I plan to connect the base of my mast to a keel bolt as part of my efforts to protect from lightning (I understand there are lots of theories and methods about how to do this but this is the one that makes the most sense to me.) My question is regarding potential problems from connecting a steel keel bolt to an aluminum mast that is grounded to the boat's DC system. As I understand it the shield on the VHF coax is DC negative and since it connects directly to the metal housing of the Shakespeare antenna, which is directly mounted to a metal bracket screwed into the mast, I assume the mast is therefore DC negative also. I'm a bit fuzzy on this stuff but wouldn't there be possibly be issues with the potential of the keel bolt being different from that of the main DC ground (battery negative tied to engine)? If so, anyone have ideas for what can I do to prevent this (for example a way to remove the DC ground from the mast) while still maintaining the lightning ground? Thanks in advance...
Lightning Ground Question
- Thread starter Ryan L
- Start date
exoduse35
Sustaining Member
The ground the antenna is really a non issue. Even I you isolate it completely, there is sill a direct connection from the antenna to the circuits in he radio you are trying to protect. The best protection I have found is to unplug the radio and all other sensitive electronics and store them in the microwave oven, then wrap one end of a chain around the base of the mast and throw the other end over the side. Edd
Thank you for the reply but I'm not trying to protect the radio per se, I'm trying to protect the hull and the people in it. Primarily from side flashes etc. My thought behind grounding the mast to keel bolt is to provide safest low resistance path to ground. My concern regarding the VHF DC ground to mast is having another ground w/ different potential formed by mast-to-keel (is ground loop the right word?) and/or possible electrolysis issues? From an article about grounding by West Marine:
"[h=2]Lightning Ground[/h]The lightning ground needs to be a direct DC connection to the keel or to a ground plate to handle currents due to lightning strikes. So how do we keep the keel or ground plate electrically isolated as required in "Bonding and Electrolytic Corrosion..." above?
The solution is to connect the keel or ground plate directly to the mast, but make sure the mast is not electrically connected to the boats DC ground system. If your steaming light, masthead light, tricolor, Windex light etc. are wired carefully and correctly, they each will have their own DC return wire; there should be no ground connection between their wiring and the mast itself. Make sure that this is the case.
This should also be true of your masthead instruments. The unintended DC connection between mast and DC ground is typically made by the masthead VHF whip, which connects the shield of the coax to the bracket connected to the mast. That shield also connects to the VHF radio which is DC grounded by its power connection. The easiest solution is to insert what is called a "inner-outer DC block" into the coax.
This RF device puts a capacitorin series with the center conductor, and another capacitor in series with the shield. This device is transparent to the VHF RF signals in the center conductor and shield, but blocks any DC current in either the center conductor or shield. This device can be made by a good radio technician, or purchased from radio supply houses, pre-fitted with any kind of coax connection on both ends. The commercial units look like a coax "barrel" connector. A vendor is listed at the end of the article.
Once the DC connection from the mast to the VHF is broken, check for any other connections with an ohmmeter, and straighten out any other wiring errors or unintended connections. If your metal fuel tank is also bonded to the lightning ground system (per ABYC) then make sure that it does not have DC connections either to the engine via the fuel line or to the electrical system via the fuel level sensor. A piece of approved rubber fuel hose in the fuel lines to the engine solves that connection, and a well designed fuel level sensor will not make electrical contact with the tank.
When you're done, there will be heavy conductors running from the external keel or lightning ground plate to the mast, stays, and to the metal fuel tank, but there will be no DC connections to the engine or to the yacht's electrical system. See Figure 3."
Anyone have any thoughts? I can't find an "inner-outer" block with correct fittings etc and I've never heard of this before reading the article... Thanks.
Tom Metzger
Sustaining Partner
For lightning protection all large metal components should be tied to the keel bolt. If this means that your DC ground is tied to sea water so be it. Most boats already tie the DC system to the sea through the engine, shaft, etc.
You should not be using the mast for conducting any normal operating currents.
