How to install Ultrasonic Antifouling and why I chose it to beef up my protection

The Baltic’s nutrient-rich waters forced Sam Fortescue to consider an eco-friendly alternative to conventional antifouling paint… Ultrasonic Antifouling

You might recognise the feeling – a sense of deepening anxiety as the moment for your boat’s annual haul-out finally approaches. What state will the hull be in? And what exotic forms of marine fouling will have taken up residence this time round?

For me, this had become a form of annual humiliation where I keep my boat in the River Schlei on Germany’s Baltic coast. The locals here gaily slap on their favourite shade of eroding antifoul every spring and suffer nothing worse than a bit of slime. But the Coppercoat on my Sadler 34 just couldn’t keep up with the brackish, nutrient-rich waters, where a riot of sea squirts, weed and barnacles would take up residence within a few days of my boat going back into the water each year.

To be fair to the Coppercoat, I had excellent results for years in UK waters and during a year-long Caribbean circuit. I know the product works well. But there is something a bit funky about the Schlei, and even the locals admit that it presents a ferocious test.

It had turned my boat into the laughing stock of the marina, with other berth holders gathering round to offer, by turns, sage advice (usually ‘Oh, I wouldn’t use Coppercoat here if I were you…’) and dry commentary to the effect that I should open a seafood restaurant.

The joshing was one thing, but the fouling also entailed a lot of work. There was no question of turning the pressure washer onto the hull until I’d scraped every square inch with a specially adapted metal tool. It was very effective at dislodging the barnacles, which rained down with a spattering of fishy slime, but it often left behind discs of calcium where they had adhered themselves to the hull.

A good blast from my Kärcher pressure washer was the next step, eliminating perhaps 90% of the discs, but the final act was a painstaking all-over scrape with a spatula blade or some kind of trowel. All in all, it constituted two to three hours of work on a day that was inevitably cold and wet.

So, a couple of seasons ago, I decided that this state of affairs would have to change…

Alternating pressure

Having found Coppercoat to be very effective for many years, I was reluctant to abandon it. Because the coating is based on finely milled copper held in a suspension of epoxy resin, it is hard and durable and can last more than a decade. I therefore wasn’t prepared to simply paint over it with the same eroding or ablative antifouling that everyone else appeared to use.

In fact, it seemed a waste to cover the Coppercoat with any sort of coating. Instead, I decided to beef up the protection by adding ultrasonic antifouling.

Installed inside the boat, ultrasonic antifouling has been fairly well documented over recent years and can’t be described as being a particularly novel solution, but I had read that it can be an effective way of turbo-charging an exterior coating in high-fouling areas.

It works using transducers which are carefully bonded to the inner surface of the boat’s hull, whether carbon or glassfibre, wood, steel or aluminium. They use high voltages to generate waves of high-frequency ultrasound (20-200kHz) which reverberate through the hull, dislodging nascent fouling, just as it is starting to bond.

It’s a little more complex than simply wobbling the fouling off. As the soundwaves spread into the water around the hull, they create bands of alternating high pressure and low pressure. During the low pressure phase, miniscule vacuum ‘bubbles’ are generated, which then collapse violently – just like cavitation around a propeller. This generates tiny areas of very high pressure and heat which disrupts the fouling. Both algae and larger organisms are prevented from getting their first foothold on the hull.

Multiple transducers

Measuring up my boat and the area of the hull, it became clear that a single transducer wouldn’t be enough to give full bow-to-rudder coverage. I was starting to incline towards an Australian brand called CleanAHull, which seemed to blend the best functionality, with a relatively low current draw of 230mAh per transducer and a reasonable price of around £510.

I also liked the fact that it could draw its power from the 12V house battery bank, and that there were preset sleep modes which would switch the system into standby mode, drawing just 15mA, for three or six hours at a time. Since plant fouling is most active during the day, I was keen to be able to switch the transducers off at night in case the noise was more audible than the marketing blurb suggested.

It was also encouraging that the system could monitor and report on its own temperature and identify faults.

Each transducer output has its own fuse while the main unit and the transducers were sealed to IP65 and IP68 respectively.

Although described as simple to fit, I had a number of problems straight off. The first entailed a hefty customs payment to see my shipment from Australia released. The manufacturer had underestimated the tenacity of the German customs officials, who refused to accept the AU$100 (£50) valuation cited on the declaration.

Ultrasonic Antifouling Installation challenges

The second issue was the fact that Sadler Yachts employed a unique building technique that makes it hard to reach the inner skin of the outer hull. The boat’s interior is a separate fibreglass moulding sealed to the inner hull using expanding foam. The idea was to provide both insulation and buoyancy, allowing Sadler to describe the yacht as unsinkable – a reputation they enjoy to this day.

