Now I am in a position to answer my own question (or maybe not), thanks to an article in this month's Yachting Monthly.
The ultrasonic device tested was the
Ultra 10. It sells in the UK for £949.00 incl. 17.5% VAT. The unit, according to the manufacturers, has a power consumption of 0.7 amps maximum @ 12vdc (and probably minimum too), which corresponds to the article's claim of "12A every 24 hours", which is a rather strange way of expressing power consumption.
The test, performed on one boat only, came to the conclusion that, "there was a clear reduction of slime around her hull, rudder, prop shaft and bow compared with the previous year". However, as I see it, the test results are not valid as not only was the Ultra 10 used but the boat was also anti-fouled and anti-fouled with a different anti-fouling than previously. All-in-all, a very inconclusive and unscientific test. One should also take into account the fact that the test was carried out in the English Channel rather than in really high-growth areas, i.e. warmer waters with strong sunlight.
Finally, the article states that Ultrasonic Antifouling's director, David Sothcott, says: "Traditional antifoul is still a good idea". So, why do we need this product if we still have the glorious job of anti-fouling, power-washing and scraping?
Aye // Stephen