FAQs

Q. Does alternating current cause metal corrosion in boats?

A. This topic has been debated for decades and, in fact, can get some pretty heated arguments going. Unfortunately, there is no simple and easy answer to the question. Perhaps the best answer is to break the question into two parts.

1. Can alternating current cause metal corrosion?
2. Can alternating current as distributed at a marina be a cause for underwater metal corrosion, particularly to a boat's underwater metal components?

I am quite confident in saying that yes, there is enough evidence from field reports and, in fact, my own experiments to support the idea that alternating electrical current can cause metal corrosion. Everett Collier, in his book "The Boat Owner's Guide to Corrosion which the ABYC currently uses as a text for its Corrosion Certification Course, says it this way: "There is a school of thought that says stray AC cannot cause corrosion. The logic is that since alternating currents are equal and opposite—that is, positive and negative half-cycles—any metal that was ionized during one half-cycle would be redeposited on the other half-cycle. This is a purely theoretical view and assumes 100% efficiency in the reactions during both half cycles. In the real world, however, not all the metal particles dissolved into the electrolyte on the anodic half-cycle are recovered on the cathodic half-cycle. The result is a net loss of metal particles, or corrosion, due to AC stray currents."

Certainly it has been the ABYC party line for some time that AC is not a contributor to the metal corrosion we typically see on boats. There is no doubt, and certainly not much debate, however, that common causes of corrosion are distributed by the AC system at a dock, via the grounding conductor, or the green wire. These causes can be either at a galvanic corrosion level, or possibly by a stray DC current, induced at battery level potentials. Proper wiring, galvanic isolators and isolation transformers are the front line of defense here. But, the question is still not answered. Collier seems quite clear in his description, but the debate continues.

Perhaps the real answer comes from the underground piping industry. In the current version of the ABYC Corrosion Study Guide, I've included a copy of a white paper that was created by a Mr. Roger Ellis and based on a considerable amount of scientific research done by Shell Oil in the UK to determine the causes of corrosion to their underground oil distribution pipeline system. The paper states that under laboratory conditions, it has been demonstrated since the 1960's that AC can cause corrosion to steel. I have demonstrated to myself and attendees of the Corrosion Certification Course that I can induce corrosion to aluminum plate stock, and have also demonstrated its effects on stainless steel. I have not been able to demonstrate noticeable effects on bronze castings, however with my current demonstration model. As for the British pipeline analysis, one of the key discoveries is that high current density is needed to induce corrosion with AC. In fact, that report states "corrosion is unlikely at AC densities of less than 20 amps per square meter." In my experiments, I have easily exceeded that threshold.

So, the big question is, can AC leakage in a marina environment exceed these 20 amps per square meter threshold in the water? I am of the opinion that it is unlikely, but am open to scientific proof that indicates otherwise. I do believe that AC can cause corrosion; I've made it happen quite conclusively, my question is still one of current density, and the ability of the water in a marina to sustain that level of current density. I have personally never seen a case of boat corrosion that could not be explained by something other than an AC induced fault.

When an issue generates interest and questions on a specific area, a Request For Interpretation is often the result. The following is the product of the PTC committee's determinations. RFI from the July 2004 Electrical Committee Meeting:

E-11.12.2.4 calls for overcurrent protection in the primary conductors of a transformer that also provides protection for the "secondary windings(s)." This section only references the "secondary windings." It does not identify the secondary conductors feeding power to the AC distribution system.

Q . Is circuit protection necessary for the secondary conductors if the transformer is a simple "1 to 1" transformer and the secondary conductors were sized to accommodate the level of current necessary to trip the primary circuit breaker?

A. The committee decided that secondary circuit protection is NOT necessary when a "1 to 1" transformer is used and the wire is sized properly. E-11.12.2.5 calls for the installation of a circuit breaker on the secondary of a 120/240 VAC transformer but provides no direction on the location of the circuit breaker.

Q. Is the secondary of a transformer considered to be a source of power for the purpose of E-11.12.2.9.1.2?

A. No, the transformer secondary is not considered a source a power. The committee stated that a circuit breaker is does not have to be located at the transformer as long as it is located somewhere between the transformer secondary and the load distribution panel.

Q. If the secondary conductors in a 120/240 VAC transformer installation were sized to accommodate the level of current necessary to trip the primary circuit breaker, AND additional simultaneous trip circuit protection was provided in the secondary conductors at the main panel, in addition to branch circuit protection, would it be necessary to provide overcurrent protection in the secondary conductors within 7 inches (40 inches if in a sheath) of the transformer? (Does this scenario satisfy the intent of the section?)

