About Boat Lightning Rods

The boat lightning rods at the masthead are ultimately connected to the ground plane which is seawater. It is important that strike energy is dissipated to ground with a minimal rise in ground potential through a low impedance grounding or earthing system. Steel and alloy boats use the hull as ground.

In GRP and timber sailing boats conductors are grounded on keel bolts. On most boats you will have to install a large separate ground plate, such as a radio ground shoe (Dynaplate, Wonderbar or Seaground), preferably the largest ones in the 50-100 sq.ft range. It must be noted that NewMar state that their ground shoes are not intended for lightning protection. This will ensure that there is a large and efficient ground area.

Some quality shoes use a gold-based grease that is used under the bolt heads to ensure a good low resistance connection. Do not use the radio RF ground plate as the lightning ground. Under no circumstances use soldered joints alone, as they will melt during a strike causing further havoc. Boat lightning protection starts with lightning rods!

Always crimp connections and ensure that all bonded connections are clean and tight. All connections must be bolted. Bridge out the two terminal bolts with a stainless steel link to spread the contact area. In other cases a large ground shoe can be used, or up to 3 smaller ground shoes can be configured in what is called a crows foot principle. These are a copper based X grounding plate, a spider like configuration.

This radial system lowers the overall impedance to allow energy to diverge as each conductor and ground shoe takes a share of current. In a lightning strike, the water permeating the sintered bronze ground shoe will literally boil increasing local resistance, so any increase in surface areas will reduce this effect. The voltage gradients around the shoe will also be lower.

The ground point must have sufficient area to adequately dissipate the lightning strike energy. Links can also be drilled and used to bolt the ground cable connector, as many ground shoes have relatively small bolts designed for RF grounds only. Boat lightning protection starts with lightning rods.

Boat Lightning Rods and the Bonding System

Some recommendations call for bonding of rails, stanchions and all large metallic equipment such as stainless water tanks should be bonded to the lightning ground. It is only necessary to bond internal metallic equipment within six feet of the down conductor and bonding point, and should be made at the point closest to the main conductor.

Ground plane potential equalization bonding between systems is designed to eliminate earth loops, differentials and reduce the level of potentially destructive transient currents that can flow when potential differences exist between unbonded grounding systems. In practice there have been common incidents where bonding of the cathodic protection system, power supply negatives and grounds and RF grounds have resulted in the vessel sinking as through hulls have been blown out, and all electrical and electronic systems destroyed.

Considerable care must be taken when bonding various items of equipment into a lightning protection bonding system. On steel and alloy vessels the hulls are the one ground plane for all equipment and all grounds are held at the same potential. In GRP and timber vessels it can be more complicated, but problems may arise where indiscriminate bonding of through hull fittings and other items is carried out.

It is easy to create differences of potential between various items creating a corrosion nightmare. After connecting up a lightning protection system, it is prudent to monitor the corrosion rate of anodes, and observe any underwater bonded items for adverse effects. Lightning protection starts with boat lightning rods and grounding!


Ideally all electrical systems should have lightning surge suppression devices fitted. Surge protection methods are as follows:

Radio Antennae Aerials can draw a strike or have an induced current flow through the coaxial conductor to the radio. All antennas ideally should have arresters fitted, although it is rare on boats. Antenna cables can be fitted with a two-way switch, one side to the radio, one to ground. You can buy remote and manual coax switches from NewMar.

During a storm or if the vessel is left unattended, place the switch to ground position. Ideally an arrester, (Hy-Gain or Dynapulse) or spark gap device can also be used. Coaxial cable surge protectors (Dynadiverta or Polyphase Corp) can also be used. Coaxial cable surge protectors via RF feeders are used even in shielded cables and tri-ax cables, which will confine most current. Some induction can still occur due to magnetic and capacitive coupling. Boat lightning protection starts with lightning rods!

DC Power Supplies Power supplies should have double pole isolation on both positive and negative supplies. Additionally surge suppression units can be installed which will clamp any over-voltage condition to a safe value, typically around 40 volts.

All equipment can have what is called a transient protection device installed across the input power supply connections. These are generally metal oxide varistors (MOV's), and are available at electronics warehouses. Boat lightning protection starts with lightning rods!

AC Power Supplies Efficient clamping and filtering at the power supply point requires surge diverters. The purpose is to limit residual voltages to a level within the immunity level range of the equipment. In 230VAC RMS systems damage can occur with just 700-volt peaks. Typical tolerances of battery chargers are under 800 volts. Some shunt devices can clamp the voltage at less than these voltages but they do not limit the fast wave front of the strike energy (dI/dt) before clamping action starts.

In a lightning strike, the rate of current rise can exceed 10kA/millisec, and this can be greater in multiple strikes and re-strikes. Low pass filter technology primary shunt diverters will reduce the peak residual voltage and reduce rate of current and voltage rise reaching equipment. Surge Reduction Filters (SRF) will provide multistage surge attenuation by clamping and then filtering the transients on power input circuits, and these include MOV's. Boat lightning protection starts with boat lightning rods!

About Boat Lightning Rods and Lightning Protection Devices

The Lightning Protection Device (LPD). This was an Italian development of some 10 years ago, and consists of a high performance varistor. The device is designed to interact with the electrical charges of the initial stepped leader where current values are relatively low and avoid the return strokes. Charges accumulate on the atmospheric electrode and varistor poles. The varistor conducts and the charge condition on the electrode alters. These charges leave when some streamers form to meet the leader. There are similar devices about. Boat lightning protection starts with lightning rods!

About Lightning Dissipators

A range of dissipation devices has come onto the market. These devices are typically brush or "bottle brush" type arrangements. The principle is that all the spikes "bleed" off or dissipate electrons or ions so reducing the differential that may cause a lightning strike. They do not protect the boat in the event of a strike like a lightning rod, and safely carry strike energy to ground. This is to stop development of a stepped leader forming and minimize the strength of any that do occur. Lightning protection starts with boat lightning rods!


Do you want more lightning protection and related boat electrical systems, such as corrosion? This article was extracted from The Marine Electrical & Electronics Bible and this is available through West Marine and Amazon.com Boat lightning protection starts with a boat lightning rods!