Antifouling that reduces the need for regular stripping, sanding and repainting has broad appeal. But is ultrasonic antifouling the answer?

N.B. This article was first published in the January 2015 issue of PBO and some of the products below have now been discontinued.

Ultrasonic antifouling has been around in the leisure market for nearly 10 years now. The idea sounds promising: an electronic device mounted to your boat’s hull produces sound waves that discourage fouling from establishing a hold.

When PBO last tested ultrasonic antifouling in 2009 there were only two companies on the market, but interest has grown in the technology and by spring 2014 there were five main companies offering systems of varying price and specifications.

Ultrasonic is not billed as a replacement for antifouling paint, rather as a companion that will extend the paint’s life and reduce the fouling that builds up. But does it work? We acquired systems available on the UK leisure boat market and, with the helpful assistance of Hamble School of Yachting, put them to the test over the summer of 2014.

This was a bad year for fouling on the Hamble, with hot weather and warm water making ideal conditions for fouling growth. It would be a tough test for the ultrasonic devices. Fouling consists of waterborne micro-organisms and bacteria attaching themselves to a hull and then multiplying to create a bio-film layer of slime.

Algae cells attach in the same way and develop into seaweeds. This ecosystem provides an ideal food source and environment for other growth including colonisation of barnacles and other fouling.


How ultrasonic antifouling works

Ultrasound has many uses – from pre-natal scans to cleaning metal components – but using it to keep fouling off leisure boat hulls is a relatively new development.

Ultrasonic antifouling systems work by emitting a low-powered, pulsed ultrasonic frequency from a digital control unit, via transducers that are attached to the inside of the hull.

The hull acts as a sound board, carrying the sound waves and creating a microscopic environment of moving water molecules over the underwater profile of the hull. The cell structures of the algae and micro-organisms cannot survive in this environment and as a result fouling growth is discouraged.

The systems comprise a control unit and a number of transducers. The transducers must be bonded to the hull, which in practice means grinding away paint from the relevant area and epoxying them in place.

Some use a gel or a liquid to create a contact between hull and transducer, while others rely on a bubble-free layer of epoxy paste. Wires are then led back to the control panel which is connected to a power source – either the boat’s batteries or a switchable battery/shore power system.

All the systems we tested will switch themselves off to protect your batteries if voltage drops too low. Current draw while pulsing was between 0.6A and 0.8A. The average is much lower as the pulses are of short duration. All are designed for DIY installation.


PBO’s Ben Meakins dons a wetsuit and aqualung to study the effectiveness of ultrasonic antifouling in the underwater environment

How we tested the ultrasonic antifouling

With many variables involved – sunlight, boat use, location – it would be tricky to test the ultrasonic antifouling systems on an even footing in exactly the same conditions.

We minimised variables as much as possible by enlisting the help of Hamble School of Yachting, which has a fleet of 14 yachts based at Mercury Marina on the River Hamble, and who kindly allowed us to kit out some of their boats for the test.

We selected near-identical yachts, all GRP-hulled, fin-keeled modern cruisers – Dufour 36, Jeanneau Sun Fast 37 and Jeanneau 37s. We then invited the ultrasonic antifouling manufacturers to visit the Hamble and install their system on an assigned boat.

Two boats were kept ultrasonic-free, and we monitored these as control units for comparison. All the boats were antifouled with the same antifouling paint – Hempel Cruising Performer, in blue or black, and applied within three months at the start of 2014.

Note: We may earn a commission when you buy through links on our site, at no extra cost to you. This doesn’t affect our editorial independence.

Once the ultrasonic units were installed in early June, we cleaned each boat’s bottom using a soft foam attachment to a Brizo hull brush to remove any pre-test slime.

Each yacht received broadly similar use over the summer of 2014 – typically Solent use during the week for charter and tuition, followed by a rest/turnaround period back on its mooring at Mercury Marina. There were occasional trips further afield on charter.

Come 6 November, and after five months in use, we went back to see how each ultrasonic antifouling system had performed.

Accompanied by underwater photographer Tristan Jones, I donned a wetsuit and scuba kit and dived to examine each boat and the fouling present on the hulls, before checking out each system’s installation and characteristics in use.

We measured the noise of each system’s transducers with a noise meter placed at head height in the sleeping cabins.


Thick green weed was present along the boats’ waterlines

The control boats – no ultrasonic antifouling installed

Our control boats for the ultrasonic antifouling test were the Dufour 36 Chill Factor and the Jeanneau 37 Orinoco Flow. These two boats had the same antifouling paint applied at the start of the season as the boats fitted with the ultrasonic systems, and received broadly similar use over the summer.

