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Yachts at sea, at anchor and ashore (e.g. in a dry dock) are vul- of all lightning strikes with impulse currents lower than 200 kA
nerable to lightning strikes. The probability of a lightning strike and higher than 3 kA.
depends on the local ground flash density N g which specifies Class of LPS III is often used for yachts (see example in Figure
2
how many lightning discharges occur per km and year. The 9.34.1). This figure allows to assess the risk of a lightning
closer the yacht approaches equatorial waters, the higher the strike to the mast. The information provided below also applies
risk to be struck by lightning at sea. In general, the ground to multi-masted yachts. The points where the rolling sphere
flash density is frequently higher ashore (at anchor) than at touches the yacht are potential points of strike and must be
sea.
If lightning strikes the mast of a yacht, lightning currents travel protected.
to the deck. Since several cables are routed on the mast, e.g.
to navigation lights, the radio antenna or the anemometer, the Lightning protection
lightning current enters the inside of the ship through these To implement lightning protection measures, a distinction
cables and spreads over the entire cabling of the on-board sys- must be made between metal and non-metal masts / bodies
tem supplying the depth sounder and log. This can damage of the yacht.
these systems and result in the ingress of water since these
devices are located under the water level. While the ingress Metal yacht
of water is noticed at sea and can be eliminated, this often If the yacht has a metal body which is conductively connected
remains unnoticed when the yacht is at anchor in winter and to a metal mast, no additional measures for discharging the
the yacht may sink. lightning current must be taken. If lightning strikes the mast
To determine potential points of strike, the electro-geometric
model (rolling sphere method) is used. It describes the flash of such a yacht, most of the lightning current is discharged via
(centre of the rolling sphere) which strikes an object after a the mast and partial lightning currents are passed through the
certain distance (radius). The smaller the radius, the more ef- stays to the body / bottom and to the water (Figure 9.34.2).
fectively lightning strikes are intercepted. In the lightning pro-
tection standards, different radii r are assigned to classes of
LPS I to IV. Class of LPS I provides maximum protection from
lightning strikes. In this case, the system safely handles 99 %

r

Figure 9.34.1 Determination of the lightning risk for a yacht using Figure 9.34.2 Lightning current distribution on a yacht following a
the rolling sphere method in case of class of LPS III lightning strike to the mast

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