Bonding of brass/bronze skin fittings

[ebbtide40]

I have recently aquired a yacht fitted with Blakes seacocks which were in a sorry corroded state. Some of the bolts had failed and the plywood backing pads had delaminated and in one case had pretty much disintegrated. Each of the seacocks was bonded by a common earth wire conected to other fixtures and fittings throughout the boat.
I have now replaced all the seacocks with new Blakes units and bonded in new plywood backing pads. The question is whether to connect up the bonding wire or not as I have read that good quality bronze skin fittings in a GRP hull do not need to be bonded if not in direct contact with a disimilar metal - see quote below from a website that I spotted:

Zincs and Protection from Galvanic Corrosion
Use zincs to protect against the galvanic currents that are set up by dissimilar metals on your boat that are immersed and that are in electric contact with one another. The best example is your bronze propeller on a stainless shaft. The best protection is to put a zinc right on the shaft next to the propeller, or a zinc on the propeller nut. An isolated bronze thru-hull doesn't need protection because it is not in electrical contact with another immersed dissimilar metal. If electrically isolated, high quality marine bronze, is electrochemically stable in seawater; nothing good can come from connecting wires to it.

I understand Blakes seacocks are no longer bronze but now good quality brass - any advice greatly appreciated.

 

[345000]

Hi Ebbtide,

You're quite right; modern Blake's seacocks are not made of bronze, but DZR brass. They shouldn't need bonding. In fact, I'd say it's likely that the earth wire you speak of has at least contributed to your problem.

My boat is 42 years old, and has all of her original blake's seacocks. I'm not saying they're perfect but they're all still structurally sound, and none of them are bonded.

Best Regards,

Dave.

 

[Tranona]

Would be very surprised if there is any corrosion in the seacock bodies or cones, although fastenings are another matter. The proper Blakes ones are hideously expensive and they could be non original. Consider cleaning up the old valves and re grinding the cones. The only advantage of the latest ones is that they have a built in grease nipple which makes maintenance a bit easier. Also as noted they are now DZR, but neither the original bronze or DZR need any form of anode in a GRP boat. For your backing pads you can use epoxy coated ply and bed the valves and fastenings in sealant - a polysulphide such as Boat Life or Arbromast is suitable.

 

[dombuckley]

Bonding seacocks to anodes is a practice which seems to have originated in the US, as method of dissipating the effects of lightning strikes. Some bright spark then imported the practice, thinking it would counteract galvanic action in seacocks (probably thinking that cheaper seacocks could be used). Unfortunately, it only works if each seacock is individually bonded to the anode; connecting them in a series just creates galvanic cells between each seacock and its neighbour. Even if the seacocks are OK, the increased electrical currents cause electrolytic decay of any surrounding timber. Bad enough on a GRP boat with timber backing pads, but I have lost count of the number of wooden boats I've seen with serious planking issues caused by this practice (iroko seems particularly susceptible).

In short, no - do not connect them up.

 

[ebbtide40]

Thanks all for your replies which collectively confirmed my suspicions - the plywood backing pads did indeed apear to have been subject to some sort of electolytic action rather than general rot.
The new seacocks are now fitted with glassed in plywood pads and the bonding wire disconnected.