Typical hull potential test set-up

We received an interesting comment regarding our earlier post, "To Bond or not to Bond". It's worth sharing so that people will understand some of the dynamics involved with that case. So, here is the comment:

Ed- "The comment I have on this situation is that given the original installation was done poorly, what is to guarantee the addition of a bonding cable would have been done any better? Regardless of how it was installed (bonding or no bonding), for my money, I would have expect that testing with a silver/silver - chloride cell being thrown over the side was done to make sure there was nothing amiss. A relatively simple and relatively cheap task given the risks.

In a similiar situation and a previous life, I demanded that a megohm test be done on all cabling pulled through metal conduit to make sure that the insulation on the cabling was still intact. I was burned once and doing this testing afterwards never let me down again.

To my knowledge, there is no type of QA checks like this in the ABYC standards. Should something like this be considered or have I missed it?"

Thanks,
Ken

So, let's walk through this step-by-step. First of all, many of you may not be familiar with what Ken is referring to when he mentions the silver chloride cell. In the photo above the black cylinder at the end of the long red cord plugged into the multi-meter is a silver chloride reference cell. Many corrosion analysts use these cells to provide a constant and reliable sensor for detecting low level milli-volt readings with the meter. The cell is put into the water the boat being checked is sitting in and plugged into the "com" lead socket on the meter. The + lead on the meter is connected to the boat's grounding system. The meter gets turned to the DC volts scale and a reading of hull potential is measured. This potential reading is an indicator of whether or not the boat is equipped with enough anodes to protect the underwater metal on the boat. It's important to note that the acceptable readings will vary depending upon what the anode material is made of, either zinc, aluminum alloy or magnesium alloy, and what the salinity level of the water the boat is floating in. Keep in mind here that this is all a measurement of galvanic level voltage potentials. Typically 1.6 VDC or less.  Now in the case we are discussing here, there was a situation where battery level voltage potential was leaking out through the light fixture. So, would that have shown up if the boat had been checked prior to the boat being delivered to the customer? Yes it might have as long as the light was activated at the time of the test. But to Ken's point that this should be a routine test that a boat builder performs as part of a general quality assurance test, that may not be practical in the real world. Let me explain why.

The actual hull potential reading for a boat is going to vary depending on a variety of environmental factors including water salinity, temperature and water flow (tidal currents). So, to be accurate, these tests need to be made in the location where the boat is going to be spending most of it's time in the water. In most cases, the boat builder has absolutely no idea where the boat is going to end up living, so precise levels of cathodic protection (what we are talking about here) are difficult to achieve at the factory. So in my view, cathodic protection levels really should be part of a delivery inspection once the boat arrives at its home port. I don't know of any builder that actually does this. Most builders install a few anodes on the boat in the usual places and hope for the best.

As for Ken's thinking that questions whether or not bonding the subject boat would have made any difference in the first place, I can say definatively that had the boat been bonded in this case, the boat would have been saved.

As to whether the ABYC has QA test requirements within it's standards at all, yes we do. ABYC standard H-24 for example, requires a boat builder to pressure test every gasoline fuel system as installed to ensure that no leakage is present.

You see, the ABYC standards are generally classified as safety standards. So, this begs the question about what constitutes proper levels of safety. Ken is quite right that we require no such broad based safety checks for electrical systems as installed. In this case, the fellow that bought the boat which sank was intending to do a lot of offshore sport fishing. If the light fixture had failed while he was motoring along 30 miles offshore and the boat filled up with water and sank, that's a potentially horrific safety issue in my view. But, I can tell you that in my time sitting in on electrical committee meetings with the ABYC, I've never heard this issue brought up.

Finally here, to Ken's mention of megohm tests for boats. Again, for those of you not familiar with such testing, this type of test is used to test the integrity of electrical insulation and involves shooting a pretty high voltage through the system and determining if there are any breakdowns in electrical resistance or insulation that could cause a problem. In commercial boating environments, this procedure is actually a requirement as part of periodic inspections. It is really quite an involved procedure to perform a comprehensive megohm test on a modern boat's electrical system. Statistically, I can tell you that the number of system failures like the one described in "To Bond or Not to Bond" is so small, that in spite of the severity of that situation, the number of actual incidents like it in a given year are so small, I could never sell the concept of a required megohm test or cathodic protection level test to the committee that governs these things. Plain and simply, there is not enough of a demonstrated needs for such tests.

