Grounding and Bonding

Grounding and bonding are not the same thing. Most boat owners and some professionals who install boat wiring don't adequately understand marine grounding and bonding. Some good sources of information can be found online. One, though quite old, is a Marine Surveyors Association article on electrical systems with good coverage of the topics of grounding and bonding. West Marine has republished some articles that originally appeared elsewhere, such as Practical Sailor. One is Marine Grounding Systems Nigel Calder's book "Second Edition, Boatowner's Mechanical and Electrical Manual" is a comprehensive reference that includes electrical wiring and grounding and bonding considerations.

DC Ground Wire Sizes

The wire sizes for the battery module ground wires will be the same as size of the battery module positive side for that module, AWG #2 gauge wire for load side and #6 gauge wire for charging side. Each module has a load side and charge side grounding wire and so a small grounding loop is formed. A grounding bus bar will be near the modules to reduce the size of this loop and the load and charge grounding wires may be twisted together to reduce induction effects. Wires from the load side ground bus will be AWG #2/0. Ground wires will be led to distribution bus bars and sized appropriately for the loads to that distribution bus. These will be AWG #2/0 for total loads of over 150A, AWG #2 for loads of over 50A to 150A. For total loads of 50A or less AWG #6 will be used. The total load will be assumed to be the sum of the circuit breakers on the positive side.

Corrosion and Electrolysis

Corrosion of underwater metals can occur due to galvanic effects or electrolysis. The two are different.

Galvanic corrosion occurs where dissimilar metals are in the water and a current can flow between them, usually due to bonding. Sacraficial zinc anodes are used so that the metal being eaten away is always the zinc and the bronze, stainless steel, and aluminum is protected.

Electrolysis occurs due to stray currents. For example, if two pieces of metal are grounded but a corroded connector in the ground wiring between the two is causing a high resistance, then a voltage difference is created whenever a current is passed through that ground circuit.

Isolating DC Ground and AC Ground

An old approach, now discouraged, was to connect DC ground, AC ground, and all bonding together. This can have disasterous consequences due to undersized wire or corrosion leading to leakage to the DC and AC ground. On land grounding the AC to the actual ground is considered a safety requirement. With today's GFCI built into inverters and GFCI outlets and in newer boats complete system equipment leakage circuit interrupter (ELCI) this is no longer the case.

AC grounding and DC grounding are separate systems and should be floating. The AC can be grounded to the shore power for loads that can be connected to shore power. In this case only the shore power battery charger is connected to shore power and so the shore power ground wire and the hot and neutral are connected to this charger but no where else. This avoids problems due to faults in the dock or marina wiring.

The DC ground is common among the 48V, 24V, and 12V DC systems but otherwise floating. These could also be independent but there is little or no risk in interconnecting them.

Bonding

Traditionally, though no longer recommended, the bonding system, usually a set of green wires, attaches all underwater metal together. The bonding system should not be connected to any ground wires. Zinc sacraficial anodes should be put on some metals such as props, shafts, and other mechanical systems in the water.

Bonding of underwater metals is no longer recommended. The issue with bonding is that a stray current through the water due to other boats, dock, or marina fault creates a voltage difference in the water itself. At some marinas this occasionally gets bad enough to electrocute swimmers. If two pieces of metal are connected together through bonding and there is a voltage difference in the water, one of those will corrode quickly.

Using an engine block as a DC grounding bus bar is one of the worst grounding offenses. It is unfortunate that many starter motors and (fewer) alternator assume that the DC ground will be the engine block. Another problem is that the engine and propeller shaft are often coupled with an all metal coupling. Couplings are available which electrically isolate the shaft and the engine.

The electric auxilliary being used electrically isolates the shaft from the motor. This makes it easier to keep DC ground separate from underwater metal. Since bonding is not used on this boat, electrolysis cannot occur. Not bonding the through hulls and seacocks means that the bronze is on its own, not protected by prop zinc anodes. This has been the case for the boat's lifetime and there has been no evidence of galvanic corrosion of the type that can occur with stainless steel underwater.

Lightning Protection

Lightning protection is highly recommended. The masts to should be connected to one keel bolt and not connected to any other underwater metal. Then using a charge disipator at the top of each mast can prevent most but not all lightning strikes. Making a boat survivable in the event of a lightning strike is more difficult. It involves creating a Faraday cage where the lightning flows around the boat.

One way to create a Faraday cage is to bond the chainplates to the keel. The stainless steel stays are not good enough conductors. This risks melting and snapping a stay under load and dismasting. A better solution is to use a Faraday cage that is installed as needed. A tall pig stick on each mast can be used to raise a set of AWG #8 wires, preferable bare wires, held outboard and dropped into the water with 1-2 sqft copper plates in the water. One at the bow and one at the stern and at least one pair outboard of each mast creates an effective Faraday cage. This takes forsight and some work to install when lightning is threatenning.

RF Grounding

RF systems benefit from a good RF ground and SSB (or HAM) tranceivers absolutely require a good ground plane. A typical ground plane is copper tape inside the hull from the stern to the keel bolts, using the metal keel as a ground. A capacitor should be used as DC isolation of the ground plane while passing the RF signal. A common recommendation is to cut the copper tape and leave a small gap bridged by a set of small capacitors soldered to the tape. This gap also better isolates the antenna tuner from a lightning strike.

When an SSB is installed, I will very likely have the antenna and RF ground professionally installed or at least seek professional advice. With any luck Space-X Starlink will become more affordable and SSB will go the way of Loran but that remains uncertain.