(Primarily addressing potential needs for small boats under 26 ft in length)
Lightning appears to be like that rogue wave—it's not very predictable, and just as soon as the scientists come up with new theories, lightning strikes at some 'typically safe' area, and more prominent targets around the area are spared. But still, things have been learnt over time, so let's see what we can do.
The first thing to ask yourself is whether you really need protection or not, and that will certainly depend on your area and how 'attractive' your mast top will be.
For some small boats covered by this website, one primary form of protection would be to simply lower the mast for days when strong thunderstorms are predicted. But that is not always possible.
Glance at this NASA map of lightning frequency. I would suggest that if you live in an area that is yellow or orange, then lightning protection is certainly something to consider and if it's red (as in Florida and much of the southern hemisphere), then protection or at least precautions, become a must.
Beyond that, it could depend on whether your sailboat will be moored out in the open or in a marina, where other masts are close and perhaps higher than yours. A study of past hits has shown that boats moored out alone, are many times more likely to be hit than if secured within a marina with other masts close by—such as is possible with monohulls in a tightly packed marina. However, a wide catamaran will open up a greater width of unprotected area within a marina—giving more justification for protecting a wide beamed multihull than a narrow monohull, all else being equal.
Much has been written in the past about 'the cone of protection', wherein conductive items achieve good protection within an imagined inversed 90° cone sitting on the highest mast top, if that mast is adequately grounded.
For each case, one thing to consider is how well one is grounded, as negatively charged lightning will more likely discharge through a positively charged leader that's well grounded. This could explain why a relatively 'low profile' golfer with metal studded shoes on wet ground, might well be at higher risk of being the channel through which lightning passes, compared to a tall tree not far away, that may have greater internal resistance, depending on many factors too complex to analyse.
But for a small sailboat, what sort of protection can help? A while back, it was considered good practice to simply mount a bunch of 'metal whiskers' at the mast head and connect that to a ground wire led to the keel that was underwater. But this has since proven quite unreliable and not only have 'the whiskers' proven ineffective but also a ground passed down through the center of a boat to the keel, has even proven to be dangerous. This is because numerous hits have shown that lightning has a greater attraction to the water SURFACE than to points deep underwater—with the result that side flashes can occur and strikes can jump from the ground path directly to the nearest water surface—often blowing a hole through the hull skin. I have personally witnessed this, as my trimaran Magic Hempel was once hit at night while moored out alone, waking me with a huge bang, to find out the following morning that a hole was blown through EACH hull at the waterline, when the easiest grounding path proved to be from the chainplate for each of the 3 wire stays, directly to the water. As the forestay was of larger diameter than the shrouds, this took most of the discharge, as the hole through the main bow side was about three times the diameter of the holes in the amas—big enough to pass my thumb in fact! Also of interest, was that the main 'discharges' came from inside the hulls (where the chainplates finished almost a foot away) and blew small holes through the composite shell, with the fibres of the outer layer, clearly opening outwards towards the water. All 3 compartments were flooded in the morning, so the holes were actually made just below the waterline, not above it. Further, there was a large tree at the shoreline only 100 ft away and at least twice the height of my mast and with roots certainly going below the water table, but this was not hit.
This experience gave me a chance to see exactly what happened. While there was some minor arcing (flashing) from the mast base to the cross beam, neither came very close to the water, so the main discharge had passed through the 40ft alloy mast to the heavier forestay and then to the forward S/S chainplate that was bolted down inside the stem. But that plate finished about 150 mm above the water, so the discharge sparked right through the side panel of the bow—not only blowing a clean hole of about 18 mm diameter but also creating a mass of cracks in the laminate that looked like a spiders web of about 350 mm diameter. The center of this was also just below the waterline and only appeared on the outside skin and not inside. So in this case at least, the main discharge came from the inside and not from the water (ground) upwards. There was obviously a bunch of smaller stray discharges as the only electronic piece on board (wired to a sensor below the waterline) was also burnt out. In fact, there appears to be NO practical way to give 100% protection to electronics on ANY boat that is hit—although pre-disconnecting them from all wires will give you the best odds—but don't fool with that if lightning is already occuring in your area. It's good to remember that there are 1000s of deaths each year from lightning worldwide so it's important to put your own safety ahead of any equipment that can be insured and replaced. For yachts, I know a delivery skipper who'd put all the electronic pieces in the (steel) oven—providing a 'Faraday Cage' for protection of the delicate electronic circuits!
Experts like "Marine Lightning Protection Inc." (www.marinelightning.com) now say that a system that creates multiple discharge paths as far to the OUTSIDE of the boat as possible, will be the preferred configuration—as shown in this sketch above, of a motor boat. They also now recommend fairly small permanently-installed (patented) grounding electrodes located just above the static water surface, but using a number of them, each connected to grounding loops.
But this is typically for permanent installation on large boats and we are looking at relative small boats here—so what to do? Well, if you're in the green or blue areas shown on the opening map, you might be OK to just forget all about this, as your chances of being hit are probably less than 1 in a million provided you personally take minimum precautions of staying dry and away from conductive material during a storm. But in the yellow, orange and most CERTAINLY in the brown areas or worse, some sort of protection plan is worth considering.
For small multihulls, with alloy masts that are large enough to be left afloat, I suggest to do something like I did for Magic Hempel after she was hit hard in the 1990s. ( BTW: This boat was in a 'yellow area' at the time.)
