For years many boaters have avoided bonding and grounding their masts and rigging
pointing to those boats which were properly grounded that were hit anyway. First of all there is a logical problem with this reasoning in that bonding and grounding does not guarantee that one will not be hit. proper grounding WILL, however, tend to minimize damage yet will not prevent the inducement of voltages and currents in any conductors on a vessel caused merely by the magnetic flux rate of change caused by a discharge.
In addition, there are at least two types of lightning discharge: static and dynamic. Both types create magnetic flux intensity and rate of change that can induce damaging voltages and currents in isolated and insulated devices.
The so-called "cone of protection" geometrically does tend to protect a volume of space within the cone created by the top of a grounded rod (translation: mast) due to VOLTAGE GRADIENTS that build statically in a relatively uniform medium of air and humidity. Voltage gradient measuring devices have been in use for decades to warn of potentially imminent strikes. For example, if 5 Volts per millimeter voltage gradient is measured then a bilge-stepped mast
50 ft off of the ocean surface can "see" over 76,000 Volts. Depending upon the standoff voltage rating of the hull
separating the mast foot from the ocean a punch through may or may not occur because that voltage is delivered right down to the bottom of the mast. Obviously a 50 V per millimeter voltage gradient will cause sufficient potential to shoot through anything on the way to the sea. This is why grounding the mast is a good thing. As the voltage gradient builds in the vicinity of the vessel the mast top bleeds off the potential directly to "ground" and possibly no strike will occur.
I have worked on several boats which were not grounded and showed various carbon paths on the hull
from the mast and/or rigging
to the sea after such strikes. If sufficient energy is available a hole may be created and that is the big concern if it is below the waterline, naturally.
Dynamic discharges can occur from various phenomenon related to the movement of air carrying charge and inducing charge by the relative movement and mass. These can be huge, violent, and capricious in that one cannot predict the development or path of the discharge. I believe that it is these types of discharges that feed the negative arguements against grounding a vessel's structural components.
I spent some time at sea in the convergence zone near the equator. Every evening the same scenario occured: beautiful day with puffy clouds and variable winds (if any) give way to dusk and gathering black clouds everywhere. Reef down, because soon 35 knot
winds blast you with heavy rain and lightning all around. After several days of this we had to adopt an attitude resigned to the fact that one may or may not get hit and there just isn't much you can do about that. The BIG deal was, though, many times lightning struck right near the boat without ever striking the mast or rigging towering over the space in the vicinity of the strike. This was dynamic lightning discharges at play with all that action. So, merely the fact that a conductor is above the surface of "ground" does not mean that lightning is "attracted" to it. That is a myth with such dynamics at play. With statics, thought, one's rig had better be grounded.
When I finally was "hit" it was at anchor in an outlying island off of Panama
. I counted 5 boats within 3 months that got hit in the same general area. All of us sustained at least some damage to electrical
devices totally insulated and isolated. For example, the speed control for the sewing machine
, stowed with the machine in one of those plastic carrying cases, was destroyed. The lightning obliterated the VHF antenna
, traveled down the coax shield (glad I had the large diameter RG-8 at the time which has a lot of copper in the shield) ruining it as well and "jumped" over to the depth sounder
shield blasting the outer layers of the depthsounder into the sea. No leaks
resulted but I then wished that I had a lead-line.
One interesting thing to note was that the masthead windspeed transducer
used a Hall-effect transistor to measure a rotating magnet. One lead to the device was "fused" off. I was able to solder it back on to the base of the transistor package and the windspeed worked again! Wow!