Ultrasonic Antifouling

Engine keel cooler fouling problem shown after one year in the water.

Earlier in the winter I got a very interesting email from my friend Bob who lives for much of the year on a Nordhavn trawler yacht, having retired from skippering and then running a fleet of fishing boats.

Bob also had a sailboat that he commuted between Maine and the Caribbean and the Bahamas on for many years. Bottom line, Bob has serious experience and is worth listening to:

Ever since I purchased this Nordhavn I’ve been plagued by serious fouling on my two coolers. The main engine cooler, a Fernstrum, can’t be coated with anything to prevent fouling for fear of compromising its primary function, that of cooling the John Deere [main engine]. The other cooler [generator I think] is made from titanium tubes and does the job even when completely fouled…

…One Sonihull transducer didn’t do much but adding a second did the trick.

After installing the sonic antifouling unit. Same time in the water, hauled at the same boatyard.

The units Bob used are from Sonihull and are available from the good people at PYI.

Before anyone gets too excited and starts thinking this is the end of antifouling, do note that Bob still uses antifouling on the rest of the hull.

From everything I hear, protecting a whole boat with these units is a different case with varying results. Charlie Doane tried this some years ago on his last aluminum boat with inconclusive results. Not sure if he has installed it on his new Boréal 47. Charlie?

My admittedly limited reading seems to confirm that ultrasonic units are not a replacement for antifouling paint, but it does seem that for special situations, like Bob was faced with, they have their place. They might also make paint last longer on the whole hull, or at least keep shell off.

However, I do wonder about unintended consequences. In this case possible stray current issues (probably solvable with good installation) and the effect on welds of ultrasonic vibrations for many years.

And, no, I have nothing to base that on other than I have been bitten on the ass so often that I always wonder about unintended consequences.

If you are interested, Googling “ultrasonic antifouling boats” (without the inverted commas) will yield enough information to keep you occupied until the current lockdown is but a distant memory.

And, no, I did not analyze much of it. I have a refit budget series to write and I’m deep into shorefasts, too, so too busy to take on another in depth project.

Update 4th April: Member, Stein has done some really good research into this and shares that in an excellent comment, don’t miss it.

Talking of which, I’m deeply grateful to all or you AAC Members who are keeping us afloat through this crisis by continuing to renew and, at the same time, giving me something meaningful to do—Phyllis and I are hugely fortunate.

Update 6th April 2010

We now have solid reports from members Steve D and Peter F that confirm that while this technology, at least in it’s presently available state, is useful in the way Bob is using it to keep a relatively small area clean, it would take an impractical number of transducers to be effective for an entire hull.

Further Reading


Anyone out there have any first-hand experience with these things? If so, please leave a comment.

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James Greenwald

Hi John,
You are a great help to many like myself who during these times cannot get out on the water and really need your articles to keep us going. Also open your window and give a shout out to Chris for his most recent podcasts.

Richard Ritchie

Thanks for raising this. I worry about the wider consequences of antifouling including ultrasonics. For this reason I have standardised on Coppercoat for my last two boats. And it works as well as anything.
My concern with ultrasonics is for wildlife (before even considering whether it works) I am not willing to add to noise pollution of the undersea. Logically, the dolphins and crustacea won’t like it.
I have tried on several occasions to get a scientific response on this, but silence from the vendors, which adds to my concern.
Has any member evidence that this is sea-life friendly other than for growths?

Richard Ritchie

Agreed. In pristine environments I don’t want to be deafening the wildlife! I just can’t find out whether it does.

Stein Varjord

Hi John,
I’ve been interested in this and investigated the options the last three years. Last summer I actually planned to paint the bottom of out 40 foot cat with a (very slippery and hard) silicate paint called Mcoating and install Sonihull ultrasound. Time and money prohibited that, so we used a normal bottom paint, but I’m still exploring the alternatives for next time. Poisonous bottom paints and frequent renewals obviously isn’t a future proof system.

As Richard above, I worry that adding ultrasound in the marine environment could be a problem. Sound travels many times faster and further in water than in air, which means that humans have made parts of the ocean a much more noisy place than a factory floor. Many whales have apparently become deaf from the noice of boats and ships. I have no special knowledge about this, but it seems alarming.