But for my purposes, it added an extra layer of complexity because you have to bond the transducer cleanly to the outer hull. In my case, that would mean cutting away a big section of the interior moulding around my chosen installation site, with no certainty as to whether or not I would find a suitable substrate.

In fact, at the best spot for the aft transducer, near the P-bracket, it was far from clear whether there was any interior moulding at all, or whether it was just very heavily laid up.

A tap test indicated hollow space behind the GRP, but my reluctance to start hacking away at the boat’s interior led me to put off the work until after she was relaunched. I therefore didn’t pluck up enough courage to do the job until after she was actually in the water – at which point the clock was ticking…

Cutting into the hull

I began with a fine 2mm drill bit, reasoning that I could quickly undo the damage if it did turn out that I was piercing the hull itself. After half a centimetre, I began to sweat. Then a great drop of liquid welled up like a tear around the hole and I stopped abruptly. But it didn’t herald an endless jet of water, indicating that I hadn’t holed the boat after all. What’s more, it didn’t taste salty. Cautiously I continued and discovered that this was simply a little pocket of condensation that had been squeezed up from under the foam layer.

CleanAHull recommends that you cut a 150mm hole through any inner moulding to give you enough space to prepare the surface and fit the transducer. I had the appropriate hole saw ready for my cordless drill, and applied it with a little more confidence.

It still made a hellish noise and tore up great strands of glassfibre.

As the circular section came away, it was evident just how uneven the original lay-up had been – just a few millimetres thick at one edge and 5-6mm at the other.

Site preparation

With huge relief, I was able to prise away much of the foam with the loose disc of GRP. The rest I could remove with a Stanley knife and chisel, quickly revealing the inner skin of the outer hull.

CleanAHull is quite clear that the success of the installation rests on the quality of the surface preparation, which must be sanded with 60-grit paper to remove any surface oxidation and gelcoating. This is where access becomes absolutely critical, because it’s best to use a disc sander for the job.

In my case, this just wasn’t possible, which meant that I had a much tougher time of hand sanding.

After you’ve revealed the GRP, you need to clean and degrease the surface. The next step was to mix up the J-B Weld two-part epoxy adhesive which bonds the transducer to the hull. CleanAHull supplies a 50mm metal disc with a threaded stud for installing the transducer on a sloping surface. It is lightweight and doesn’t risk slipping or sagging while the epoxy cures over 24 hours. But if you’re installing near the horizontal, it’s recommended that you don’t bother with the disc.

Either way, it is essential that you let the epoxy stiffen for 10 minutes before lathering it onto the hull. You then twist and press the transducer into the adhesive to squeeze out any air pockets that might weaken the sound waves later on. I then taped it down to bond it. The second transducer was easier to fit in a locker under the saloon seating, although I had to go through all the same steps again.

Electrical connection

The electrical connection was relatively straightforward. Ideally, you want the unit to work even when you’re away from the boat – it must bypass the main battery switch – so connect it directly to a battery terminal or busbar. You must make sure the positive cable is fused (10A), and don’t connect up the power until the transducers are wired in using the watertight glands.

As ever on an old boat, running the wires neatly took the bulk of my time.

I decided to mount the control unit in the port lazarette next to the shore power inlet and battery charger. It had proven to be dry enough and gave relatively easy access for cable runs under the galley and through into the aft bilge. The last job was to paint the exposed foam around the transducers with a sealant, in case of water spills.

The results

In the first full sailing season post-Covid, we made good use of the Sadler 34 with lots of sunny weekends and a longer cruise around Denmark’s incomparable South Fyn Archipelago.

I dived several times under the boat to check for hull growth and found little to be concerned about – just some slime around the waterline and on the rudder. There were no barnacles to be seen.

There was perhaps a six-week period at one point where the boat didn’t get much use, and that did have an effect. By the time we hauled her out, there were a few small barnacles, but no sea squirts. It was the quickest and easiest hull clean that we’ve had since leaving Poole Harbour back in 2018.

But it hasn’t all been problem-free.

If there is any kind of interruption to the marina power supply, or a selfish visitor takes a chance on unplugging my shore power for a day or two, it gives the fouling a foothold on the hull. By the same token, a battery in poor condition will not provide the power that the system needs and will generate ‘low voltage’ errors.

And if the boat spends long weeks lying untouched in a marina berth, it allows superficial fouling to build up, offering a way in for the more persistent stuff. In particular, it is worth giving the waterline a quick scrub a couple of times during the season.

With regular use, the boat will now keep a clean bottom with just a quick scrub every now and then along the waterline. A decent boat speed also helps to scour light fouling off the hull. But beware of long periods lying idle in the berth. For boats kept in British waters, strong tides and a swinging mooring would probably improve matters in this respect. Unfortunately for me, however, the River Schlei got its revenge in a year when the boat sadly saw very little use.

My final verdict? Overall, good – but with caveats!

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