 |
 |
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
 |
by Bill Laudeman
Over the past couple of hundred years, as science has learned more about the nature of nature, lots of folks have tried to find ways to protect their homes and businesses against damage from lightening. But even after Ben Franklin's famous kite proved that lightening is an electrical force, and discoveries by such famous scientists as Volta, Ampere, Ohm, and others, lightning still remains something of a mystery. And we are still looking for ways to minimize damage.
Several
years ago in preparing an article on "Lightening Protection
Systems" (Practical Sailor, December 1993) I interviewed
one of the engineers responsible for preventing lightning damage
to the U.S.'s largest power utility, the Tennessee Valley Authority,
plus others at firms making lightning damage prevention systems
for high-risk operations such as petroleum refineries and tank
farms.
The conclusions reached were fairly simple, and within a short
time after the article was published, advertisements began to
appear
for devices called ion dissipators, like those previously offered
only in high-tech industrial systems. These are stainless steel
wire brushes, typically about two inches in diameter and about
eight
inches long. You can buy a similar brush for a few dollars
from a supplier of diary cleaning equipment; the ions probably
can't
tell the difference, but you will have to build your own mount.
Lightning Striking Your Boat?
The
following statistics are based on all of the BoatUS Marine
Insurance claims for lightning damage over a five-year period.
The percentages suggest the chances of the various types
of boats being struck in any given year.
Auxiliary
Sail
.6%
Six out
of 1000
Multi-hull
sail
.5%
Five out
of 1000
Trawlers
.3%
Three
out of 1000
Sail Only
.2%
Two out
of 1000
Cruisers
.1%
One out
of 1000
Runabouts
.02%
Two out
of 10,000
Source:
BoatUS Marine Insurance Claim Files
Although there remains some controversy about how well the lightning protection systems (LPS) function (Seaworthy, April, 1995) , there is little or no argument about the fundamentals of good lightning protection: (1) It is better to avoid being struck in the first place, but (2) if your boat does get hit, you will suffer less damage if the strike is grounded as directly as possible. With these basics in mind, let's review and reiterate some general science that every boat owner can and should understand so that you can make the best use of the devices that are being sold. Armed with a few facts, you can better analyze your boat's lightning risk and make the best LPS installation or improve your present protection.
FACT It is better to avoid being struck, so you do not want to "attract" a strike to any part of your boat. The power of a direct strike is likely to heavily damage anything in its path and nearby electronics will suffer corollary damage from a side-effect of the strike, the magnetic pulse. Therefore, I suggest that the primary goal of an LPS must be to allow the accumulating "ground charge" (the build-up of energy in the earth or sea below a thunderstorm) to drain away at a low voltage level and prevent the initiation of a strike. This is what the "ion discharge" or "dissipator" air terminals are designed to do. For these to be effective, however, they must be part of a well-designed system..
FACT No LPS is any better than its worst component. Although the most obvious part of an LPS is the air terminal (or dissipator) mounted atop the rigging, it actually starts with the water beneath the boat. What is needed is the best possible ground, so that the charge building up in the water can get to the air terminal. You can guarantee the best ground by having a bare metal hull.
Oh well, so much for the best. What's second best?
Second best is to have many square feet of metal wetted by the water. If this is still a problem, it is possible that connecting all underwater metal thru-hull fittings to the LPS may be helpful, but you run the substantial risk of having a fitting blown apart in a strike, leaving a big hole in the hull of your boat.
In practical terms, it comes down to this: Were I a boater in Florida's "lightning alley," I'd make sure the boat was well grounded. One way to do this is to make a grounding plate from 1 1/2" to 2" wide bare copper grounding strap, as long as can be fitted below the waterline, epoxied to the hull exterior, or as an alternative make a "buss bar" using a 1 1/2" - 2" copper strip under the boat running from the base of the mast to the engine or, as an alternative, several square feet of copper sheet epoxied to the hull, with a substantial grounding lug extending into the bilges directly below the main LPS cable (which, in turn, leads straight to the air terminal). If it were not feasible to run this cable - 4 AWG is a good conductor-straight down from the masthead to the bilge, I'd make up a jumper cable long enough to reach from the overhead to the bilge, to be clipped in place when a storm threatened. This means that access must be provided to the cable where it enters the cabin overhead, and to the ground lug, with sturdy (bare!) connection points to which you can clamp the jumper. Do you think that this is too much trouble? Ask someone whose boat has been struck.
