Beware the Unprotected Saildrive
Paying close attention to anodes and your sailboat’s bonding system will help prevent corrosion that can quickly cripple these power units.
On several boats I’ve inspected after the damage was done, lead or cast-iron keels had been exposed to seawater due either to faulty paint systems on the metal keels or, in one case, because the owner was racing the boat and had wet-sanded the epoxy barrier coating on the lead down to bare metal. In any of these cases, the anode-to-cathode surface-area relationship had been altered, and insufficient anode area caused the factory-installed anodes to dissolve quite quickly, in a matter of weeks instead of months. Remember that the next bit of metal in the corrosion food chain after the anodes is the drive casing itself, which, made of aluminum, is far more susceptible to anodic action than bronze through-hulls or other fittings.
The Cathodic Protection System
To fully understand the cause of this corrosion, you need to understand how and why a properly designed cathodic protection system works. The system begins by tying all of the underwater metals together electrically via a bonding system. On most boats, this is accomplished with green insulated wire, sized 8 gauge or larger. What happens next is an electro-chemical process known as polarization of the metals. You see, all metals when submerged in an electrolyte solution (such as salt water) have a voltage potential. Potentials vary depending upon the specific alloy used. By electrically connecting all of the various alloys, once polarization occurs, the voltage potentials equalize. Anodes need to be tied to this same bonding system, and the anode material must have a voltage potential at least 200 millivolts more negative than the rest of the system. The anodes then become the sacrificial component within the system; enough anode mass needs to be added to ensure satisfactory service life, which is generally considered to be six to nine months. At that interval, you need to install new anodes.
Hey, It’s the Neighbors
Essentially what’s being created with this bonding system is what we call a galvanic cell, with an anode and cathode connected electrically and both submerged in the same electrolyte solution, in this case, the water around your boat. Think of it as a giant battery. Low-level current flows from the more negative anode to the cathode, but because each element of the cathode has undergone polarization, their potential voltage is equal, so none experiences corrosion. Instead, the anode slowly wears away.
Remember that there has to be a difference in potential for current to flow. That’s the operating premise behind bonding; if all of the potentials are equalized due to polarization, there can be no current flow between the individual elements. So a boat with proper anodes, sitting in a lone anchorage or on a mooring, isn’t likely to suffer from galvanic corrosion.
But what about boats in a marina? That’s a whole nother story. The issue at hand is that the green wire that ties the metals on a boat together is a part of the overall grounding system and, so, is also connected to the boat’s shore-power grounding system. The significance of this is that whenever your boat is plugged in to shore power, it means that you’re electrically connected to all of your dock mates via the green wire, which is continuous throughout the entire dock.
Because of these continuous links among all the boats, your boat’s anodes may actually be helping to protect one of your dock mates from corrosion, putting a heavy demand on your boat’s anodes and causing accelerated anode depletion. In other words, the boats around you are causing your anodes to wear away faster than you might expect; hence, your saildrive is unwittingly put at risk.
To mitigate this problem, you must have a galvanic isolator installed in your shore-power system. In essence, this device is installed in series with the green wire in your shore-power wiring, in accordance with the American Boat & Yacht Council’s Standards, Section E-2: “Cathodic Protection Systems.” The galvanic isolator works by blocking low-level galvanic current, but it allows the passage of AC fault current in the green wire if called into action due to an AC-system short circuit. It’s important to remember that by removing any one of the components of a galvanic cell, which needs an anode, cathode, a hard-wire connection between the two, and an electrolyte, you stop the corrosion process. The galvanic isolator blocks the hard-wire connection to others in the marina, leaving your anodes to work for your boat alone.