Zinc Anode - Protection for your Boat
13/09/14 13:03 Filed in: Maintenance
When we launched our first boat, S/V Two-Step, a Classic 37, and started sailing her in the Great Lakes back in the 80s I didn't know exactly what to do about our zincs. I understood we needed one on the propellor and apparently there was one in the engine, but what did they really do? And when should I be changing or inspecting them?
"What the heck is a zinc anyway and why?"
I quickly learned the answer when we later sailed in salt water. At the most basic, if you have a mix of different metals and add salt water you will get corrosion. Without getting too technical, zinc protects other metals on your boat exposed to the harsh underwater environment by being the "most likely to corrode".
Note that Zincs only work when they are electrically connected to the items they are intended to protect. So the prop-shaft zincs are attached directly, as are any engine zincs and our bowthruster zinc. The main zinc below will be set up as part of a ships bonding system and will be connected inside the boat via the long bolts you see below. If you are worried your boat is set up incorrectly you might like the VERY technical description of bonding and lightning ground system here or testing procedure here.
There are a variety of metals on your boat's hull with salt water all around. A typical boat probably has some stainless steel (prop shafts and other components), different varieties of bronze (propellor, thruhulls etc) and possibly aluminum (outboard motor or outdrive components). With these different metals in salt water you get what amounts to a battery action. The different metals have different electrical charges and the most active will corrode - pieces of the metal dissolving and depositing on the less active metals.
The idea of using zinc is that the zinc becomes the anode in this battery and it will slowly corrode away protecting the other metals. We can call this a sacrificial anode since it sacrifices itself to protect the other metals. (Note other metals like Aluminum and Magnesium also can perform this function in fresh or brackish water - see below or this link)
How fast will this happen in salt water? Warm water speeds up this reaction, as does salinity and even pollution in the water. Hopefully your zincs will last at least 6 months, but until you have a chance to check them and find out, it is best to check them after a couple of months.
Change a Zinc when it is half gone. If you're in for a swim you can check zincs underwater and plan out the next change. The zinc on the engine (if it has one) will be easier since you can check and change it in the comfort of your engine room.
On Distant Shores II we have one large main zinc connected to the bonding system, plus 2 on the prop and shaft, and another up on the bowthruster. Here is the bowthruster zinc compared after 4 years. It has lasted well since there isn't much metal to protect - the bowthruster blades are plastic composite.
Our Autoprop has this custom Zinc and I replace it every year.
I also added a "donut" zinc to the prop shaft as extra protection in case the prop zinc fell off. When installing these shaft zincs you need to clean the shaft before attaching the zinc. Then tighten it up and give it a few gentle taps to settle it into place and retighten (just gentle :-)
And what about our first boat in the Great Lakes? Well, fresh water is a much less corrosive environment and corrosion is much slower. But, Zinc anodes do not work well in fresh water. Magnesium or aluminum are recommended, but be sure not to mix anodes. You can check with your engine or prop makers for the correct sacrificial anode for your cruising areas.
It is pretty easy to check and change the zincs - and absolutely important to the underwater health of your boat. The marine environment is hard on metal components, and you need to protect them.
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