Reverse Current Keeps Ferry Afloat
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A news story with a title about a boat and reverse current is more likely to be referring to water flow in a river or stream than about electrical current in a conductor. Having grown up in a neighborhood next to a tributary of the Chesapeake Bay, I spent quite a bit of time around boats, both large and small. Salt water is particularly destructive to metal hulls due to cathodic corrosion, exacerbated by the salt water's conductivity. While working as an electrician in the 1970s, I installed electrical supplies for a few dockside cathodic protection systems that probably functioned like the one described in this 1965 issue of Popular Electronics magazine. The principle is fairly simple whereby anodes are placed in the water around the hull and a counter-current is induced to cancel the natural current flow. Evidently the systems needed to be fine tuned so as to not harm the hull's paint. Reverse Current Keeps Ferry Afloat
Converted ferry was stripped of machinery and engines, and connected to city water, gas, electricity, etc. Problem was to prevent the hull from rusting away without yearly dry-docking. By William P. Brothers When the San Francisco firm of J. Walter Landor ran out of space, it simply bought one of the last of the Bay ferryboats. The company tied the boat up to a dock and converted the topside into offices. Keeping the steel hull of the 40-year-old ferry from scuttling the studios and staff was a problem.
Electrical potential of steel hull is measured every month. Brass studs brazed to hull give solid electrical contact, accurate reading. Amount of d.c. current required depends on exposed hull area. A permanently moored ferryboat is corroded by the electrolytic action of sea water. One ampere of d.c. will wear away 20 pounds of steel each year. To overcome this action, designer Alexis Tellis dropped four carbon anodes overside and fed them d.c.-reversed to the corrosive action of the hull. A solid-state rectifier supplies 0.85-0.95 volt to the rods. More voltage damages the paint; less voltage corrodes the hull.
Designer Alexis Tellis examines one of four carbon anodes dropped around steel hull. Anodes, charged with d.c. equal to that ferry would ordinarily lose by electrolytic action, counteract current flow.
Posted May 22, 2018
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