The short answer: a bonding system electrically links underwater metal fittings so they sit at the same potential and share sacrificial anode protection. It suits some boats very well and harms others. Whether your boat needs one depends on its hull, the metals below the waterline and how its shore power is protected.
Bonding is often treated as automatically good. It is not; it is a design choice with trade-offs.
A bonding system uses heavy conductors to connect underwater metals, such as through-hulls, the propeller shaft, rudder fittings and the engine, to each other and to one or more anodes. Once linked, the anodes corrode instead of the fittings, provided the anodes are sized and positioned properly.
Boats with several dissimilar metals below the waterline, and particularly metal hulls, usually benefit from coordinated protection. Bonding also plays a part in lightning protection, where it limits voltage differences between large metal masses during a strike.
Linking isolated bronze fittings into a system can expose them to stray current from elsewhere on the boat or the marina. On shore power without galvanic isolation, bonding can connect your underwater metals to other boats through the earth wire, accelerating anode loss and sometimes fitting damage.
That is why a boat with isolated fittings in good condition is often best left alone.
On shore power, the earth wire connects your boat to every other boat on the pontoon. A galvanic isolator or isolation transformer breaks the galvanic path while keeping the safety earth. Any bonding decision should be made alongside how shore power is protected, not separately.
Before adding or changing bonding, the existing arrangement should be surveyed: which fittings are bonded, the condition of conductors and joints, and anode consumption. A reference electrode can measure whether fittings are adequately protected. Guesswork here is expensive.
Bonding conductors must be heavy, well terminated and routed out of bilge water where possible. Joints should be accessible for inspection. Anodes must be matched to the water, since salt, brackish and fresh water call for different alloys.
Safety note: bonding interacts with the AC safety earth. Changes should be made by a qualified marine electrician who understands both corrosion and shock protection.
Anodes that disappear much faster than usual, or not at all, are the classic signs. Pink or discoloured bronze fittings, flaking antifouling around metal fittings and corrosion on the propeller or shaft all suggest something is wrong with protection. Bonding conductors that are green, loose or disconnected at a fitting are another giveaway during a haul-out inspection.
Each of these symptoms has more than one possible cause, so they are prompts for investigation rather than proof of a bonding fault. Measurements with a reference electrode give firmer answers than visual clues alone.
Older boats often have bonding added piecemeal over decades, with conductors of different sizes, joints in the bilge and some fittings connected while others are not. Before adding more, it is worth mapping what is there and deciding what the system is meant to achieve. Sometimes the right answer is to rationalise and repair; sometimes it is to remove bonding from fittings that are better isolated.
Any change should be followed by checking anode consumption at the next haul-out, so you can confirm it is doing what was intended.
Stray current is different from ordinary galvanic corrosion. It comes from a DC fault leaking current into the water, either on your boat or a neighbour's, and it can eat fittings and anodes remarkably quickly. A bonding system can spread the effect of stray current to more fittings if the fault is on board. Tracing stray current involves measuring leakage with circuits switched on and off in turn, and it is one of the reasons we check the whole DC installation, not just the bonding conductors, when corrosion is unusually fast.
No. Many GRP boats with isolated fittings are better left unbonded. It depends on the metals, hull and shore power arrangement.
They are related but different. Earthing is about electrical safety; bonding here is about equalising potential between metal fittings.
Stray current, a shore power galvanic path or a bonding fault are common causes. It is worth investigating rather than simply fitting more anodes.
We assess existing bonding, shore power protection and corrosion before recommending anything. Free assessment, written quote.