What on earth should I do?
QI had previously earthed my boat’s domestic batteries to the hull by mistake and this has caused the bottom of the boat to be damaged. I have now earthed them to the engine block using the same earth terminal that was fitted by the manufacturer on the starter battery. I am still getting a reading from my positive point to the boat’s hull indicating that I am still doing something wrong. Can you help? NEIL SMITH, via email
AI don’t think you have done anything wrong. If you use a shoreline without a galvanic isolator or isolation transformer, that would be a far more likely suspect for hull damage. Think about it: you have bonded the domestic battery negative to the engine block but the block is connected to the hull via the exhaust pipe, control cables and to some degree the shaft, so of course you will get a voltage reading between the battery positives and the hull. This is normal.
Technically, you should only have one negative 12v bond to the hull. But if the engine is flexibly mounted, a fault can in some circumstances cause burned control cables. To minimise this risk it is common to bond to the engine (which carries the heavy starter current) and then run another heavy bond cable between the engine bond point and the hull, usually on the engine bed (which allows fuses to blow or MCBs to trip in the event of a short to the hull).
If this is a newish boat I would suspect the hull ‘damage’ is either mill scale falling off the steel, or rust inclusions within the plates if it’s poor quality steel.
Your anodes would first show signs of excess erosion – you do have them fitted?