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GALVANIZED BRASS PROPELLERS

2002

Dear Sir,

I have a twin engine boat. Each engine is wired separately on an individual battery wit -ve ground. LAST year I bought a 2kg precast sacrificial anode which I REMOLDED INTO A CUSTOM INGOT. The anode was then bolted through hull behind the keel(not to loose knots)and connected via 2 6mm copper wire to both engines. THIS seemed to protect the brass propellers (all other underwater fittings are stainless steel).

HOWEVER this year I bought a rod of zinc which I AGAIN MOLDED AS THE PREVIOUS ANODE BUT TO MY SURPRISE, A WEEK AFTER I LAUNCHED THE BOAT IN THE WATER, BOTH PROPELLERS DEVELOPED A GREY COATING WITH A SORT OF SALTY MATERIAL ON IT.

IT TOOK ME ABOUT 4 HOURS SCRAPING AND WET SANDING THE PROPELLERS (IN MY OPINION ZINC HAS DEPOSITED ON THE PROPELLERS).

Could you please give me an idea of what's going on?

Godfrey Baldacchino
- B'Buga, Malta

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That seems highly unlikely. The zinc should preferentially go into solution, not plate out. As long as the zinc is connected to the brass, there should be no way for this to happen. More likely may be that the zinc is coming from another boat if there are some kind of stray currents about.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002