Let Us ZINC About It

Written by Capt. Juan Watson
Written by Captain Juan Watson

My trusted diver has finally got an opportunity to get to one of my client’s vessel in Annapolis to change out the zincs. This particular client decided not to haul the boat this year, but instead leave it in the water at a marina. As part of the winter plan we decided to have the anodes (zincs) replaced in late December. Is that really necessary one would ask? I would have to say yes.

Understanding the basics of galvanic corrosion is important to any boat owner. As a less than stellar science student asking me to provide any lengthy and in depth explanation might be doing all of us a disservice. Instead, I hope to maintain reader interest and to not confuse the masses.

Galvanic corrosion is a process where dissimilar metals found in an electrolyte creates a DC current. With this current there is a migration of electrons and the anodic or less noble metal suffers damaging corrosion. It must be noted that the further the metals are apart on the chart the more severe the corrosion can be. So this is where the sacrificial anode or “zinc” comes into play.

Without the sacrificial anodes the various metal components below the waterline such as shafts, struts, rudders, props and thru hulls would become damaged through this corrosion and fail. This could in some cases cause the thru hulls to fail and your vessel to sink. This is something that might not be covered under your insurance policy and leave you fitting the bill of replacement or repair. With the anode in place, it suffers the corrosion and the remainder of the immersed metals is not affected. Remember once the zinc is worn the corrosion will move to the next least noble metal which could be the props or thru hulls as the shaft made of stainless steel which is more “noble” than the bronze.

It is not uncommon for boat owners to experience zincs deteriorating at a faster rate when in marinas as opposed to being on a mooring or at anchor. This is likely to be the result of being plugged into shore power. The reason is because the ground that is found in each plugged in shore power cable effectively connects each vessel to the next. This directly “connects” all the dissimilar metals together, accelerating the galvanic corrosion.  Another consideration is that vessels next to you might not have any anode protection which results in your anodes dissolving at a higher rate than if other vessels had good anode protection. The best way to address this is to have a galvanic isolator installed for each shore cable on board. This attaches to the ground wire and interrupts the galvanic flow while maintaining the flow of AC ashore. This will effectively interrupt the circuit and limit any added galvanic corrosion.

Here are a few points for consideration with regard to anodes:

  • Use military grade Zinc or Aluminum anodes in salt or brackish water only and not Magnesium anodes as those are best suited for freshwater use. Aluminum alloy anodes can be used in all types of water and can last longer than traditional ones.
  • If an anode is at 50% its original size it should be replaced with new ones.
  • When attaching zincs there should be good, clean tight direct contact to the metallic surface or they will not function correctly. Zincs also should never be painted.
  • Make sure that all your anodes are all the same material and not a combination like Zinc and Aluminum. The less noble will do all the work with the other doing none.
  • Stick to the existing size zincs. If the anodes are too small they will not have enough electrons to disperse and the more noble metals will begin to corrode. Conversely if your zincs are too big they will disintegrate at too slow a rate and have marine growth form on them which will render them useless.
  • Make sure that all your fittings below waterline are well bonded. These fittings are protected by the transom anode plate which should fit snuggly to the studs on the transom. If you do not see much wear on your plate zinc check install as your thru hulls might be deteriorating.
Like many things in life forgetting to address the simple things often result in the biggest headaches. Anodes by comparison to many things in boating are not high cost items and a reputable diver hired is a good investment to ensure they are installed correctly. There is more to galvanic corrosion than is covered here so I do encourage you, as with any topic I cover, to read up and understand this in more detail.


Juan Watson is a 14-year mega yacht veteran. Currently, he owns and operates Pelorus Yacht Consulting, LLC in Annapolis, MD. PYC focuses on educating yachtsmen and owners.