A. The committee felt that, as long as the secondary conductors were protected from overcurrent by the secondary breaker there is no opportunity to overload the transformer secondary due to an unbalanced 120/240 volt load condition. In the event of a short circuit between the secondary connection and the secondary breaker, the short circuit current would be reflected in the primary and trip an appropriately sized primary (shoreline main) circuit breaker. If you have any questions regarding the application ABYC Standards that would be of general interest to the membership, we would like to hear from you. Please submit your questions for publication to jramsey@abycinc.org. Those members whose questions are selected to answer in the FAQ column will receive a small token of ABYC appreciation.

Q. There has been a lot of talk concerning CO poisoning and boating. What can we do to educate our customers to help avoid CO poisoning?

A. There are many precautions one can take in order to mitigate exposure to CO.

1. Know the symptoms - the first signs of CO poisoning are headache and nausea, too often this is contributed to sun and motion, keep in mind it may not be.
2. Recommend a MARINE RATED CO detector to customers with boats with a cabin and an inboard gasoline engine or generator set. The MARINE RATING is important, this means the unit has been built to withstand the harshness of the marine environment.
3. Locate and point out the exhaust terminus for your client/customer. Let them know that this is the source for CO production and that consideration should be given at all times to it's proximity to:
   1. Swimmers
   2. Other boats
   3. Wind direction (e.g. transom exhaust, following wind)
4. Open ports, canvas configurations and wind conditions can all contribute to the location of CO onboard a boat. These are unique to different makes and models, builders of newer boats have indicated susceptible areas by labeling, older boats may not have this advantage.
5. Make a copy of TH-22 "Educational Information About Carbon Monoxide" available at your shop. The document is a FREE download located at www.abycinc.org. Just drop us a line and let us know you are using it! There is also end-user information located at www.uscgboating.org available to you free of charge.

Q. I'm trying to add some 120V AC outlets to my boat for more powering options at the dock. Can I just go down to the local hardware store and pick up any backing box for the outlet?

A. The most important thing to look for when purchasing an outlet box for your boat is the UL mark. In a 120V system you will want to look for the UL 514 mark on the outlets and the boxes. Using products with this mark eliminates the chance of an un-marked product failing and causing an unsafe situation.

Q. Our marina is thinking about re-wiring our docks. How do we do it so we are ABYC compliant?

A. ABYC standards stop addressing systems at the shore power plug, so we don't have anything in our book that targets this issue. However, NFPA 303 Fire Protection Standards for Marinas and Boatyards, covers this exact topic in accordance with the National Electric Code. Contact information and document ordering information can be found at the FPA website at www.NFPA.org.

Q. I am replacing the navigation lights in my 1985 boat. I have heard of some recent regulations that have stated that all navigation lights on boats have to be marked "USCG approved. When I go to my local marine store I see lights that are labeled both USCG and A-16. Can I use the models that don't have the USCG marking?

A. The regulations you speak of are true. However, in your case (aftermarket applications) you may use either type. The new standard is strictly for OEM boat building. When replacing the lights in older boats, either style may be chosen, but it is good to look for the new certification.

Q. Our company is designing a rather large boat and has questions about voltage drop. We know that essential items like navigation lights and equipment must be wired to a 3% voltage drop. What about our bilge pumps? The run for the wire is fairly long and to achieve 3%, we must use a large wire to connect to the provided pigtails. The wire we would like to use achieves 6%. What is the voltage drop requirement for the bilge pump setup?"

A. In general a less than 3% voltage drop for all systems on board would be a preferable. However, when cost, weight, and wire size become issues, some sacrifices must be made to fit the build of the boat. The issue of the bilge pump however leaves us with only one choice: a 3% drop. As with most motor operated equipment on board, improper voltage can harm the performance of the unit. With this fact, bilge pumps become an "essential" item on the boat and should be factored into the 3% drop category.

Q. Does the fill tube need to extend to within 1" of the bottom of the tank? I read this in a book written back in the '40's by Sam Rabl. He explained that this would greatly reduce the likelihood of an explosion if there is a spark when fueling, as it would only allow the vapors in the fill tube to ignite, instead of the entire contents of the tank. I just looked in my copy of the ABYC Standards, but did not see any mention of this requirement.

A. You are correct that ABYC does not require the fill tube to extend to within one inch of the bottom of the tank. One reason is that would make it very difficult to achieve a nine gpm fill rate. In addition, ABYC requires all components of the fuel system to be grounded so that its resistance to the boats ground is less than 1 ohm, which greatly reduces the chance of a static spark.

