For many years I made a living in the telecommunications outside plant area--and I can assure anyone who thinks lightning
protection a waste of time should consider that public utilities consider it mandatory and for very good reasons--it saves them lotsa dough!
Whenever I have examined any unprotected structure which received a moderate discharge the total destruction of all of the equipment
and usually the structure itself is so badly damaged it requires replacement.
A full lightning
discharge involves many thousands of amps, or even megamps, flowing for up to a few seconds. This produces tremendous heat and a magnetic pulse, as has already been explained. The heating
effect is the square of the current
multiplied by the resistance of the structure--and no boat afloat will survive a major strike, nor will its crew.
Effective lightning conductors do two major things quite well. They conduct to earth minor residual discharges, and they reduce the charge potential on the air surrounding the collection point of the protection.
One of the problems involved in explaining how lightning conductors work is the way tiny-tots versions of transient events
which are very complex get explained. These explanations are full of holes because it is not a simple situation whi8ch produces a discharge and it varies constantly as the storm or disturbance continues.
There are a few ideas one might consider.
The air is not a conductor unless ionized. Ionized gasses are conductors.
Ionization requires a high charge potential. If this charge potential can be collected and safely conducted to earth, atmospheric ionization is reduced in the area surrounding the structure which is protected.
This reduction in the conductivity of the atmosphere does reduce the potential for a "strike", either because there will now be some other structure elsewhere which will be a better conductor, or the atmosohere will not permit
a discharge at the present electrostatic potential of the charged storm clouds and surrounding air. This charge emanates from the friction of the jet stream on the slower moving air beneath, is always present, and a strorm situation provides additional turbulence which can provide an ionized discharge path to earth. Once this pathway is established, a current
will flow further ionizing the air and reducing its "resistance" to the flow of electrons.
Once this path is established, rapid reduction in the electrostatic potential of one portion of the atmosphere can lead to other charges areas discharging into the same pathway--and the "strike" can be prolonged. Sometimes electrons may flow backwards and forwards along the path initially established.
I have heard the phenomena explained as the air being a dielectric between two plates of a capacitor, one plate being the earth and the other the charged upper layers of atmosphere. It is not that simple. Capacitance is an electrostatic effect in its ability to hold charge--and a charged atmosphere could be an electrostatic charge--but it is NOT a static system, and many factors apply.
The jury is still out on some aspects of the reasons lightning strikes occur--but somethings we do know from practical experience and we can take a bit of an educated guess at others.
Lightning protection properly fitted will reduce the severity of a discharge, and may well prevent most discharges from occurring. Once a discharge does occur, the severity of it will depend on whether or not the charged area from which the discharge emanated is not re-supplied with charge by some other area of higher charge potential. One can see lighning flashes from one part of the sky to another--so this is not an uncommon thing.
Photographs of lightning running from the sky to the top of a lightning conductor show small to medium discharges. There is no lightning conductor made which can conduct the high currents of a major strike without vaporizing. I once searched for a mahor fault inn a lead-sheathed multiconductor copper wired cable which had simply vanished after an unprotected terminal was damaged by a discharge I would class as of medium intensity. There were four hundred pairs of 0.9mm copper star quad cable, but there was no cable remaining. The entire cable had vanished, diffused into the surrounding soil. Copper, lead, steel
armour--the lot had simply boiled away, staining the surrounding dried out earth with small particles of condensed metal droplets.
Before a strike the air must become ionized--or it is not a conductor. Moisture is partially responsible--but it is the ionized gasses that carry the current--as in a neon tube. Streamers of illumination sometimes appear as the ionization takes place--the St Elmo's fire reported by sailors in the rigging
I would rig my own lightning protection using the methods for protecting power pylons. I would buy a Mu metal cabinet in which to lock away vital navigation equipment
top protect it from electromagnetic forces. If you can not buy a Mu metal cabinet second hand, a soft iron or even a steel
box is a poorer substitute but better than nothing.