Earthing and bonding are two different systems that happen to share overlapping terminology, and confusing them is one of the more common misunderstandings owners bring to us. Earthing is about AC electrical safety, giving fault current a path back to source so a protective device trips. Bonding is largely about DC and galvanic issues, connecting underwater metal fittings together so they share a common potential and corrode evenly, or protectively, rather than attacking each other.
Both matter, they are wired differently, and treating one as a substitute for the other leaves a real safety or corrosion gap. This guide separates the two clearly.
On a boat with a shore power or generator AC supply, the earth conductor provides a low-resistance path back to the source so that if a live conductor faults to an exposed metal part, a washing machine casing, an inverter chassis, current flows through the earth conductor rather than through a person touching it, and trips the RCD or breaker protecting the circuit.
This is exactly the same principle as household earthing, but the marine environment, damp, salt, movement and the shared shore power supply on a pontoon, makes a properly maintained earth and an RCD considerably more important than in a house. A boat's AC earth should be bonded to the boat's engine and to the DC negative system at a single point, following the wiring diagram for the specific installation, rather than connected at multiple points which can create stray current paths.
Bonding connects underwater metal fittings, the engine, prop shaft, seacocks, rudder stock, keel bolts where fitted, together with a bonding conductor, usually to a common bonding point that also connects to an anode. The purpose is to ensure these fittings sit at the same electrical potential in the water, so that if a small stray current or a galvanic effect is present, it acts on the anode, which is designed to be sacrificed, rather than corroding a shaft, propeller or through-hull fitting.
Bonding is not primarily a shock protection measure, though on some AC systems it can play a supporting role; its main job is corrosion management, working alongside anodes and, where fitted, a galvanic isolator or isolation transformer on the shore power supply.
Treating the DC bonding system as a substitute for a proper AC earth, or vice versa, leaves gaps. A boat can have excellent underwater bonding and anodes, and still have no functioning AC earth or RCD protection, leaving a real shock risk if an AC appliance develops a fault. Equally, a boat with a sound AC earth can still suffer rapid galvanic corrosion if the underwater bonding is missing, broken, or was never connected to the anode.
Multiple, uncoordinated earth or bonding connections are also a known cause of stray current corrosion, since they can create unintended paths for current to flow through the water via underwater metal rather than through the intended conductors. This is why bonding and earthing should follow a single documented scheme for the boat, not be added to piecemeal.
Where a boat is regularly on shore power, a galvanic isolator fitted in the shore power earth conductor blocks the small DC currents responsible for galvanic corrosion between boats sharing a marina supply, while still allowing AC fault current through in the event of a genuine fault, preserving the safety function of the earth. An isolation transformer goes further, electrically separating the boat's AC system from the shore supply entirely, which addresses both galvanic corrosion and some stray current risks, at a higher cost.
Neither device is a substitute for proper underwater bonding; they address the shore power connection specifically, while bonding addresses potential differences within the boat's own underwater fittings.
An owner can reasonably check that bonding conductors are physically intact and connections are not visibly corroded, and that an RCD trips when tested. Diagnosing whether a bonding or earthing scheme is correctly designed, testing for stray current, or making any changes to how the systems are connected together requires test equipment and a clear understanding of the boat's specific wiring diagram, and is professional work.
If anodes are disappearing quickly, or you are unsure whether your boat has a functioning AC earth and RCD at all, get it checked rather than guessing, since the consequences of each fault path are different but both are serious.
No. Earthing is about AC shock protection, giving fault current a path back to source. Bonding is mainly about connecting underwater metal fittings to manage galvanic corrosion.
It is possible for a competent owner, but the bonding scheme needs to follow the boat's documented wiring diagram and connect correctly to the anode and the rest of the system, so it is worth having a professional confirm the scheme before making changes.
No, a galvanic isolator addresses the shore power earth connection between boats; underwater bonding within your own boat is a separate and still necessary system.
Correct bonding is part of the answer, but rapidly wasting anodes usually point to a stray current or marina-wide galvanic issue that needs proper diagnosis rather than assuming bonding alone will resolve it.
Yes, a missing or faulty AC earth combined with no RCD protection is a genuine shock risk and should be treated as a priority safety issue, not a minor defect.
We test and correct AC earthing and DC bonding systems to a documented scheme, and can advise on galvanic isolators where needed. Get a free assessment.