Should I ground my mast to the keel?

Bill Roosa · Mar 8, 2014

There is lots of new thought on lightning grounding.
MY opinion is lightning clouds induce a charge on the SURFACE of the water (<1/4" deep). The keel is deeper (and at a higher potential )than that so grounding the mast to the keel just brings lightning into the boat with nowhere to go except out the side of the hull. The electrical potential of the mast to keel wire has a very odd (for a wire) voltage distribution in this situation. High at the mast end, zero at the level of the water surface and higher (not as high as the mast but higher then the water surface) so the lightning has not reason to continue to the keel and tries to find the shortest route to the water surface, in this case it is (as the insurance folks will tell you) through some metal part near the hull and at the water surface level.
Better to leave the keel disconnected and ground the shrouds and stays over the gunnel and down to the water surface and not bring the stuff into the cabin at all.
You are going to find there is NO consensus on this topic so I'd take my info from the building industry which NEVER runs the lightning arresting wires into an occupied space and has not had much of a problem with frying stuff in the last 100 years or so. The only (IMHO) difference between a boat and a building is the actual "grounding rod" and the fact that earth is not as good a conductor as sea water.

Stu Jackson · Mar 8, 2014

Read this and then make up your own mind.

http://www.westmarine.com/WestAdvisor/Marine-Grounding-Systems

Good luck.

Benny17441 · Mar 9, 2014

Ask the boat manufacturer the reason for which they do not bond the boats at the factory. If their answer is, as per recommendations of their attorneys you may take that to the Bank. The reality is that no one knows and opinions keep changing. Some seem to think that bonding not only increases the odds of getting a strike but also the severity of the strike. The awesome power in a lightning strike means it will go where it will go and not necessarily where we may try to steer it to. There is actual data that indicates that more strikes are recorded in boats near land than those at sea. Is that due to some interaction with land or just because there may be more boats docked than at sea at any given time. The legendary cone of protection afforded by the mast seems to work but then it could be just so by mere accident. I guess it just boils down to those that feel the need to be proactive will be happier if the bond and the rest of us will spare the time and effort for something else. There are two positive observations ; the risk to life and limb is minimal based actual count of casualties and a good insurance coverage will help mitigate equipment losses.

CarlN · Mar 9, 2014

If side flashes are the threat that Bill suggests (and it makes sense), it would seem pretty easy when at anchor or dock to clamp a piece of lifeline wire to a shroud and drop it in the water. Even better, attach a large copper bolt to the wire at the water surface.

I've seen a short length of chain used but copper with sharp edges would seem a better dissipator

Why not?

MrUnix · Mar 9, 2014

Everything you ever wanted to know about sailboats and lightning can be found here: http://edis.ifas.ufl.edu/sg071

Produced by the University of Florida as part of NOAA's Sea Grant program. Great read for those wanting to know more about how lightning strikes form and move through a sailboat and what can be done to prevent substantial damage and injury to crew.

Cheers,
Brad

PS: My H23 has, from the factory, a ground wire running from the base of the mast, down through the cabin and attached to a keel bolt.

psheppard · Mar 10, 2014

I just use car jumper cables. One end clamped to each side shroud, and then toss in the water. Nothing yet, knock on wood. When not being sailed, my boat sits on an elevated lift, on wooden slats covered with old carpet, so this would be the only contact with the water and hopefully give any lightening strike the "Path of least resistance". 1.21 gigawatts, Holy Crap Batman!

SailingHarry · Mar 10, 2014

Every time the issue comes up, you hear the "some sort of wire over the side" argument. Seriously? You folks are sailing along, a thunder storm is coming up, you are working in a reef, and you have somebody clipping jumper cables onto a shroud and throwing it overboard? Or tie a chain around the mast and drag that over the waterway into the water? While making 6+ knots in a heavy sea? I just can't get my mind around that. Or, worse yet, you're going to make a connection with a giant alligator clip on a high resistance stainless steel shroud?

While the commercial building industry does use a perimeter lightning grid with outside down-comers, they have no issues with how to get that to "ground" and it is as much a matter of convenience than anything.

