Lightning, Galvanic Bonding, and ABYC Standards
PHIL FRIEDMAN?
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A WHILE BACK, PETER SWANSON OF "LOOSE CANNON" RAISED SOME INTERESTING QUESTIONS CONCERNING ABYC STANDARDS FOR LIGHTNING PROTECTION...
About a year ago, in an edition of Loose Cannon, Peter Swanson raised some questions about the American Boat and Yacht Council (ABYC) safety standards for lightning protection. Citing the independent work and opinions of marine lightning researcher, Ewen Thomson (Marine Lightning Protection, Gainesville, FL), Swanson pointed out that the current ABYC recommendations may very well be deficient, particularly when it comes to small vessels.
According to Swanson’s article, the currently propagated ABYC lightning protection standard overlooks the danger of a lightning strike to crew in the bow or stern of a small vessel because of the failure to enclose them within the “cone of protection” afforded by the ABYC-recommended single air terminal placed amidship. And whether or not you agree with the conclusions of the Loose Cannon article, the facts of the situation described raise questions — not for the first time — about the organization’s procedures and protocols for generating engineering and construction standards for recreational small craft.
I’ll leave it to you to read first-hand the details of the lightning protection discussion (*1,*2) because they are not my focus in this article. What gives rise to this discussion is the similarity, in form, of Swanson’s concern about the ABYC lightning-protection standard to a concern I’ve had for a long time about the organization’s approach to bonding a boat’s low-voltage DC electrical system — a similarity which begs the following two questions:
Bonding a Boat’s DC Electrical System
The ABYC standards for bonding of a vessel’s DC electrical system have been propagated, nominally, to:
a) protect critical parts of dissimilar metals from accelerated deterioration due to natural galvanic currents,
b) prevent the existence of an electrical potential on exposed metallic enclosures of electrical equipment,
c) provide a low resistance path to ground for voltages that may be considerably in excess of those for which the system is designed, as might occur when lightning strikes, and
d) minimize radio interference. (*3)
ABYC recommends using, when available, a metal hull structure as the central conductor. When a metal hull is not available, ABYC calls for using a main trunk+ branch conductor network (which does not normally carry current) to connect all of the vessel’s major metal parts and equipment together in single circuit. The main idea is to place all of the boat’s metal parts and equipment cases at the same electrical potential and in circuit with anodic protectors (e.g., sacrificial zinc anodes).
However, assuming I understand the ABYC position on bonding, my own experience in the engineering and construction of steel and aluminum yachts — not to mention everything I’ve learned and know about galvanic corrosion — tells me their recommendation may be precisely the wrong advice. It also tells me that, when it comes to fiberglass hulls, it is also questionable advice. Why? Because in my experience, the most effective way to prevent galvanic corrosion in the presence of mixed (i.e., galvanically incompatible) metals is not to enhance the conductivity of their connection(s) to one another, but to break the circuit by completely isolating those metals from one another.
No conductive circuit, no galvanic current. No galvanic current, no galvanic corrosion.
Consider that case of stainless steel seacocks fitted to an aluminum hull. Why would you choose first to enhance the conductive connection between these two metal parts, then seek to counter the resulting deleterious flow of natural galvanic current by introducing a sacrificial anode — a third metal — into the circuit to protect the aluminum? Far better, as I see it, to electrically isolate the body of the s/s seacock from the aluminum hull by a system of non-conductive, plastic bushings. No electrical connection, no circuit. No circuit, no galvanic current. No galvanic current, no galvanic corrosion.
Speaking bluntly, about the only more egregious error I can think of is to transform a metal yacht’s critical structure into a bonding connector that is in circuit with metals which are significantly higher on the galvanic scale than the vessel’s structure.
If you’re a glutton for punishment, why not just go out onto your boat’s foredeck during a half-gale and urinate into the wind?
Moreover, similar considerations apply in the case of non-conductive composite vessel structures — because they represent a prime opportunity to electrically isolate any mixed metals used from one another.
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Consider that your bronze seacocks cannot induce galvanic corrosion in your stainless steel propeller shaft unless there is an electrically conductive circuit formed between the two. So why create that galvanic circuit by connecting two significantly dissimilar metal parts together with a heavy wire conductor? Yet, that’s exactly what ABYC recommends you do in an FRP hull — again assuming I understand the recommendations concerning bonding.
Remember, no circuit, no galvanic current. No galvanic current, no galvanic corrosion.
There may be situations in which it is almost impossible to avoid direct contact between two metal parts that are galvanically incompatible — for example, when you fit a Nibral (r) or bronze propeller to an Aqualoy (r) or stainless steel propeller shaft. And in such cases, you may choose to employ localized anodic protection, such as a zinc shaft collar clamped to the shaft immediately forward of the prop. But that is significantly different from bonding every metal part in the vessel together in a circuit that literally facilitates the destructive flow of galvanic current.
I suspect but cannot confirm that the real motivation in doing so may be an underlying desire to have the “bonding” do double duty as a lightning protection circuit. To be sure, bonding all of the metal fittings and parts in a yacht together, will work to avoid the potential for side flashes during a lightning strike when the current seeks a path to ground.
Is this a clue as to why the ABYC standard for bonding took the form it did? Indeed, in a couple of places, the ABYC standard openly suggests that a galvanic bonding system can do double duty as part of a lightning protections system. Which, once again, is perplexing to me because quite a few lightning protection systems incorporate air gaps that prevent the system from conducting currents at low voltage (< 50 VDC), but which gaps can be jumped by the ultra-high voltages developed during a lightning strike.
And by the way, it should also be noted that it’s possible to provide “grounding” to the low-voltage DC system without bonding all the metal parts of the boat together into a single circuit.
Can You Really Serve a Multiplicity of Stakeholders at the Same Time?
This leads us to shaking hands with the 800-lb gorilla in the room: We all know — or should know — that ABYC standards come from volunteer committees who study identified issues, often for several years, while they work to develop a consensus on a set of voluntary standards. These standards are nominally propagated to protect the boat-buying public. However, as a former boat builder and boat manufacturing manager, I have come to understand that, due to active lobbying and the high numbers of industry employees present on the relevant committees, the adopted standards are frequently skewed by concessions to the goal of manufacturing “efficiency” and away from what would be best or even just better practice from a purely objective engineering perspective.
Understand this is certainly not to say anyone actively conspires to achieve this result. Rather, it is to question a system of voluntary “regulation” that is so strongly motivated to achieve consensus in order to be accepted. And I suggest that voluntary standards only work as they should when the people who develop the standards are independent from those who are intended to accept and follow those standards.
— Phil Friedman
Copyright ? 2024 by Phil Friedman and Port Royal Group — All Rights Reserved
Footnotes:
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