LIGHTNING...!

Why Should We Care...?

According to published reports, in the US, out of the annual average of about 100 deaths due to lightning, 13 are aboard boats. 

If lightning is a hazard where the boat will be used, a plan should be developed to deal with the possibility of a strike. In the Pacific Northwest, lightning is relatively rare. In Florida, strikes are measured in numbers annually per square mile, with some areas having more than ten!

The electrical potential built up before a strike may be over 100 million volts. A lightning strike may carry something like 20 to 50 thousand amps, and generate temperatures of some 55 thousand degrees. Fortunately, it lasts only a fraction of a second, but within that time it can be deadly and / or very destructive.

 

What Can We Do...?

It seems undesirable, at best, to invite a lightning strike to preferentially pass through the interior of our boat. A lightning protection system aboard a boat has a dual purpose:

It will primarily serve as a lightning prevention system, the purpose of which will be to continuously shed any charge built up by the boat, thereby rendering the boat 'invisible' to lightning.

It will secondarily be asked to serve as a lightning strike protection system, to safely conduct a direct strike to ground.

(Ewen Thompson's results are published in a paper entitled, A Critical Assessment of the US Code for Lightning Protection of Boats, presented to an international conference of electrical engineers during May 1991.  For an up to date review of Ewen M. Thompson's excellent research on Lightning, please see the links at the end of this article...)

It is critical that the primary conductor of the lightning protection system, which takes the path from the top-most end of the system to the water be as robust as it can be made, be as direct as possible, and use long radii, rather than sharp bends along the primary path to the grounding plate. The connections must offer low electrical resistance or the energy of a strike may instantly heat and melt the connection.

 

Details...

Ewen M. Thompson's research suggests that all conductors be a minimum of #4 AWG copper (21.2 mm2 or 0.0329 in2). Possibly as a result of his research, the new ABYC rule recommends a minimum of a #4 AWG copper wire for the primary lightning protection system conductor, and a minimum of a #6 AWG copper wire for the secondary conductors. Tinned wire is recommended, as usual.

A boat with a metal mast which is stepped on the keel will present an ideal situation. Here, the primary conductor may be the mast itself. An aluminum mast will be an excellent conductor, and will mainly require a substantial direct connection to the air terminal at the top, and to the grounding plate at the bottom. With a wood mast, it may be necessary to rout a channel for a substantial copper conductor.

A bronze sail track on a wood mast might be considered to be a substantial enough conductor, but it would have to be made continuous via mechanical, brazed or soldered connections. If used, it must have the same sectional area as a #4 AWG copper wire.

In any case electrical continuity will have to be assured from the spike at the masthead to the copper ground plate in the water.

Other types of conductors need to be considered: Per the ABYC, a carbon fiber reinforced wood mast or a carbon fiber reinforced composite mast will not be treated as though it is a conductor. Stainless is not a good conductor. A stainless sail track will not usually be large enough to be adequate. The ABYC states: "every metal shroud and stay shall be connected from the chain plate directly to the ground plate or ground strip with a conductor equal to at least #6 AWG copper. Where the system consists of multiple shrouds, stays, and mast, they shall have an aggregate conductivity of no less than a #4 AWG copper conductor."

A traditionally rigged vessel, having fully insulated shrouds due to serving and possibly the use of deadeyes and lanyards, will likely be a separate case.

The ABYC rule states: "Large metal objects such as tanks, engines, deck winches, stoves, etc, within 6 feet of any lightning conductor shall be interconnected by means of a lightning conductor at least equal to #6 AWG copper. To minimize flow of lightning discharge current through engine bearings, it may be preferable to bond engine blocks directly to the ground plate rather than to an intermediate point on the lightning protection system. To minimize side flashes and the induction of high voltage to the boat's wiring, lightning conductors in proximity to the boat's wiring shall not be routed in parallel to the boat's wiring."

The ABYC further states: "...a lightning system conductor shall not form an bend of less than 90 ° and it shall not have a bend radius less than 8 inches."

For the grounding plate, an area of about one square foot is considered by the ABYC to be sufficient. The grounding plates should be located as close to the base of the primary conductor as possible to minimize any horizontal runs in the primary conductor. The edges of the external ground plate or strip need to be sharp, exposed, and not caulked or faired into the adjoining area.

The ABYC suggests the use of a grounding strip, rather than a plate. The ABYC rule states: "A grounding strip shall have a minimum thickness of 3/ 16 inch (5 mm), and a minimum width of 3/4 inch (19 mm)." A strip approximately one inch (25 mm) wide and 12 feet long (3.7 m) has nearly six times the amount of edge area exposed to the water, which will improve the dissipation of charges. "The grounding strip, if used, shall extend from a point directly below the lightning protection mast, toward the aft end of the boat, where a direct connection can be made to the boat's engine." 

