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How to protect the steel hull and the ecology?

Please share your views. I have a 180 ft. x 36. ft flat bottom, steel hull barge which sails in the inland brackish waters of Goa, India. We generally use a marine zinc based primer coat and then marine paints on the vessel including it bottom. This however does not prevent heavy corrosion leading to replacement of the bottom and side plates of the vessel in around 5 years inspite of annual painting. I'd like to know how I could do better with life of the hull. Would the use of zinc anodes help and if yes, then how many will I need? There is also the ecology that I need to be concerned about - the rivers in which these vessels operates are fringed with a lot of mangrove forest. These are the breading grounds for a wide range of marine species and thus need to be protected.

Joseph D'Souza
Transporter of iron ore - Mapusa, Goa, India
January 1, 2011

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Hi, Joseph. I believe that zinc anodes will make a huge difference. I would start with a dozen and see how fast they are exhausted. If you search for "zinc anodes for ships" and read up for 15 minutes, though, you'll probably know more about it than me :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 3, 2011

Zinc anodes will protect the submerged portions of the hull. Some further research online should provide you with the necessary values to calculate how many anodes are required. The calculations are based on surface area to protect, type of material to protect (in this case steel), coating efficiency factors (how complete is your coating), and type of anodes. Different anode materials can produce different amounts of protective current, and consume at different rates. Consumption rates are dependent on both material and current output. For example, a small anode protecting a large surface would be depleted much faster than a larger anode of the same material, for the same given surface.

Sean Quigley
Corrosion Consulting - Concord, California, USA
January 31, 2011