With an alloy mast, the main current path will most likely pass that way. So this now needs to be 'connected' to the water surface in the most direct way and if possible, EXTERNAL to the boat. [The ABYC (American Boat & Yacht Council) has suggestions in their TE-4 Lightning Protection Report, but I believe that was last updated in 2006.] Various past sources commonly indicated that the discharge plate in the water needed to be about 1 ft² (900 cm²), but in the 1990s I used a small tube that still offered about the same area but was more compact to store on aboard, and I would drop this in the water with the upper connection just above the waterline. This was streamlined enough to even sail with it at speed and this I would do when caught out in a storm with thunder around. More recently, the preferred ground plate has changed, to be longer but of much narrower strips that give more exposed edge. So I now suggest this. Still using a 300 mm length of alum or copper tube of approx. 50 mm diam and 2 mm thick, make many saw cuts up its length, to increase the exposed length of edges, just leaving about 30 mm at the top for physical connection. Another option I've sketched out, is to cut a 2mm plate like a star (adding to edge length) and then bond this to a foam slab, to keep the plate at the water surface. To attach either of these plates to the mast (or forestay), buy a heavy gauge welder's spring clamp and a length of Gauge 4 (5 mm) copper wire (preferable with insulation in this case). If you cannot get wire thick enough, use a duplex wire (2 x Gauge8 (~3 mm) and connect both ends firmly to the clamp—or even use two separate clamps. With the clamp on the base of the mast (or to some part solidly bolted to it), see what wire length you need to run over the deck (hence the insulation) and down to the water surface in the shortest path with the minimum of bends. Then bolt on the metal ground plate or split tube, so that the attachment is close to the water surface when you throw it over the side. This is something that is inexpensive to make and certainly lower your risk of taking a damaging lightning hit, although as always, you can still expect some smaller currents that can destroy delicate electronics in the area. Make a habit of clipping this cable on the mast base and throwing the tube over the side each time you leave your boat and as mentioned earlier, you can even sail with this unit dragging in the water when stuck out in a thunderstorm. Personally, just keep as dry as possible and away from large metal parts and you've probably improved your odds of not being injured or your boat damaged, by a large margin.
If you prefer to purchase something already made, there's now a somewhat similar unit developed by Wally Hall for small sailboats (see zenpole.com) that uses a patented electrode developed by Ewen Thomson of Marine Lightning Protection Inc.
As for cars, boats on trailers are less at risk due to being on rubber tires—unless there is a ready path to wet ground through a metal trailer support. In such a case, one can again clip on the mast clamp and now lay the metal tube on some moist grounded area to at least offer some protection to the boat itself.
But what about a carbon fiber mast? Well, carbon fiber IS a conductor, though apparently not as good as aluminum. As to how effectively a carbon mast will conduct such high voltage discharges, I think it will depend on the actual construction of that particular mast as the continuity of the fibers will play a part and also how tightly they are compacted. Professionally constructed CF masts, built using resin infusion or vacuum-bagging, are probably nearly as effective conductors as aluminum and Hall Spars (for one) report that they've not seen a greatly increased number of masts damaged by lightning just because they are made of carbon fiber. Now, I'm not sure how far spar companies will go with such statements to protect potentially lucrative sales, but one risk with CF must be the possible softening of the resin due to the heat created by a lightning strike. Perhaps it will reharden again and not record any damage—I just don't know at this time, so I'd be pleased to see more data on this. But I can imagine that a CF spar with a relatively higher proportion of resin to fiber might be a higher risk. In such a case, it might be smart to provide some alternative way for high voltage lightning to travel to/from the mast top .. and this could be a heavy gauge (5 mm) wire from a short, rounded-off rod at the masthead. This wire could be loose inside but then you'd have its weight always present. An option could be to haul up a wire to the spinnaker mount with a fitting to assure good mast surface contact, and then bring this wire to the bowsprit and from there to the water surface ground plate or electrode, to keep it as far from the core of the boat as possible.
Another option, could be to electrically connect the upper mast to the forestay and then connect the lower end of the forestay to a grounding plate. This might ultimately sacrifice the strength of the upper part of the mast but at least the main part below the hounds would have some protection. Also, instead of using a copper wire, a small copper tube might be even more effective as it provides more surface and should stay cooler—and there are new but complex theories that indicate material surface area is as important as cross sectional area.
Anyway, I hope these thoughts and suggestions will help, and at least give some ideas to work on for your own solution. Keep in mind, I am hardly an expert on this and these suggestions are really for small boats under say 26 ft, but the ideas offered are based on my own experience and from what I have researched and both seen and heard. Larger sailboats will need either two or more discharge plates and possibly more protective grounding loops to fend off flashes from rigging at the stern—such as from backstays, runners and metal arch structures, so better to contact specialists for this.
The classic inexpensive solution for many cruising sailboats, is to mechanically clip on a length of chain reaching from the water to each chainplate—but of course, the rigging must then be all metal from the mast to the chainplate—no synthetics here. They also need to be installed immediately you hear thunder in the distance, as this could otherwise be a dangerous operation. An arrangement that permits attachment with one gloved hand could be preferable—with the other one by your side not touching anything, just to lower the risk from an unlucky strike.
Past lightning strikes have shown that NOTHING will give you totally GUARANTEED protection, but at least find a solution that provides you with some comfort and personal peace of mind.
In closing, here are some things that can help when making decisions:
Lightning doesn't like to make sharp turns.
It seems to prefer a 'surface' path, so tubes may perform better than wire, other things being equal.
Lightning will not always hit the tallest structure, but seek the path of least local resistance.
If you want to read more about Lightning, here are some references that might help:
www.colutron.com/products/cosmos.html Scroll down to 'Atmosheric Electrostatics'
The National Agricultural Safety Database (out of Florida)
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