Over the past 16yrs cruising in lands of lightning, we have taken one direct strike, and at least 4 indirect strikes with various damages in the form of lost equipment. I've learned a lot from observation and taking things apart about how lightning surges move about the boat, and how equipment and structure are damaged by them.
1. Give lighting a robust direct path to the water.
This is more than just a small wire assuring equal potential. It needs to be able to carry a strike load and provide a surface designed to dissipate the energy. This surface needs to be more than just a metal keel - it needs edge surfaces.
Our boat that took the direct strike had a LPD system that consisted of a 4/0 cable with a tinned copper electrode on the end of it containing 20' of edge surface. This part was dropped in the water whenever at anchor or dock, and the other end was clamped to the mast with a significant clamping mechanism attached to a permanently installed plate on the mast. This is a catamaran, so clamping to the mast and dropping into the water through the trampoline is the most direct path. While we lost almost all electronic equipment from the strike surge, there was no structural damage, and the LDP electrode had its tin burned off the edge surfaces, some pitting of those edges, and melting of the heat shrink on the connection points - so it did its job.
Unfortunately, this system is no longer sold, as it was a one-man operation by someone steeped in lightning background who just got too old to continue. So for our current boat, each hull has a large ground plate with sharp edges installed in them, with the shrouds and mast connected to the plates with 1/0 wire. It isn't ideal, as the wires are longer and have more bends in them than the other system, but it does keep the mast at ground potential, and gives lighting paths to the water more favorable than just bursting through the fiberglass.
2. Clamp/shut down main electrical paths into the boat, and protect overall circuits.
All of the damage from our strike and indirect strikes was from energy on the DC(-) and grounding system side of things. Radios with fuses on both Pos and Neg leads had only the Neg fuses blown. Damaged circuit boards had only Neg and ground traces and components damaged. Equipment whose breakers were turned off suffered damage through the common negative bus. If possible, it is best to use double pole breakers that isolate both legs of a circuit when off.
Next, use MOV surge protectors on the AC and DC main electrical buses. We have them on the shore power inlet, the AC and DC distribution panels, and directly on the battery main buses. Ones like this - you will find AC and DC versions and need to have the correct ones:
https://www.amazon.com/dp/B07PFVQBL2?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_1
For the VHF coax, a coax surge protector that is more than just a gas discharge tube - it also needs a static bleed and a DC block like one of these:
Coaxial Cable Lightning Arrestor, Coaxial Cable Surge Arrestor
3. Protect individual devices.
We consider our NMEA2000 network and VHF radio to be most important, so they get isolated from the rest of the devices using isolated power supplies like this:
https://www.amazon.com/dp/B01LYZZRBQ?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_1 . An additional advantage of using these is that they are buck/boost, so the output voltage can be set to a constant 13.8V regardless of the house battery voltage. This is better for the equipment and protects them from surges and varying voltages. The N2K network supplies power to all the devices on it, so they are all isolated and protected at once. The VHF is a real problem in lightning strikes, as its coax shield is grounded to the main DC bus. Even if the coax surge protector is overwhelmed, a dedicated isolated power supply keeps the VHF isolated from the rest of the circuits.
Other devices than the above are protected by TVS diodes across their power inputs - like these:
https://www.newark.com/littelfuse/p6ke33ca/tvs-diode-600w-33v-do-15/dp/17H1873?st=p6ke33ca . I even put them across the DC-DC power supply inputs and outputs to help protect the power supply device itself.
Does all the above work? Maybe. That large catastrophic strike the boat next to us took last summer caused no damage to us, while the boat on the other side of us, and the boat on the other side of the struck boat, both had electronic damages. In the past before these efforts, we would take some electronic/electrical damage any time lightning hit close to us. This is still in the category of coincidence, however.
Mark
