Officially, summer is here, and with it, summer thunderstorms. When a fire-breathing storm rolls over you out on the water, it is hard not to feel that you are at great risk. We all know better than to seek shelter beneath a tree, yet here we sailors are huddled at the base of an aluminum "tree" that reaches defiantly toward the clouds. A metal mast towering overhead would seem to heighten the danger (no pun intended), but as it turns out, that mast is actually your protector. It's powerboaters who most often get electrocuted by lightning.
Cone of Protection There is one requirement, however, in order for the mast to save your bacon—it must be grounded. The lightning rod—which is what your aluminum mast becomes when it is grounded—was first conceived by Ben Franklin to protect wooden buildings from fire. A grounded metal rod extending above an adjacent structure offers lower resistance than a wooden roof or another 100 feet of air, and this is the path the charge takes. Wooden masts should be fitted with a grounded lightning rod—called an air terminal in this application. Without it, the mast's high resistance can generate enough heat to instantly convert moisture in the wood to steam, which tends to blow the mast apart.
The idea of attracting lightning makes some sailors reluctant to ground the mast on their vessels. The logic behind this fear is fundamentally flawed for an aluminum mast because grounded or not, the mast is a better conductor than air. Ground the mast and a strike has a safe exit route. Leave out the ground and the strike will leap across the interior of the boat to some other metal component that is grounded, or it will simply blast through the hull to reach the water. There is no evidence that grounding the mast increases the likelihood of being struck, but compelling evidence that doing so reduces the risk of damage or injury if a strike does occur.
Any time the top of the mast is closer to the lightning than the ground, the lightning tends to divert to the mast. That protects the area below the masthead and inside a circle around the base with a radius approximately equal to the mast's height. This area is known as the cone of protection, and as long as the boat is entirely within the cone, there is little risk of anyone aboard being struck directly. You are at risk, however, if you place yourself in the "circuit" by bridging two metal components—the steering wheel and a stanchion, for example. And if the mast isn't grounded or if it is poorly grounded, a leap from the mast to some other metal component—called a side flash—could get you. And even with a good ground in place, you should stay well away from the mast during an electrical storm.
Static Dissipaters Look around in your marina and you will see aluminum masts topped with a pointed rod or what looks like a bottle brush. The owners of these boats have installed these devices with the hope of warding off lightning strikes. The theory is that the point or points of these devices—called static dissipaters—bleed off the charge from the grounded mast, thus lowering the voltage differential below what is required to "spark" lightning. Dissipaters probably do bleed off static charges, but trying to bleed the ocean's charge into the air with a dissipater on your mast is like trying to lower the ocean's level with a soda straw.
Now the concept of electrical grounding with boats is something that every sailor should understand. Almost any metal component that is in contact with the water constitutes a ground on a boat, but for lightning protection, the requirements are more exacting. You cannot just ground the mast to the closest through-hull fitting. Such a ground provides inadequate conductivity, and a powerful strike is almost certain to generate enough heat to melt the fitting right out of the hull. You can guess what happens after that.
If your boat has an external keel, you are all set. Don't worry about the bottom paint; it will not present a significant barrier to a strike that has gotten this far in its quest to reach the water. However, if the keel on your boat is encapsulated, a proper ground for lightning protection will require a copper ground plate. Lightning dissipates from the edge of the plate, so the perimeter of the plate should measure at least four feet if you sail in salt water. Use bronze bolts to fasten the plate to the hull and for attaching the ground cable.
|"Sintered bronze plates designed for grounding radios are a poor choice for lightning protection."|
Sintered bronze plates designed for grounding radios are a poor choice for lightning protection. These are less effective than solid copper at dissipating the strike, and reportedly they tend to explode when heat from the strike turns trapped water to steam. I also discourage routing the strike through the boat's engine. While the propeller probably has enough edge length to do the job in salt water, passing such high voltage through the engine is bad for the bearings.
A Florida Sea Grant study found that when the mast was grounded, only one boat in 10 struck by lightning in salt water suffered damage (excluding electronics), while six out of 10 in freshwater suffered some kind of hull damage. Given the less efficient conductivity of freshwater, this disparity is almost certainly due to inadequate grounding. In freshwater the ground plate should have at least 24 feet of edge, typically accomplished by attaching a 12-foot length of inch-wide copper strap to the hull. Such a strap is six times as effective at dissipating a strike as a square plate with the same area.
Because lightning travels on the surface of the conductor, a solid copper strap is the best choice for connecting the mast to the ground. I use flattened, half-inch copper tubing. If you want to use wire, it should be No. 4 AWG or larger.
When you go to install your connection, remember that lightning doesn't like to change directions, so make the leads as straight as possible to the ground. Any unavoidable turns require a radius of at least a foot. Even if your mast sits directly on the keel, perfect the electrical connection with a copper strap from the mast to a keel bolt.
An aluminum mast offers lower resistance than steel stays, but a powerful strike can still induce current flow in stays and shrouds. To provide this current a safe path to ground, cable the chainplates to the ground terminal using No. 6 AWG wire. Again, the route to the ground should be as straight as possible.
I recommend that boat owners make cable connections with mechanically attached terminals because solder will melt. Drill attachment holes in the ends of the copper strap. Use washers to increase the contact area—stainless steel on the mast connection (to minimize corrosion) and copper elsewhere. All connections must be clean and tight. Coat the assembled connection with an anticorrosion spray and periodically measure the resistance from the mast to the ground plate with an ohmmeter.
The mast will have a potential of 30,000 volts or more during a lightning strike, while other metal components inside the boat are essentially at 0 volts. To minimize the risk of a potentially deadly side flash, I recommend that you connect all significant metal masses within six feet of the mast (like water or fuel tanks or even an oven) to the ground plate with No. 6 AWG wire.
Electronics The harsh truth is that the lightning protection system described above can nearly eliminate personal risk and significantly reduce damage to the boat, but it offers little if any protection for electronics. About half of all boats struck by lightning will experience damage to some or all of the electronics aboard, and it doesn't even require a direct hit. Lightning induces current flow just by passing near a conductor (near being a relative term where lightning is concerned). The closer and more powerful the strike, the more current that is induced. Such currents often exceed the capacity of the tiny, low-current components inside most on board electronics. Electronics need not be connected to anything to be affected, so disconnecting them may not prevent damage.
This doesn't mean you should be entirely fatalistic about your expensive electronics. You should ground all metal housings; this can protect internal circuits and components from directly induced currents. Store portable electronics in a grounded metal box for the same reason. (The oven can serve in a pinch.) Twist all electronics power leads so induced currents will tend to cancel. Be sure electrical wiring crosses bonding wires at 90 degrees to minimize the inductive effect of current flowing to ground. However, recognize that despite every protective effort, if your boat is struck, your electronics have only one chance in two of not becoming toast. Your best protective measure in this regard is keeping your insurance paid up.
There is a lesson to be learned here. Don't flatter yourself that you can be a cosmic player: lightning doesn't know you exist, and it doesn't care. The way to deal with lightning is just to stay out of the way. On a sailboat, staying out of the way primarily means giving the lightning an unimpeded path to ground.