As mentioned in the introduction, the summer of 2014 was a bad one for fouling on the South Coast – the often sunny conditions and warm water proving ideal for promoting fouling growth.

It was no surprise, then, to find fouling was present, with a stubborn slime on all underwater surfaces of the two control boats. To remove the slime, it needed fingernails or a scourer to get back to the paint.


Extensive barnacle growth was present on both propellers

There was additional green weed growth along the waterline, and in particular at the bow down to the forefoot. The rudder and keel had some weed growth on their trailing edges. The propellers were fouled with some shell growth and heavy slime.

The saildrives and P-brackets also had some weed and shell growth. Compared with the boats with ultrasonic antifouling systems installed, the control boats had thicker weed along the waterline

Thick green weed was present along the boats’ waterlines and at the bow. The slime was also more stubborn to remove and there was more shell growth on the propeller and stern gear.


There was heavy green weed present around the forefoot


The rudder and keel had thick slime with sporadic weed growth

4 of the best ultrasonic antifouling systems tested


The control box was unobtrusive

Sonihull (NRG Marine)

The Sonihull system comprises a single combined control box that runs on both 12V and 240V. In this case it was mounted behind the boat’s switch panel, driving two transducers, one mounted in the forepeak and another aft of the engine, close to the centreline in the owner’s cabin.

The transducers are low-profile, which means they can be mounted in a shallow bilge as found on most modern production boats like the Dufour 36, and produced a buzz-click sound when activated.


The forward transducer was mounted next to the speed/depth transducers

The peaks were 11dB above ambient noise. This system was less intrusive in operation than others, but was still found to have been switched off by charterers for part of the test period.

The results

Classic Dream had light slime covering the whole hull, with some brown weed on the trailing edge of the rudder and keel. There was some shell growth on the propeller.

The bow had some green weed growth on the waterline and down to the forefoot, and there was some waterline growth further aft. There was no shell growth on the boat’s hull.


Slime was light – note the bare patch, caused by a gloved wipe


On the keel and rudder the slime was slightly thicker


There was some barnacle growth on the propeller boss and blades


The weed at the bow was slightly lighter than average on this boat

Price as installed: £1,338
Boat: Classic Dream (Dufour 36)
Peak current draw: 0.6a

Buy it now on Amazon


UltraSonic system has a pair of control boxes which can accept AC power as well as DC

UltraSystem Series II

This installation had two control boxes mounted underneath the chart table to run the system on 12V or 240V shore power. The control boxes drive two transducers simultaneously, and all three – the two transducers and control box – emitted a regular Morse code-like clicking when engaged. The peaks were 14dB above ambient noise.

Charterers and skippers reportedly found this to be disruptive through the season – Hamble School of Yachting staff sometimes found that the wires had been unplugged when boats were returned. They therefore made sure to always plug them back in to re-enable the system.


One of the UltraSonic transducers fitted to test boat Impala

The transducers are the lowest-profile of all on test and could be mounted in a shallow bilge if required.

The results

Impala had a light slime, below average, covering the hull, which wiped off easily with a gloved hand. There was some weed on the trailing edges of keel and rudder, and some slime and shell growth on the propeller boss. There was some heavier green weed along the waterline and at the bow.


Keel and rudder had some slime, with occasional weed


There was heavy weed growth near the waterline at the bow

Price as installed: £1,378
Boat: Impala (Dufour 36)
Peak current draw: 0.7a


The Citadel Products control box

Citadel Products

Citadel Products’ system comprised a control box and a two transducers. The box had a bright LED that lit up when activated, as did the transducers. The system will run on 12V DC and 240V AC. The transducers were fairly tall and needed a reasonable depth of bilge for installation.

This system had a very different sound to the others on test, an ascending scale followed by a quiet click – but we found it was too quiet to measure with our noise meter.


The transducer needed some depth of bilge to install

And despite the range of sound generated, crews found it to be by far the least intrusive of all the units we tested – with the result that it stayed switched on for the whole season.

The results

Quantum Leap suffered a sheared seacock and had to be lifted out in an emergency two weeks before the end of the test. This gave us a good opportunity to observe the bottom. It was covered with slime and some weed growth – at the bow, especially – but this had stuck to the bare gelcoat above the antifouling line.

Along the waterline was some tough green weed, but less than average on the Citadel Products control box test. Keel and rudder were relatively clean, with some weed. The propeller, a shaft and P-bracket type, had moderate weed and slime growth. Generally though, the boat had average growth for the test.

