Mounting lightning discharge plate

[Cyrus Safdari]

I am not happy about the idea of making holes in the boat below waterline to discharge lightning BUT Without getting into the pros and cons of doing so, lets assume I installed the ABYC recommended lightning grounding plate. Again, leaving aside the other debates about lightning, what would be the best way to mount the grounding plate to the hull? A strip of 1/4" (6mm) thick, 2" wide copper or bronze, secured by two 1/2" thru-bolts at 48" long will add up to the min 1sq ft surface area required ...

How are these thru bolts glassed-in? Id assume the penetration area is built-up well with additional glass layers but still, those bolts willnget red hot. Can they be insulated so as to not cook the hull? Hi temp, ceramic epoxy?

And is it better to mount the strip with a standoff gap withbthe hull, or must it be glassed onto the hull.side? Seema like a gap will allow more edge effect. I guess buildinf up the exterior of the hull, in the area justbunder the strips, with additional layer of glass wont hurt either...

[Frankly]

I will suggest what I did, not that gurantees that it is the best or only way. The attaching bolts are part of the electrical circuit so they should pass through the hull and be the connecting point for the mast wire(s). Although silicone bronze is more than 95% copper it has no where near the conductivity of copper (like 20%). Making up for this poor conductivity means large cross section and multiple parallel paths. I used 4 ea1/2" flat head bolts and soldered them to the plate. The idea of the solder was to seal this interface between the bolt head and the copper plate.

Mounting the plate proud of the hull would expose more area but the inside would become fouled and be difficult to clean. Don't think copper will not attract the critters. Also make a great catcher of things. I put mine on 14 years back and check it at every haul out. Still looks pretty good but shows some surface pitting. I paint mine and scrape the edges before launch.

A picture in my album if you want to see.


Frankly

[Orion Jim]

Just buy a Dynaplate. It comes with the mounting hardware. It’s made of sintered bronze...mine is 36 years old and will last forever.

[coopec43]

I use X4 1.5M long discharge strips approx 5mm X 19mm. Each is bolted with X3 countersunk bolts through the hull.

All the stanchions as well as chain plates are connected to the strips. There is a copper strip directly from the main mast to a discharge strip directly below.

Lightning Attenuation Onboard

[Cyrus Safdari]

Originally Posted by Frankly

4 ea1/2" flat head bolts and soldered them to the plate. The idea of the solder was to seal this interface between the bolt head and the copper plate.

.

A picture in my album if you want to see.


Frankly

Thank you this was helpful. I have read that solder melts but i guess theres no harm in soldering over mechanical fasteners and can indeed help the connection

[Cyrus Safdari]

AN SSB dynaplate as lightning grounding plate is not a good idea as it will explode. It is porous, and so trapped qater inside it will turn to steam and make a big hole in your hull. Im nervous enough about the thru bolts for a grounding plate poking thru the hull and what will.happen to them and the glass theyre in once superheated ...

[Frankly]

I think you may be a little over concerned about the heat generated by a very high but very, very short current pulse passing through a good electrical conductor. That white hot lightning strike passing through the air is not quite the same once it is flowing in a conductive medium. The point in making the connections large and multiple is to minimize the impedance and there by the heat (the real destructive force). Also there is a wide range in amplitude of strikes. While a reasonable strike is most likely, there may come a time when your number really comes up and any reasonable LP system will be overcome.

Yesterday I watched a strike on a transmission line a mile from my house. One of them big fat ones that looks like an interstate highway in he sky. I did the best I could in protecting my yacht, but i don't want any part of what i saw. "You pays your money and you take your chances".

The solder was an attempt to seal that surface between the bolt heads and copper plate. It might melt but hopefully by that time it is all over, for better or worse.


Frankly

[Cyrus Safdari]

Originally Posted by Frankly

I think you may be a little over concerned about the heat generated by a very high but very, very short current pulse passing through a good electrical conductor. That white hot lightning strike passing through the air is not quite the same once it is flowing in a conductive medium. The point in making the connections large and multiple is to minimize the impedance and there by the heat (the real destructive force). Also there is a wide range in amplitude of strikes. While a reasonable strike is most likely, there may come a time when your number really comes up and any reasonable LP system will be overcome

Indeed, but there is not much more work to taking extra steps to protect the bolt penetrations by, for example, using hi heat resistant epoxy resin. FR4 (not G10) laminate is also fire resistant.

There is now a sailboat in the same yard that had been struck and has multiple holes punched through it, barely made it back to the travel lift. That convinced me to install a long grounding strip starting right below the mast and additional shorter strips each side midship near the chainplates.

Theres the question of attaching the mast to the grounding strip on a deck-stepped boat with a compression post. There are a few stainless bolts that pass through the deck, that can carry the current feom the mast to the below-deck compression post but they would present a lot of resistance. A through bolt of copper?

Another question, do you connect the DC negative bus and/or engine to the grounding strip from the mast? Your supposed to connect any largish metal object but that creates other complications.

[Nicholson58]

Originally Posted by Orion Jim

Just buy a Dynaplate. It comes with the mounting hardware. It’s made of sintered bronze...mine is 36 years old and will last forever.

I second that. Mine are 1983 and clean up nicely each haul out.

[Nicholson58]

Originally Posted by Cyrus Safdari

AN SSB dynaplate as lightning grounding plate is not a good idea as it will explode. It is porous, and so trapped qater inside it will turn to steam and make a big hole in your hull. Im nervous enough about the thru bolts for a grounding plate poking thru the hull and what will.happen to them and the glass theyre in once superheated ...

I think this is urban legend. I have two Dynaplates and we have been struck three times in ten years. The last hit was direct and destroyed a lot of stuff.

[Frankly]

First on the DP. I have always been suspicious that the bigger issue (vice sintered bronze) is the typical single (gold plated bolt) electrical connection. Like I said silicone bronze is a great marine alloy but not so great in conductivity. If a strike blows the plate off and holes the boat might be poor electrical path via the small bolt and hard to determine after the wreck what actually went bad. Also jumbo DP ain't cheap (other than the one I sold to A64 for $50).

A suitable chunk of solid copper plate and 4 SB bolts can be had for $100 or so.

The aft end of my plate is the single point ground and there is a conductor to the 12V negative which is also connected to the block. Keep in mind that the idea of connecting all the other metal to this point is to maintain everything at a common potential. This is to eliminate side flashes and these conductors usually do not conduct a large current.

I had a 30' Hunter with a deck stepped mast with a copper ground plate (and external keel). I actually ran parallel conductors from the lower end of the mast. One to the keel bolt and one to the ground plate. Also tied the compression post to the plate but did not intend this as the primary current path.


Frankly