"An equalization bus on the inside of the boat, paralleling the grounding strip on the outside of the boat, may be used as the lightning ground conductor." ABYC encourages use of two bolts at each end of the strip, extending between the external strip and the internal equalization bus, a metal strap inside the boat substantially parallel to the exterior lightning ground plate, and connected to the lightning ground plate at each end. Secondary lightning conductors can be connected to the equalization bus.

A grounding plate, if used instead, should be solid, rather than the sintered bronze type often used as radio grounds. The sponge-like structure of the sintered bronze plates may, in the event of a strike, allow the instant formation of steam, which could blow the plate apart, resulting in possible severe damage to the surrounding hull.

 

Grounding Plate

The ABYC states: "An exterior grounding plate of copper, copper alloys, stainless steel, or aluminum may be provided by means of a plate which has an area of at least one square foot."

Ewen Thompson's research indicates that in salt water, a grounding plate of one square foot in area may be sufficient, but that in fresh water, even two square feet or more may not be enough to provide a sufficiently low resistance in the event of a direct strike. Thompson argues in favor of a generous grounding strip.

If the grounding plate or strip is not large enough, the inevitable result will be that a lightning strike will seek additional pathways to ground, and the danger of side flashes will be dramatically increased, along with possible severe damage to the hull.

The grounding plate must not be painted, and therefore should not be integrated with the boat's corrosion protection system. If the grounding plate is attached to a zinc anode, the natural antifouling qualities of the copper will be eliminated, and the plate will quickly foul.

On a boat with an external ballast keel, the keel itself might seem like an ideal lightning ground. However, if an external ballast keel is used, a substantial area of the surface of the ballast would have to be kept bare in order to bleed off the gathering charge to help avoid a strike. That strategy would not be entirely practical even with a lead keel, since the exposed lead area will foul fairly quickly, especially if the lead keel is provided with zinc anodes for corrosion protection (as it should be).

A coating of paint is too much insulation, not necessarily in the case of a strike, but in order to help prevent one. Therefore, even if there is external lead ballast, it will be probably be preferred to use a copper plate or grounding strip, which, if it is not connected to zinc anodes, will provide its own anti-fouling.

Thompson states, as does the ABYC that the actual form factor of the ground plate is important. A ground plate with sharp corners or points will initiate streamers at a lower voltage and result in a lowering of ground resistance at a lower current than will a smooth or round edged plate.

 

Side Flashes

The possibility of 'side flashes' has been mentioned. Lightning strikes may involve side flashes to other metal objects. Side flashes are encouraged by an insufficient path to ground, or an insufficient area of the grounding plates. Side flashes can create a chain from one item to the next on the way to ground. Side flashes are most prevalent in fresh water.

In fact, Ewen Thompson's findings suggest that side-flashes are inevitable in fresh water, but they are much less likely in salt water, assuming that in both cases there is a lightning protection system aboard. His survey results show a much higher incidence of serious hull damage in fresh water than in salt water.

To minimize the potential for side flashes, the secondary protection system involves all the metal objects aboard a boat which are larger than about one foot in any direction. These are to be connected to the lightening protection system equalization bus via the secondary conductors.

The ABYC states: "Seacocks and through hull fittings, if connected to the lighting ground system shall not be connected to the main down-conductor. They shall instead be connected to the grounding strip or plate, or to the internal equalization bus." Connecting these many metal components to the lightning protection system will serve to equalize the built up potential among them and reduce the likelihood of a side flash.

The by-words for lightning conductors are robust, direct, and having no sharp bends.

 

Bonding...  Oh No!

We are faced with a problem here!  The bonding system on any boat hardly qualifies as a lightning conductor. Neither is the bonding wire or strap adequate to handle the intense discharge during a direct strike.  

We also know that if the bonding system is asked to participate in the lightning protection system order to help prevent side flashes, the copper grounding plate will be stripped of its natural antifouling qualities.

 

EMP

A lightning strike involves an extremely rapid change in an electric current, generating a momentary but very powerful magnetic field. This electro-magnetic pulse (EMP) can readily induce currents in adjacent wiring.

Currents induced in wires by the EMP from a lightning strike may do some very weird things, such as fry every piece of electronics aboard. A strike nearby or on another boat can fry the electronics aboard your boat without even requiring an electrical connection. 

A direct or nearby strike may have a radical and permanent influence on the compass, and may require that it be completely re-calibrated. Strikes have been known to erase all the cassette tapes on a boat! Consider what would happen to other magnetic media, such as your hard drive!   Sensitive electronics hardly stand a chance.

 

Faraday Cage

Lightning tends to prefer to run along the outside surface of objects. A logical approach might take advantage of this natural tendency. For lightning protection, airplanes use the strategy of external shielding to create a 'Faraday Cage' around instruments.

