Ultrasonic antifouling for cruise ships and passenger vessels: Technology deep-dive

Marine biofouling is the gradual build up of marine organisms on submerged surfaces. On passenger vessels, this typically means biofilms, algae, barnacles and mussels on the hull, thrusters, propellers and other underwater areas.

The performance impact is significant. The GEF-UNDP-IMO GloFouling project, summarised in the GESAMP report Marine biofouling: non-indigenous species and management across sectors, notes that even a biofilm layer as thin as 0.5 mm covering up to 50 percent of a hull surface can increase fuel consumption by around 20 to 25 percent, with heavy fouling pushing fuel use close to 50 percent higher compared with a clean hull. You can see this data in the GloFouling report via the IMO site here.

These drag penalties quickly translate into higher fuel bills and more greenhouse gas emissions. They also make it harder to comply with port state biofouling rules that require ships to arrive with clean or low fouling hulls. For example, New Zealand’s mandatory Craft Risk Management Standard requires all international vessels, including passenger ships, to arrive with a “clean hull” and to provide evidence of biofouling management before entry. California and Australia likewise require ships to document biofouling management practices for hull and key underwater areas and to submit biofouling information prior to arrival in their waters.

Ultrasonic antifouling is one of the few non toxic technologies that can prevent this build up before it starts while helping passenger vessel operators stay aligned with tightening antifouling and environmental regulations.

In this guide you will see how ultrasonic antifouling works on passenger vessels and how systems like Cathelco’s USP DragGone can support cleaner and more efficient operation of cruise ships and ferries.

Understanding ultrasonic antifouling technology

Ultrasonic antifouling uses high frequency sound to stop organisms from establishing on the hull and other wetted surfaces.

A typical system for a ship includes three main components:

• power control units that generate ultrasonic frequencies
• junction boxes that distribute power and signals to multiple transducers
• ultrasonic transducers bonded to the inside of the hull or structure

On passenger vessels, the transducers are installed on the inside of the hull plating. They transmit vibrations through the steel structure into the wetted surface. These vibrations create alternating pressure that makes it hard for microorganisms to attach and form a biofilm. Without that first biofilm layer, larger fouling organisms find it much harder to settle and grow. This principle is explained in Cathelco’s ultrasonic antifouling buyer’s guide, available at https://evac.com/ultrasonic-antifouling-guide/.

The GESAMP technical report on marine biofouling describes acoustic antifouling systems as operating in the ultrasound range above 20 kHz, using local vibration and microbubble formation to disrupt fouling films at the surface. Studies reviewed in that report show that ultrasonic devices can significantly reduce settlement of fouling organisms, including a sea trial on the shell plating of a large ship over a four month period.

Advanced systems such as Cathelco’s USP DragGone use guided wave and heterodyning technology so that energy travels inside the hull structure instead of dissipating into the water.

Heterodyning means the system generates and combines two or more ultrasonic frequencies to create additional mixed frequencies at the steel and water interface. Different fouling organisms and life stages respond more strongly to different frequency ranges and pressure changes, so using a wider spread of frequencies increases the chance that, at any given moment, conditions at the hull surface are disruptive for their attachment.

As a result, a broader range of fouling species finds the surface unsuitable and is prevented from settling.

In Cathelco’s own data, each guided wave transducer location can protect a radius of around 25 m, approximately 2,000 m² of hull area, which is up to 40 times more than traditional point ultrasonic devices that cover only about 50 to 80 m² around each unit.

Environmental benefits for passenger vessels

Reducing harmful biocides

Traditional antifouling paints rely on biocides. The GESAMP report notes that the majority of ship hull coatings in the global fleet are still biocidal, meaning they continuously release metals and organic biocides into surrounding waters with potential toxic effects for marine organisms.

The International Convention on the Control of Harmful Anti-fouling Systems on Ships (AFS Convention) was adopted by IMO in 2001 to ban organotin systems such as TBT and later restrict other harmful substances, including the biocide cybutryne. Passenger ships are directly covered by this convention. A short overview is available on the IMO site here.

Ultrasonic antifouling does not release chemicals into the sea. For cruise ships and ferries that regularly call at ports with strict environmental standards or sensitive areas, this helps reduce the overall biocide footprint and can support a move to lower toxicity foul release coatings where operational profiles allow.

Alignment with biofouling management guidelines

The IMO Guidelines for the control and management of ships’ biofouling to minimise the transfer of invasive aquatic species encourage every ship type, including passenger vessels, to maintain a biofouling management plan and record book and to use effective antifouling systems on hull and underwater appendages.

The guidelines recognise that, in addition to coatings, other technologies such as ultrasonic systems can play a role in keeping hulls clean. For operators facing biofouling inspections in regions like New Zealand, California or parts of Australia, an ultrasonic system that helps keep the hull closer to a clean state can make it easier to demonstrate good practice based on visible condition and documented management.

Benefits for sustainable passenger vessel operations

Lower environmental footprint from drag and fuel use

The GloFouling study results show that:

• a biofilm only 0.5 mm thick covering up to 50 percent of a hull can cause roughly a 20 to 25 percent increase in fuel consumption
• heavy macrofouling can increase fuel consumption by nearly 50 percent

These figures apply to ships in general and therefore to cruise ships and ferries. Since fuel use is the main source of CO₂ emissions for passenger ships, any reduction in drag directly reduces emissions.

By preventing the formation of early biofilm, ultrasonic antifouling slows down the rate at which roughness and drag increase between dry dockings. This helps passenger vessels operate closer to their intended energy efficiency over the coating life and contributes to the decarbonisation goals reflected in the Fourth IMO GHG Study .

