Boat ElectricsMetallic through hull fittings turn from light to dark green and ultimately to purple, before dissolving to dirt.
by Brian Gatt*

The Bonding System

The bonding system is one of the most crucial parts of non-metallic hulled vessels and should be treated like any AC or DC electrical connection.
Many people seem to forget about the bonding system as it does not play music, show you the depth, speed or temperature, charge batteries or make the boat go faster.
However, a boat without a rudder does not steer and corroded propellers will not perform – which can both be due to galvanic corrosion caused by a failure within the bonding system. Apart from the obvious safety implications, it is extremely costly to fix these things and generally insurance does not cover corrosion or electrolysis to a vessel.
I have seen some older boats that have been beautifully cared for but the bonding system is no longer working, which is often only noticed once the damage is done. I have seen some new boats with fantastic bonding and others with little to no bonding systems.
In New Zealand marine electricians who fit anodes also check and test the bonding system and accordingly fit the correct anodes to suit that vessel. This guarantees that the vessel is not over or under-protected and the chances of galvanic corrosion are markedly reduced. Regrettably, this is not the norm in Australia.
There are many things that can be done to care for the bonding system. Make sure all connections are clean, tight and covered in a lanolin or a non-corrosive coating. Replace all the connections every three to four years.
The bonding cable must be very fine strand tinned copper between 4mm² and 10mm² for most vessels. When new anodes are fitted, ensure that there is good internal connection and that the cable is connected throughout. We are working with just millivolts and milliamps, that do amazing amounts of damage in a short time.
High quality skin fittings will last for years. Using connected fittings of the same material e.g. Stainless-316 gate valves with Stainless-316 through-hull will ensure that components last much longer. Mixing materials and using cheaper materials will cause galvanic corrosion, resulting in reduced longevity.
There are different views on what should be bonded. Within a wooden hull vessel minimal bonding is ideal – e.g. just shaft, shaft gland, skeg, rudder and raw water inlet. For fibreglass vessels the rule for the bonding system is “the more the better”.
One problem often encountered is that the bonding system is connected in a daisy chain, therefore there is no redundancy if there is a cable breakage or high resistance. A bonding system should be designed using good quality buss bars, tinned copper fine strand cable and taking all fittings back to bussbars as per the diagram below.
Atmospheric corrosion is just as dangerous as galvanic corrosion. This is where we have metallic fittings housed in an environment where we have some degree of moisture or salt solution.
The metallic through hull fittings turn from light to dark green and ultimately to purple, before dissolving to dirt. There are plenty of ways to protect these valuable fittings. Clean them thoroughly and once they are clean and back to their original colour, coat them with a Lanolin spray.
When a fitting is suffering corrosion and it is bonded into the protection system, your sacrificial anode will work harder to keep it protected and eventually the anode will reduce to nothing, which can have further consequences. I have seen a brand new anode disappear within a month due to incorrect anode selection or failure within the bonding system.
The importance of a sound bonding system cannot be overemphasized, and boat owners are well advised to ensure that their system is adequate and checked on a regular basis.

* Written on behalf of aquavolt (02) 9417 8455 by specialist contributor Brian Gatt of Electralogic (02) 4381 0790 | |