Proper lightning protection is not easy to retrofit on an Ericson. Tying the mast to the keel is a reasonable start. My previous boat, built on the east coast, was well protected from the factory with all chain plates, mast step, and the engine tied to the keel.
You should not be using the mast for conducting any normal operating currents.
Proper lightning protection is not easy to retrofit on an Ericson. Tying the mast to the keel is a reasonable start. My previous boat, built on the east coast, was well protected from the factory with all chain plates, mast step, and the engine tied to the keel.
Yeah Tom, thank you, I agree but maybe I'm not doing a good job explaining my question...
My DC negative (battery) is tied to sea water via a connection to engine. The concern I have is having an additional connection to seawater through the aluminum mast tied to a keel bolt. I'm not using the mast to conduct operating current but the boat's DC negative is connected to the mast via the shielding on the VHF coax being connected to the mounting bracket which is connected to the mast. I assume this is the case for many people but few people are probably aware of it and it's probably not usually a problem because relatively few masts are connected to keel. Does this make sense? Maybe what I need to do is tie engine to keel as well to keep everything at same potential? This is stuff I have a fuzzy understanding of but the West article got me thinking... Am I explaining it any more clearly?
Tom Metzger
Sustaining Partner
OK Ryan, you said the magic words... West Marine. I looked at their words of wisdom. I can't argue with what they say, and don't recognize the scope of their concern. I suspect that you would have a problem finding a boat that is compliant with their scheme. How many people isolate their fuel tank or antenna from the DC system?
Oops! I can argue with them:
"Do not bond any thru-hulls or other immersed metal that can be electrically isolated. Specifically, keep your metal keel/ballast, your metal rudder shaft, your engine/prop, and all thru-hulls electrically isolated, from each other, and from the engine."
They isolate the engine/prop from the engine. That's hard to do.
They say to isolate the radio antenna cable from the mast. I'm not sure what their concern is. My concern is lightning, not corrosion, which has not been a problem for me. If corrosion becomes a problem it can be dealt with. Others may go in other directions.
I don't think connecting the engine to the keel is a problem or a solution in the real world. Up to you.
If I understand correctly their concern is having the two different potential grounds (ground loop). But the odds of me understanding that correctly are slim. As for isolating prop from the engine depending on how it's accomplished at least you wouldn't have to worry about lightening! Although a submerged boat might have additional issues with corrosion? Thanks again for the replies.
Joliba
1988 E38-200 Contributing Member
conundrum
Ryan,
This is indeed a perplexing problem. The answer is that common sense and practicality have to win over electrical perfection. One can follow the West Marine advice (if the isolating fittings they describe exist), but the real question is: Can true isolation be achieved practically? When we rebedded Joliba's keel, the experienced yard owner was planning to electrically tie our mast AND OUR ENGINE to the keelbolts to bond the whole boat. I had just read an article similar to the West Marine adviser, warning not to do that. Though the yard boss, Larry, said that most other boats are bonded that way, I decided not to do it. However, I did connect the mast to the keel. As for grounding of DC instruments, I am certain that there must be some stray connection between the mast and some lighting, instrument, radar, or other ground wire, not to mention the VHF coax ground. The practical side is that there will be corrosion and one needs to know where to anticipate it so that it can be monitored in order to refresh contacts or replace vulnerable fittings periodically.
