Galvanic Voltage & Water Salinity Measurements
Many of our customers have been surprised at haul-outs by corroded underwater hardware (props, shafts, struts, sail drives, etc.). This in turn leads to very expensive repairs that can unexpectedly strain a boating budget. If you’ve ever been through this awful experience, you know it’s definitely not something you want to go through again. The truly frustrating thing is that this is completely avoidable.
Many people think Galvanic Corrosion and the how, when, and why anodes should be replaced — leans more towards voodoo witchcraft than science. Thankfully, this thinking is absolutely wrong. The condition of your underwater anodes and whether or not they are effectively protecting your expensive underwater hardware from corrosion is absolutely measurable and quantifiable.
Incidentally, most people commonly refer to anodes as “zincs”. Zinc is actually a type of anode. Anodes can be made of zinc, aluminum, or magnesium alloys. The correct term to describe them is “anode”.
As part of our First Mate bi-weekly vessel checkout, we measure the galvanic voltage on your drive system(s). These may be traditional propeller shafts, outdrives, sail drives, or IPS units. We use a special reference cell that goes into the water next to your boat. The lead from this reference cell plugs into a multi-meter, and the other meter lead is connected to your drive system. By measuring and interpreting these readings, we are able to determine if your drive system is properly protected from galvanic corrosion. If your boat has equipment that is isolated from the boat’s DC ground (like a sail drive or bow thruster), it may be necessary to measure at several important locations.
As the anodes do their job, they will physically erode over time. As they erode, the voltage readings we measure will decline. Once the voltage reading dips below the recommended protected range for your type of drive system (it varies by the type of metal the anodes have to protect), anodes will need to be replaced. The idea is that once new anodes are installed, the voltage readings should be at or near the upper range limit. This will give them the maximum lifespan (amount of time before they erode enough to reach the lower limit), and the longest interval between replacement. Sometimes additional anodes need to be installed so the system starts out at the proper upper limit voltage.
A very important misconception is what type of anodes should be used on vessels in and around the Clear Lake area. You will consistently hear people in the area refer to them as “zincs”. This is probably because most people, including most contractors in our area, sell and install zinc anodes. Unfortunately, this type of anode is not the correct fit for our waters.
Clear Lake is unusual in that the water salinity can vary greatly. Heavy rains, extended periods of time between rainfalls, or storm surges can cause the water salinity to vary greatly. However, it almost always stays with the salinity range that would define it as brackish water. If you keep your boat in Galveston, say at Pelican Rest, water salinity levels are much higher, and zinc would be appropriate. Here are the classifications of water based on salt content (in parts per thousand):
Fresh Water: 0 to 0.5 PPT
Brackish Water: 0.5 to 30 PPT
Salt Water: Above 30 PPT
Each of the above water classifications requires a different anode material, as metals can be more or less active as salt water salinity changes. Below are the recommended anodes for the three water types:
Fresh Water: Magnesium
Brackish Water: Aluminum
Salt Water: Zinc
What happens if you use zinc anodes on Clear Lake? Several things take place… First, with newly installed anodes, your galvanic voltages (whether you measure them or not) start out well under the upper range limit, giving you less time before those voltages dip below the lower range limit, meaning a shorter lifespan between anode replacements. Also, since zinc is less active in our brackish waters, they will physically erode at a slower rate. If you are using a visual inspection method to determine when to replace them, you can easily inadvertently go too long. The end result is your underwater hardware can be chronically under protected, resulting in corrosion damage.
If you have ever hauled your boat and have seen bronze parts (props, struts, rudders, etc.) with a pink hue to them, this is a definite indication of chronic under protection, meaning the galvanic voltages at the drive system sat for extended periods of time below the lower limit of the protected range.
Why do we record water salinity readings during First Mate Checkouts? These figures enable us fully interpret your galvanic voltage readings. Sometimes after a period of heavy rainfall we may see a sharp drop in water salinity levels. This, in turn, may cause galvanic voltage readings to dip below the recommended protected range. By understanding average water salinity for your area, we can determine if the out-of-range reading is a temporary issue that will correct in a matter of days, or if anodes actually should be replaced.
In summary, during each visit to your boat as part of our First Mate Checkout program, we monitor and track these important galvanic voltages in order to provide definitive, closed-loop indication of when your anodes require replacement. The benefit to you is the peace of mind that no corrosion is happening while you sleep, and you don’t spend any more or any less on anode replacements than is required to properly protect your equipment.