Sonic Antifouling: the Hum of a Persistent Myth

DESPITE A CONSPICUOUS DEARTH OF INDEPENDENT THIRD-PARTY TEST DATA, THIS IDEA FOR KEEPING CRITTERS FROM GROWING ON YOUR VESSEL'S BOTTOM CONTINUES INTO ITS FOURTH DECADE OF MYTHIC MARKETING

Dating back to the 1990s, various patented electronic/sonic antifouling systems have been marketed, but if you actually look at the scant performance data, you find that these systems just don’t work well enough to constitute a viable alternative.

I recently received a query from a FYBBO subscriber in Canada asking if I had any experience with ultrasonic antifouling systems and, if so, do I have any recommendations as to which are the best. Well, it just so happens that I have and I do …

In the Beginning …

My first exposure to sonic antifouling came in the late 1990s, when I contracted with a major marine marketing and advertising firm to write copy for a newly patented sonic anti-fouling system for boats and yachts. The manufacturer of the system made strong claims for its patented system and referred liberally to a “supporting” study performed by the testing lab at Florida Institute of Technology (FIT). I no longer remember exactly what it was that triggered my ongoing curiosity, but when I had completed the copy writing for the marketing campaign in question, I decided to track down and actually read the FIT lab report.

I know, I know — you might reasonably ask why I didn’t check those results first. It’s because in that particular case, my brief was to produce interesting, compelling marketing copy, not to evaluate the claims the company was making on behalf of its product. And it’s common in such circumstances to assume that the information coming from the company involved is accurate. Unfortunately, as you might guess, in the end it wasn’t — or to be more accurate, the interpretation by the product manufacturer of the test results employed some seriously tortured logic.

While the repeatedly touted “landmark” FIT study showed that sonic antifouling did indeed discourage the squatting of barnacles and some other crustaceans and mollusks, it fell far short of supporting the claim that sonic anti-fouling does anything to discourage the formation or attachment of slime, algae, or grass. Moreover, the FIT study concluded (if you read it closely) that the sonic anti-fouling tested worked consistently to keep only 70% to 85% of the surface free from barnacles and other critters. And I could not, at the time or later, find any other notice of independent third-party studies or tests to support the claims being made on behalf of sonic anti-fouling systems manufacturers.

The main problem with the results reported in the FIT study is that even 85% clear of barnacles and other growth means you still have to haul regularly to clear those areas which are not kept clear by sonic antifouling. Not to mention that you’d also likely have to haul annually or biennially to inspect your prop(s), zinc anodes, Cutless bearing strut(s), through-hulls, immersed rudder pintels and gudgeons, and so on. None of which even yet addresses the issue of growing greenery (slime, algae, and grass). So, when you start to work the overall cost numbers of keep your bottom clean, there is, in fact, little if any dollar savings to be attained with sonic antifouling.

I Think I Can, I Think I Can: The Little Theory That Couldn’t …

The theory behind sonic antifouling is that it puts a vibration into the hull skin at a frequency that is irritating to barnacles, with the result that they decide not to build their protective calcium shell and instead simply move on.

The main problem with the theory is that a single anti-squatter’s vibration frequency seems not to work in every case. Which means that somewhere between 15% and 30% of barnacles don’t move on. And since the proteinaceous adhesive secreted by barnacles (“barnacle cement”) is hellishly tenacious on just about any underwater substrate, you’re left with patches of barnacle shells, even if the barnacle itself eventually vacates and moves elsewhere.

The other problem with the “theory” is that it seems to have remained mostly conjecture, focused initially on lower frequency sound waves in the region of 16 Hz up to 20 kHz, but later switched to ultrasonic waves in the range from 20 kHz to 1 GHz — a difference that, IMO, calls into question just how much science is involved in the development of these antifouling systems and how much is marketing hocus pocus — a theory still looking for confirmation.

Time Marched On and Claims for Sonic Antifouling Evolved …

Anyway, some 15 years later (which was likely 20 or more years after the issuance of the original patents), there was a distinct proliferation in the number of competing brands of sonic and ultrasonic antifouling systems. During this round of expanded marketing and promotion, the claims as to the effectiveness of sonic/ultrasonic antifouling systems were dialed back to proposing they were most useful when used in conjunction with more traditional biocidal and algicidal coatings.

The nominal reason usually given for this change is that such coatings take care of the algae and grasses that do not respond to the sound/vibration based discouragement that induces critters like barnacles to move elsewhere. However, I suspect that if such hybrid combos really work better, such really has to do with putting an ablative coatings layer on the substrate that makes it easier for barnacles and barnacle residue — as well as slime, algae, and grasses — to slough or be scrubbed off the underwater surface of a boat’s hull when sonic antifouling alone proves insufficient.

Let me digress for a moment to tell you a story about the relative effectiveness of various antifouling coatings. An old dock rat whom I once knew was fond telling fellow boaters that he found acrylic latex exterior house paint (at $40/gal) to be just as effective as high-priced biocidal marine anti-fouling coating (at $400/gal) … If you paint two hulls, each with one of them and keep them in adjacent slips, at the end of the season, the hull with the house paint is fouled just the same as the hull with the expensive antifouling coating (but at 1/10 the cost)!

All kidding aside, the point is that, in the case of marine antifouling for boats and yachts, an 85% solution is not really a solution at all. Because you often need to perform an annual or, at least, biennial haulout for inspection and maintenance, any bottom line cost savings will be far less than the marketing hype for sonic antifouling would lead you to believe — and may amount to nothing at all. Consequently, you’d likely be better saving your money to apply against your next bottom job.

Riding the Magical Marketing Train …

An interesting marketing spin has emerged in the last few years, namely, an emphasis on the application of ultrasonic antifouling to metal external drive train components and to internal raw water intakes and system components. Since these are both composed, for the most part, of metal parts, claims of efficacy for sonic systems is more plausible than otherwise. After all, metal is much better at transmitting sound and vibration in the relevant frequencies than are wood or plastic composites (whether solid laminate or cored). So maybe the marketing spinmeisters have finally found better holding ground for anchoring their efforts on behalf of sonic systems.

However, that said, heading now into the fourth decade, I recently, once again, reviewed the available product literature — as I’ve done at approximately 10-year intervals since I was first introduced to sonic antifouling systems. And I still don’t find any credible, non-anecdotal, independent third-party testing data out there to confirm the efficacy of such systems for antifouling boat or yacht hulls. Just a lot of theorizing and wishful thinking. Which after more than 30 years rings — or perhaps hums — with a tinny overtone.

— Phil Friedman

Afterword — some differing views on sonic antifouling and … a challenge

1. The following are some links to what purport to be test results, supplied to here in the pursuit of editorial balance for this article:

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   a) https://wavetrain.net/2011/11/06/ultrasonic-antifouling-full-season-test-results/ (2011)

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   b) https://www.pbo.co.uk/gear/ultrasonic-antifouling-tested-hull-72549 (2015)

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   c) https://drakemarine.com/ultrasonic-antifoul-test/ (2020)

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2. The Challenge:

   I fully expect to get some hate mail over this article, including some accusations that this kind of sweeping indictment of a product type is unfair and, indeed, unsupported. My defense to this is that it is impossible to provide objective evidence for a negative. So, to forestall meaningless bickering, I offer the following to any manufacturer or vendor of sonic or ultrasonic anti-fouling systems — supply FYBBO with links to non-anecdotal, independent, third-party testing results that either controvert what I’ve said here or confirm the efficacy of any sonic or ultrasonic antifouling system(s), and I will append those prominently to this article. Waiting to hear from you. — PLF

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