In my experience, lightning protection is a myth. I do not know of any boaters killed by sailboat lightning strikes, though I have seen several boats that were struck and have spoken to many who have had the experience. I have seen lightning strike the water yards away from my boat in open water, and strike nearby boats in an anchorage and marina. Fortunately, I have not been hit. Though tying the mast to the keel is common advice and practice (I have done it) I believe that it is akin to assuming the the "crash position" as we were taught to do in elementary school in case of nuclear attack. The practice of Lake Michigan sailors before 1960 was to drop chains over the side from the shroud chainplates during a thunderstorm. The thinking was that the boat is generally heeled over and the masthead, down the shroud to the water path is more direct for the lightning than down the mast. One might think that a wooden mast, composite mast or even aluminum would be a poor conductor for the lightning...hence the heavy copper conductor tying the mast to the keel. If you have ever watched a tree struck by lightning, even a dried out dead tree, you know that a huge static electrical discharge will not care much about a curving horizontal wire when it only needs to jump a short gap through some humid bilge air to complete a path of least resistance to the water. Boats I have seen struck by lightning were bonded from the mast to the keel or a ground plate. Nonetheless, electronics, even disconnected in the cabin will generally be fried. I have seen a through hull exploded out of a boat cruising in Panama, that survived by plugging and pumping until they could be hauled. Another boat had the entire balsa deck core destroyed. So, I don't have a great answer, but I truly doubt that even the experts have a true solution that works. A Faraday cage around the boat... like a car or an airliner would do it. But we have to do our best to guide the lightning through the vessel or try to keep it away altogether. Good Luck!
Mike Jacker
Ryan,
This is indeed a perplexing problem. The answer is that common sense and practicality have to win over electrical perfection. One can follow the West Marine advice (if the isolating fittings they describe exist), but the real question is: Can true isolation be achieved practically? When we rebedded Joliba's keel, the experienced yard owner was planning to electrically tie our mast AND OUR ENGINE to the keelbolts to bond the whole boat. I had just read an article similar to the West Marine adviser, warning not to do that. Though the yard boss, Larry, said that most other boats are bonded that way, I decided not to do it. However, I did connect the mast to the keel. As for grounding of DC instruments, I am certain that there must be some stray connection between the mast and some lighting, instrument, radar, or other ground wire, not to mention the VHF coax ground. The practical side is that there will be corrosion and one needs to know where to anticipate it so that it can be monitored in order to refresh contacts or replace vulnerable fittings periodically.
In my experience, lightning protection is a myth. I do not know of any boaters killed by sailboat lightning strikes, though I have seen several boats that were struck and have spoken to many who have had the experience. I have seen lightning strike the water yards away from my boat in open water, and strike nearby boats in an anchorage and marina. Fortunately, I have not been hit. Though tying the mast to the keel is common advice and practice (I have done it) I believe that it is akin to assuming the the "crash position" as we were taught to do in elementary school in case of nuclear attack. The practice of Lake Michigan sailors before 1960 was to drop chains over the side from the shroud chainplates during a thunderstorm. The thinking was that the boat is generally heeled over and the masthead, down the shroud to the water path is more direct for the lightning than down the mast. One might think that a wooden mast, composite mast or even aluminum would be a poor conductor for the lightning...hence the heavy copper conductor tying the mast to the keel. If you have ever watched a tree struck by lightning, even a dried out dead tree, you know that a huge static electrical discharge will not care much about a curving horizontal wire when it only needs to jump a short gap through some humid bilge air to complete a path of least resistance to the water. Boats I have seen struck by lightning were bonded from the mast to the keel or a ground plate. Nonetheless, electronics, even disconnected in the cabin will generally be fried. I have seen a through hull exploded out of a boat cruising in Panama, that survived by plugging and pumping until they could be hauled. Another boat had the entire balsa deck core destroyed. So, I don't have a great answer, but I truly doubt that even the experts have a true solution that works. A Faraday cage around the boat... like a car or an airliner would do it. But we have to do our best to guide the lightning through the vessel or try to keep it away altogether. Good Luck!
Mike Jacker
Tom Metzger
Sustaining Partner
FWIW, way back when, I designed electric substations and keeping lightning out of the station was a primary concern. The primary protection from direct strokes was tall masts strategically placed around the yard. These masts were grounded, as were all structures on the property, and each had a 1" vertical rod on the top that was brought to a point. The purpose of the point is to bleed off the electrical charge from the ground into the atmosphere neutralizing the atmosphere around the station.. The lightning stroke requires a charged path so the stroke goes elsewhere.
I believe the antenna and/or the sharp edges on the grounded mast head act the same way greatly reducing the probability of a direct stroke. This is my explanation why sailboats aren't hit more often.