The ABYC states: "Wherever possible, electronic equipment should be enclosed in metal cabinets that are connected to the lightning grounding system. Surge suppression devices should be installed on all wiring entering or leaving electronic equipment."

With a metal boat, the hull itself makes an excellent Faraday Cage as well as an excellent conductor. Still, it may not serve as an adequate lightning grounding surface due to the hull being completely enclosed within an epoxy skin. Even considering the zincs as ground plates, lightning protection will not be as intended, since the path to the zincs may not be straight, or the area may not be sufficient.

The ABYC states that aboard a metal boat, "If there is electrical continuity between metal hulls and masts, or other metallic superstructures of adequate height, then no further protection against lightning is necessary."  However ideally, on a metal boat there will be a pointed spike at the top of an aluminum mast, bolted to a #4 AWG copper jumper connected in a straight line from the mast to the grounding plate immediately below the mast.

With this arrangement, a lightning strike can proceed unhindered in a straight path. 

 

Corrosion Protection

Primarily in order to have control over stray currents, i.e. escaped electrical currents generated on-board or existing in the water at a marina, it may seem most desirable aboard a boat of any material to have the ability to disengage a lightning protection system when the threat of a strike does not exist, particularly if the lightning protection system is elaborate. It may then be intentionally connected prior to an electrical storm.

Suggestion has been made that, in salt water, a robust (minimum #6 AWG) and directly routed bonding system may be kept separate from the primary lightning protection system. The rationale being that the salt water itself is sufficient as a conductor to allow the charges to be equalized. However, this may not be effective in fresh water, and there is no support for this strategy within the ABYC rules.

Aside from an external ballast keel, the propeller probably represents the largest underwater mass of bare metal, and consequently an excellent ground. It is therefore desirable for the engine block to be connected to the lightning grounding plate via a heavy and direct conductor. This should be done in order to reduce the potential for a side flash to the engine in the event of a direct strike and consequent damage to the engine.

Strictly for the sake of corrosion prevention, however, it may be desired to isolate the engine electrically from the underwater metals. In this case, it would be best to have the ability to selectively engage a lightning ground conductor during an electrical storm. 

 

During an Electrical Storm

In an electrical storm we will be safest if we stay inside the boat, away from all metal parts, such as metal masts, shrouds, lifelines, engines, metal tanks, stoves, water pipes, faucets, sinks, electronic gear, inside ballast, spare anchors, or chain. We should especially stay away from any area between large metal parts.

Aboard a metal boat, the boat itself forms a protective Faraday Cage. The best strategy aboard a metal boat will be to encourage the hull itself to be integral with the primary conductor.

The ABYC notes that even if their new Lightning Protection Standard is employed, complete protection from equipment damage or personal injury is not implied. And that a lightning protection system offers no protection when the boat is out of the water, and is not intended to afford protection if any part of the boat comes in contact with power lines.  

The ABYC further notes that protection of persons and small craft from lightning is dependent on a combination of design and maintenance of equipment, and on personnel behavior. The ABYC guide is general in nature, and pointedly states that in view of the wide variation in structural design of boats, and the unpredictable nature of lightning, specific recommendations cannot be made to cover all cases.

Needless to say, aboard a boat, any strike is to be considered dangerous. Protection requirements for a boat should be more stringent than those for buildings, since a failure of the protection system is more likely to result in personal injury aboard a boat. A failure rate which may be acceptable on buildings is clearly unacceptable on a boat in mid-ocean.

This discourse on lightning is not intended to recommend a specific approach. It is instead presented here simply to introduce the magnitude of the problem. Experts and standards bodies have not reached consensus on the subject. It is up to your own best judgment to choose what will work best board your own boat!

During an electrical storm, we had best observe Bre'r Fox' advice closely: "Lay low, an' don't say nuffin!"

 

Summary

From the above, we can readily see that the lightning attenuation issues aboard a boat of any material are not at all optimally addressed by a bonding system, which in itself is not designed to adequately handle lightning protection, nor radio grounding, nor even potential corrosion due to stray currents. Clearly, if we are to employ a bonding system aboard our vessels, we must choose to do so on the basis of an informed rationale, rather than to expect a bonding system to perform duties for which it is inadequately suited.  

Similarly, if we are to install a lightning attenuation system, the above details are quite important.  A system that is poorly done or possibly compromised by a failed connector could actually make the situation worse, for example by not allowing an accumulated charge to dissipate, thus even attracting a strike...

Michael Kasten

Port Townsend, 1998 - 2003

Rio Verde, 2005 - 2007

  Ewen Thompson's Lightning Website  |  Non-Technical Article on Lightning Grounding  |  More Technical Lightning Grounding Information