Why Bond Metal Parts on a Boat?

Purpose of Bonding:

The major purpose of bonding vessels is to equalize the potential of all the dissimilar underwater metal parts by connecting them together with copper straps and wires.


Caution Required:


Anodes must be used when bonding - When two or more dissimilar metals are connected together in water galvanic corrosion will commence. They will therefore need to be connected to a sacrificial anode to provide cathodic protection. Note: Even two pieces of metal of the same alloy can develop slightly different voltages and start corrosion.  Bonding all these metal parts together with a sacrificial anode ensures that they are all protected so long as the anode is not depleted.


Bonding is not always essential however. It can sometimes be better to use bronze fittings which are naturally protected and do not require protection from an anode, particularly on a wooden hulled boat which can be damaged by the low voltage provided by the anode.


How to Bond:


The main bonding conductor should be run well above the bilge level to the boat's common ground point at the engine block (and -ve on the battery). A minimum of #8 insulated stranded wire should be used. Individual bonding conductors (wires) should again be at least #8 and run out to the individual metallic components. The bonding system is usually connected to a sacrificial anode mounted on the transom with through-hull bolts.

Internal electrical systems through which water passes (engines, pumps, air conditioners etc) should also be connected to the bonding system along with metal fuel tanks, fuel pumps etc.

Do not daisy chain from one fitting to the next.

Do not connect through a device, i.e wire into a through hull bolt on one side and out a different bolt the other side.

Do not use bonding wires as current carrying conductors.


Electrical Safety:


The bonding system also provides a path for harmful charges from any malfunctioning ac appliances to be bled off to the supply ground, since it is connected to the common ground point.


Miscellaneous Tips:


We do not recommend bonding a wooden hulled boat, but if you do, make sure the bonding system is insulated from contact with the wood.  Any current flowing through the wood (especially if damp) causes rapid deterioration.

We do not recommend bonding trim tabs so they will require their own anode protection.

We recommend using shaft anodes for the protection of propellers and shafts. The electrical connection through the engine/transmission is a very poor one since it passes through gears and bearings that are immersed in oil which is a non conductive solution. Shaft brushes, usually made of graphite, iolite bronze or copper are again very poor conductors and in our opinion do not work. A slip ring assembly (much more expensive) made using silver brushes is effective however.

Metal Hulls - In this case the hull is part of the bonding system. Fittings made from a dissimilar metal should ideally be insulated from the hull material to prevent local galvanic corrosion.


Other Advantages of Bonding:


  • Internal collection of stray currents that may find their way into wet bilge areas that contain metal hull fittings.
  • Lightning protection
  • Radio noise suppression.