Heads/seacock corrosion

[Conachair]

This is almost a crosspost from PBO so apologies but I figured I'd try here as well as you always get good resposes.
Just had the boat lifted out (33' steel ebbtide) after 10 months. I've got quite heavy anode wear around the heads area and the bronze seacock has some green dust on the flange and also metal parts of the heads (baby blake) itself have a green sheen. Other seacocks seem fine, little bit of green but minimal. All seacocks appear to be isolated from the hull, reading between 1.5 - 2 megohms (does this sound enough?). Some corrosion of the inside of the hull around the seacock as well Any ideas what could cause this? Only thing I can think of was a drip which I didn't notice for probably quite some time so that area had seawater on it.
Or could it be stray current? Boat has spent a lot of time in marinas.
Muchos gracias in advance.

 

[tcm]

If you are worried about stray currents in marinas, install a galvanic isolator, findem on google, fifty quid or so.

Bit of green around bronze fittings ain't unusual?

 

[Conachair]

Not so worried about current from shorepower, only connection is through battery charger (stirlng) which says it is isolated and seems to be he case. More concerned about what might have been going on around me but don't know enough to know the symptons or effects. And would dearly like to know why one seacock is much worse than the rest when they all seem to be isolated from the hull. I did just notice a barnical bridging the gap between the seacock and the hull - do they conduct lx? Grasping at straws here!
Ta

 

[tcm]

um, if the battery charger is knected to the shorepower, then so is the earth? It's not the current you draw which is the issue - even if you draw zero current the adverse effects arise from being part of the common earth for all the other boats on the same pontoon/circuit.

 

[Conachair]

[ QUOTE ]
um, if the battery charger is knected to the shorepower, then so is the earth?

[/ QUOTE ]
Apparently not with the stirling unit I have according to the manual. Shorepower comes in to a 4 way block, charger plugs into that as does laptop, everything else is 12v. Just checked the resistance between the earth on the 4 way block and engine (battery neg attached to engine, everything else is 2 wire) with charger turned on it shows 3.2megohm. Oddly enough it's less with the charger unplugged, 2.8megohm. Shows 15megohm between the earth on the charger plug and batt neg.

 

[FAITIRA]

Very important not to have any areas where water is 'pooling', heaps of limber holes in frames and stringers, a chum with an EBBTIDE 40+ish, 6yrs old had to replate an area, coincidentally by the heads. He was paying me to help him, in Tortola, bu##er of a job. Bill.

 

[roly_voya]

Sterling confirmed to me that the output is isolated when I was worried about the same thing, should not be a problem unless there is and earth connection from the case to ships earth. In any case it would be an odd place to get stray current corrosion not usually much electrical stuff in that area. Is it possible you have been getting sea water leaking, possibly when flushing, which has bridged the insulation? Might only show up when wet and the rest of the time insulation tests ok. You could then be looking at galvanic interaction between tiolet parts and hull

 

[Conachair]

[ QUOTE ]
Is it possible you have been getting sea water leaking, possibly when flushing, which has bridged the insulation? Might only show up when wet and the rest of the time insulation tests ok. You could then be looking at galvanic interaction between tiolet parts and hull

[/ QUOTE ]
there was a drip in that area which i didn't notice for a while so could be that. But wouldn't the bronze be ok and the steel rusting? Which it is, actually, some piant on the inside of the hull lifting and some rust in the inside next to the seacock. Doesn't look too bad and a friend has an ultrasound meter so I'll be able to check the thickness. Fingers crossed.

 

[vyv_cox]

Don't assume that all seacocks have the same composition. Even if they were bought in the same shop at the same time, their composition could be very different, even made from a different class of alloy. For example, I was pretty certain in my mind that Blakes seacocks and skin fittings historically were made of tin bronze. Reference to their website shows that they are now made from a DZR brass.