Weed growth was above the waterline of the antifouling paint

Slime looked fairly heavy – but was easy to remove

Fouling was fairly heavy around the prop and P-bracket

Some green weed had colonised along the waterline

Price as installed: £599
Boat: Quantum Leap (Jeanneau Sun Fast 37)
Peak current draw: 0.8a

Buy it now on eBay

Pressing a switch disables the SonicShield system for 12 hours so crew can sleep without the periodic ‘click’ of transucers (below)

SonicShield III

The SonicShield system comprised a control box, mounted in an aft cabin, and included a mobile device that means it can be remotely monitored. Three transducers were installed; one in the forepeak and one in each of the aft cabins.

This system has a momentary rocker switch which disables the system for 12 hours, to allow crew to get a good night’s sleep. Its transducers also proved to be the second-quietest on test, with peaks 8dB above ambient noise.

The results

Shalingar’s Luck had light slime present – slightly less than average for the test – and I found this slime could be easily wiped clear with a gloved hand. There was some weed present on the waterline, but less than average. Some weed was also present on the keel and the rudder.

The propeller was clean, but we later found that it had only been installed a week prior to the end of the test after the old one was damaged. The transducers are a similar design to UltraSonic Antifouling’s and are epoxied to the hull.

SonicShield’s manufacturers offer this system on a ‘no risk trial’, where customers can return the system after 6 months for a refund if they’re not satisfied with the results.

Light slime was present on much of the hull but could be wiped off

Keel and rudder had a similar level of slime – note the glove wipe mark

Heavy weed was present at the boat’s bow and forefoot

Price as installed: £1,198.80
Boat: Shalingar’s Luck (Dufour 36)
Current draw: 0.7a

PBO verdict: Which was the best ultrasonic antifoul on test?

None of the ultrasonic systems produced a completely clean boat at the end of the test – but then we didn’t expect them to in what is a particularly high-fouling area. These sailing school boats received a higher usage than your average sailing cruiser, which the manufacturers claim will have reduced the difference between test boats.

As it was there was a noticeable, if slight, difference between the two control boats and the ultrasonically-protected ones – the control boats had thicker slime and slightly higher waterline weed growth.

On the ultrasonic boats the slime was noticeably lighter and easier to remove – a plus point for motor boaters, whose higher speed might therefore shift the residual fouling more easily than a sailing boat. Manufacturers also say that hard antifouling is beneficial to the performance of the system.

On all the hulls – protected ultrasonically or otherwise – there was no barnacle or other shell growth, although there was some growth on all bar one of the propellers, which turned out to have been replaced at the end of the season.

Stern gear is always likely to prove more of a problem as it is remote from the hull – the ultrasonic waves propagate through the hull laminate. All boats had green weed growth at the waterline, particularly at the bow from the waterline down to the forefoot.

Slime coverage was slightly lighter with SonicShield III and UltraSonic Antifouling, but Sonihull had marginally less weed growth on the bow. The Citadel Products system was relatively unobtrusive in terms of noise, had comparable levels of weed and slime, and has a substantial price advantage.

Sound levels

What was interesting was that skippers, crew and charterers complained of the noise made by most of the devices. In some cases they disconnected them to stop the clicking and get a good night’s sleep (the SonicShield gets around this issue with a simple switch that disables the system for 12 hours).

Temporary switch-offs will, of course, reduce the efficacy of the system, but having people aboard is a useful ‘real-world’ test. Hamble School of Yachting checked and, where necessary, reactivated the systems after each charter.

Worth the cost?

Whether or not you should shell out for ultrasonic antifouling instead of one of the best antifouling paints will depend on your circumstances and the depth of your pockets.  The results show that the systems seem to make a small difference, and make removing slime from the hull easier.

Whether the reduction in slime and weed fouling seen in our test justifies the cost is up to you, but a further test in a higher-risk area for barnacle growth would be informative. Reports suggest that they are more effective against barnacle growth.

Power requirements mean you’ll need to be on shore power or, if on a swinging mooring, have enough solar charging capacity to keep them going all season.

Many thanks to Rob Gaffney and Chris Rushton at the Hamble School of Yachting who allowed us to use six of their 14-strong fleet of yachts at Mercury Marina on the River Hamble. Hamble School of Yachting run a full range of RYA courses and offer boats for skippered and bareboat charter in the Solent.

Why not subscribe today?

This feature appeared in the January 2015 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

Subscribe, or make a gift for someone else, and you’ll always save at least 30% compared to newsstand prices.

See the latest PBO subscription deals on