Q. Why do all references to carbon monoxide appear to pertain only to gasoline powered equipment? Does not the combustion of Diesel fuel produce CO?

A. You are correct, diesel combustion does produce levels of CO, however, not the levels that are seen in spark ignition engines. This is why the concern has focused on gasoline engines. Less than 100% efficiency in any combustion process (including diesel) will produce CO.

Q. Did ABYC make a rule change that changed the color of the DC negative wire from BLACK to YELLOW?

A. No. Included in E-9 DC Electrical Systems on Boats since 1996 has been the OPTION of using YELLOW as an alternate color for the DC negative lead. If there is an AC system installed on the boat then the BLACK wire can also be the ungrounded (hot) current carrying conductor. An increasing number of builders are using YELLOW as the alternative to black in the DC conductor to avoid confusion. Confusion, I might add, that could result in a very bad situation when the AC hot is confused with the DC negative.

Q. ABYC E-10 requires that I have a 12" "Dielectric shield" above my battery for the purposes of gassing. I understand this requirement when installing traditional "Lead Acid" type batteries, however, I am installing gel-type batteries and do not see the need for this spacing.

A. First, E-10 does not differentiate between traditional (lead-acid) and newer types of batteries (gel) that minimize or eliminate hydrogen gassing. Keep in mind that a battery is not permanent. At some point in time, the unit will have to be replaced. Generally these batteries are replaced with what the owner can afford or what the owner finds is available at the time. The suggested method of installation found in E-10 takes into account this common situation. Since hydrogen gassing can affect most materials used in component construction (e.g. aluminum cases on chargers/inverters, fuel lines) the standard calls for the 12" of dielectric shielding.

Q. Are devices installed in a diesel engine compartment required to be "Ignition protected?"

A. No. E-8, AC Electrical systems on boats and E-9, DC Electrical systems on boats both describe the need for ignition protection in spaces containing gasoline powered machinery, gas tanks, fittings or other connections between components of a gasoline system. Written below this requirement is an exception for boats using diesel as the only fuel source. For example, you may install a non-ignition protected device (any electrical component that is not labeled "Ignition Protected") in an engine compartment containing a diesel engine, however, add a gas generator set to that same compartment and then all electrical items must be ignition protected.

Q. Is there anyway that I can take a look at your standards book without buying it?

A. Yes! As BoatU.S. stores are renovated and remodeled, copies of the ABYC Standards manual are being displayed. To find out if the ABYC Standards are available at your local BoatU.S. store, give them a call.

Q. While rewiring my boat, a guy at the dock told me to make sure that I meet the CFR. What is it and how do I meet it?

A. The CFR are the Code of Federal Regulations. Title 33 & some of Title 46 CFR requirements pertain to recreational boats. As far as complying with the CFR, NO WORRIES!! If you meet ABYC standards you meet or EXCEED the requirements of the CFR.

Q. Can a diesel fuel tank have openings below the top of the tank?

A. Yes; ABYC Standard H-33, Diesel Fuel Systems is silent on the issue of location of diesel fuel tank openings and therefore does not prohibit openings below the top of the tank.

Q. Is a fixed fire extinguishing system required in the engine compartment of an inboard boat?

A. ABYC Standard A-4, Fire Fighting Equipment requires that inboard and sterndrive boats (both gasoline-fueled as well as diesel-fueled) with engine compartments shall have either a fixed fire extinguishing system installed in the machinery space or provisions for discharging a suitably sized clean agent portable fire extinguisher directly into the space immediately surrounding the engine without opening the primary access. Requirements for the discharge port, as well as labeling requirements, are given in the standard.

Q. Are there engine compartment ventilation requirements for diesel-fueled boats?

A. ABYC Standard H-32, Ventilation Of Boats Using Diesel Fuel states "power or natural ventilation is not required on a diesel boat, but may be used to control compartment temperature." The standard also provides guidance on other ventilation considerations such as supplying necessary combustion air, venting hydrogen gas from batteries when appropriate, and removing the discharge from fixed gaseous fire extinguishing systems.

Diesel vs. Gasoline Engines

Q. Why does a diesel engine operate more economically than a gasoline engine?

A. There are two basic reasons why a diesel engine operates so much more economically than a gasoline engine. First we must take a look at the fuels themselves. Diesel fuel is much heavier than gasoline… with densities of around 0.85 for diesel and about 0.74 for gasoline. The higher density of the fuel the higher the heating value. This translates to a net heating value of about 130,000 BTU per gallon for diesel, and about 115,000 BTU per gallon for gasoline. Therefore, diesel has about 12% more energy per gallon.

