What on earth should I do?

Canal Boat - - Back Cabin: - TONY REPLIES:

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?

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