Protecting Metal Rudders With Sacrificial Anodes

If your system ain't broke, don't fix it. Trying to save money on zincs can be penny-wise and pound-foolish.

18th July 2013.
By Ed Sherman

Question: I just had my boat hauled to get the bottom cleaned and repainted with antifouling paint. I am of course going to check all of my anodes and replace as needed. A question here, related to the best way to cathodically protect my rudders, which are solid metal.

rudder idsk anode

The disk anode shown on the rudder is part of a system that needs to be balanced. Err on the side of more anode protection than less.

The way things are set up now, each rudder has its own disc zinc installed as you can see in the photo I sent in. The boat also has two transom zincs installed. A friend of mine says that as long as the rudders are connected to my boat’s bonding system, all I really need is the transom zincs. What’s your take on this?

Answer: I’ll give that a “maybe.” It’s important to understand that the amount of anode material needed on any boat is related to two very important factors; first, the amount of exposed metal that needs protecting, and second, the expected service life of the anodes. The level of protection is established by the amount of anode surface area exposed related to the amount of exposed metal, and the longevity of the anode(s) is determined by their mass.

So in your case, the transom anodes may offer enough protection in terms of the desired voltage levels generated by the anodes, but the question will be, is there enough metal mass in the transom anodes to provide an adequate level of protection for the desired service life, typically between six and nine months. Those rudders on your boat present a pretty large exposed surface area if left unpainted as shown. That fact will put a pretty heavy load on your bonded cathodic protection system, dictating more anode surface area required to achieve the desired voltage levels needed to provide proper cathodic protection. Elimination of the rudder anodes as shown will force the transom anodes to deplete faster, even if they do generate adequate protection levels in terms of voltage potential.

My advice is simple here. If your anode service life is acceptable and you are not having any underwater metal corrosion issues, don’t change anything. Spend the extra money for the disk anodes you show. They are considerably less expensive than buying new rudders!


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About the author:

Ed Sherman

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Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.
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http://www.EdsBoatTips.com

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