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Updated: Jan 22, 2019

During an impromptu Sunday Boat Brunch party on D-Dock, I was asked by some long-time friends: “What’s the story about zincs? My diver put two on the propeller shaft, where there was only one before.” They asked, adding, “Oh, by the way, great Bloody Mary’s - love the horse radish!”

Great - Finally, a technical, boat maintenance question - my next Krabby article was writing itself. This question is specifically about zinc anodes. What are they, how do they work, and is more better?

Dear D-Dock Boat Brunchers,

Sacrificial anodes are the little zinc collars, buttons, or plates that are attached to propeller shafts, rudders, struts, or grounding studs underwater on our boats.

Zinc is less noble on the Galvanic Scale than the stainless steel, bronze, and aluminum that make up the important and expensive parts on a boat – therefore Galvanic corrosion will erode the zinc anode away first, protecting the more noble metals that it is attached to (or the cathode).

Now, here is where it gets more complicated. To be scientific about this, we would need to go back to basic college chemistry. I must admit, I didn’t pay much attention in that subject after my thorough research in the fermentation & distillation processes, therefore I would be hard pressed to speak intelligently on the subject of electrochemistry. Let’s keep it simple, so I can get back to my favorite chemistry experiment… cocktails.

“The bloodies I made for brunch are a good analogy to answer the ‘how much zinc’ question”, I explained to my friends, “You said you like the horse radish? Well, what if I added a bunch more because you liked it – it wouldn’t turn out too well. It takes just the right amount to be tasty, without putting your sinuses into spasms. The same is true with zinc anodes: too much zinc, and the eletro-chemical reaction between the zinc anode and cathode becomes too strong. This reaction is a form of electrical energy that has to go somewhere – it’s not picky either – it will find its way to the saltwater by de-laminating gel-coat, blistering fiberglass, paint, and de-lignify wood. On the other end of the spectrum, if you don’t have enough zinc, then your expensive metal boat parts will be susceptible to galvanic corrosion.”

So, if you notice your zinc is eroding away, and your propeller is getting pink spots – you need a little more zinc. If your bottom paint is showing black blister spots, you have too much zinc. Warmer water temperatures, and using your boat often both will speed this process up – you might add a little more zinc, or replace the anode more often. With some careful observation you will get it just right.

See, I snuck a little chemistry lesson in there after all. Now, where did I put that bloody down, I’m thirsty.

Happy Cruising,


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