Mailboat Letters

Lightning Protection

I enjoyed the article on lightning protection (we have a Catalina 320 insured with BoatUS). The size of a grounding plate or other structure always seems to be based on saltwater which is a very good conductor relative to freshwater. What are the recommendations for freshwater?

Author James Coté responds: You bring up a key point, as very little research has been conducted with respect to the effectiveness of lightning grounding plates on nonmetallic boats in freshwater environments. The jury is still out.

It is my opinion that the size of the plate is not so much of an issue as its location. At the instant immediately before the strike, the electric charge that is of opposite polarity to that in the cloud resides on the very top surface of the water. If our grounding plate is located several feet below the surface of the water, then the lightning currents must travel through the lightning down-conductor to the plate and then back up to the surface through the freshwater. Therefore, the most effective solution may be to install the grounding strip at, or near, the surface of the water.

Keep in mind that sharp edges or points are believed to be more effective at discharging lightning than flat surfaces.


I have a 53-foot pilothouse style powerboat located in Cape Coral Florida. Since I have been at this location, I believe the boat has taken two lightning hits. I say "believe" because there has been unexplained damage to electronics, but no physical damage to the boat. That is to say no blown-up antennas or even a burn mark. I'm just assuming lightning because several things failed at one time.

The second event was about 6 months ago. I was not on the boat, but when I got back, some things were not working. This time I lost the electronic engine controls, the autopilot computer, the chartplotter, and two bow thruster switches. Both times the boat was tied up at the dock with shorepower on. Power to all damaged items was switched off at the breakers. No burn marks or signs of a hit to the boat or antennas. Antennas were up but are not the highest point on the boat and did not appear to have been hit. Any thoughts about protection — other than moving the boat out of Florida?

James Coté responds: Unfortunately, it's hard to reliably diagnose an event that occurred months or years ago, based solely on your recollection of events. That being said, were there storms in the area on the dates of loss? How closely did you examine the metals aloft for lightning entry points?

I have my doubts with respect to the first event being lightning-related, but the second has some of the earmarks of the boat grounding a dockside strike through the shorepower cord. Shorepower-system surge-protective devices may have prevented some or all of the damage.

It is likely that your DC circuit breakers are single-pole devices, so turning these off will not disconnect the negative [i.e. grounded] conductors. Again, surge-protective devices would provide additional protection.


I just read James Coté's very interesting article, and a question came to mind. One thing that he did not address in describing recommended lightning protection for a sailboat is the issue of grounding a deck-stepped mast.

My question is this: How important is it that the connection between the deck-stepped mast and the grounding plate be in a straight line? I have often read that if it is not, then the charge may jump from the wire and go straight down through the hull.

James Coté responds: Your question reminds me of the enduring joke in lightning-protection installation. Marine electricians get a good laugh from the notion of installing a lightning down-conductor away from all other conductors and in as straight a line as possible; fully compliant aftermarket installations are nigh impossible, further amplifying the need for lightning surge protective devices.

The complication that is unique to sailboats is where to install the lightning grounding plate [the strip or point]. Whether installed port or starboard, a given plate may be fully submerged, at the waterline, or out of the water dependent on how the boat is heeled. In the perfect world, at least two plates should be installed and interconnected with the keel.


I disagree with the statement in the January 2016 issue: "(Note that wiring inside of the mast will be protected due to the Faraday effect.)"

I once owned a Heritage Yachts (Charlie Morgan) West Indies 38. The aluminum mast rested on a steel step bolted to the exposed lead keel (not encapsulated). Perfect ground!

But the VHF antenna has to extend higher than the mast top to effectively transmit and receive. My boat was struck twice by lightning and both times it melted wiring inside the aluminum mast. The Faraday effect can't prevent that.

James Coté responds: Thank you for your thoughtful response, but I believe that you are missing a few key points.

Aluminum in contact with steel and steel in contact with lead makes a good mechanical contact, but may not be a "perfect ground." If any moisture comes between the metals, galvanic corrosion could introduce electrical resistance.

Also, I note that your VHF antenna is the highest point on the mast and therefore acting as your lightning rod — and sacrificial lamb! I am not surprised that lightning strike energy passing through a coaxial conductor would melt this and adjacent conductors.

The aluminum mast Faraday cage effect is dependent on a robust lightning rod (Franklin Rod) installed as the highest point on the mast, and a properly grounded mast step. All lightning energy passes through the mast, which did not occur with your VHF lightning rod.

This is additional proof that partially installed lightning-protection systems are often wholly ineffective.

Finally, communications is not my area of expertise, but I suspect lowering your VHF antenna 10 feet will not significantly reduce your range.

More On Lubricants

I recently received the latest issue of Seaworthy and noticed an error. Lubrication of bolt threads does not make "the reading on the torque wrench much lower than it actually was." Simply put, a rusty bolt and nut will take lots of torque (twist) to develop the desired tension in the bolt — which translates to the "squeeze" between the pieces to be mated. Conversely, a well-lubricated bolt and nut, with clean threads, can quite readily be over-tensioned, because it is so easy to over-tighten the connection. So, the statement about "applying a lot more torque to a fastener," due to lubricating the fastener, is not true. What one is doing is tensioning the bolt excessively, perhaps beyond its design strength, because the lubrication allows this to happen even though the torque applied is less than specified.

Sorry, I deal with this almost daily and couldn't let this one pass.


Apparently the owners of some marinas (this one in Bayville, New York) don't receive your magazine. It may be worth the postage to send them a free copy. This is just a small portion of boats blocked in a similar fashion there.

Editor: While we've written about this subject before and condemned these kinds of blocks, it turns out that the American Boat & Yacht Council (ABYC) doesn't prohibit all blocks, just cinder blocks. Here's the skinny from the ABYC: "... cinder blocks, and other masonry products not intended to bear weight should not be used for blocking." It's hard to tell a cinder block from a concrete block, so we can't tell you if this arrangement is prohibited or not.

Honestly, we'd much rather see wooden blocks used — there's virtually no chance of failure.


I enjoy reading your magazine, and in the past I have been able to print a PDF copy. Am I missing the link on the page or has it been discontinued?

Editor: PDFs are still available. Go to the Seaworthy homepage, click on archives at the upper left, and you'll see PDF versions available on the dropdown.

— Published: April 2016

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