Hi Brian
It sounds like your boat has probably developed a stray current issue. I have some experience with this having bought a
steel hull in 2010 that had problems which are now fixed. Stray current is present when there is a positive or negative leak/connection from your
battery to wet metal (submerged metal). This could be the entire boat in a steel or aluminium
hull but can also be via your
propeller in a GRP vessel or even through damp,
salt water impregnated timber. If your vessel has even just a little then your boat is not isolated from the stray current effects from other "hot boats" is "contagious" when you are in a marina. When your boat is completely electrically isolated then it is immune to the cumulative effects of the boats around you. In
marinas steel boats often wrongly take flack for this from uninformed boat owners that are looking for someone else to blame but I have worked on GRP vessels to electrically isolate them that were putting as much current into the water because their
engine - reputed to be marinised - had a negative lead (from the battery) permanently connected to the
engine. Nearly all
commercial fishing boats and ships have electrically isolated engines (starter motors and alternators with two big leads) so I'm not sure why most recreational boat engines are not isolated. One problem here is that if your boat is in a marina in a line of other "hot boats" then each non-isolated boat thats added causes an accumulated effect, just like adding an extra plate in a
battery. If your vessel is isolated then you are immune to these effects. Your engine is not the only place that stray current can come from. It can also be generated by changes in your domestic system or anywhere that
electric current is present.
Wiring and
equipment can break down over time and begin transferring current especially where it is in a wet
environment such as a wet
bilge.
I was fortunate when my boat presented stray current issues to find an extremely sensitive electronic detector that can diagnose the source of the problem and allow the owner or
marine electrician to resolve it. I had one permanently mounted on my vessel ever since that picks up problems as they happen. They are not inexpensive but
work out a lot cheaper than having to fix the resulting corrosion. Anodes cannot overcome the effects of stray current which can be thousands of times stronger than the galvanic current generated by the union of dissimilar metals that anodes are installed to counter. Please don't let people tell you that you just need bigger or more anodes, they are wrong. The manufacturer of the detector I installed offered for me to be a distributor. I live in
Queensland Australia but if you are interested in finding out more you can contact me at
tim@solaexchange.com.au