I believe the antenna and/or the sharp edges on the grounded mast head act the same way greatly reducing the probability of a direct stroke. This is my explanation why sailboats aren't hit more often.
EGregerson
Member III
lightning strikes
I read an article years ago about grounding the mast to the keel boat; I couldn't believe this was effective; what? the photons are going to ground thru the gelcoat/ epoxy that encapsulates the keel? I called ABYC in Hollywood, MD; spoke with a guy; he said yeah, it might burn the gelcoat but it's supposed to work. This article also said to run a cable from the keel bolt to the motor; I thot, this is the path to ground, thru the prop shaft, not the (lead) keel. But who am I to question? So I have a battery cable already to connect the mast to the keel boat...for 3 years. I just can't connect it because I still have doubts. So your thread has re-opened the subject for me again. I googled 'ground plate'; saw a Guest Dynaplate; What does it say on it? "Do Not Paint" i.e. the mere painting of the grounding plate will electrically isolate it. Well what will gelcoat / epoxy do? So I've decided to get a Guest Dynaplate when I get the money; til then I'll attach battery cables to the shrouds and toss the loose end over the side.
ps: another item maybe to consider is a 'static dissipator'. I rolled my eyes when I first read of them back when; like, what is this? a divining rod? But maybe there is some valid science behind them.
I read an article years ago about grounding the mast to the keel boat; I couldn't believe this was effective; what? the photons are going to ground thru the gelcoat/ epoxy that encapsulates the keel? I called ABYC in Hollywood, MD; spoke with a guy; he said yeah, it might burn the gelcoat but it's supposed to work. This article also said to run a cable from the keel bolt to the motor; I thot, this is the path to ground, thru the prop shaft, not the (lead) keel. But who am I to question? So I have a battery cable already to connect the mast to the keel boat...for 3 years. I just can't connect it because I still have doubts. So your thread has re-opened the subject for me again. I googled 'ground plate'; saw a Guest Dynaplate; What does it say on it? "Do Not Paint" i.e. the mere painting of the grounding plate will electrically isolate it. Well what will gelcoat / epoxy do? So I've decided to get a Guest Dynaplate when I get the money; til then I'll attach battery cables to the shrouds and toss the loose end over the side.
ps: another item maybe to consider is a 'static dissipator'. I rolled my eyes when I first read of them back when; like, what is this? a divining rod? But maybe there is some valid science behind them.
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Tom Metzger
Sustaining Partner
Don't do it! the Guest Dynaplate is designed for radio grounds. It will explode if exposed to a lightning stroke.
I don't believe in the "static dissipater", nor does West Marine. I think WM is wrong about dissipating the charge; I think the masthead does do that. I just don't think the bottle brushes do it. BTW, we are talking about electrons, not photons. :nerd:
"Given that you have grounded your mast solidly to the ocean, your mast will be at exactly the same electric potential as the ocean. There is no chance that you can dissipate the charge between the ocean and the atmosphere, so don't bother with a static dissipater at the masthead. Wire "bottle brush" static dissipater may be useful to dissipate seagulls, however, but that is beyond the scope of this article."
My math skills are too far out of date to do the calculations, but several years ago I got in this discussion with an engineer named Kim on the sailnet/Ericson email list that claimed she had done the math to justify the bottle brushes. She was a nice lady so I hate to say it... she never convinced me.
