What Is a Bonding System?
A network that links the underwater metals — seacocks, shaft, rudder — with heavy conductors and to the anode, the unseen layer of corrosion defence.
Bonding is the linking of underwater metal parts on the boat — seacocks, shaft, rudder stock, trim flaps — with heavy conductors and a central anode arrangement. The aim is to keep all metals at the same electrical potential and let the sacrificial anode protect them all.
A metal left unbonded becomes a battery pole in its own right and cannot benefit from the anode. The classic cause behind the 'everything looks fine except that one valve has gone pink' case is a broken or oxidised bonding wire.
Bonding check in the yard
Look at the terminals: The bilge environment is the enemy of copper wires, and terminal connections need a yearly visual check.
Check continuity: Use a multimeter on a few random points; the target is resistance close to zero.
Don’t forget new equipment: If a valve or thruster fitted later has been left out of the bonding circuit, it is unprotected.
If the anodes are wearing unevenly, with one exhausted and another still looking nearly new, the first suspicion should always be a break in bonding continuity. The fix is not to fit a bigger anode, but to repair the wire. The system is invisible, which is why the fault creeps in quietly and is only spotted in the yard.