Anode bolts

[Chartwell]

Hi I have a 1992 GB 36 currently in the South of France in a boatyard.

Today I took my port hull anode off and found the bolts are corroded such that they look like they have been acting as an anode.

The starboard side bolts look fine. I am obviously going to renew the defective bolts but would like to know what may have caused this if anyone has any idea's so that I can remedy.

I will try and attach pictures.

Thank you for your assistance.

Regards Fred

 

[diver dave]

If those are (were) stainless, use silicon bronze. That's what I used for decades on a transom mount Divers Dream.

PS; make sure the bonding wires are intact. The install looks a bit peculiar. I use studs, that are fastened with nuts and washers on each side of the hull. Then the zinc is attached with nuts 5 and 6.

 

[Chartwell]

Dave thanks for the reply. I think that they are stainless and these where studs as you describe. They where fastened with nuts and a fixed washer each side of the hull.

If those are (were) stainless, use silicon bronze. That's what I used for decades on a transom mount Divers Dream.

PS; make sure the bonding wires are intact. The install looks a bit peculiar. I use studs, that are fastened with nuts and washers on each side of the hull. Then the zinc is attached with nuts 5 and 6.

 

[Comodave]

Agree with Diver Dave. In addition I would check your bonding system if only one side corroded away. The side that didn’t corrode may not be connected well so it did not corrode. Or maybe the bolts that didn’t corrode may be bronze.

 

[diver dave]

Forgot one thing. Is the transom wood or composite or ?? If wet wood, I can see that as a problem with long term contact with some SS grades. Dry and bed the new stuff.

 

[Chartwell]

The hull is GRP. Will check the continuity on the other side tomorrow.

 

[Comodave]

It should be less than 1 ohm resistance in the bonding system.

 

[Chartwell]

Hi guys I put a magnet on the anode studs today to check composition. They are not stainless. Should I now consider changing the corroded ones for stainless and also change the as yet undamaged starboard ones.

Also checked continuity between props and anode studs . It is below 1 ohm.

Regards Fred

 

[diver dave]

fyi, a lot of stainless alloys are magnetic, some are slightly magnetic, some are non-magnetic. Some will change magnetic properties upon stress, heat or mechanical.
In any case, try hard to find a threaded rod in SiBronze. My last install took 1/2", but 3/8" may be more appropriate for your case.

https://www.albanycountyfasteners.com/silicon-bronze-threaded-rod-3-foot-lengths.html

 

[Comodave]

Stainless steel can corrode in the absence of oxygen so it isn’t the preferred metal for below the waterline. Look up crevice corrosion.

 

[koliver]

Stainless steel can corrode in the absence of oxygen so it isn’t the preferred metal for below the waterline. Look up crevice corrosion.

Those corroded bolts look like the bolts holding my swimgrid supports onto the transom, NOT bonded to anything, when I removed them after~25 years.
Below the waterline, stainless and suffering from crevice corrosion.

The replacements are above the wl, though after 10 to 15 yrs I had a reason to remove a couple of the bots attaching the swimgrid to the supports, from the top of the swimgrid, and found some that were already suffering a significant amount of crevice corrosion.