Next, we look at how the fuel is combined with air and ignited. There are some fundamental differences:

1. A gasoline engine mixes the ambient air and fuel before drawing it into the cylinders. The diesel forces the air (turbo charger) into the cylinder, compresses it, and then injects the fuel into the cylinder.
2. The gasoline engine ignites the fuel/air mixture with a single spark. The resulting flame front grows during the compression and power cycles. The diesel engine uses the high temperature of compression to ignite the injected fuel. This method causes combustion to occur over a much larger surface of the fuel and providing more complete combustion. The charge of fuel can be controlled to generate more power during the power stroke, and for a longer duration. Due to these differences, a diesel engine can achieve thermal efficiencies above 50% while a gasoline engine is only about 30% to 33% efficient

Q. Is a lightning protection system required to be installed in a boat?

A. No, but if one is installed it should comply with E-4, Lightning Protection. This standard provides guidance on the proper size of conductors, the types of components to be connected to the system, and the method of discharging the lightning currents into the water.

Q. Should the bilge pump float switch be installed in the positive or negative conductor?

A. The positive conductor. The placement of the switch takes into consideration the possibility of stray current corrosion. With two electrical components in the bilge water, it is best to minimize the number of components that carry the voltage continuously. With the switch in the positive conductor, the pump only sees the voltage when the switch is activated. This helps reduce the effects of stray current corrosion should the pump become damaged.

Q. What are the limitations for the use of stainless steel in fuel tank applications?

A. Stainless steel can be used for both gasoline and diesel fuel tanks. However, for gasoline tanks, the tank cannot be any larger than 20 gallons and it must be cylindrical with domed ends. There are no restrictions on size for diesel tanks.

Q. Can welding cable be used on boats?

A. Welding cable is often used for battery cables on boats. However, the welding cable's insulation must be oil and water-resistant. Cable that is not oil and water resistant may become impregnated with the water or oil and reduce the insulation's effectiveness. ABYC E-9.15.2 covers the requirements for the construction of cables and conductors.

Q. Is it necessary to provide separate ventilation openings for combustion air and natural ventilation?

A. No. If it was determined that 10 square inches of intake and 10 square inches of exhaust opening was necessary for natural ventilation (20 square inches total), and 24 square inches was needed for combustion air, two 12 square inch openings would satisfy both requirements.

Q. Why is the LPG system pressure gage installed on the cylinder (high-pressure) side of the pressure regulator and not on the appliance (low-pressure) side?

A. The gage is installed in the system to provide a quick and easy way to test the LPG supply system for leakage. With the gage in the high-pressure section of the supply system, a leak anywhere between the tank valve and appliance valves will be detectable more readily than if the gage were in the low-pressure section. A supply system leak test should be conducted every time the cylinder supply valve is opened for use, and after any event that might effect the integrity of the system, such as grounding, fire, or collision. The leak test is conducted by pressurizing and then isolating the supply system. With the supply system pressurized, the pressure gage reading should remain constant for at least three minutes. Any drop in pressure would indicate a leak somewhere in the supply system. The requirements for the gage and leak test are given in ABYC A-1.5.1, A-1.5.2, and A-1.10.3.1.

Q. Is the 12 VDC negative conductor suppose to be black or yellow?

A. For boats sold in the United States either color can be used. Yellow was added as an option to allow the differentiation between the AC systems line conductor and the DC negative on boats with both AC and DC electrical systems. Even if the boat does not have an AC electrical system, yellow may be used for the DC negative conductor.

Q. Can a fuel pump be installed in, or near, the gasoline fuel tank to help eliminate vapor-lock?

A. Both ABYC H-24, Gasoline Fuel Systems, and the Code of Federal Regulations do, within certain parameters, permit the use of a remote fuel pump. The parameters are: the pump must be located within 12 inches of the engine that it supplies, and the hose from the pump output to the engine cannot exceed 48 inches. The objective of these requirements is to limit the amount of pressurized fuel hose in the boat. Any pressurized fuel hose that develops a leak can cause a significant fire and explosion hazard by spraying fuel into the compartment. The USCG has provided a grant of exemption to some boat manufacturers to permit the installation of the fuel pump in the tank. These exemptions are granted on a case by case basis and are based on a demonstrated ability of the system design to provide at least an equivalent level of safety. These designs may include a hose-within-a-hose design where the outer hose captures any fuel that may leak from the inner pressurized delivery hose, and a sensor causes an alarm to indicate that there is an inner hose leak.