Every now and then the topic of bonding a boat's underwater metal comes up. Sometimes pretty heated discussions can get going among the marine systems geeks that get interested in this topic. In any event, its worth sharing some insight into this topic with all, especially in light of my post last week that discussed the poor fellow's new boat that sank due to an issue with electrolytic corrosion.

When we talk about a bonding system on a boat we are discussing a system, usually made up of minimum 8 AWG wire and or copper strap that interconnects all of the underwater metals and also large metal objects on board like metal fuel tanks and such. The system can also become integral with a cathodic protection system. This all gets tied into the boat's grounding system. A line drawing, which comes from the ABYC Electrical Standard E-11 is shown below and it illustrates all of this inter-connection.

The area to focus on here is the lower portion of the diagram where you see the zinc (think anode, it may actually be made of aluminum alloy or even magnesium in fresh water). You can also see bits of underwater metal, like seacocks and such tied into this system.

The system is also tied into the boat's grounding bus and ultimately the boat's AC and DC grounding bus bars. Which are connected to the boat's battery(s).

The idea here is simple, but really does confuse most people. It all boils down to a basic electrical fundamental that states that if there is no difference in potential (think voltage here)from one point in an electrical circuit to another, then there can be no electrical current flow. Well, with that thought in mind, if all the metals are tied together via the bonding system, then they become equalized electrically and even if there is an induced ground fault leak from the battery as happened in my underwater lighting case, if all the connected bits of metal are at the same electrical potential then there will be no current flow. No current flow= no corrosion of underwater metals.

The bottom line here? The underwater lights on the sunken boat had not been bonded. Current leaked right out of the fixtures into the water and found an alternative path back to ground, actually via a nearby transom anode in this case. The anode was connected to the bonding system, and ultimately the batteries on board. The bonding system brought the fault current back to its source. Unfortunately it took out the light housings as a part of the process in this case.

Today's installment is a sad one. Its the story of a brand new boat, the buyer's dreamboat if you will, that sank in its slip about 1 month after he took delivery. The photo below shows the hole in the boat that caused the sinking.

Hole where under water light fixture was fitted

The white ring you can see around the perimeter of the hole is where the flange for the fixture was fitted. The white stuff is the 3-M 5200 used to seal the fitting. So the question is, where's the flange and the light?

This my friends is a sad tale of electrolytic corrosion, or to most people, stray current corrosion. This is of the DC variety and I can tell you that what you see here did not take to much time to occur. Although we can't be sure of the exact amount of usage on the underwater lights, what I can tell you for sure is that whenever they had power to them there was also a low level short circuit from the DC power feed to the light that was touching the bronze housing for the light. Why? Simple, the installer did not use an insulated terminal. But further the inside of the housing was not provided with any insulation by the light manufacturer, it was just nice raw bronze, totally uninsulated.

I can also tell you that the light was of the halogen variety and was therefore rated at a fairly high DC current, a little over 8 amps to be exact. So, what was going on here is that whenever the light was turned on, a portion of that electric current was exiting the fixture through the flange of the light, forcing the flange to act as an anode. The bottom line? The flange face simply fizzed away un-noticed and ultimately popped off. The light fixture then fell backwards inside the boat and the rest is history. This fellow's $400 K dreamboat sank in its berth.

So what's the lesson here? Make sure that there is no way possible for the power lead or terminals supplying power to any underwater light fixtures can come in contact with one another. That point of contact is a guaranteed recipe for disaster!

Xantrex has been busy. I just received one of their newest inverter chargers and Freedom Sequence units to check out and I'm totally impressed. This is a cool system. The inverter charger is shown here:

OK, so I know some of you that are regulars here are sitting back saying Ed's losing it.....what's so exciting about an inverter charger? Listen up, I'm not losing it. This thing is really part of a nicely designed system that has some functionality we just don't see all that often. Not that what this system does hasn't been done before, it has. But, I think the level of sophistication Xantrex has brought to this sort of system is really pretty special. To begin to get a feel for what I'm talking about here, check out the possible system design shown in the diagram below:

Not shown is the optional generator auto-start controller. Also, the Freedon Sequence is something you would probably not understand without a little explanation. Basically it is an automatic AC input source controller and programmable load shedding device.