FACT Lightning strikes are radio frequency (RF) events. It is true that the build-up of energy is a direct current (DC) phenomenon, and the current flowing during a strike is unidirectional, but as each spark starts and stops, a great deal of high frequency RF power is generated. This is why you can hear distant lightning storms on your AM radio-and when a strike does hit your boat, everything in the path of the strike becomes part of a wildly varying, enormously complex network of tuned RF elements. What you may find hard to believe is that a length of solid metal conductor is a perfect insulator at certain RF wavelengths. This may cause very high voltages to appear between two points (perhaps several feet apart) on a mast, a wire, or a length of metal trim. These voltages are the source of a sometimes deadly effect called "side flashes" that flicker between various parts of a boat -and its occupants.
To avoid becoming part of a side flash, you should understand the second purpose of the LPS: Martyrdom (self-sacrifice).
When the LPS fails to prevent a strike, it must sacrifice itself in such a manner as to best prevent damage to the boat and occupants. You encourage this vital role of the LPS by visualizing the path(s) that a lightning strike might take, then make the LPS conductor the one path that is as short, as straight, and as low in resistance as possible. Then, during any lightning threat, you and your crew should stay as far away from this "favored" path as you can.
If you've done a good job on these basics, from ground plate, to conductor, to air terminal; and if you keep as far from the primary protection conductor as possible, then you've taken practical steps to guard against one of nature's mightiest forces.
Only one more suggestion: Monitor weather reports and avoid being caught out in a thunderstorm!
* Copper straps, bars, and plates designed for lightning protection are available from Thompson Lightning Protection, 901 Sibley Hwy., St. Paul, MN 55118-1792. Telephone: (612) 455-7661.
Bill Laudeman spent more than 30 years working as an industrial techologist and engineering project manager before becoming a marine surveyor. This spring his first book, Sailboat Wiring will be published by International Marine Publishing Bill lives in Chattanooga, Tennessee.
A Direct Hit-
|
Unfortunately, this 1963 Columbia 28' did not have a Lightning Protection System when it was struck by lightning last August at a marina in Indiana. When the owner came aboard he noticed his stereo and VHF radio were not working. He then noticed his masthead VHF antenna was missing, indicating a direct strike. Several leaking through-hulls prompted him to haul the boat for repairs, whereupon the spider exit cracks and holes were discovered.
The flow of electric current through a conductor is analogous to the flow of water current through a pipe: water flows easily through large, straight pipes. The surveyor determined that the electric current flowing down the Columbia's large, straight mast was blocked at the base because there was no continuous conductor leading straight to an external ground plate-it hit an electrical dam. Not enough of a dam, however, to hold back a flash flood of current. Pushed by such pressure, the current came crashing down and "overflowed" the dam, so to speak, by arcing over to other conducting objects nearby (known as side flashes). Fortunately, no one was inside the boat, or they too might have become a conductor.
The numerous exit points along the boat's bottom show how lightning current can seek and find alternative paths to ground when its path is blocked or constricted, but the leaking through-hulls indicate how risky this can be; a large spike of current can blast through small conductors, like water bursting a pipe, and sink the boat. Had there been a continuous large conductor leading straight down and out the boat (e.g., #4 AWG wire), the current would have more easily flowed through with minimal damage.
Instead, the surveyor speculated that the current arced several feet across from the mast step to a nearby keel bolt and traveled down into the keel before finally blasting a hole through the fiberglass skin. Another path through the wet bilge found the #8 bonding wire connecting all the through-hulls to ground; yet another path, perhaps down the backstay, found the rudder shaft, the last underwater ground terminal. Still, all these underwater ground exits were not enough to dissipate the charge of a direct strike; the current divided at the masthead, came down the headstay, and jumped down to a mass of chain piled in the chain locker, before finally blasting through the stem of the fiberglass hull - all in the blink of an eye.
The total cost of repairs was $1,900. Larger boats outfitted with radar and other expensive electronics can cost much more to repair. If a boat is left unattended during storm seasons, it is a good idea to completely disconnect as many electronic processor and display units as possible. That, coupled with a proper LPS and bonding system, may save you a lot of down time and headaches during the peak boating season.
BoatUS : my.BoatUS : Membership : Boat Insurance : Towing Services : Boat Loans : Online Store : Boat Graphics : Classifieds : Site Map : Contact Us
©2008, Boat Owners Association of The United States. All Rights Reserved.
