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Subject: [worldcruising] Lightning Protection for Multihulls
From: Financiera Maldonado (Uruguay) S.A. (financieramaldonado@XXX.XXX)
Date: Sat Mar 10 2001 - 16:21:19 EST
Dear Friends:
I friend of mine from a spanish speaking nautical forum "posted" this article this evening, which I found very interesting, therefore I took the liberty of copying same, I do hope it will be of some utility............
Papa Bravo signs off
Lightning Protection for Multihulls
By Earl Hinz
There is a plethora of writings on lightning protection criteria and concepts for monohulls, but little on multihulls which have some unique implementation problems. Statistics gathered from BOAT/US Marine Insurance claims indicate that lightning damage is somewhat influenced by boat design and suggests that the vulnerability of multihulls is close to that of monohulls. A five-year accumulation of lightning damage claims showed the following incident percentages:
• Auxiliary sail – six out of 1000 boats
• Multihull sail – five out of 1000 boats
• Trawlers – three out of 1000 boats
• Sail only – two out of 1000 boats
• Power cruisers – one out of 1000 boats
• Runabouts – two out of 10,000 boats
The probability of a lightning strike is dependent on climatological factors which vary widely with location, time of year, and time of day. It has been estimated that lightning discharges occur at the rate of 100 times per second around Mother Earth, just pick your time and place.
Since lightning is the product of thunderstorms, it will be more prevalent in those areas where thunderstorms commonly spawn, such as Florida, the Isthmus of Panama, the "horn" of Africa, Madagascar, Singapore, Darwin, Papua New Guinea and the Solomon Islands. Although these geographic areas tend to specialize in thunderstorms, thunderstorms do occur worldwide over land and sea in all latitudes between 70°N and 70°S. As far as boating in the United States is concerned, the lightning threat is greatest along the west coast of Florida, but it is also somewhat of a threat along other Gulf Coast shores and the Plains States to the north. In fact, all inland areas that carry a tornado threat also have a lightning threat. Consider where you do your sailing.
THE THREAT
The frequency of thunderstorms is at its maximum during the hot summer months of July and August and is usually inconsequential during the winter months. Daily, it peaks in the late afternoon and early evening. Unfortunately, those peak periods are also the best day-sailing times making lightning considerations an important factor in planning water recreation.
Lightning is a byproduct of thunderstorms, those nasty dark cloud formations that show up so often in the afternoons of otherwise great sailing days. Everyone can recognize the approach of a thunderstorm by noting the dark cumulonimbus clouds forming a weather front in the distance. The front may be a cold front, where an incoming cold air mass wedges itself beneath a warmer air mass, forcing it to rise; or it may be a warm front in which a warm air mass pushes itself over a cold air mass. In either case, ominous clouds form along the front as the warm air rises and its moisture condenses.
Scientists generally agree that cloud electrical charges are created by friction between the two air masses. The upper portion of the frontal cloud becomes positively charged while the lower portion becomes negatively charged. When a sufficiently large electrical buildup has taken place, we have the makings of a thunderstorm with its booming noise and magnificent (and threatening) lightning displays.
Thunderstorm cloud complexes have a radius of six to ten miles. The electrical activity is found within the cells, making up the complex, but rarely is there simultaneous activity in all of the cells within a complex. Electrically active cells measure a few miles in diameter and the activity within them lasts between 30 and 60 minutes. Other cells becoming active within the complex may cause the activity to seemingly extend over longer periods.
Statistics show that over half of all lightning discharges occur within or between clouds. Such lightning discharges, hidden within distant cloud banks, appear as diffused flashes of light and are sometimes referred to as heat or sheet lightning. They illuminate the cloud with a soft glow and when they are farther than about 10 miles away, they do not have a perceptible thunder clap. Sheet lightning poses no distinct threat to boaters because of its remoteness. It is the closer-in discharges, commonly referred to as bolt lightning, that pose hazards to boats and other objects extending above the local earth’s surface.
