Generally, the DC electrical system is grounded to the battery negative terminal. The AC ground would be a bit different, but the 12 VDC ground is the battery.
There are three types of grounding systems on a boat, and they should generally be kept separate. One is a electrical system ground either AC or DC. Another is the RF ground, and it doesn't require a connection to the ocean—a large grid of copper or bronze in the bilge of the boat will often work quite well for it. The last is the lightning grounding system, which is often done using a large flat copper strip, rather than heavy wire, as the copper strip is better at conducting the large, high-voltage static electricity charges involved in a lightning strike.
The RF ground can also use a dynaplate attached to the hull. The lightning ground often uses a large SOLID copper or bronze plate or strip attached to the hull. The Dynaplate is sintered bronze, and made up of a great number of tiny spheres, and will disintegrate if used in a lightning ground system.
Attaching the lightning ground or DC system ground to the keel is generally a bad idea. This is especially true if the keel is encapsulated in fiberglass, as many are...as the lightning discharge will then have to make its way through the fiberglass, burning pinholes at a minimum, through it as it passes out of the the keel and into the seawater.
Last edited by sailingdog; 05-09-2006 at 06:46 AM.