Airboats
Three
ratchet-mount rods on this USGS airboat provide a zone of protection that covers
the whole craft
Summary
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Air terminals provide strike attachment
points
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Protective zone can include all occupants
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Ratchet mounts allow for quick drop
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No additional grounding needed for metal
hull
The problem with being the highest point in a salt marsh
Owing to its shallow draft design, the typical airboat
tends to spend much of its active life in low-lying marshy areas. This type of environment is not
particularly suitable for tall trees, high bluffs or other high objects
that might provide alternative attractive targets for a lightning
strike. Add in to this mix the
preponderance of prime airboat territory in steamy southern states such as Florida
and Louisiana, and the
potential for personal injury becomes obvious. To make matters worse, the raised
driver's seat, while great for improving the view over marsh grasses,
further increases the odds of a direct lightning attachment to the
operator. Passengers near the bow
are also exposed. This is a
particular concern for airboat tour operators as it raises yet another risk
– that of liability exposure.
The USGS airboat above is used by scientists to gather
wildlife and geological data at all times of the year so that braving
summer thunderstorms is just a part of the job. While running for cover from threatening
weather is a pragmatic course of action, it is not feasible to abort every
experiment at the first sight of a towering cumulus. Invariably a storm blocks the exit path
and the only course of action is a white-knuckled run to safety. This does not have to be a
life-threatening situation as well.
The attractive solution offered by lightning rods
Principle
The place where lightning connects to an object on the
ground is determined by a process called the attachment streamer. This hundred-foot spark originates from
the ground and carries charge of opposite sign up towards the downward
extending lightning channel. When a
person is struck by lightning, this attachment streamer invariably starts
at the top of the head and involves current flow throughout the length of
the body. Once the conducting path
is formed by the attachment streamer, a peak current of tens of kiloamperes
follows down the same path after the streamer connects with the main
lightning channel. About half the
time, this current flow results in cardiopulmonary arrest and death. The function of a lightning rod is to
initiate this attachment streamer and conduct the current safely to ground.
Rod design & placement
The ideal lightning rod should be the optimum shape for
its job. Obviously its top needs to
be higher than all other objects on the boat but there are other important
factors. Recent scientific research
has shown that blunt rods are much more effective than pointed, and the
most effective diameter is between 3/8"
and 5/8".
While the "cone of protection" afforded by a single rod
has been touted as the way to calculate the zone of protection, several
lightning rods around the perimeter of a boat are much more effective than
a single rod in the middle. Note
that NASA has recently changed the design of its lightning protection
systems at its launch pads from a centrally-located rod to a ring of masts
around the perimeter of the complex with catenary cables strung between
adjacent masts.
Ratchet-mount for convenience
One problem with placing lightning rods on the corners
of an airboat is that some activities, such as docking, require open deck
space. Fortunately, this problem has
already been solved, albeit for vhf antennas, by using ratchet-mount bases
that can be locked in either up or down position. We customize these bases to accept
UL-listed aluminum rods, with appropriate attention being given to galvanic
and conductivity issues. Since a
metal-hulled airboat requires no further grounding, these air terminals can
be simply attached to any metal structure to provide the required
continuous conducting path to ground.
We call this new product the ZzapRodTM. It can be used not only on an airboat,
but also on any craft wherever a collapsible lightning rod is needed, from
jon boats to superyachts.
Easy implementation
Determining the zone of protection
Following NASA's lead, we no longer recommend the use of
the "cone of protection" for planning where to place lightning
rods on small boats. Instead, we
adopt the "rolling sphere method" used by lightning protection
authorities such as the National Fire Protection Association. The main difference as far as
implementation goes is that more than one lightning rod is required. Simply stated, the protective zone is adequate
if a sphere with a radius of 100' contacts only lightning rods when it is
rolled over the boat. For a small
boat there is very little difference between the arc of a 100' sphere and a
straight line. Hence in the USGS
airboat at the top of the page we have placed three lightning rods, two at
the bow and one behind the driver, with sufficient heights that the hatched
surface is above the top of the heads of all occupants while standing.
Installation
In a metal hulled boat, no further grounding is required. Hence all that is required is to secure
each ZzapRodTM to either
a flat surface (using a surface-mount fitting) or a tubular structure
(using a rail-mount fitting) The
surface-mount fitting require four holes while the rail-mount fitting
merely bolts on with no holes required.
See the inset views in the illustration at the top of the page.
Precautions
While the ZzapRodTM is designed to
significantly lower the risk of injury by diverting the lightning strike
point from an occupant to itself, it cannot be expected to be 100%
effective and being this close to the strike is still extremely
hazardous. Hence reasonable
additional care and precautions are recommended. Enhanced protection can be obtained by
adding catenary wires to interconnect the tips of ZzapRods.
Products
Overview
The ZzapRodTM comes with various mounting
options to allow for attachment to deck or the motor cage. All versions have ratchet-mounts to
enable the rod to be dropped out of the way if needed.
Surface-mount air terminal
Our surface-mount air terminal (SMAT) is designed to be installed
on a flat horizontal surface, typically the forward deck, to extend the protective
zone over any passengers and enhance the zone over the driver. For docking, trailering, or cloudless days
it can be lowered to deck with its tip secured in a rod clip on the deck or
inboard.
Rail-mount air terminal
Our rail-mount air terminal (RMAT) is meant to be
mounted on a 7/8"-1" or 1.25"-1.5" diameter tube at any
orientation, such as are typically found in the motor cage. Since Florida
law requires an orange flag at height of 10' from the bottom of the boat,
this lightning rod can double as a flag pole. A convenient location is just to the
right of the driver where it can be lowered vertically down for trailering.
Flexible coupling
After talking to several airboat builders it became
obvious that contact between foliage and airboats is a very common
occurrence. In response to this, we are working on a flexible coupling that
can be added to both surface and rail mounts to absorb the occasional impact
with vegetation. To accommodate more
serious interactions, we plan to combine the flexible coupling with
replaceable rods that can be simply screwed into the coupling.
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