Steve -

The zincs on my boat need replacing very often. Is there anything that can be done?

It all depends on what you mean by “very often”. Unlike most things on boats that wear out and need replacing, zincs are intended to wear out. That’s why we call them sacrificial. They sacrifice themselves by electrically oriented corrosion, called electrolysis, which prevents much more expensive electrolysis damage to propellers, rudder shafts, and through-hull fittings.  

If you ever notice having a zinc that doesn’t erode significantly, start worrying.  It probably means that the zinc is not electrically connected to the equipment that it is supposed to protect. If the zinc is not eroding, then the protective electrolysis is not occurring.  

If your zincs are lasting less than several months, then you may have an electrical problem with your boat, or more likely, with the dock you are connected to. A common internal boat problem is using the bonding system for carrying ground current for on-board systems. A bonding system is the array of green wires or copper straps that connects all of the through-hull gear together. Using the bonding system to carry current will cause voltage drops that will force a small voltage difference between various parts of your running gear, accelerating electrolysis. The bonding system should be independent from your power distribution system, and connected to the power distribution system at only one point (normally the engine). 

Another high-speed zinc killer is shore power. There are many common problems with shore power (or your neighbor’s boat) that result in a small voltage on the safety ground wire.  If you have an isolation transformer on your boat, the safety ground can and should be disconnected.  It would be nice to solve this and many other problems with an isolation transformer, but the cost is often prohibitive. A far cheaper solution is a galvanic isolator (or “zinc-saver”). A galvanic isolator is a simple device that puts a “loose link” in the connection between the shore safety ground and your boat. It will allow a few tenths of a volt to exist between the shore power ground and your ground, without allowing damaging current to flow. If a safety fault occurs, the voltage rises and it will make the connection that is needed for safety. 

 Galvanic isolators do not solve serious shore power problems. They certainly will reduce the natural zinc consumption that is the result of protective current flowing from your zincs to even a proper shore ground. If the voltage is over a couple of tenths of a volt, the problem is too severe for them to help. You can get an idea of whether there is a problem that is too extreme for your galvanic isolator by measuring the voltage across it when everything is hooked up and operating. The voltage should be very low, using the AC and DC scale on your meter. I hesitate to quote an appropriate voltage, because you will be measuring the average voltage at one point in time. There can be peaks in the voltage that cause the galvanic isolator to conduct and of course your neighbor may turn on his miss-wired air conditioner, after you put your meter away. Two or three tenths of a volt might be OK. It the voltage is in this low range and you want to make sure, put a milliamp-meter in series with the isolator, to check for zero current flow. 

Occasionally, docks or neighboring boats can have faults that don’t show up on the shore power safety wire.  Sometimes the offending current is flowing through the water, in on one side of your boat and out on the other. You can check for this problem by using two identical “fish” zincs from your local chandlery. These are meant to be grounded and hung over the side, and usually come molded in the shape of fish. Just connect the ground wire from each of the “fish” to your low-voltage capable digital meter, and start probing in the water around your boat. If you find that the AC or DC voltage increases as you move the “fish” apart, then there is a problem. Continue moving the fish around and you should be able to find the offender by searching for maximum voltage.  

If you don’t have a problem with shore power, internal power distribution or a neighbor, then continue on. 

Electrolysis occurs anytime two different metals are in the same body of water, and are electrically connected together.  Basically what we have is a battery that is shorted out. The one metal is always eroded, and can be considered the fuel for the battery. The other metal, is undamaged or even protected from other less powerful dissimilar metal combinations that it might be involved in. I could go into a long sermon about the “galvanic series of metals”, but the important thing to remember is that zinc and magnesium are at the most corrosive end of the table, so they always end up being the fuel for the battery, thereby protecting your running gear. 

If you are still unhappy about replacing zincs, this can be reduced or even eliminated. If your boat is like most boats, with barely enough zincs, you can use large “fish” zincs that you hang over the side, while at the dock or at anchor. You can replace these with far less trouble and cost than a haul-out or a diver, for replacing attached zincs. If you choose this option, make sure that the “fish” ground wires have excellent contact with your ground “bonding” system. I have seen far too many zinc fish that are old and completely un-eroded, meaning that they haven’t been doing any good, due to poor grounding. If your zinc replacement is far over-due, definitely use the “fish” until you replace the zincs. To totally eliminate zinc replacements, invest in one of several systems that use carefully metered battery power as a source of protective current, rather than zincs. However, most of us pass them up as violations of the KISS principle. 

Stephen Sommer is a degreed electrical engineer with extensive experience in electrical, mechanical, refrigeration and air conditioning systems and holds a USCG Masters license. He consults in all areas of yacht systems, which include all the equipment on board yachts beyond a basic hull and motor or sails.

757 SE 17th Street, Suite 237, Ft. Lauderdale FL, 33316

Boatek.com has written articles for  Dockwalk and is a regular monthly columnist in South Winds magazine. To see a current list of our published articles

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