I’ve spoken to the people of Sonihull at the METS trade fair in Amsterdam, where I live. In addition to being likeable people, they seemed just as interested in exploring consequences as I am. They mentioned that one of their significant markets is fish farms. Sonihull transducers can help reducing growth on the nets a bit, but not very efficiently. What it does very quick is make the water clear. It makes debris and algae sink to the bottom and make a better environment for the fish. This they say has proven to make the fish grow better and be more healthy. They had two tanks and filled them (while I was there) with he same very murky water, mud and algae. They put a transducer in one of them. The water became completely clear in about half an hour. The sample with no transducer showed no visible difference from when it was filled. They said it was also useful in garden ponds with still water or small lakes with pollution. Quite impressive, and interesting that farm fish seem to benefit from it. Still no proof that it doesn’t do other damage on marine life, of course, but it’s an indication that the damage might be limited, if any.

Their belief was that the functionality is a bit unclear yet, but that the actual sound might be above the audible range of sea creatures too. It seems like the effect comes from the direct vibration energy. It creates microscopic bubbles on the surface of the of the protected item, which when they collapse, creates turbulence that bumps off the larvae of some growth, especially things that become hard growth. The bumping off is only effective when they are trying to settle on the bottom. It won’t bump them off or reduce their growth when they got a grip. It seems to not help much with most soft growth.

None of this seems conclusive, but it does make me think that ultrasound might be harmless. It also seems like it’s well suited for protecting specific items, like a sail drive, but that it alone cannot protect a whole hull well. A Norwegian guy I met runs an initiative called “Grønn Marina” (green marina), http://www.gronnmarina.no. They work to make pleasure boating poison free, as an idealist project, but also trying to make a living from it. They now also work with commercial shipping. They’ve found that Sonihull have the best ultrasound equipment on the market and that when combined with a special coating it works well for protecting the whole hull. The boat needs to move at more than 6 knots fairly frequently. More than a month or two at rest means it must probably be brushed to remove slime. High speed boats can get it off by going fast. Without the ultrasound, the coating just makes it a bit easier to brush off growth. It doesn’t reduce growth at all.

The coating they work with is the one I mentioned, Mcoating. It’s made in Italy and they have a not so good web page, yet. Mcoating.it, new one coming at Mcoating.net. The owner, Maurizio Magnano, lives in the Netherlands and is another sympathetic and eager guy. I’ve spoken to him a couple of times on the phone. His product is not the same as a silicone coating. Those are slippery, but fairly soft and a bit vulnerable to physical abrasion. Mcoating is epoxy with silicate and some other ingredients. It’s very hard, thus more resilient. It’s meant to last the life time of the boat. Damages on the coating, like on the keel, can be repaired.

I really don’t know yet. In 2021 I think I might try out this combo, Mcoating and Sonihull. Ultrasound and this type of silicate coating seems to compliment each other. The strength of the one seems to help the weakness of the other. I’m still far from certain that the ultrasound is harmless to the marine environment, but I have the impression that the damage might be less than the poison added from the alternative anti fouling paint. I also like that I can paint once, and then it’s perhaps done for that boat.

Marc Dacey

When you mentioned “the direct vibration energy. It creates microscopic bubbles on the surface of the of the protected item, which when they collapse, creates turbulence that bumps off the larvae of some growth”, I thought of cavitation, which can occur with props that are poorly pitched. It’s interesting to consider what is a problem with props could be a solution to slime!

Stein Varjord

Hi John,
Thanks for the approval!

Hi Marc,
It seems as the ultrasound is more efficient on barnacles and other larger fouling, and not too efficient on slime and other soft growth, but this is just my impressions from what I hear and read, not my own experience.

As another environment friendly alternative to ultrasound and the others mentioned here, I thought it might be interesting to mention is FINSULATE, (https://www.finsulate.com/en/). A Dutch initiative which is in production. I’ve seen it applied, professionally, but don’t know how well it works. It’s a thin self adhesive flexible foil that is put onto the clean hull. The surface has microscopic structures that apparently mimics the skin of some sea creatures. This allegedly stops all types of growth from establishing and is said to last at least 5 years. For commercial shipping, the same product is called Micanti Antifouling.

Brian Smith

If you do go for the ‘slippery’ paint with the silicates and you find it doesn’t work as you hope do you have to grind it all off ?
I would imagine that if you don’t then any paint you apply may not stick. Removing epoxy is a much harder job than removing paint.

Stein Varjord

Hi Brian,
I’m pretty certain that you are right. I haven’t asked about it, but i strongly doubt that anything will stick to that coating.

To apply the system, the hull needs to be stripped of most other coatings, as they’re probably not durable enough to last the life time of the boat, which the coating is meant to do. There’s a specific epoxy primer in two layers, and then the coating in two layers. they’re both relatively thin. The coating is reportedly an unusually hard surface, as opposed to silicone, which is noticeably soft.

