LBSALE[100]LBSALEHOW DANGEROUS IS LIGHTNING?
In the United States, lightning kills, over time, more people than do hurricanes, floods or tornadoes. Although lightning rarely reaches the catastrophic dimensions that make headlines, it consistently kills over a hundred people a year in this country. Not everyone struck by lightning dies: researchers estimate that two-thirds of those struck by lightning survive, and often people apparently "killed" by a lightning strike can be revived by cardiopulmonary resuscitation. But risks remain high for those who spend time outdoors - farmers, fishermen or boaters. Because water conducts electricity so well, and because a boat - or even a person - may present a high profile on water, boaters need to take special precautions. Records show that lightning has killed people who were standing in or fishing from boats or starting outboard motors. One study shows that out of 494 people killed while involved in outdoor recreation, 200 were in, on or near open water. This same study listed an additional 177 who were injured while on or near open waters. Water and lightning are a natural combination. WHAT CAN YOU DO?
To protect yourself in a boat, the important thing is to give lightning a ground. Boats made of steel, such as naval vessels, have an automatic ground in their metal hulls; but most small boats, usually constructed of fiberglass or wood, prevent the lightning easy access to the water and pose a grounding problem. Small boats may also lack tall objects which could deflect lightning, and even boats that do have tall spars - like sailboats - can run into problems if their spars are not properly grounded. The following grounding system will minimize the risk of lightning damage: On a sailboat, make a lightning rod using a piece of aluminum which sticks about 1 foot (30.48 centimeters) above the mast. Sharpen the rod to reduce resistance at the top. A radio antenna can also condluct electricity but may lead lightning to the radio and may well vaporize during a strike. From the rod, lead a wire down a wooden mast; an aluminum mast can serve as its own conductor as long as a wire is led from its base to direct the charge to the ground. (A wooden mast with a metal sail track can be grounded as though it were a metal mast.) The Coast Guard finds a #8 wire adequate, though a larger gauge, say #4, further reduces heat and resistance.
http://www.mdsg.umd.edu/CB/lightning_diagram.gif
Grounded spars or antennas offer a 60 degree cone of protection. Note that this cone measures from the highest point off the water, not from a mast's full height. Since boats usually heel during storms, the cone of protection will shrink proportionate to the dip of the mast. A bending or slanting whip antenna will considerably lesson the protected zone, as the illustration shows. Adapted from the National Fire Protection Code.
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Attach this wire to a copper ground plate mounted on the hull beneath the waterline or dangled underwater. A square foot of copper flashing, easily obtained at many hardware stores, will make an adequate ground plate. If you decide to attach a permanent ground, use large stainless steel bolts, say 1/2 inch, to prevent their cracking under large electrical discharge. To increase safety, some recommend all stays be grounded to the plate. On a motorboat which has no high spar, use a metal radio antenna (not a fiberglass one), attaching a wire to a ground plate, as described above. If there is a loading coil on the antenna, use a shunt of 31-strand, 17-gauge, bare lightning grounding mesh to bypass it - this will make the whole height of the antenna an effective ground. Quick-release clamps will allow an easy temporary attachment. (On a fiberglass antenna you can clamp the wire to a metal rod, letting that act as a lightning interceptor.) Use a "lightning arrestor" to protect your radio. When leading the wire to a ground, avoid sharp bends or turns. Any bend in the wire should have a radius of at least 60 degrees. One alternative calls for leading the grounding wire to a metal strip which runs down the bow or stern and maintains constant contact with the water. This offers the advantage of carrying the charge, at a gradual angle, safely away from the engine and the helm. Both the mast and the whip antenna will provide a "cone of protection" with a radius of approximately the same dimension as the rod's height. For most small boats, this will include the entire deck area. There may be, however, induced electrical surges created by the lightning, and for this reason large metal objects - engines, for example - should be avoided during a storm. (Remember that an engine will have its own ground in the propeller and the shaft.) Induced electrical charges can cause arcing and electrical shocks strong enough to knock you unconscious, perhaps producing dangerous heart arrhythmias. A lightning strike can also magnetize a keel or other, metal fittings, rendering your compass useless. FIRST AID FOR LIGHTNING
After a shipboard lightning strike, check to see that everyone is all right - it is, of course, perfectly safe to handle someone who has been hit by lightning. Check for burns, Which should receive normal first aid treatment. (Don't put ointment on severe burns; cover them with clean cloth or plastic to keep out air.) If someone has been knocked unconscious, act immediately: Check for breathing and heartbeat. If you feel a pulse, but no breathing, begin mouth-to-mouth resuscitation (a handkerchief over the mouth will help the squeamish). If there is no heartbeat, begin cardiopulmonary resuscitation, a technique every serious boater should know. PRECAUTIONS
Grounding your boat and unplugging radios and electrical equipment during a storm are good ideas, but the best precaution against lightning is avoidance. Especially in small craft, keep a weather eye out for the coppery haze and building cumulonimbus clouds that signal thunderstorms, heading ashore well ahead of the turbulence. Remember that lightning can lash out for miles in front of a storm, and it can strike after a storm has seemingly passed. Remember, too, that, storms can bring high winds and waves, making a last minute trip to shore a dangerous dash. The best maneuver of all is to think ahead.
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发表于 2005-9-10 10:03:00
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