A slight expansion on what ssobol commented. The "Green wire" in a boat is often DC ground (somewhat different than AC ground). It is for shielding of various electronics cabinets, and in some boats it also ties the thru hull fittings together.
For true RF shielding such as for an auto pilot when running high power SSB rigs, we use copper foil (about 1 mm thick and 2 to 3" wide. This gives far better RF shielding than just a wire.
As ssobol notes these all also go to the negative of the battery.
The only C-Dory I used this type of ground on was my Tom Cat 255, where I had a 100 watt SSB radio.