Bolt lightning of immediate concern to boaters is the cloud-to-ground discharge. Ninety percent of this lightning is initiated by the negatively charged cloud, which has gained its negative charge from friction between the moving air masses or by rainfall within the clouds. A positive earth charge is believed to follow along under the negatively-charged cloud as a ‘shadow.’ The mechanisms are far more complex than can be put into simple words, but the results are awesome, to say the least. Lightning bolts may emanate from a thunderstorm cell at a rate as high as ten per minute, but they average more like two or three per minute.
A flash of lightning between cloud and earth takes place in about one-half second and involves an electrical potential measuring in excess of several hundred thousand volts. The current transmitted during that flash averages near 20,000 amperes, but may peak as high as 10 to 20 times that amount. Within the flash channel, temperatures may reach 30,000° Kelvin. We are talking about an extreme level of energy transmitted in a minuscule time frame. Scientific evidence shows that the bolt begins with a leader moving downward from the cloud along a very erratic path, giving it a stepped look. When within a few hundred feet of the earth, or a high object, a positive discharge called a streamer rises upward to meet it, completing what we see as a bolt of lightning. The myth that the high object has attracted the lightning is false. The lightning was already on its way to earth at some point within a few hundred feet of the high object and the positive streamer rose to meet it.
The thunder that is heard during a thunderstorm is caused by lightning discharges in and around the clouds. The lightning stroke rapidly heats the air to a high temperature, creating a series of expansion shock waves which move through the atmosphere at the speed of sound. Noting the time interval between the lightning flash and the arrival of the thunder gives an indication of how far the strike was away. The distance is about one mile for every five seconds of time difference between the lightning flash and the clap of thunder.
LIGHTNING PROTECTION SYSTEM CONCEPTS
The fundamental goal of a lightning protection system (LPS) is to provide a low resistance path for the lightning to follow in its journey from the masthead to the water. This path starts with an air terminal at the masthead, a down conductor which connects the air terminal with a proper grounding plate which can disperse the charge directly into the water. In making this connection from air to water, we must (1) keep the route as nearly vertical as possible, since the electrical charge seeks to get to the water by the shortest possible route; (2) provide heavy conducting materials to handle the high energy electrical charge; and (3) minimize electrical resistance encountered in connections along the controlled pathway.
A properly installed LPS offers a zone of protection around the boat based on the height of the lightning protective mast. Although two different means are used for estimating the size of this zone based on mast height, both define zone sizes compatible with the known variability of the striking distances of lightning. Inside the conservatively defined zone of protection the risk of a direct lightning strike is minimized.
The air terminal at the top of the mast is a sharpened copper alloy spike1 which extends at least six inches above the highest masthead device and is firmly fixed to the masthead. If mounted on an aluminium mast, it can be bolted directly to the masthead cap. The resulting joint should be externally coated with a sealant to prevent corrosion because of the dissimilar metals involved. If the mast is wood, the air terminal should be rigidly bolted to the masthead using an appropriate bedding compound. Good bedding will not only help preserve the wood from moisture but will minimize electrical conductance down the mast, itself.
The primary down conductor for an aluminum mast is the mast itself. For a wood mast a heavy copper wire (#4 AWG stranded wire) is run down the side of the mast2. The down conductor for the wood mast, #4 AWG wire, is fastened directly to the air terminal. It must be solidly and continuously affixed along the side of the wood mast in a manner which prevents it from becoming vulnerable to damage from sailing or maintenance activity.
At the lower end of the down conductor is the critical water-immersed ground plate. I say ‘critical’ because like RF grounding it is more often than not trivialized with the result that it is less effective than it could be, thereby defeating its purpose for being used. Minimum design criteria for this ground plate by ABYC3 standards are:
• Minimum area – one square foot totally submerged for salt water (more for fresh water)
• Thickness – minimum of 3/16"
• Width – minimum of 3/4" (for a grounding strip)
• Materials – copper, copper alloy, stainless steel or aluminum.
Other important guidelines relative to designing the external ground plate are:
(1) the ground plate should be located as close to directly under the down conductor (mast) as possible;
(2) the edges must be sharp and exposed4 and not caulked or faired into the adjoining hull surface;
(3) the grounding strip, if used, should extend from directly below the mast towards the stern and be electrically connected to the aft end of the engine (a minimum strip length of 48" is recommended, with an appropriate width to yield at least 1 ft2 of area); and
(4) a pair of thru-bolts should be installed at each end of the strip to prevent it from twisting. Intermediate bolts may be used as necessary.
