I think if one was to use immersed metal fittings in a plastic boat as the sea end of a lightning
protection system I think it would likely be the shaft and prop I would go for using too as the collateral damage to the hull
should be less, as you say. And I suspect it could be done with the shaft insulated from the engine
In our own case we have a shaft insulated from the engine
but the shaft is bonded to its bronze stern tube liner in our case because we are steel
, so is done for galvanic reasons rather than for lightning
but on a plastic boat could very well be used for lightning.
The shaft seal
is a lip seal based one and floats on the end of a rubber hose much like traditional packing gland
ones. Inside the seal housing is a long metal bearing which centres the seal on the shaft (the bearing looks like it is made from something like white metal to me just from the little of it I can see if I pop the seal out, but the manufacturers state it is actually a bronze). There is a copper strap back from that over the rubber hose to the stern tube liner to electrically bond them together.
Despite the metal bearing being water
lubricated it always seems to have zero resistance through it to the shaft (which surprised me). I suspect if the same arrangement was used in a lightning protection system to bond the system to a shaft insulated from the engine it would handle the current
as well as anything else, maybe with the result of welding the bearing and causing damage due to steam explosion in the stern tube (if a bearing at each end of it like our own has, but I might be dreaming about enough heat to boil the water
in the stern tube