Marine Lightning Protection

Patrol boat

Overview

These craft may be constructed from either aluminum or fiberglass, each material having its own set of advantages and disadvantages from a lightning protection perspective. A common requirement for both is that the protective zone covers the whole deck area. Careful attention is paid to bonding of metallic fittings to prevent hazardous voltages from forming. Enhanced grounding minimizes the absolute potential reached by the craft during a lightning strike. This lowers the risk of a sideflash to a nearby floating object. Given the rigorous use expected, all components are heavy duty and designed for the marine environment, and careful attention is made to interconnections to minimize the risk of overheating.

Special hazards

Whether the patrol involves interception or rescue, being able to operate in all types of weather is a prerequisite for this type of craft. Since operations are possible in the middle of severe storms, safety for the crew is paramount, and survivability of engine electrical systems and communication electronics is highly desirable.

Layout

A generic system for an aluminum hulled boat is shown below. Additional lightning conductors and grounding electrodes would be required for an fiberglass hull.

This system has the following features:

multiple external air terminals that provide a protective zone over the whole deck

four grounding electrodes

aluminum hull used for contact ground

DC ground connected to lightnig system via isolating air gap

Surge suppressors on communication antennas improve the survivability of these key systems. In order to place all crew members in a zone of protection, the tips of the external air terminals are higher than the head of a crew member. This provides far superior protection to a single rod, and the air terminals can be much lower. provide external paths for the lightning current. The aluminum hull provides the necessary electrical connections between the air terminals and the grounding electrodes. The two electrodes at the stern of an aluminum vessel provides alternative discharge paths to the engine outdrives to lower the risk of engine damage while the two electrodes amidships offer a direct path to the water surface whether the craft is sationary or moving at top speed.

System features

Our recommended lightning system is shown in the above figure with the features discussed below.

Whole boat coverage
By placing air terminals externally and at several points on the hardtop and rails, we develop a protective zone that, according to the rolling ball model, covers all members of the crew. See our Air terminals page for more details on protective zones.

Energy from a lightning strike diverted efficiently to the water
Observations of surface discharges from lighting strikes and high voltage discharges indicate that the most efficient way to divert the lightning electrical energy from the boat is towards the surface of the water where the charge to be neutralized resides. See our discussion of the discharge mechanism for this process. Our approach to the grounding issue is to add electrodes to the hull just above the waterline to intiate these surface discharges. For this application we recommend our nickel-plated copper electrodes in a stainless through hull, with two aft for enhanced protection of the engines, and two amidships directly below the hard top and air terminals.

Surge protectors for all antennas
Even when the lightning attaches to an air terminal, upward discharges can be expected off any exposed antennas, even those that are inside the theoretical protective zone. As indicated in this specification, surge suppressors are needed. The"grounding" connection from these is connected to the lightning protection system or anywhere on the hull. The geometry for this connection is important to minimize inductive voltages.

Grounding both at rest and while moving
The advantage of Siedarc ^TM electrodes in this application is that they are designed to form a grounding discharge in the air above the water surface. Hence it makes little difference if the interceptor is stationary or at a full plane.

Bonding to electrical system ground

Theoretically (section 4.3.2 in our Grounding Concepts page), any internal conductor (such as electrical circuit wiring) inside an external conductor (such as an aluminum hull) is at the same potential as the external conductor. However, once large lightning currents flow inductive and resistive volatges can form, so that interconnection is a good idea. However, if this is not allowed, we address this by adding a connector with an air gap between the hull (that is, the main lightning conductor) and the electrical system "green" wire