The Offshore Voyaging Reference Site

Protecting Our Boat’s Underwater Metals From Corrosion

There are few boat maintenance subjects surrounded with more bad information and general confusion than “electrolysis*”.

*In most cases the above common term is used incorrectly, which is why I put it in quotes.

Anyway, here’s what we really do need to be concerned about:

Stray Current Corrosion:

Current flowing through our boat’s underwater metals, either coming from our boat’s own batteries, or surrounding boats in the same marina that are in some way connected to our boat, usually via the ground wire of the shorepower cable.

Dissimilar Metal Corrosion:

The immersion of two metals in a conductive liquid (think salt water) with a connection between them in some way (think the boat’s bonding system) generating a current (amperage) flowing through the water between them resulting in the less noble metal corroding away.

Speed of Damage

In extreme cases stray current can turn our underwater metals to mush in days. That said, dissimilar metal problems can be just as destructive, but it does take longer.


It’s near-impossible to totally eliminate immersed dissimilar metals, so as we all know, our boats must be equipped with sacrificial anodes made from a less noble metal than those we are protecting, most commonly zinc, but magnesium for boats in fresh water and increasingly aluminum, which will work in both brackish and salt water.

The right anodes will also protect against stray current, although not for long since that modality will eat them up quickly (think days, or weeks) and when they are gone the next least noble metal starts to erode.

Anyway, sacrificial anodes are a good thing but the trouble is a huge number of boats have them installed incorrectly, the wrong type, or poorly maintained, so they don’t work.

How many times have you heard:

My zincs have been on the boat for years and are perfect, so I don’t have a problem with electrolysis.

The deluded boat owner

Just the use of two incorrect terms—can you pick them out—is a warning flag, and the fact that the anodes are not eroding is most often a warning of problems, not a cause for complacency.

Does your head hurt yet?

What Not To Do

Maybe it would be best to turn to the “professionals” at our local boatyard, or perhaps a marine electrician, to help sort this out? Or maybe a surveyor.

Generally, not a good idea. In my considerable experience (30 years of aluminum boat ownership), the vast majority of these “professionals” are woefully ignorant about:

  • how immersed metals corrosion works,
  • how to test for it,
  • how to fix it.

So they tend to fall back on FUD (fear, uncertainty and doubt) by repeating oft-heard bogus memes, rather than digging in and properly analyzing a problem.

Don’t believe it’s that bad? Check out the next two photos:

The “professional” boatyard in Connecticut where our new-to-us J/109 was stored, had painted over the one and only zinc on the boat, rendering it ineffective.

Over the years I have seen this done a bunch of times, and on several occasions the boat was launched in that condition.

Some other bright spark (pun intended) left a live 12-volt positive cable lying in the bilge under the water hose to the right (I moved it for the photo) “protected” by a few turns of white insulating tape.

I don’t know for sure who was responsible for that one, but given the boat’s maintenance history I suspect another “professional”.

(The reason that person disconnected the cable in the first place, together with the breathtaking ignorance indicated by that action, is a subject for another article. Spoiler: the original screw-up goes back to when the boat was built 17 years ago, and then was compounded later by someone else who clearly couldn’t read a simple wiring diagram.)

What To Do

So what do we do?