The down conductor should be kept as near vertical as possible. If it has to be bent, it should be done with a generous radius, never less than 6 inches. The down conductor should never run horizontal. Any deviation from vertical for the down conductor gives the lightning charge a chance to "jump the rail" resulting in dangerous side flashes. Keep the following rule of thumb in mind when routing down conductors and bonding wires: The lightning down conductor wires are best run vertically while the boat’s bonding system wires are best run horizontally. The down conductor connections must be able to carry high currents in a lightning strike with as low an electrical resistance as is possible and they must also be rugged. Lightweight ear-type terminals may not be sufficient. Consider the use of bronze, split-bolt electrical clamps (obtainable from industrial electrical supply houses) for connecting the down conductors to shrouds, stays, ground plates, et al.
TRIMARAN ISSUES
Because it has a central hull, the trimaran down conductor from a deck-stepped aluminum mast can connect in a relatively straightforward vertical manner with an external ground plate mounted on the bottom of the main hull under the mast step.
The trimaran design practice of stepping the mast on the cabin top over a load-carrying bulkhead also means that a single vertical down conductor cannot be used because it would interfere with the passageway through the bulkhead. One solution to this problem is to use a bifurcated down conductor made up of two identical #6 AWG5 down conductor wires to electrically connect the mast step with a single external ground plate. The two conductors (of equal resistivity) should be routed well away from any possible personnel contact, avoiding sharp- bend radii and horizontal runs. They should also be covered to prevent accidental personnel contact with them during an electrical storm.
To complete the lightning protection system, the fore and aft parallel lightning paths created by the forestay and backstay should be connected to the same main hull ground plate through external fore and aft grounding strips as described earlier.
Shrouds provide similar parallel paths and they should be grounded to separate plates on the floats to eliminate horizontal runs of down conductors if you attempt to connect them with the main hull ground plate.
CATAMARAN ISSUES
The design of a lightning protective system for a catamaran is a first order conundrum. Excepting metal-hulled cats, the $64 question is how to complete the vertical down-conducting path from mast to water. A down conductor segment connecting the mast step with a ground plate on each hull would, most likely, have too much of a horizontal run to prevent the strike from jumping off the wire in its quest for a more direct path to the water. An impossibly high bridge-deck bulkhead would be needed to obtain a suitable sloping run of the down conductors to the ground plates.
That is not to say that a central ground plate cannot be used. One has only to observe that the majority of recreational boats (of any description and location) are inactive the majority of the time. This allows consideration of a temporary, detachable centrally-located ground plate which would be no more a problem in using than slipping dock lines when setting off for a day of sailing. The idea is similar to the practice of dropping a zinc overboard at the dock to help neutralize stray electrical currents while in a marina berth.
Considering the metal mast as the primary down conductor, the lightning circuit to the water can be completed with a detachable submersible ground plate connected to the mast down connector with a heavy clamping device. The boat can thus be protected from lightning strikes during its inactive life (which is most of the time) and the skipper will plan his sailings to protect it the rest of the time.
To provide a catamaran with reasonable lightning protection all of the time, a design scheme emphasizing the parallel lightning paths may be considered to offset the absence of a central hull. This consists of carefully grounding the cap shrouds to make them the electrical path of least resistance. Lower shrouds on a metal mast should also be included, but only after providing good grounding of the cap shrouds. In using shrouds for lightning down conductors, they must have assured minimum electrical resistance at all joints through which the lightning current is expected to flow. This may require placing copper jumper wires around the mast tang-to-shroud wires as well as around turnbuckles to assure good conductivity.6
Each of the parallel lightning paths from the common air terminal to their ground plates should be designed as the equivalent of a single-path installation. It would be fallacious to expect lightning with its unpredictable nature to arbitrarily split its charge between routes which may have different electrical resistivity. A safer assumption would be that the lightning strike will take the route of lesser resistance which will then have to carry the full discharge load. It should also be apparent that when the catamaran is underway with a hull lifting, there may be momentary lapses in the electrical contact between that hull’s ground plate and the water.
