What is a bonding system?

ABYC recommends bonding all metal parts, which come in contact with seawater.  Using  #8
gauge green, or green with yellow stripe stranded wire, each part is connected to a common
metal so if any electrical charge is built up, it will be harmlessly grounded back to the negative
terminal of the battery.  This includes any type of electrical charge at the fuel fill.  But just as
important, think of a bad bilge pump float switch that is leaking 12 volts of DC current into the
seawater in the bilge.  All that electricity is searching for a pathway back to the battery
negative, so it finds a brass through-hull sea cock and flows out into the salt water to the
propeller shaft back up into the boat to the negative battery terminal.  That through-hull will
become an anode and eaten alive in just a few hours.  Or a less noble aluminum outdrive will
be swiss cheese in a few weeks. The boat sinks!  The bonding system would have been an
easier pathway back to the battery and avoided damage to the through-hull.  Note:  Bonding is
not meant to protect from lightning. Picture below illustrates stray current corrosion in a
saildrive. Notice the holes in the aluminum casting. This was caused by a broken grounding
wire.
From the Surveyor's Notebook