I have a 1979 Bristol 29.9, the boat does not have a grounding plate on the hull. About half the sailboats I see while on dry dock do have ground plates mounted to the hull. Does anyone know what triggers the need for a grounding plate?
Grounding plates in your boat size are mostly for lightning.
The DC grounds through the engine/prop shaft.
All 8 of your stays/shrouds, the 7 bronze thru hulls, the engine, and the mast are all bonded together. The wires are visible on all except the mast, where there is a wire-attached metal plate under the base.
With your internal ballast, you can't exit lightning through the keel as there are no bolts. Bristol exits the lightning through the shaft and 7 thru hulls. It also keeps the rig at the same electrical potential as the ocean.
would agree with Skipper, they are for (and Don) so-called lightning protection. That method has not been conclusively confirmed to be effective BTW. Also there are better high frequency (HF) radio grounds then a sintered bronze plate a few feet below the surface. Your HF radio will push out a stronger signal if your ground crosses the water surface during transmit. (think fin attached above the water line but projecting into the water) Your VHF does not need a radio ground for its signal. It does need one for the chassis ground though but that can be the normal bonding wire.