As part of our First Mate Bi-weekly Vessel Checkout program, we perform a brief but valuable check for AC stray current that may be affecting the safety of your vessel.
What is AC stray current?
In a nutshell, all of the AC electrical current that comes from the dockside electrical system, through the shore power cable and onto your boat should return via the same path. The net result of this current flow should be zero.
For example, say you have a single 30 amp shore power cable supplying your boat. If you have an air conditioner running and a water heater heating, you’re probably pulling about 20 amps. All of the amperage your appliances are consuming is supplied through the 120v AC black “hot” wire of your shore power cable. All of that current should return to shore via the 120v AC neutral “white” wire in the same cable.
For safety reasons (in order to protect against possible shock or electrocution dangers), the 120v AC green “grounding” wire is connected on board your vessel to the DC ground system, which is in turn usually connected to a bonding system of the engine block. These are ABYC standards that define how your boat is wired. Long story short, if there is an electrical fault or problem of some sort, electricity has a path to ground through your boat and into the water. This means it is not returning to shore via your shore power cable.
How do we check for stray AC current?
We use a special AC amp meter and clamp it over your shore power cable. No matter which AC appliances are running on the boat, this reading should be at or near zero amps. This means that the net result of all current flow entering the boat minus all the current flow leaving the boat through the shore power cable is zero.
Some small amounts of AC stray current is acceptable. The accepted threshold is 30 milliamps (mA). This mirrors the trip point of residential GFCI 120v AC outlets you likely have in your house in kitchens and bathrooms. Anything under and up to 30 mA is not considered lethal to humans.
What are the risks of excessive AC stray current?
For people on your boat – hopefully the wiring and supporting systems are in good shape and this stray current is travelling through it, going harmlessly into the water. However, scenarios could easily arise where people on board could be at risk of experiencing electric shocks when touching certain metal objects (faucets, thru-hulls, etc.).
For people in the water around your boat – While the probability of this may seem remote, numerous drownings have been attributed directly to AC stray current in the water originating from electrical faults on adjacent boats. If levels in the water are high enough, muscles can uncontrollably contract, resulting in drowning. Divers regularly can detect a “tingling” sensation when levels are high enough. This is one of the reasons no swimming should be allowed in marinas. Children especially are susceptible to this. Keep in mind also that boating is a leisure activity, often with alcohol involved. An accidental trip or slip resulting in a fall into the water could be catastrophic.
What is the recommended course of action if excessive stray current is detected?
If our First Mate Checkout technician detects stray current above the 30 mA threshold, we first confirm our reading with a second meter. Once the reading is confirmed, we need to make the determination as to whether the electrical fault is originating on the boat or from the dock (it could be from another vessel in the area or a problem with the dock’s wiring). The next step is we turn off all main AC breakers on the vessel. If the stray current goes away (returns to levels under 30 mA), we know with certainty there is an electrical fault on the boat. However, just because it doesn’t go away doesn’t mean we can assume the boat is not the source. At this point, it requires a more in-depth inspection and analysis by one of our ABYC certified Marine Electricians, and the use of special break-out shore power cable adapters to isolate the problem. If the determination is made the problem is on the boat, we can continue troubleshooting and make recommendations for repairs once issues are identified. Depending on the complexity of the vessel’s AC electrical system, this could be brief or very time consuming. If we find the problem originates from the dock, we inform the marina of the situation and they have their electrical contractor investigate further (we regularly work with marinas to solve these types of problems). In any case, this situation should be taken seriously. Bad things can happen very quickly.
The presence of AC stray current can be affected by numerous factors. What appliances on board the boat that happen to be running or cycling at the moment AC stray current is checked is a major one, for instance. Also, what AC equipment other boats in the area may have in operation can also significantly impact the situation. Weather conditions, tide levels, ambient temperatures, etc. can all play a part in the overall equation and make electrical faults more or less present and detectable. The important thing to remember is clamping the shore power cable(s) with a meter takes an accurate reading at that point in time (a snap-shot, so to speak). We cannot guarantee there is no AC stray current present under different conditions at other times. However, by checking for AC stray current as part of our First Mate Checkout Program every two weeks, we provide you the best chance to identify, catch, and correct potentially dangerous issues that could be present.
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 (commonly incorrectly called “zincs”…) 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.
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):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:
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.
Was your vessel ready for Tropical Storm Cindy last week?
Put your mind at ease, protect your investment, and focus on other priorities by utilizing
True North Marine’s New Storm Support Services – Another service offered under our Captain-Lite Yacht Management Program
Our Storm Support Services consist of the following:
True North Marine is now a dealer for FCI Watermakers and Pompanette Air Conditioners
Check out more details below!
Being a dealer for these products gives TNM direct access to factory engineers, factory training, and repair/applications support. To our customers, this translates to a professional, reliable, and efficient installation that will work when you need it most!
Now is the time to get your boat ready for the Summer Season!
Have you been thinking about replacing those old sun-faded sailing instruments, or that chart plotter that refuses to turn on? Raymarine could be your solution!
In this Issue:
- Why choose Raymarine electronics?
- Why purchase a turn-key Electronics Installation from True North Marine
- Raymarine Special Rebate Offers
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Fortunately, there are solutions to these problems and they are not just another trade-off you have to make as part of boat ownership. Continue reading
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Why Buy Your Raymarine Electronics from True North Marine?
We realize there is stiff competition from online websites and other retailers when it comes to purchasing your electronics. Here are the Pro’s and Con’s when purchasing your equipment through True North Marine: Continue reading
Your engine is likely the single most expensive piece of equipment aboard. However, it seldom gets the attention that its replacement cost should warrant.
With the summer behind us, and boating schedules less demanding, the off-season is a great time to catch up on preventive maintenance. If you own a sailboat in the 28-60’ range, chances are you have a Yanmar, Volvo Penta, Westerbeke, Universal, or Perkins engine. While you might have something different, the discussion points to follow will probably apply just the same.