I don't have an encapsulated keel, however a guy in FL has written extensively about lightning protection for sail boats after doing research. I would read what he has to say. I see he has a company now and has changed some of his ideas. I haven't read all of this: http://www.marinelightning.com/index.html
I just want to share a first hand lightning experience or should I say pre-lightning experience. This is going back many years ago - mid 70's, so I can't give you any brands, but the jist is I was cruising with my parents on their Tartan 34. We were anchored in a cove enjoying a typical Chesapeake summer afternoon, e.g. there always seems to be a thunderstorm around the corner. As we sat there, a hum/buzzing like sound started up that seemed to come from the rigging. My Dad went below and disconnected the VHF antenna lead from the radio and connected it to a lead that went to a plate through the hull, I'm thinking something similar to the Dynaplate. When he made that connection to the plate, two things happened. The buzzing stopped, and he got knocked back hard - no permanent damage, but I think it knocked him off his feat. As luck had it, he happened to be doing some business with someone who did lighting protection on towers (think I remember this right). The feedback from this guy is that it sounded like a field or charge (my words may be a bit off - like I said, long time ago) was building up in the rigging and we were indeed about to take a lightning hit. When my Dad grounded out the antenna, it dissipated it. So, not sure where to go with this, but I'd say that having some sort of ground path on the radio seems to be important, but we also would (and did at this point) hang big jumper cables from the stays overboard into the water (I've also hear of wrapping chains around the stays to hang over) - the thought here is that a strike will go down the stays and hopefully decide the jumper cables offer a path of least resistance that entices the lightning to go that way versus blowing a hole out of the bottom of the boat as it goes straight down the mast. Honest truth is I think lightning will do what it wants....
So, not sure I have done anything but help make mud, but I've recounted this best I can remember.
So, not sure I have done anything but help make mud, but I've recounted this best I can remember.
Maine Sail
Member III
This is how I isolate my VHF whip from the spar...
Yeah, Emerald, whenever lightning got close we used to pull out big jumper cables, clip them on the shrouds, and dangle them in the water. No idea if it works, but very comforting.
Becalmed 100 miles off New Jersey, we were surrounded by a thick atmosphere of yellow gloom. No thunder. On and off stupendous rain. Then the sky lit up with bombs. Loud explosions overhead and all around. Balls of fire and light which seemed to make no connection with the sea, and didn;t have a vector, as lightning does. Way cool--sort of. Never so scared before, except when the Concorde used to pass over supersonic and blow up the morning watch with its boom. Man, no warning at all for that.
I no longer carry jumper cables, now I just pull out the bottle of gin.
Becalmed 100 miles off New Jersey, we were surrounded by a thick atmosphere of yellow gloom. No thunder. On and off stupendous rain. Then the sky lit up with bombs. Loud explosions overhead and all around. Balls of fire and light which seemed to make no connection with the sea, and didn;t have a vector, as lightning does. Way cool--sort of. Never so scared before, except when the Concorde used to pass over supersonic and blow up the morning watch with its boom. Man, no warning at all for that.
I no longer carry jumper cables, now I just pull out the bottle of gin.
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EGregerson
Member III
more data...
Thanks Tom for the heads up on the Dynaplate. Even though they advertise lightning protection, an article on Boat.com describes the apps of the 3 models; small is for bonding thru hulls; the medium for loran and the large for ssb; no mention of lightning protection. One web site carries both Guest and a comparable Newmar; the newmar verion of the plate states 'NOT for lightning protection'. ABYC and Coast Guard info state 1 square foot of ground plate for lightning protection. I suppose if you had an array of Dynaplates, that would be ok. ( I still have reservations about tying the mast to a keel bolt). An anecdotal case in Chicago has a boat hit (with mast grounded to keel bolt) and it blew a thousand tiny holes in the bottom of the keel. I found a CG doc, Lightning: the Cone of Protection. It says to ground the mast to "...a submerged ground plate or to an exposed metallic keel". This supports my contention that gelcoat (and epoxy and CF bottom paint) insulates the lead keel and so, does not serve as an effective ground. So, for my boat, I'll pick up copper plates from McMasterCarr (total of 1 sfqt) and tie the mast, the shrouds and the backstay to them. Then I'll pop the cork.
