Whether underwater metals are protected cannot be seen; it is measured. A reference electrode reading shows in numbers whether the anodes are doing their job, whether there is over-protection, and signs of stray current.
How the reading is taken
- Boat afloat. The reading is only taken with the boat in the water.
- Electrode in the water. The reference electrode is lowered into the water beside the boat.
- Connection points. The meter goes between the electrode and the bonding system, or individual metal parts.
- Each metal separately. Shaft, propeller, saildrive, rudder stock, through-hulls; each is recorded.
- Shore power on and off. A difference between the two is the sign of current coming from outside.
How the values are read
- Under-protection
- A value outside the range for that metal means the anode may be undersized, spent or disconnected.
- Over-protection
- Harmful especially to aluminium hulls and saildrives, and around metal fittings in wooden hulls; it can lift paint and soften timber.
- Value changes with shore power
- A sign of stray or galvanic current; a galvanic isolator or isolation transformer is considered.
- Metals read differently
- One part reading unlike the others shows a broken or corroded bonding wire.
What we often see
- Spent anodes and low protection: Saildrive and shaft anodes largely wasted; protection measured with the reference electrode is inadequate.
Readings under different conditions
With the boat afloat, the reference electrode is lowered into the water near the boat and a meter reads the voltage between it and a point bonded to the underwater metals. The reading is repeated for each bonded metal and in different conditions: shore power connected and disconnected, main systems on and off. The difference between conditions points to the source of any problem.
What the readings mean
Each metal has its own potential range in seawater, and anodes shift it in the protective direction. A reading close to that of unprotected metal means the anode is inadequate or the bond has failed. A reading showing excessive protection can itself lift paint, especially on aluminium legs and hulls. Readings are interpreted against the ranges in the standards.
Suspected stray current
A clear change when shore power is connected suggests galvanic current from the marina; a change when an appliance aboard is switched on suggests a leak in the boat's own wiring. The source is then traced by switching equipment off one item at a time.
Choosing anodes
The anode material is chosen for the water the boat lives in: salt, brackish or fresh. The wrong material either does not work or wastes very fast. The number and position of anodes are set by the metal area to be protected.
Records
Readings are recorded with date and conditions. When repeated a year later, the change shows how the system is ageing.
As part of the survey
The readings are read together with the condition of anodes, propeller, shaft and saildrive seen at haul-out; together they explain what is happening below the waterline.
The reading matters most on aluminium hulls, boats with saildrives or aluminium drive legs, and boats that sit on shore power for long periods, where corrosion can progress unseen and damage costly parts.
The reading takes minutes, can prevent the loss of an expensive part and sets a reference value for next year's check.