Since the complete system supposedly is relatively thin, I’d guess a normal orbital sander (connected to a proper dust sucker) could do a reasonably good job with complete removal, but not if any of the layers “melt’ and fill the paper. I have no idea if that is the case here, but some primers tend to do that a bit. The epoxy silicate coating itself will most likely not be hard to get off with a good sander. Perhaps the primer is a good foundation for other coatings too?

If all needs to be removed, and/or the sander doesn’t do the job well, a scraper will always work. I like the “Pro scraper”. Very durable blades, nice to work with and connection to a dust sucker, which is mandatory for boat bottom jobs, to collect the poisonous ingredients.

My conclusion: Stripping the bottom and putting on the hard silicate coating is a big job, and far from cheap. If it doesn’t work well enough, that would suck enough, no matter how easy it would be to go back to a normal antifouling. As mentioned, I think the silicate coating needs to be totally removed, while the primer could perhaps be left. My guess is that this is not a too hard task.

Ernest E Vogelsinger

This is a bit off topic so I apologize in advance, but with regards to environmental damage the question might fit in here. I heard a lot of good things about CopperCoat and I was wondering about the environmental issues it may cause, if any. Do you, or perhaps Stein as he has investigated in this area, have any information on this? (yes, I searched AAC and found the article on CopperCoat https://www.morganscloud.com/2007/01/01/coppercoat-antifouling-paint/ but this doesn’t touch the environmental aspect)
Thanks, Ernest

John Cobb

Coppercoat claims to use a water-based epoxy. Thinking about putting it on my boat. As you said: A complicated decision.

Steve D

I have been following ultrasonic anti-fouling since the first salesman walked into a yard where I worked 30 years ago, I recall he had a broken arm. He sold a system to one owner, and it failed miserably.
I’ve had many clients try these products through the years, as each new and improved generation was released. Early on they essentially all followed the above course, none remained aboard for long. As time passed I’ve seen incremental improvements, however, none I’ve encountered, directly or anecdotally, have proven to be a replacement for anti-fouling coatings. The one exception seems to be hardware, again as in Bob’s Nordhavn case, and the keel cooler on these vessels presents a special challenge because the manufacturer advises it not be coated. When transducers can be properly attached to struts, shafts (when not running of course) rudders and coolers, it seems it can be an effective deterrent to fouling, and metal components like this are the most difficult to anti-foul, so it may finally be a good utilization of this product.

James Peto

You may find the write up on the Nordkyn Design site of interest, especially as it is based upon actual experience. Ultrasonic Antifouling System – Part 2, Fighting Algae Growth additionally too his system is being used upon an Aluminium Sailing Yacht.

James Peto

John, You are quite right though this is a “New/Revised Article – July 2019 and from a technical as well as real-life documented point of view has much to recommend it.
You might be interested too in the additional paragraph concerning the resultant side effect for Fish Farms of cleaner nets and clearer water.
At the moment in Total Lockdown so have time to peruse some pretty esoteric reading!!
Keep Safe

Peter Forbes

My aluminium hulled yacht was based in the Middle East where the summer fouling is severe. I antifouled with Trilux 33 but within days of launching it was apparent that the Trilux would not make the grade. To extend the time to liftout I installed an ultrasonic system from CleanAHull. Four transducers were installed as recommended for a boat of around 40 feet on the waterline.

After a year the antifouling had failed but around the transducers the hull was clean of all growth out to about a 3 foot radius. There was only light growth of barnacles and hard organisms out to about a five foot radius. Beyond that there was no discernible reduction in growth.
In my opinion to effectively combat growth over my entire hull, at least 12 transducers would be needed. However the proper positioning of more than one or two more transducers would not be possible on my yacht due to internal tanks and structure.

So I would have to agree with Steve D – ultrasonics can work well in localised areas but are not practical as a whole hull fouling defence.

Chuck Batson

I’ve been researching minimal-harm antifouling solutions, including ultrasonic, for several years. The option which seems most promising is Selektope (https://selektope.com/about/) which I’ve been watching excitedly for years. It’s not yet available in the U.S. so I haven’t been able to try it personally, but will as soon as it comes here or I’m in a country in which it’s available.

Paul Van Slyck

I worked at a yacht charter company in Washington state, in the San Juan Islands, with cold water and lots of growth. There was a 57’ Symbol that installed a Sonihull system with two transducers, the system was purchased from PYI. Typically in this region bottom paint will last 12-18 months. After three years the bottom paint was still in good condition. The hull would grow some green slim at the waterline, however, that would fall off after the yacht was out for a day. The cost of the Sonihull system was half the cost of a bottom paint job. The Sonihull was quick to install, the most difficult part is running the wires from the transducer to the controller. The system will keep things from sticking to the hull but will not remove what is there. So you need to start with a fresh bottom paint job.