Lightning protection for a boat is important since damage to the boat by lightning may directly lead to the risk of crew's life. The surge current when the boat is struck by lightning and the countermeasure was considered based on the results of numerical analysis in this paper. There is a risk that onboard equipment on the boat is damaged by surge current, even if the boat is 1km away from the lightning strike point. The amplitude of the surge current on the boat depends on the direction and the distance of the lightning strike point from the boat. When some grounding electrodes or metal parts are lined up in the same direction as the surge current direction in the seawater, the surge current which invades to the boat is large. The electrode that is installed in parallel with the direction of the grounding electrodes as a countermeasure can reduce the surge current on the boat. However, if the electrodes as a countermeasure cannot be placed into the point close enough to grounding electrodes of the onboard equipment, it is safer to install the countermeasure on annular shape.
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