intervention becomes necessary.

2. Beyond the hull, where can fouling cause the greatest problems?

The consequences extend well beyond increased hull resistance and fuel consumption. Fouling in sea chests, gratings, intakes and strainers can restrict seawater flow, impairing cooling capacity and the operation of essential machinery.

These areas are also more difficult to protect using conventional coatings alone. Fouling on propellers presents another risk, increasing surface roughness and affecting propulsion efficiency.

The concern when a vessel's operating profile shifts in ways that increase biofouling exposure isn't simply how much additional fuel it might consume. Fouling in critical seawater systems can compromise its ability to return to service reliably, potentially creating further delays and maintenance requirements.

Owners therefore need to assess their exposure across all submerged surfaces and seawater systems, rather than treating biofouling primarily as a hull-maintenance issue.

3. If a vessel is already caught in an extended delay, what can its crew and technical team realistically do?

The starting point is the vessel's biofouling management plan. Crews should document the duration and location of the delay, review coating limitations and confirm that existing marine growth prevention systems are operating correctly.

Direct inspection using divers or ROVs provides the most reliable evidence of the vessel's condition. This should include the propeller and critical niche areas, not just the hull.

Operational data can provide additional indications of deterioration. Cooling water temperatures and flow, together with differential pressures across heat exchangers and coolers, can help identify problems in critical seawater systems.

Once mature fouling has developed, no operational adjustment will simply make it disappear. If cleaning is necessary, owners should use an approved service provider and – preferably – an appropriate method that captures and disposes of the removed material.

Biosecurity is another consideration. Organisms accumulated during a disrupted operating period may be transported to another region as the vessel continues trading, potentially introducing invasive species into a new marine environment.

4. If periodic cleaning restores vessel performance, why is continuous prevention necessary?

There's an important distinction between a vessel being operationally acceptable and operating close to its optimum performance.

A vessel may complete its voyage without triggering any alarms, yet relatively light fouling can increase surface roughness and drag, requiring additional power and fuel to maintain the same speed.

Owners can track this deterioration by comparing speed, power and fuel consumption against an established clean-condition baseline, allowing for operational and environmental variables. Propeller slip is another useful indicator.

The limitation of periodic cleaning is that efficiency can deteriorate progressively between interventions. By the time the vessel is cleaned, the owner or charterer may already have incurred months of additional fuel costs and emissions.

For charterers, who often bear the fuel costs, that deterioration has a direct financial impact. Owners and technical managers also need to consider the implications of fouling for equipment reliability, maintenance and vessel availability.

The real value of continuous prevention is therefore not simply reducing cleaning frequency. It is maintaining the vessel closer to its clean-condition performance throughout its operating cycle, rather than repeatedly allowing efficiency to deteriorate and then restoring it.

5. What difference can continuous prevention make when a vessel is unexpectedly forced to remain idle?

The important difference is that preventative protection is already in place when the disruption occurs.

Some conventional antifouling coatings rely on the vessel moving through the water to help prevent or remove marine growth, so an unexpected period of inactivity can change the conditions under which that protection is expected to perform.

Sonihull operates continuously, whether the vessel is sailing or stationary. By inhibiting the formation of biofilm and the attachment of marine organisms, it helps keep protected surfaces cleaner for longer, reducing the risk of fouling-related performance deterioration.

For an owner facing an unexpected delay, this can mean a lower risk of additional maintenance requirements and a better prospect of returning the vessel to service without unnecessary intervention.

The extent of protection depends on where the system is installed and the vessel's existing condition. But having suitable preventative measures already in place means the owner is less reliant on responding to fouling after it has developed.

6. Does this change how owners should approach coatings, cleaning and biofouling protection?

I believe owners should consider the overall protection strategy rather than looking at each technology in isolation.

Sonihull is a complementary solution, not a remedy for mature fouling or a replacement for all conventional coatings and maintenance. Surfaces with established growth need to be cleaned before installation, and periodic inspection and cleaning remain necessary.

The extent of protection should reflect the vessel's operating profile and the areas where fouling presents the greatest risk. On a large commercial ship, targeting the propeller, sea chests, bulbous bow and aft flat bottom may provide a better return than protecting the entire hull.

