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E38-200 Lightning happens to other people

When I unstepped the mast a few weeks ago and pulled up the mast plate, I was dismayed to find no grounding connection between the mast and keel. In fact, hunting around my boat I couldn't find grounding or bonding wires anywhere. Is California weather so perfect that a lightning protection system is completely unnecessary?

Two months ago I was wrapping up my tour of the Caribbean in my Pearson 10M. I sailed from Abaco Island (Bahamas) direct for NYC, chasing a low pressure system out over the Atlantic. The first night I found myself beneath a squall line, on my knees in the cabin praying that the crashing lightning all around would not find my mast. It dawned on me that my wet, sweaty body was nearly as good a conductor as anything else on that boat. Without warning, I experienced a blinding flash of light, accompanied by what sounded like a shotgun being fired off on-deck. I was certain the boat had been hit, for 5-10 seconds afterwards I could hear rhythmic hiss and crackle - I still have no idea what that sound was. Luckily, my boat was not hit, but that was the closest I've ever been to a direct strike. Lightning happens to other people, but not always.

I don't expect to have a complete lightning protection system in place at once. As I build it out, I will continue to document and post updates here.

Information
Before we can solve the problem, we must first understand it. Perusing through various sailing and boat owner forums online I found a large number of conflicting statements and opinions. One of the most frustrating sentiments I saw out in the wild was something like: lightning is an unpredictable phenomena, so don't bother trying to control it.

While true, lightning is unpredictable, it is incredibly well-understood and it's damaging effects can be mitigated. My goal here is to protect myself and my crew first, the hull second, and electronics third. Research shows that #1 and #2 are quite achievable, #3 could be a bit more difficult. Still, let's not throw out the baby with the bathwater. Just because we can't protect the radio from a lightning strike, doesn't mean we shouldn't strive to keep people safe onboard and maintain hull integrity.

Here are some resources I found very useful in gaining a deeper understanding of lightning and how to protect a sailboat:
Plans
What my lightning protection system will look like will probably evolve as I get to know my boat better, and understand the problem more completely. For now, the key components I would like to achieve are:
  • Grounding the mast to the keel
  • Bonding system (connect engine, deck and cockpit hardware to ground)
  • Loop conductor at deck level
Electrical conductivity of some common materials (source):

Materialσ (S/m) at 20 °CConductivity
Silver6.30 × 10^7
Copper5.96 × 10^7
Gold4.10 × 10^7
Aluminum3.5 × 10^7
Zinc1.69 × 10^7
Lead4.55 × 10^6
Stainless steel1.45 × 10^6
Sea water4.8
Drinking water5 × 10^−4 to 5 × 10^−2
Wood (damp)10^−4 to 10^-3
Deionized water5.5 × 10^−6
Glass10^−11 to 10^−15
Hard rubber10^−14
Wood (oven-dry)10^−16 to 10^-14
Air3 × 10^−15 to 8 × 10^−15

Grounding the Mast

This is the first, and arguably the most important step. If lightning hits your boat, the first thing we need to deal with is a large number of electrons rushing down the mast, seeking a low impedance path to ground. I believe the biggest risk of not grounding the mast, is that we leave that path to ground up to chance. That could mean arcing over to the engine and out the prop shaft, through the electronics, or god forbid a sweaty crewmember.

To ground the mast, I drilled a ½" hole through the mast plate and base, which nearly perfectly accepted a 1/0 AWG wire. The bottom termination is made up of a ¼" thick strip of aluminum that fits over my keel bolts. When ordering replacement keel nuts, I also ordered a few 1" locknuts to hold this aluminum plate in place. The cable terminal was heated with a blowtorch and bent to accommodate a vertical wire termination.

While planning for how to dissipate charge safely out of the bottom of my mast, I also needed to think about how to handle water coming down the mast. Corrosion under the mast plate was evidence that the current system of water dispersion was not working well. I sealed the underside of the plate where it meets the TAFG, and also sealed the mast base to the plate, to prevent water from finding its way between these surfaces. A side-effect of sealing these surfaces is a poor electrical contact between the plate and mast. I ran an additional wire from the underside of the plate, just aft of the mast to a keel bolt.

plate assembled.jpg


A challenge for designing this is finding a balance between electrical conductivity without creating a rusting mess due to dissimilar metals corrosion. I suspect that my combination of copper, aluminum, and stainless steel are not ideal. What could perhaps also work would be:
  • Seal the mast plate bottom and fasteners to prevent galvanic corrosion between the plate and screws.
  • Leave the mast base/plate mating surface bare so they make electrical contact.
  • Seal the 4 fasteners connecting the base to the plate to prevent galvanic corrosion.
  • Create several C-shaped brackets, where the lower end just fits over a keel bolt, and the upper end just mates to the underside of the mast plate. Screw the bottom down with a 1" nut, and fasten the top section to the plate using aluminum rivets.
This plan would pull copper out of the equation, and the dissimilar metals in the bilge (stainless keel bolts and C-shaped aluminum plate) are easily replaceable without having to pull the mast.

I will be slathering up my connections with a lot of lanocote, and see how they do. The primary drainage hole I fitted with a 1" barbed hose adaptor, so I can route the water down into the bilge below the aluminum ground terminal.

Screenshot 2024-08-17 at 8.41.09 AM.png
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