ABYC recommends a heavy grounding system (rig and mast) with a clear set of rules on how to do it. Most if not all boat builders follow that guidance. I'd be really leery about coming up with my own (or some other non-study based idea from someone else who has no engineering/electrical/study/lab basis).

But as someone else mentioned, this is a contentious issue with about 1000 different opinions. So anything you do will meet with at least one approval!

Back to the OP. I'd get the biggest wire I could, bond it to the mast as best I could, run as nearly vertical as I can, to the biggest keel bolt I can find. I'm about to upgrade my Sabre 34 mast ground from #10 (factory in 1979) to #1 (it's what I happened to have on hand) and I think that's a big improvement. The shrouds will still be #10 bonding -- hey, you can't do everything!

Harry

Maine Sail · Mar 11, 2014

First: THERE IS NOTHING YOU CAN DO TO PREVENT A STRIKE.. LIGHTNING WANTS YOU, IT GETS YOU..

You can try to minimize hull damage by providing a clean low resistance path to ground, and this does tend to minimize hull damage.

I do lightning damage surveys/consults and repairs and also meet with the insurance adjusters on behalf of the owner who hired me to get them a non "screw job" by the insurance company.. As a result I see LOTS of lightning damaged boats. I think I was on-board 11 last summer alone, with 7 of them being full lightning damage assessments..

The only good data that we have suggests that a bonded boat gets struck LESS, not more. This data is slightly less but still the data suggests less. In my own n=X pool of lightning damaged boats I also see more non-bonded boats get hit.

About a year ago I had a long talk with Beth Leonard of Boat US, this reminds me I need to call her again

, to see what type of data they collect. I was shocked to find out they really collect very little.

When non-bonded boats get hit I see:

More hull damage, considerably more on average

More on-board wiring damage

When bonded boats get hit:

I see less hull damage

Much rarer wire damage above 14GA or so

When carbon spars get hit:

YOU DON'T WANT A CARBON SPAR, IF YOU CAN'T AFFORD TO REPLACE ONE.......

I personally go well beyond ABYC standards. Our spar is bonded to the keel via 2/0 wire and these connections are kept clean and coated with terminal grease.. When we were hit our strike exited the bottom of the keel where there was a void in the barrier coat. There were also some spiders starting from the exit area.. the exit was approx 5' below the surface. We suffered ZERO hull damage yet nearly everything on-board was toasted even multiple items not even plugged in. A few items survived, miraculously, with no rhyme or reason as to why..

The same night we were hit this non-bonded boat was hit..

Right behind this group of holes was the VHF wire.... This boat was riddled with pin holes anywhere a wire got close to the hull. Mast was not bonded to keel...

IStream · Mar 11, 2014

Thanks, Maine. There's just no substitute for real-world data and experience. I was toying with the idea of removing the bonding on my boat because it can make things worse in an electrically hot marina but now you've given me something to think about. We don't get much lightning here in the great PNW, but hopefully we'll someday be taking the boat to where it's more of a threat ;o).

Bill Roosa · Mar 11, 2014

So MS
Why do you think the lightning has a propensity for leaving the hull in some location other than the well bonded keel? I've been paying attention to this for years and it seems that lightning generally exits near the water line more often than not.

Blitz · Mar 11, 2014

Maine Sail said:
I personally go well beyond ABYC standards. Our spar is bonded to the keel via 2/0 wire and these connections are kept clean and coated with terminal grease..

Mainesail - we know you have pictures of your bilge....

What have been you experience with encapsulated keels? Does the glass around the keel just blow off?

I have a keel stepped mast which I'd like to bond. Keel bolts are normally in my wet bilge (since it's shallow, rainwater down the mast, and pumps don't get the last bit out). Curious if you just bolted a terminal to the mast. How did you attach to your keel bolts?

On your boat that was hit were all the stanchions and pulpits bonded?

MrUnix · Mar 11, 2014

What have been you experience with encapsulated keels? Does the glass around the keel just blow off?

If you check out the University of Florida/NOAA Sea Grant publication I linked to, which is based on real life assessments of over 1800 boats struck by lightning here in Florida, it says:

If the lead is either painted or encapsulated in fiberglass, minor repairs may be needed after a lightning strike. However, the paint or fiberglass does not seriously compromise the ballast lead as a lightning ground.