Thanks Tom for the heads up on the Dynaplate. Even though they advertise lightning protection, an article on Boat.com describes the apps of the 3 models; small is for bonding thru hulls; the medium for loran and the large for ssb; no mention of lightning protection. One web site carries both Guest and a comparable Newmar; the newmar verion of the plate states 'NOT for lightning protection'. ABYC and Coast Guard info state 1 square foot of ground plate for lightning protection. I suppose if you had an array of Dynaplates, that would be ok. ( I still have reservations about tying the mast to a keel bolt). An anecdotal case in Chicago has a boat hit (with mast grounded to keel bolt) and it blew a thousand tiny holes in the bottom of the keel. I found a CG doc, Lightning: the Cone of Protection. It says to ground the mast to "...a submerged ground plate or to an exposed metallic keel". This supports my contention that gelcoat (and epoxy and CF bottom paint) insulates the lead keel and so, does not serve as an effective ground. So, for my boat, I'll pick up copper plates from McMasterCarr (total of 1 sfqt) and tie the mast, the shrouds and the backstay to them. Then I'll pop the cork.
Thank you! An excellent solution! I went on your Compass Marine site (http://www.pbase.com/mainecruising/boat_projects) and made a donation in thanks for your post as well as the other incredible articles and information you post.
As for the suggestions and feedback in the other replies, thank you too! The points raised were pretty much the same I've found in many other threads, books, etc but it's nice to have them all in one place for easy reference. I especially appreciated the discussion regarding relative risk and the almost philosophy of lightning protection.
Lightning protection and SSB installation are borderline voodoo magic.
My 2c:
Ground plates - better than nothing but I don't like the thought of "helping" the lightning to what is basically a through hull with a bolt in it (albeit a small diameter one).
Keel - I tend to agree with the many experts (and ABYC) who seem to all recommend the keel as the ideal ground unless it's encapsulated. I do not think the gel coat etc provides enough insulation to prevent lightning from finding the "ground." If lightning can find it's way to water through a hull I assume it can easily punch through a much thinner layer of gel coat and/or paint. The surface area of a keel is so much greater than that of an external ground plate and with multiple large keel bolts I believe it is more structurally able to 'survive' a strike. If there are some pinholes in the gel coat afterwards (or even a large chunk missing) than so be it. Much better than the potential alternative.
Static dissipators - I've read many articles online and in print stating they simply do not work in this application, or at least not as advertised. That said, if you already have one I can't imagine any reason why not to use it. If nothing else maybe it will help keep birds away?
Chains clipped to shrouds and thrown overboard - Many reasons I do not like this idea.
Doing nothing and hoping for best - I personally feel that despite the lack of certainty regarding this topic there is enough anecdotal evidence and reliable observation for me to believe that a lightning ground designed like the ABYC mast-to-keel standard provides a greater protection from the most destructive effects of a direct strike (side flashes, seeking ground through hull, etc).
My decision - Now that I have a design that will allow me to isolate the DC ground from the mast (thanks again Maine Sail) I am going to connect a thick cable from my mast in the most direct path possible to a keel bolt. My thought is to drill and tap as thick of a stainless or bronze nut as I can find and mechanically attach the cable to the nut. I already purchased the nuts from McMaster and will try to remember to post a follow up after it's done.
One other thing I plan to do is fly a kite using 18AWG wire as kite line connected directly to the battery. I figure this should provide enough power to allow me to run the watermaker during storms. Any thoughts?
Rick R.
Contributing Partner
One other thing I plan to do is fly a kite using 18AWG wire as kite line connected directly to the battery. I figure this should provide enough power to allow me to run the watermaker during storms. Any thoughts?
Ben Franklin would say Bravo!
Joliba
1988 E38-200 Contributing Member
If you run the kite wire to red and green globes mounted on port and starboard spreaders, St. Elmo's fire will make a very good substitute for your running lights and further save your batteries. Not to mention that use of the kite for propulsion will save wear on your sails during the thunderstorm.
Maine Sail
Member III
Thanks!!!
An easier method is to buy some 2" X 1/4" flat stock, drill it to keel bolt OD & drop it over the exposed keel stud. Now drive a new SS nut on top of it bonding it to the keel bolt. The other end of the flat stock gets tapped for 5/16 X 18 and receives the 2/0 wire with 5/16" ring terminal. Keep some terminal grease on it to maintain conductivity and keep corrosion at bay.