Marine Surveyors Lexicon


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Bonding provides a low-resistance electrical path between metals in contact with seawater. This path protects against self-generated galvanic corrosion, is helpful against other types of corrosion, provides a lightning ground and also acts as a radio ground.

Although a bonding system normally will only carry the protective current of the sacrificial anode (under 1 amp) it is recommended to use #8 wire or larger in order to ensure a low resistance path. Over 20' length, use #4 wire. Over 40', use #2 wire.

If the wire is insulated, the insulation should be of a green color to designate its purpose. solid or heavy stranded wire is best to resist corrosion, but will break if allowed to work. Stranded wire does not break as easily as solid, but care must be taken with sealing connections as stranded wire corrodes much more easily. Copper braid is not recommended. Bonding may be painted with non-metallic paint.

Connect all metallic parts touching water inside the hull, including; bilge pumps, intake strainer, etc., together in a neat, short route. All connections should be secure and made to a #8 or larger wire, copper strap or similar bonding conductor. Connect the system to the engine block and to the sacrificial anode.

Soldering is a good method to secure the bonding wire to metal fittings but is rarely possible as the water acts as a heat sink. Another method is to thoroughly clean a spot on the metal surface, use an all-stainless hose clamp to mechanically secure the wire, and seal the connection against the environment with a long last, low water absorbing two-part dielectric removable epoxy. All connections should be soldered and sealed and wiring under clamps should have a heavy solder piece or wire end affixed to provide a secure grip for clamps.

Props and shafts are not always electrically connected to the engine block as the transmission oil acts as an insulator. A high quality motor brush (preferably metal graphite) riding on the shaft with a wire attached to the bonding system is a good method of connecting the prop and shaft to the bonding system.

Rudder shafts sometimes do not have a good electrical connection to the rudder bearing. A flexible loop of wire should connect the shaft and rudder to the bonding system.

Exterior mounted metal touching water must be bonded also. Swimsteps, trim tabs, boarding ladders, etc., should have their own zinc if difficult to put in the bonding system.

The engine, containing a separate body of water, will not be protected by the boat's bonding system even though the bonding system is attached to the engine block.

Most electrical installation manuals will recommend fuel tanks, fuel pumps, fuel fill fittings, etc., be added to the bonding system as a spark deterrent.

Never use bonding wire or the bonding system as a current carrying wire on the DC system. A positive ground DC system should be changed to a negative ground system. Shorts to seawater on a positive ground boat causes the entire grounding system to be anodic (more positive) and the entire bonded system to quickly corrode.

In a twin engine boat system with a crossover (or emergency) starting method, be sure to attach the engines together with a heavy enough bonding cable to carry a starting current. If the common ground should fall off the battery, the engine would try to start through the fuel lines, etc.

Metallic base paints and preservatives can aggravate galvanic corrosion on a hull, especially if the hull screws and fittings are far apart on the Noble scale. Current flows through metal in paint.

Fresh water systems may require a higher voltage and less current than saltwater systems. Pollution, minerals and current can help to destroy metals in fresh water.

Sacrificial anodes not connected to a metal will not protect that metal.

Wood boat mounting bolts, zincs and bare metal bonding straps should be insulated from contact with the wood as electricity from a bonding system attempting to flow through the damp wood cause the wood to foam and soften much like wood rot.