The Offshore Voyaging Reference Site

Q&A: Should I Use Coppercoat Antifouling Paint On An Aluminum Boat?

Question: We are considering applying Coppercoat to our aluminum boat and would be interested in your thoughts. I have attached some correspondence that I had with the makers of Coppercoat that may help:

Question to Coppercoat [edited for brevity]: I’m seriously considering Coppercoat for my aluminum cutter. I am confused by one group of advisors that strongly advise me to stay away from copper (other than copper thiocyanate) and another group that says an insulating barrier between hull-bottom and Coppercoat suffices to be safe. Could you please explain to me how Coppercoat manages to be safe on an aluminum hull, even when the hull is scratched by a rock and bare aluminum is showing under water?

Answer from Coppercoat [edited for brevity]: Unlike many paint companies, we do not manufacture different types of anti-fouling for different sorts of boats and particular locations. Instead, Coppercoat is only available in a full strength, hard wearing version. The epoxy carrier is filled with the maximum amount of copper allowed by law, and consequently this coating will keep fouling at bay in virtually all conditions (including warm and cold, fresh and salt waters). Please note that Coppercoat only uses pure copper, and not the weaker and less effective alloys such as copper-nickel. Furthermore, while the non-eroding epoxy can easily withstand use by high powered motor-boats and racing yachts, this coating is equally effective on less frequently used leisure vessels.

Over the years we have supplied Coppercoat to numerous aluminum boats. You must remember that Coppercoat is not a copper anti-foul in the traditional sense, in that it is not a water miscible paint. Instead, Coppercoat has all the copper bound within a waterproof epoxy. Coppercoat is non-conductive and does not cause or promote electrolysis or cathodic decay. However, when treating aluminum it is important to prime the hull thoroughly with a good layer of pure epoxy primer before over-coating with the Coppercoat. This distances the copper rich epoxy from the aluminum hull, and, to all intents and purposes changes the hull from being aluminum to being plastic (as far as our anti-fouling is concerned).

I understand that you have reservations concerning a potential breach of this epoxy primer (if scratched by a rock), thereby exposing fresh aluminum to the sea which is also in contact with the Coppercoat. However, the relatively poor conductivity of the sea-water and the incredibly weak field emitted from the tiny amount of exposed copper powder combines to mean that this is not a problem. Remember that although Coppercoat contains a large amount of copper, at any given time only a very small percentage of this is actually exposed as the vast majority is held within (and each individual copper sphere encapsulated by) two-pack epoxy resin.

Answer: I can’t really advise you on what to do on your boat since I’m not an expert, or even close, on dissimilar metal corrosion. Also, we all have different levels of comfort with such things. What I can say is that after giving it quite a bit of thought I would not apply Coppercoat to Morgan’s Cloud, or at least not without a great deal more research, for the following reasons:

  1. I’m just plain uncomfortable about mixing more noble metals with aluminum on our boat. I’m sure Coppercoat are correct in everything they say, but suppose there is something strange about my boat, or suppose the problems only manifest after 10 years? If Morgan’s Cloud was damaged by the paint, we would never be able to afford to repair or replace her; it’s just not worth the risk to us, no matter how small.
  2. There is one part of aluminum boats that can’t be coated and is always in contact with the water and that is the inside of the propeller shaft tube. I have been advised by a very experienced surveyor of aluminum boats, that it is relatively common for the tube to corrode out on older aluminum boats, resulting in a complex and expensive repair. Would Coppercoat make this worse? I don’t know, but I don’t want to find out the hard way. The tube on Morgan’s Cloud is still in good shape after nearly 20 years. I don’t know for sure why that is, but I don’t want to change anything. If it’s not broken, don’t fix it.
  3. Although we have no experience with Coppercoat, we have had poor results from hard antifouling finishes over the years. I think they can work well for high speed motor boats or boats that are underway all the time, like commercial crew boats, but for the average yacht—even one like ours that is underway a relatively high amount of the time for a yacht—soft ablative finishes seem to do better. We use the zinc-based antifouling from ePaint and have tried both their hard and soft products and found that the soft works a great deal better in our application.
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