47063

Opposing polarity engines within same vessel [New Zealand] 

November 17, 2007

I am a metal worker who has been asked to give advice regarding electrolysis to a business that is cosidering purchasing a 20+ year old fishing boat. The genset motor and main engine are earthed using opposing polarities with a manual shut off switch on the genset after start up of the main. On removal of the boat to dry dock it was evident that there was a problem with electrolysis. All zinc anodes were gone, there was moderate pitting of the 10 gauge steel hull, severe pitting of the mild steel stabalisers and the mild steel welds had been eaten away. Upon further investigation within the hull and engine room signs of electrolysis were observed on the rear bulk head and on the exterior of the engine. I would be interested in any oppinions as to how far the electrolysis may have travelled, in particular whether it could have travelled to the engine cylinders and what steps should be taken to mitigate the problem.

Dion McNab
metal worker - Whangarei, Nothland, New Zealand

Once metal atoms have become ions and have combined chemically with oxigen or other ion species there's no way to put them back where they belonged. They leave pitts or craters behind. Corrosion is believed to account for an astonishing 5% of the world gross product. To find how far corrosion traveeled into the engine or other critical parts requires to disassemble them or inspect with a borescope (this would not be feasible for the internal combustion chambers of the engine).

Guillermo Marrufo
Monterrey, NL, Mexico

If the vessel has sonar or other fish-detecting or depth-sounding gear that uses an underwater signal generator and receiver, check for electrical leakage from those underwater components. Such leakage can wreak havoc. Any leaking electric current will return via the hull and/or the prop and prop shaft - sometimes showing up as heavy etching of the prop itself. The prop and prop shaft are poorly earthed to the hull compared with the ease of current flow through the prop shaft to the transmission gear and thence to the hull earth.

Bill Reynolds    metallurgical services Melbourne, Victoria, Australia