Well, after spending the weekend crawling around with a voltmeter, I've learned some things, but not sure if any are useful.
As is, there was about -12.5MA of current
at -.35V between the fitting and the bonding system (once i disconnected it). The ball valve is definitely a different color than the through hull
, it's too soft to be stainless, and not magnetic. There's a PN25 marking on it which appears to be a european pressure standard. Some googling suggests that it might be nickle plated brass which seems to fit. They also show some available in a BSP thread which matches what Beneteau
claimed for the through hull, which explains how they avoid the mismatched threading issues and get along without a real seacock.
Long story short, i disconnected the entire bonding system at the bus, and measured the potenials between each leg and the -12Vdc system which it had been tied to. Some legs showed more current than others, varying between 4-5ma to as much as 25ma. There was about 30ma of current between the entire bonding system and the 12vdc ground.
Even after I disconnected the bonding wire and cleaned up the through hull, the through hull continues to accumulate fuzz. Measurements between the through hull and the water
with a silver:silver chloride half cell show -590MV which is about where it should be AFAIK.
So, the only thing i can come up with is that there must be some galvanic action between the ball valve and the through hull. But why it took 8 years to start being a problem is a mystery to me. I can't find any current in the water
to explain things, and the galvanic isolators should protect from that anyway. I guess I'll haul the boat and replace the through hulls and valves.
Against my better judgement, I also tasted the fuzz. Definitely not salt
. Nasty tasting stuff, a bit soapy too.
I'd welcome any insights anyone has. I'd hate to go through the expense of doing all this only to see the problem continue.