Keep Your Drive Alive........Part 1

If it hasn't happened to you, you probably know a fellow boater that has had their powerboat outdrive or sail boater that has had their engine drive unit attacked by corrosion. This is an all too common occurance that can be avoided with a little careful maintenance and installation of what I consider a must have electrical accessory for any boat that spends time plugged into a dock shorepower system.

Marine corrosion seems to be shrouded in mystery and misinformation, and every dock will have at least one boater who claims to know all the answers to these mysterious occurances. Most boaters I've encountered that have had this sort of problem will almost immediately begin their analysis by trying to pin the blame on one of their dockmates. The old wooden liveabourd at the end of the dock must be emitting stray electical current into the water, or perhaps its the guy in the next slip who regularly lets their shorepower cord droop into the water. Dock guru theories abound. The truth is, all of this can be explained with real science, there's no voodo magic involved here, just the need to understand how these things can happen and what to do to make sure they don't. I'll need several posts to explain all of this because you really need to understand the technical facts involved and at least some of the science to get you past the witchcraft theory explanations. So don't stop reading here, I'll do my best not to bore you with a bunch of esoteria that would surely knock you out. I'm going to use simple, easy to understand explanations for all of this.

First and foremost, most marine corrosion occurs due to what we know as galvanic activity. Galvanic action requires some key components to occur and removal of any one of the neccessary components will stop the corrosion. These components make up what we know as a galvanic cell, a fancy term for a battery. The components are:

  • An anode (could be your saildrive)

  • A cathode

  • Electrolyte (the water your boat is floating in)

  • A good electrical connection between the anode and cathode.

What most people are not aware of is that any time you are plugged in at a dock, whether the shore power is turned on or not, you are providing one of the components needed to create the galvanic cell mentioned above, that is a solid electrical connection between an anode and a cathode. In the diagram below, we see the connection via the shorepower system's green safety ground wire.

This all important wire is continuous throughout the entire dock wiring system. So, anytime your boat is plugged in to a dock box, you are electrically connected to all of your dock mates. Think back now to the components needed to make up a galvanic cell, you're plugged in, so the hard wire electrical connection is made, your boat is sitting in electrlyte, so now all you need to complete the chain is an anode and a cathode. Studying the diagram above you'll notice that the engines, and through-hull fittings represented by the three small rectangles at the bottom of each hull picture, and the propeller shaft anode on the right hand boat are all connected via the green ground wire. This is how things should be if the boats are wired in accordance with ABYC wiring Standards. So now, let's say the boat on the left has a conventional engine, propellor shaft and bronze propellor and the boat on the right is your boat with a Mercury Alpha outdrive, or a Yanmar or Volvo saildrive. We're talking about dissimilar metals here, exactly what's needed to create an anode and cathode relationship. All metals used in marine construction differ in their properties, that's easy to understand, some metals are anodic in their properties, some are more noble, or cathodic, in their properties. Without dragging you into the esoteria of all of this just understand here that your drive unit falls into the potentially anodic category, and the bronze propeller in this example is much more noble, or cathodic metal. So, your outdrive is potentially at risk when matched up to your dock mates bronze prop. Given enough time, your boat's anodes, and then your drive after the anodes deplete, are going to become the sacrificial component in this giant galvanic cell we've created just by plugging in to the shore power supply. So how do we prevent this activity? Enter the galvanic isolator.

What's a Galvanic Isolator?

Here's where we get into a little bit of the science involved in this situation. First of all, understand that galvanic electrical current is very low-level direct current (DC), less than about 1.5 volts DC. Next, understand that a common semi-conductor called a diode has a valuable property that gets applied to this situation. Diodes have an inherent property, that is, they have a voltage loss as DC flows through them, typically about 0.6-0.7 volts of loss. Now, if you connect two diodes in a series configuration, the loss is cumulative, meaning they will effectively "block" about 1.4 VDC of electrical current. We need to block this current both leaving and coming into your boat from a neighbor on the dock. So, a galvanic isolator uses another property of diodes and that is that they will allow some current flow in one direction, but not in the opposite direction. Using an electrical schematic, the arrangement inside a galvanic isolator looks like this:

The triangle shaped components shown above represent the electrical symbol for a diode. The arrows indicate the direction of flow, and the straight line next to the triangles illustrate the "block" if you will. Now, the additional component shown, the capacitor, adds to the mystery, what's that for? Its there to help protect the diodes from low level AC current that we know is often present in the green grounding wire in your AC circuitry. This parallel connection allows the low level AC current to pass through the isolator and leave the diodes alone. Over time we have learned that continued exposure to the low level AC in this wire can damage the diodes and cause them to fail. Capacitors allow AC to go through them, but not DC, their primary electrical characteristic, so the capacitor is there to provide and alternate route for the low-level AC current to follow, leaving the diodes alone.

By blocking 1.5 VDC of electrical current flow, we can eliminate one of the primary components of the galvanic cell, and corrosion can be mitigated. You need a galvanic isolator on your boat if you are plugged in to a dock shorepower system. Why? Because based on the science described above, if you are connected to your dock mates, then the anodes on your boat may be helping to protect the underwater metals on somebody else's boat. Now, understand this, the anodes provided on your outdirve are only sized to protect your drive, thats all. If they get forced into a situation where they have to protect other metals, they are going to deplete very rapidly, and the next bit of metal in the galvanic food chain is your drive, and for sure its going to get eaten alive. Some examples of currently available galvanic isolators are shown in the photos below, one is made by ProMariner and the other by Dairyland Industries. You can learn more about their products by visiting and respectively. Examples of their isolators are shown below:

In my next installment to this blog, I'll show you where this vital component needs to be installed in your electrical system and I'll talk about the important installation and specification considerations you need to make. Then I'll begin discussing the maintenance issues you need to consider to help keep your drive alive.