Energy efficiency and speed performance

Schultz and co authors, summarised in the GESAMP report, have shown that fouling related drag penalties can drive fuel costs to levels that are much higher than the cost of fouling control measures themselves. On passenger routes with fixed schedules, a fouled hull may require higher engine power to maintain speed, further increasing fuel consumption.

Because ultrasonic antifouling acts at the initial settlement stage, it supports:

• lower average hull roughness over time
• more predictable fuel consumption and speed performance on regular routes
• reduced need for frequent in water cleaning to restore performance

Financial considerations for cruise ships and ferries

Fuel cost influence

For high utilisation passenger vessels, fuel is one of the largest operating costs. As the Cathelco examples for cruise ships and ferries show, even percentage savings in the low double digits translate into hundreds of thousands to millions of dollars per year at typical fuel spend levels.

Actual savings for a specific vessel will depend on route, speed profile, coating, cleaning regime and how heavily the hull would foul without ultrasonic protection. However, the relationship is straightforward: less fouling over time means less drag, and less drag means lower fuel consumption for the same service profile.

Our estimates give a sense of scale for passenger vessels:

• for a cruise ship with a typical annual fuel bill of about 8 million US dollars, Cathelco estimates that using DragGone as part of a hull fouling management strategy can save up to 1.4 million dollars per year in fuel, roughly 13 percent, which over ten years corresponds to around 14 million dollars and about 61 kilotonnes of CO₂ avoided in total
• for a ferry with a typical annual fuel bill of about 5 million dollars, the same modelling suggests savings of up to 0.6 million dollars per year, again around 13 percent, which over ten years corresponds to about 6 million dollars and roughly 31 kilotonnes of CO₂ avoided

These figures are based on Cathelco’s modelling for representative cruise and ferry operations and show what is possible when ultrasonic antifouling is combined with good hull maintenance. You can use your own ship data to calculate your own savings here: https://evac.com/draggone-calculator/.

Reduced maintenance and coating wear

Conventional antifouling strategies often rely on cycles of biocidal coating application and in water or dry dock hull cleaning. The GESAMP report notes that hard mechanical cleaning of biocidal coatings can damage the coating, increase roughness and generate paint particles that act as microplastics and additional sources of residual biocides in the environment.

Because ultrasonic systems are preventative rather than reactive, they:

• slow down fouling accumulation
• can extend the time between major hull cleaning or polishing
• reduce the need for harsh mechanical scrubbing to recover performance

For cruise ships and ferries that operate on fixed schedules and tight dry dock windows, this reduction in unplanned cleaning and coating damage has clear operational and cost benefits.

Integrating ultrasonic antifouling in new passenger vessels and cruise ships

For new cruise ships and ferries, one of the most effective ways to use ultrasonic antifouling is to consider it during the design stage, so that transducer locations and cable routes can be planned sensibly.

From Cathelco’s USP DragGone configuration examples, the key technical point is that correct transducer placement is vital for effective coverage and performance on larger passenger vessels. Although installation work itself is straightforward for a yard or competent contractor, Cathelco recommends that the design of the layout and the final positioning of transducers are carried out or approved by a Cathelco certified technician to ensure that:

• hull and propeller areas are covered in line with the system design
• guided wave paths in the hull structure are used effectively
• the number of transducers is optimised for the area to be protected

This approach keeps the practical work simple for the owner or yard, while making sure the system performs as intended.

Retrofitting existing passenger vessels and cruise ships

Ultrasonic antifouling can also be added to existing cruise ships and ferries without the need for hull penetrations or hot work.

In practice, system installation has a few straightforward requirements:

• transducers are bonded to prepared steel on the inside of the hull using a suitable adhesive
• no hull penetrations are required for the transducers
• no hot work is required for transducer installation

Because the transducers are installed on the inside of the structure, the work does not depend on dry docking. Installation can often be scheduled while the vessel is alongside, as long as internal areas are accessible and the external hull is in a clean condition. This is because ultrasonic antifouling does not remove pre-existing fouling, but rather prevents new fouling attachment.

Once installed and commissioned, the system operates in the background and supports:

• reduced need for frequent in water cleaning
• less reliance on aggressive mechanical cleaning that can damage coatings
• continuous protection in port and at anchor, which are often high risk periods for fouling settlement on cruise ships and ferries

In higher fouling regions, passenger vessels can combine ultrasonic antifouling with modern low friction or foul release coatings. The coating provides a smooth, low energy surface, while the ultrasonic system disrupts biofilm formation, making it easier to maintain near clean hull conditions between scheduled dry dockings.

Are you ready to embrace ultrasonic antifouling for your fleet?

Marine biofouling has direct, measurable consequences for passenger vessels in terms of fuel consumption, emissions and compliance with emerging port state biofouling rules. GloFouling and GESAMP assessments show that even early-stage slime can drive fuel penalties of around 20 to 25 percent, with heavy fouling pushing that close to 50 percent.

At the same time, regulatory pressure on harmful antifouling chemicals has increased under the AFS Convention, and the IMO Biofouling Guidelines are being used by several jurisdictions as the basis for inspection and control of hull fouling on all ship types, including passenger ships.

Ultrasonic antifouling offers passenger vessel operators a practical way forward that is:

• non toxic and aligned with AFS Convention objectives
• compatible with modern coating strategies
• suitable for both newbuilds and retrofits of existing passenger vessels

For operators of cruise ships and ferries who are looking to cut fuel, reduce emissions and move away from heavy reliance on biocidal paints while meeting clean hull expectations at key ports, ultrasonic antifouling is directly relevant and already supported by real world use in the passenger segment.

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