The ground terminal, which represents the other end of the system, is the connection between the vessel’s lightning protection system and the sea. These must be metal and can be made from copper, copper alloy, aluminum, stainless steel, or lead, with a minimum thickness of 3/16 inches and 1 square foot of surface area.
Existing underwater appendages can fill this requirement, particularly an exposed ballast keel, struts, and rudders, provided they are nearly directly under the down conductor. In most cases, that excludes everything but a keel or a dedicated ground plate; however, these other submerged objects may be electrically tied to the ground terminal, augmenting its effectiveness.
For stand-alone, dedicated ground terminals, TE-4 calls for the edges to be square, or sharp, rather than radiused and not filleted with caulk or fairing (the leading edge can be rounded) for maximum dissipation effectiveness. Some studies indicate that the edge is what dissipates the strike, so the longer it is the better, making a rectangle more desirable than a square. My preference is for a solid ¼-inch-thick, 1- or 2-inch-wide copper strip—a band of sorts—that runs parallel with the vessel’s centerline, amidships, or beneath the mast or air terminal, that totals a minimum of 1 square foot of surface area. Thru-bolts connect this to the hull, with one serving as a connection point for the primary conductor from the air terminal, as well as for secondary conductors from other bonded gear and underwater metals.