North Harbor Diesel Blog

Are you familiar with the terms hot marina, electrolysis, and galvanic corrosion? Are you curious to know what divers actually do under your boat?  These are questions that will be answered under today’s topic:  galvanic isolators.

We’ll begin by addressing the second question first.  Divers spend their time under boats changing zincs (sacrificial anodes). These zincs are electrically connected to all underwater metals and prevent the boat metals from corroding by sacrificing itself to corrosion.  Anytime you have two dissimilar metals immersed in salt water (an electrolyte) you have galvanic corrosion.  A battery is a beneficial type of galvanic corrosion.  And as long as your zincs are working, your boat is under controlled galvanic corrosion and only the zincs corrode.  But if all your zincs are consumed by galvanic corrosion then your boat metals (most likely your bronze propellers) will be next so it’s important to ensure you install new zincs at the appropriate interval.

So what is the appropriate interval?  There are many factors that go into how quickly zincs are consumed.  Some of the factors are how much metal the zinc is protecting, the types of metals, water temperature and salinity, currents, and possibly, other boats in the marina (more on this soon).  It is best to have your zincs inspected very three months for the first year to establish a baseline of consumption. Once you establish your baseline, you can modify the inspection schedule to reflect your pattern of consumption.  If you change marinas or even slips, it’s best to establish a new baseline.  Timely inspections of zincs can prevent very costly corrosion damage to props and other underwater metals.

There is another type of corrosion called electrolysis.  This is similar to galvanic corrosion except now we have an electrical current (stray current from a problematic electrical component onboard) which greatly accelerates the effects of corrosion.  These stray currents can wreak havoc on your underwater metals and cause extensive damage very quickly.  This stray current can come from your boat in which case there is not much you can do other than recognize the problem and repair the faulty electrical component.  But stray current can also originate from other boats in your marina (hence the term hot marina).  How can someone else’s problem become your problem?  Well, if you are plugged into shore power then you are connected to every boat in the marina via the green, grounding wire in your shore power cord.  This is where the galvanic isolator comes into play.  This device blocks any electrolysis-causing stray current from entering your boat.  It prevents other boats from consuming your zincs and eventually your precious boat metals.

Galvanic isolators are recommended by the American Boating and Yacht Council (ABYC) for all boats–regardless of age or composition–that utilize shore power. More information can be found in Chapter A-27 of the 2010 ABYC Standards manual.

Zincs and a galvanic isolator can offer protection from galvanic corrosion, electrolysis and a hot marina but neither of them will protect you from onboard stray currents.  Having a bonding system survey performed will ensure your boat is fully protected.