Yacht Insurance News Lightning and Boats
Unversity of Florida

by Ewen M. Thomson

Lightning and Boats
Photo by E. Thomson

Editors Note: This page is only one of many to be found on Ewen Thomson's site entitled Lighting and Boats. The site contains much more information in the form of articles, pamplets, movies, graphs, tables and drawings than can be shown here. Go to the link at the bottom of this page for the entire site.

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 was well grounded, that multiple grounding surfaces were needed over an extensive underwater area, much more than the one square foot ground plate quoted in the code. This requirement is very difficult to fulfill in practice.

Of even more concern were some types of 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 ( USA Patent Number 6,708,638 ) and licensed solely to Marine Lightning Protection Inc.

by Ewen M. Thomson
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