Wiring Anodes To Engines On Wooden And GRP Boats

Wiring Anodes To Engines On Wooden And GRP Boats

Wiring anodes to wooden boats or boats with GRP hulls is common to help provide better cathodic protection. Bonding the anodes to the component to be protected via a cable ensures continuity. The engine is often a convenient location to connect to, but any flexible couplings could be bridged.

The purpose of a bonding system is to ensure that metal equipment and fittings on your vessel are at the same voltage.  This provides two significant benefits:

  1. Electrical safety for the above water metals
  2. Protection for the underwater metals

When bonded, underwater metals cannot be damaged by leaking electrical currents originating from within the boat (a defective pump, float switch or wire splice). When the bonding system is connected to the sacrificial anodes, all bonded underwater metals are then protected against galvanic corrosion.

The easiest way to check the continuity is via a multi-meter which will measure the ohm reading. Any reading higher than 0.2 ohms should be investigated.

For example, bonding twin engines’ starter batteries will ensure that both propellers have exactly the same voltage running through them. This will eliminate electrolytic plating and corrosion between the two which can be identified by one propeller discolouring faster than the other.

The recommended range of cathodic protection for a vessel depends on hull material and underwater metals present:

More details are available on the article on cathodic protection voltages. Cable is cheap in comparison to replacing propellers or other fittings, it’s definitely worth the investment.

When To Use An Anode Backing Pad

The backing pad was designed to stop the back of the anode from wasting away and to help keep the anode in place. On GRP and wooden hulls it is especially useful as it can help provide a barrier between the anode and the hull. The pad protects the hull from the alkalinity produced as part of the cathodic protection process. Typically, they are made of a neoprene type material or rubber for longer lasting pads. You’ll find they are usually available for the most common sizes, but there are a few which can be trimmed down to fit more unusual anodes.

Hull TypeRecommended Range
Fiberglass w/ Inboard Engine(s)-750 to -1000 mV
Fiberglass w/ Aluminium Outdrive(s)-900 to 1050 mV
Aluminium-900 to -1100 mV
Steel-800 to 1050 mV
Wood-550 to -600mV

The pads can also be used on trim tabs, although it is not always necessary if you ensure that the anode core bar itself is clamped to the trim tab, rather than the actual anode material itself.

Backing pads also work as a good stencil when applying antifoul paint, so the finish is professional when you fix the anodes in place.


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