As today's boats get increasingly more complex, in large part due to customers "appetite for amps" as I always describe it, systems like the one shown above are actually becoming commonplace. A typical RV is going to be set up much like what you see in the diagram. Swap out the land based generator shown with a marine unit and you're in. But why is source switching and load shedding important you may be asking? Several reasons come to mind. First on the source switching. Some of the equipment we are attempting to power up is voltage sensitive. As overal power grids get over-loaded, system voltages drop. Having the ability to sense this and switch to an alternate source is a good idea. As for load shedding, things like cycling loads common to air conditioning and refrigeration systems come to mind. A modern cruising boat, either power or sail will easily have multiple refrigeration and or air-conditioning systems. Block ice and open hatches are getting to be quaint memories of times gone by. But, we don't want to be tripping circuit breakers day in and day out. So, with a system like this the owner can select and prioritize circuit loads and the system will automatically shut things down if the system starts to get over-loaded for any reason.

 So, advanced power systems such as the diagram shows are rapidly becoming the norm. The programmability of this new system enables the boat owner / installer to absolutely maximize the system to achieve top efficiency and component space management; all really important considerations on today's boats.

The other hidden secret with this new system is that it can be paralleled. What this means to the boater is that if their needs go beyond what is shown in the diagram above, they can expand the system easily by adding a second paralleled inverter, effectively turning their 3 kw system iinto a 6 kw system. The goodness doesn't stop there either, big inverters need big battery banks to feed them, and these battery banks need to get recharged often too. Xantrex made sure those needs were going to be met with this set-up. The battery charger side of the inverters can also be paralleled. What that means is that with the SW 3012 as an example, by adding a second inverter charger unit in parallel the system charge capability would become 300 amps or C/5 (1/5 th the battery bank capacity in amp hours) whichever is less. That's huge! That sort of output helps to answer the fact that some of the newest battery technologies have huge re-charge acceptance rates in amps. This system output potential can help feed that need.

So, if you are looking to upgrade or working on a new build and are considering your inverter charger options, I encourage you to get a look at this system, I'm impressed, I think you will be too.

A pair of really good and pertinent questions came in yesterday that need to get shared with everyone. The photo below is of a sacrificial anode attached to the end of a string of green bonding wires which are in turn connected to some through hull fittings on the boat in question. The anode had been stored neatly in a locker located in the head on the boat.

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So, our reader had two questions, here they are:

"Is a zinc effective when out of the water but still attached to the bonding system?"

Answer: Absolutely, positively NO! Sacrificial anodes, whether they be made from zinc, aluminum or magnesium are part of a system. The system is known to tech types as a "cathodic protection" system. In fact, the ABYC standard E-2, which is entitled Cathodic Protection Systems explains all the requirements for such a system. Anyhow, this system requires both an anode and some cathodes. The anodes, or zincs as our questioner describes them are the sacrificial component. But, it must be submerged in the SAME electrolyte as the cathodic pieces (in this case the through hull fittings in question) it is intending to protect. So, that means the water the boat is floating in. This is all a part of what is known as a galvanic cell, which is comprised of an anode, cathode, electrical connection between the anode and cathode and again, a common electrolyte, in this case the water the boat sits in.

Now, it is a little bit more complicated than that because this whole system needs to be calibrated, if you will, so that voltage potentials etc. are within prescribed quidelines; it does take an experienced technician to make these adjustments. But, suffice to say that an anode sitting in a relatively dry onboard locker, connected or not to a bonding system, is doing nothing to contribute to the protection of the underwater metal.

Second question: "Assuming a boat is fresh water cooled and has marlon through hull fittings, would the engine zinc for the raw water side of the cooling system need replacement less often if the ball valve for the water intake is kept closed when the engine is not in use?"

Answer: It probably won't make any noticeable difference to anode life. In the case of engine internal anodes the things that really impact their life expectency is the rate of water flow past them, so if an engine is run more frequently, that will reduce their life. But, still water will still be sitting in the engine whether the ball valve is open or not so I really don't think any quantitative difference in anode life could be realized.