For smaller vessels such as offshore support vessels (OSVs) and tugs, which can spend prolonged periods stationary, full-hull protection may make more sense.

It comes down to choosing the combination of measures that best protects the vessel's performance and operational reliability under both normal and unexpected operating conditions.

7. Should the risk of prolonged idling (or low-speed operation) now influence how owners specify and manage biofouling protection?

Absolutely. I believe this needs to be addressed at fleet-management level, both when specifying protection for newbuilds and assessing the requirements of existing vessels.

Owners should consider whether their current coatings and maintenance programmes provide adequate protection if a vessel is forced to remain stationary, or operate a low speed, for longer than anticipated. That means understanding the limitations of their existing systems, identifying the zones most vulnerable to fouling and assessing where additional preventative measures may be justified, taking account of each vessel's operating profile.

Continuous protection, such as Sonihull, can complement conventional coatings and periodic cleaning by helping to keep critical surfaces cleaner throughout the vessel's operating cycle, including during unexpected periods of inactivity or low-speed operation.

Owners cannot predict when or where the next disruption will occur, but the combined likelihood of some form of disruption forcing a vessel outside its expected operating profile is greater than that of any single event, and they can prepare for the consequences. By combining appropriate coatings, continuous prevention and clear inspection and maintenance procedures, they can reduce the risk of an extended delay leaving a vessel with a fouling problem that compromises its performance or readiness to return to service.

FACT BOX: Australian sister-vessel study – the value of continuous prevention

An Australian shipowner initiated a comparative study involving three nearly identical vessels operating along Australia's eastern coast. Sonihull was installed on one vessel in 2024, protecting its propeller, sea chest and selected hull areas, while the other two remained unprotected by the ultrasonic system.

The owner supplied operating data, including 2025 noon reports, which was analysed in collaboration with Sonihull to compare fuel efficiency across different speed bands.

The results showed that the vessel equipped with Sonihull achieved:

• 4-5% better fuel efficiency across all speed bands.
• In the 12-14 knot range, fuel efficiency difference widened to 12-16%.
• Estimated annual fuel savings of 403 tonnes, equivalent to approximately USD 242,000 at a fuel price of USD 600 per tonne.
• An estimated investment payback period of 6.6 months.

Propeller slip measurements provided additional supporting evidence of a performance difference between the vessels. The Sonihull-equipped vessel averaged 3.4% slip during 2025, compared with 9.25% and 5.04% on the two sister vessels.

Following the study, the owner opted to extend Sonihull protection to the other sister vessels.

The results reflect these particular vessels, their operating conditions and the data available. They indicate the potential value of continuous prevention, but should not be treated as a universal fuel-saving claim, as proof that biofouling alone caused the full performance difference, or as evidence that Sonihull alone accounts for the result. Nor are they a direct measure of performance during prolonged idling.

Read the case story

FACT BOX: How Sonihull works

Alfa Laval Sonihull uses ultrasonic transducers fitted to the dry side of a vessel's structure. Controlled acoustic pulses travel through the material, creating microscopic agitation at the wetted surface that disrupts biofilm formation and inhibits the attachment of marine organisms.

The low-power system operates continuously, whether the vessel is sailing or stationary, without releasing biocides into the water. Installation requires no through-hull penetrations. The system uses a similar amount of electricity as a standard desktop computer.

Coverage and installation:

• Each transducer has a theoretical effective radius of approximately 4-6 metres, although actual coverage depends on the vessel's structure and the cumulative effect of multiple transducers.
• Sonihull's naval architects examine the vessel's structural drawings and use simulation software to determine the optimum number, configuration and positioning of transducers for each installation.
• On large commercial vessels, 20-40 transducers are typically sufficient to protect critical areas, including the propeller, sea chests, bulbous bow and aft flat bottom.
• On smaller vessels, including offshore support vessels and tugs, full-hull protection may be appropriate, requiring up to around 40-50 transducers, depending on the vessel.

The number and placement of transducers depend on the vessel's design, operating profile and protection requirements. Full-hull coverage is not necessarily the most effective investment for every vessel.

Targeted installations on propellers and sea chests can often be carried out during normal port operations, while more extensive hull retrofits depend on the vessel's layout and access requirements.

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