And like MS said, if lightning wants you, it will get you.. all that grounding/bonding does is provide a path for it to take so it will minimize damage to the boat and crew. For a boat that does not have a grounded mast, they state:

The difference is what happens where the conducting path, the mast, ends. Since current cannot flow from the ground to feed the growing attachment spark, a negative charge accumulates at the base of the mast and eventually arcs across in the general direction of the water or a nearby conductor. (For this exercise, crew members are conductors!) The result is an unharnessed electrical discharge between the bottom of the mast and the water.

And goes on to say:

The idea of the grounding system is to divert the lightning current through a predetermined path so that it does not make its own explosive path through fiberglass, teak, crew members, etc. [...] Wherever these sparks travel through bad conductors (fiberglass hull, teak bulkheads, through-hulls, porta-potties, etc.) sufficient heat is generated to explode the impeding material into a nicely conducting plasma that is hotter than the surface of the sun.

And that could definitely ruin your day

http://edis.ifas.ufl.edu/sg071

Cheers,
Brad

Maine Sail · Mar 11, 2014

Bill Roosa said:
So MS
Why do you think the lightning has a propensity for leaving the hull in some location other than the well bonded keel? I've been paying attention to this for years and it seems that lightning generally exits near the water line more often than not.

On well bonded boats I don't tend to see this nearly as often as non-bonded or poorly bonded. Remember this is only to attempt to minimize hull damage not prevent it. Lightning is unpredictable.

A Hinckley Picnic Boat got his so hard in SW Harbor that if literally drove the VHF whip, like an arrow, through the cockpit sole. About 10" of it was visible from above and the rest was shot through the deck....

I saw a boat where the teak bilge covers, about 15 pounds each, were blown clean across the cabin and took a huge chunk out of the nav station when apparently the bilge water was vaporized. The mast on that boat was not bonded to the keel. Lightning then picked up the seacock bonding system and exited through the engine, rudder and some seacocks as well as many pin holes along one of the bare copper bonding wires deep in the bilge... All exit wounds were well below water..

A customer's boat was hit on the hard in MD. The keel was blocked 2' off the ground (tar lot) and the boat was on stands. It was not plugged into shore power. The blocks were wood and the stands had the boat 100% isolated from anything. His mast was still hit, the strike came down the spar through the bonding wire and out the very bottom of the keel, jumped the 2' gap and burned a hole in the asphault. All electronics were fried but the bonding wire carried the brunt of the strike to Earth. Only the VHF wire was damaged but when the mast wires were pulled all the others tested fine. IIRC his amst was bonded with 1/0 wire direct to a keel bolt..

A J-42 customer was hit, boat is very well bonded. Strike exited through the keel about 3-4 feet down. No hull damage other than the keel patches..

Lightning just wants to find ground. I don't find it discriminates just at the waterline..

SaltyHog · Mar 11, 2014

Lightning Plate Material?

MS, What would be the best material to make a ground plate for my encapsulated ballast full keel boat (Cabo Rico 38)? I've read that silicon bronze would be best, but I can't find a supplier of flat stock. Onlinemetals.com has aluminum bronze in stock and in the right sizes, but I'm not sure about saltwater corrosion with that material. They also have copper plate and flat bar stock, but again would the corrosion be a problem. Plan is to make 2 of 3/8" x 6" x 18" plates and install them on either side of the the keel just above the ballast cap. Connect to mast step with #2AWG. Also connect forstays and shrouds to the same plates with #4AWG. As a side note, I've looked at all the boats on the hard at my marina and two others, and I have never seen a boat with a ground plate attached to the hull of an encapsulated ballast boat. I wonder why? I have seen sintered bronze plates, but not sure if they are for lightning or a counterpoise.

Another question: Should the Lightning ground be electrically seperate / isolated from the bonding circuits and DC ground? My bronze thru-hulls are bonded to DC ground and to 2 hull zinc anodes.

