Hull9, we've gone around this a few times here, and its widely discussed on the web elsewhere so wide reading is worth your time and using AAB.com Forum's Search function is worth the time as well.
before I give my opinion about your question, I'd like to say why the term
'ground' is so problematic on a boat.
#1 "There ain't none"; unless your metal boat will be used as a potato planter, the boat has no ground involved, it simply is not involved with dirt/soil/earth- hence there is no ground.
#2 All metal boats, floating, are
potentially 'charged' (no pun intended) if they have an electrical system, so reference to 'ground' is confusing when compared to the other two potentials; to (A) Battery Positive (regardless of voltage level) and potential or 'charge' in relation to the (B) water.
#3 The word ground is used in terms of the 'earth ground voltage sink' of building wiring, buried ground rods, and then used to discuss Battery Negative; this results in confusion of information at the voltage charge levels and current flow are not the same in each case- so using a word that seems to associate or conflate these conditions results in reader and discussion confusion.
#4 when isolated Battery DC neg still has a charge in relationship to the 'earth' or zero potential. So Grounding on the land and on the boat may result in different final potential voltages and therefore a more useful term is to
electrically bond the DC neg to the hull.
All metal boats, with an electrical system regardless of type (DC or AC) should have one single point of bond (electrically zero resistance) to the hull. (In the case of both AC and DC that point should be, if possible, at the same point and the initial statement stands correct in this second case.)
The DC Neg leg of the hull should be bonded to the hull at one point, that can be conveniently at the battery location or, as is common with inboards, at the DC neg of the starter motor, which will bond the engine block and all its DC instrumentation to the hull even as they should all be two wire or 'battery return' circuits and not single wire 'block common' circuits.
The bond should be the same size as the largest conductor in the load side of all electrical circuits, an the ends should be soldered on lugs that are bolted to aluminum tabs or padeyes, dedicated for this purpose (sacrificial and not structural) and the face of the two metals (copper lugs and aluminum padeyes) should be in as dry a location as possible, and should have a graphite paste between the two metals to insure no resistance layer of corrosion is built up.
The joint or metal to metal interface should be accessible, replaceable, and dry, and if possible sealed so that water cannot get to the metal to metal faces. This insures the DC Pos. of the hull is not allowed to 'charge' the hull; instead an stray current or charge has a zero resistance path, at one location, into the hull and therefore out to the water through the anodes.
Of course, all circuits should be two wire, beginning at the DC positive bus and each individual load should be fused and switched at the distribution buss. Each load (DC device) should have a load side sized conductor back (return leg NOT
ground wire!) to the DC buss and that should be un-switched to the Battery Neg terminal. The hull should not be used to return the Neg DC leg as with automotive circuits.
It is the DC neg that should be bonded to the hull's metal structure to eliminate a charge potential that would seek zero potential levels through the hull and not the anodes.
http://newboatbuilders.com/pages/elect.html Ike's (retired Coastie) site for new builders is very helpful as he's collected tons of links to lots of information, if you research this site it will show details to more thoroughly answer your electrical question. Please let us know what you learned and your decision wiring your hull?
Incidentally, bolting an outboard to the hull does not necessarily bond that engine to the hull, the best installs have a bond strap (battery cable sized conductor) with sanded metal-to-metal interface. The paint on many new outboards' mounts are truly dielectric so the transom bolts are NOT assurance of the bond.
Cheers,
Kevin Morin
Kenai, AK