The catamaran forestay could also be considered a possible parallel path but it has little merit in the common rigging designs which incorporate a crossbeam between hulls. The transfer of lightning strike energy from the forestay via either the crossbeam or the forestay bridle to the ground plates on the hulls would necessitate that the energy pass near horizontal to the respective hull ground plates. That may not be to the lightning strike’s choosing when it already is so close to the water.
BONDING
The handmaiden of a successful lightning protection system is good bonding of the major metallic masses within the hull and the electronic equipment on board. Bonding is commonly employed to counter electrolysis arising from different levels of electrical potential which may exist between metal masses having some degree of contact with the water outside the hull. This same bonding added to all large metallic masses inside the boat, whether in contact with the water or not, can help prevent side flashes during a lightning strike. It can also prevent electrical shocks to crew members should they carelessly come in contact with any of the metallic objects during lightning activity.
Good bonding of major metallic bodies inside the boat is a safety requirement as well as an anti-electrolysis requirement and is needed on both mono-and multihulls. There are no hard and fast rules to the installation of a bonding system, but there are guidelines which may be helpful in making it effective:
• Use at least #8 AWG wires for bonding connectors, running them horizontally where possible
• Keep bonding conductors out of the reach of bilgewater
• Connect individual bonding wires to an equalization bus bar running horizontally which in turn connects to the exterior lightning ground plate
• Do not use the bonding system as the negative return in any part of the boat’s electrical system.
Protecting a boat’s electronics may be the most difficult of all tasks because of the sensitivity of semi-conductors to transient electrical impulses. Besides grounding the metal electronic cases, twisted power leads in shielded harnesses should be used to furnish power to all electronic devices. Each piece of electronic equipment should be equipped with its own transient protection device at the input to the unit. In addition to good installation of safety devices, it would be prudent to disconnect the antenna and power leads prior to the arrival of an electrical storm. If, however, the storm is already upon you, stay away from them.
TO ROUND IT OFF - - -
Lightning is an extremely unpredictable act of nature. It has its own timing and follows its own path. There is no way of preventing a lightning discharge and, if you are in its path, you try to oblige it by providing a simple easy path for it to follow as it moves from the masthead to the water in a fraction of a second. You try to minimize the amount of damage that its high energy can create and to keep it well out of reach of personnel and sensitive electronic equipment. There is no guarantee that any lightning protection system will be 100% effective, but failing to provide a proper protective system greatly increases the chances for significant damage.
We have touched on the principals of the lightning protection system, but multihull owners intending to install one would do well to additionally familiarize themselves with the following references:
a) ABYC Standard E-4, Lightning Protection, American Boat and Yacht Council, 3069 Solomons Island Road, Edgewater, MD 21037
b) NFPA Standard 302, Chap. 9, Lightning Protection, National Fire Protection Association, 1 Batterymarch Park, Quincy, MA 02269
c) Lightning & Sailboats, Ewen M. Thonpson, Florida Sea Grant College, Bldg. 803, University of Florida, Gainsville, FL 32611
d) "Lightning Protection Systems", Michael Taylor, Professional BoatBuilder, October/November 1996
1 There is insufficient evidence at present to support the claim that an ion dissipater is any better than a simple sharpened spike as an air terminal.
2 A carbon fiber mast has an inherently high electrical resistance and can be severely damaged by heat from a lightning strike. There is no straight-forward solution to providing a low resistance electrical path to its base unless the crude external down conductor wire used on a wooden mast is considered. The design of a sophisticated LPS for a carbon fiber mast must be made in conjunction with the boat’s designer prior to the fabrication of the mast.
3 American Boat and Yacht Council
4 This requirement challenges the use of sintered bronze ground plates designed for RF grounding
5 The National Fire Protection Association (NFPA) recommends the following minimum wire sizes: For the single main down conductor — #4 AWG; for two parallel paths — #6 AWG; and for more than two parallel paths — #8 AWG.
6 The NFPA specifies that the electrical resistivity of a connection should not be in excess of that of two feet of the conductor.
[Non-text portions of this message have been removed]
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