Maine Sail · Mar 11, 2014

SaltyHog said:
MS, What would be the best material to make a ground plate for my encapsulated ballast full keel boat (Cabo Rico 38)? I've read that silicon bronze would be best, but I can't find a supplier of flat stock. Onlinemetals.com has aluminum bronze in stock and in the right sizes, but I'm not sure about saltwater corrosion with that material. They also have copper plate and flat bar stock, but again would the corrosion be a problem. Plan is to make 2 of 3/8" x 6" x 18" plates and install them on either side of the the keel just above the ballast cap. Connect to mast step with #2AWG. Also connect forstays and shrouds to the same plates with #4AWG. As a side note, I've looked at all the boats on the hard at my marina and two others, and I have never seen a boat with a ground plate attached to the hull of an encapsulated ballast boat. I wonder why? I have seen sintered bronze plates, but not sure if they are for lightning or a counterpoise.

Another question: Should the Lightning ground be electrically seperate / isolated from the bonding circuits and DC ground? My bronze thru-hulls are bonded to DC ground and to 2 hull zinc anodes.

Usually copper bar stock is used.

brazenarticle · Mar 11, 2014

Hello MaineSail -
If I may get back the the original question ( I also have a Catalina 28) what is best bet to get some level of protection? The mast is deck stepped on a stainless plate, I'm assuming fiberglass between that and the top plate of the compression post, and guessing there is no clean electrical connection other than wires in the mast.
Would a #2 wire from keel bolt to compression post also need a jumper between post and mast? Attaching a longer directly will have to wait until I pull the mast for maintenance.
How about the chainplates? Do they need to be connected to the keel bolts too?
One last thing - what happens, what should I do if I am on the boat in a severe electric storm (common enough scenario on the Chesapeake).
Is it just a numbers game that many more boats get hit but I have yet to hear of sailors struck? How long can we count on lightning bolts going for golfers more often than sailors?

scherzoja · May 30, 2016

I think I'm going to stir this stew a bit more to get some clarification.

Below is a plan I came up with after reading Calder's book and several lightning/bonding threads and I'd like some input/feedback.

Page 233 of Calders book has a diagram where all the negatives/grounds/bonds (incl. lightning) go to a grounding bus, which is then connected to an "immersed ground plate or strip".
However, another paragraph describes a separate lightning grounding system with external plate and no connection to common ground bus

A. separate grounding system for lightning ground; cables straight/short as possible
B. AC ground, DC negative, & bonding terminate at common ground, which has its own immersed plate

External lightning ground strip (1/4" x 1.5" x ~120", I'm in brackish water)
Through bolted about every 2 or 3 feet, wherever I can reach and get to if hole forms
I have an isolation transformer

1. mast (2/0 wire), shrouds, stays connected (biggest I can manage up to 2/0) as straight as possible to external grounding strip through bolts.
2. Aluminum fuel tank (and deck fill) & all underwater metal fitting (including prop strut) daisy chained to each other, then connected to common ground with 8 awg
3. Large on-deck metal fittings, pulpits, stanchions, anchor roller daisy chained, then connected to lightning ground strip with 4 awg
4. No other connections to the lightning grounding strip, which begins fwd of mast, ends about 10' aft under galley.
5. DC negative terminates at negative bus
6. AC ground (not neutral) terminates at AC ground bus
7. Common ground point, engine or its own immersed copper plate?

-- Questions --
What do I use for common ground for AC ground and DC negative: engine or separate, external copper bar, like with the lightning ground, but smaller?
Engine has transmission, v drive, then out to prop

The engine is freshwater cooled with raw water heat exhanger. Is there enough electrical contact in the cooling water system and the gears of the transmission and V drive use it for the common ground?
Is the engine raw water intake metal thruhull electrically connected to the engine because the hoses are filled with seawater?

What if water gets brackish? There would be no connection, if it was marginal to begin with. the water here can be salt, brackish, and sometimes sweet, if we've had a long run of rainfall.

It seems that the more I reread Calder's book, the more I think using the engine/propshaft is not as good as a grounding plate outside the hull, esp. if I install a flexible coupling/drive saver.

Is there any reason to connect the common ground to the lightning ground strip instead of its own ground plate (or engine/prop shaft)?

Do I install a zinc to the external lightning ground strip?

Thanks for the input.