When Benjamin Franklin invented the lightning rod in 1750, he noted that it could also be used to protect ships. It was not long before the first ships were to benefit from his ideas.* In the late 18th century the sailing warships of the British navy
were fitted with lengths of anchor
chain to prevent their wooden masts from splintering when struck by lightning.* Franklin himself was unsure of the actual mechanism, thinking initially that a pointed rod would discharge the thunderstorm "for if there be a rod sharpened ... the electrical fire would be drawn out of a cloud" but five years later covering all bases by adding "pointed rods would either* prevent a stroke or would conduct it so that the building should suffer no damage".** For whatever reason, this technology worked.* The discharge physics of the* lightning strike to ground would not be well understood until research done in South Africa
in the 1930's and later.
In the intervening centuries scientific opinion has come down squarely on the side of Franklin's last opinion - that a lightning rod protects a building by offering a suitable path for the current
to flow.* Still, modern day refinements for marine
protection somewhat mirror the historical record
.* Although a code developed by ABYC definitely improves lightning safety
, research continues into the underlying science.* In* a paper published by IEEE (the Institute of Electrical and Electronic Engineers) in 1991, Ewen Thomson of the University of Florida
tested this code by applying the traditional science used in lightning protection systems for ground installations.* The traditional science, reflected in terms such as "ground resistance" and "step potentials" models voltage gradients as a consequence of current
flow in the ground (or water). Thomson concluded that key changes were needed. While some changes were trivial to implement, such as upgrading down conductors from #8 gauge copper to #4 gauge copper, others were highly impractical.* In particular, Thomson noted that hull
damage to sailboats struck in fresh water
was so extensive, even when the boat had a good connection to the keel
, that multiple grounding surfaces were needed over an extensive underwater area, a requirement is very difficult to fulfill in practice.
Of even more concern were some types of boat lightning damage that were impossible to explain with the traditional scientific model. In the light of these inconsistencies, Dr. Thomson concluded, in a yet unpublished study, that key assumptions in the traditional model were invalid* Removing these assumptions and reinterpreting the fundamental science has resulted in a new model that enabled innovative technology to be developed to overcome the above practical limitations.* This technology is now covered by a* patent issued by the University of Florida* and licensed solely to Marine
Lightning Protection Inc.
Since there is no scientifically proven method to repel it, the fundamental problem in yacht lightning protection is how to deal with lightning when it strikes.* Where the lightning channel attaches to a boat is determined by the geometry of topside conductors on the boat and the location of the downward-going stepped leader relative to the boat.* For example, if the stepped leader is heading for the water behind the boat, aft conductors are more likely to be struck.* In general, the taller the conductor, the higher the probability that its upward streamer will be the one that connects with the stepped leader, thereby completing the ground channel for the lightning. The conductor in the lightning protection system intended to make this connection is termed the air terminal, or, more commonly, the lightning rod.* In this respect, research reported by Dr. Charles Moore and associates in New Mexico
only two years ago finally resolved that blunt lightning rods are actually more effective than the traditional sharp pointed rods. The tendency of a tall conductor to attract the lightning strike, by preferentially launching the final connecting streamer, has resulted in the idea of a "cone of protection".* This somewhat flawed idea holds that a vertical conductor forms an effective cone of protection, the apex of the 90 degree cone being at the top of the conductor, and protects the circular area of the cone's base. The idea is flawed in that a vertical conductor does not eliminate the electric
field on the ground within this "protected" circular area.* Any conductors inside the area, people included, may give rise to upward streamers if this electric
field reaches breakdown strength.* A better arrangement is to have conductors arranged around the area to be protected, or, better yet, forming an umbrella overhead, where the outer edges of the umbrella are connected to down conductors leading towards the water.
Hence the major concern regarding the lightning attachment is to ensure that the lightning attaches to, and causes current to flow in, only an air terminal, or other termination conductors, rather than more vulnerable conductors such as crew members, electronics, etc.
Charge dissipation into the water
is such a good insulator that it is used to make insulators for high voltage installations.* Nevertheless, the lightning voltage is more than enough to cause electrical breakdown through a fiberglass hull
if no alternative path is provided, and frequently even if one is.* Each penetration leaves a charred hole and much more extensive internal damage.* Grounding conductors (electrodes) are intended to form a bridge into the water to eliminate this hull damage.* However, a single ground plate is inadequate to prevent sideflashes, necessitating multiple interconnected conductors.* These cause a whole new set of problems:
accelerated galvanic corrosion
or loss of sacrificial zinc's;
electrolytic erosion in marinas
with ground currents leakage;
many mounting bolts and hull penetrations, each one raising the risk of water seepage;
additional drag since plates should have exposed edges.
Through-hull transducers, fittings, and all immersed metal, including outboard
drives, also inadvertently act as lightning grounds.* A typical scenario for an ungrounded smaller powerboat, such as a 20' fisherman, is for lightning to attach to the VHF
antenna (vaporizing it), spark through the electronics panel (destroying all electronics), travel* into the battery
ground or control cables
into the outboard
solid state ignition (rendering it inoperable), and then spark into the water through the drive unit. Any transducer such as a knotmeter
is also likely to be blown out, possibly leaving a hole where it was mounted.* This scenario assumes that no crew member
is unlucky enough to be bridging a gap along the way.
Carbon deposits after lightning strikes* trace out the paths followed by sparks forming from immersed conductors, both those grounded and those* that are isolated.* A detailed discussion of this effect is given in a *letter* published in Professional Boatbuilder
.* Briefly, charge accumulates on all conductors on the boat,* even when current is flowing into the water.* The charge density is largest close to the water and on sharp corners and edges of conductors, which is thus where sparks are most likely to start.* So sharp corners are highly desirable on the outside of grounding plates and are recommended in most standards.* As well as initiating current flow, spark formation reduces the grounding resistance, thereby lowering the voltage of the whole protection system.* There is experimental evidence that the eventual effective area of the sparks formed from at the water surface above-water fittings may be hundreds or thousands of square feet.* See a brief discussion of this mechanism below.
In summary, the major problem with charge dissipation into the water is how to provide the appropriate number and distribution of grounding conductors, to eliminate sideflashes,* while minimizing the corrosive effects of multiple immersed conductors that are bonded together.