P-Bracket corrosion

[ste7ve]

I have a Nauticat with a bronze p-bracket for supporting the prop shaft. When new the p-bracket was, I believe, coated in gelcoat which even covered up the bolt heads securing it to the hull.
Over the years (about 11) the gelcoat has come away and exposed bare metal. Normally I paint it all over with a couple of coats of underwater metal primer and then apply normal antifoul over the top.
Every year at lift out I find that most of the antifoul and primer has disappeared and I am back to some bare metal again. Some of the metal looks a bit pinkish which I believe is not a good sign because it indicates electrolysis action. The boat has 2 big anodes close by on the hull and also a shaft anode.

I was thinking of re-doing the whole bracket with some gelcoat or epoxy material before doing the usual antifouling.

Any advice on this whole matter?

Thanks

 

[GrahamM376]

Sounds like it's not been grounded. You say there are two large anodes nearby but don't say if the P bracket is connected to them, have you tried mutimeter to see if continuity? Shaft anode won't protect it as the cutless bearing is effectively an insulator with the nitrile bearing.

 

[VicS]

A bronze bracket should not suffer corrosion.

Pink colouration is usually a sign of dezincification but its brass that suffers from dezincification ... bronze does not contain zinc. Its an alloy of copper and tin. Never heard of de-tinification !

If the pink colour is only surface deep it not something to worry about.

look at Vyv Cox's website. Info there on dezicification and IIRC on P brackets

http://coxengineering.sharepoint.com/Pages/default.aspx

 

[Puggy]

Hi Steve. I have an NC43 and the wire that supposedly is connected to the P bracket i found didnt in fact do so anymore: it had corroded, I checked with a multimeter. I found the nuts on the p bracket bolts (under the aft cabin floor in mine) and put a new bit of grounding wire between it and the main anode. I also drilled through the p bracket and fitted a disc anode to each side of it. This seemed to stop the "blowing off" of the paint caused by electrolysis. I have to say I that I have being doing a lot of work to rewire the boat to improve the electrical systems which I believe are exacerbating the electrolysis

Best wishes piers

 

[vyv_cox]

My P-bracket is almost certainly made in manganese bronze, which of course is actually a modified brass, definitely subject to dezincification. I have been epoxying it for protection, but like the OP have had problems with adhesion of primer to the bronze.

Last season I painted my prop, a Bruntons so probably not a manganese bronze, with Hammerite non-ferrous primer followed by some Italian antifouling. Adhesion between the A/F and the primer was not great but the primer has stuck very well. This year I will try the primer on the P-bracket. It is usually the case that epoxy does not adhere well to non-epoxy primer, so I shall probably use the Italian A/F on it.

Here's the prop after a season afloat, before pressure washing.

 

[ste7ve]

Thanks for all the comments.

I will certainly check the electrical side. If my memory serves me well the p-bracket is not grounded and maybe they relied on the thing not being exposed to the seawater being covered in epoxy.
I was intrigued vyv using Hammerite underwater on a boat. I assume that is the red paint showing thru on the prop.

If I do use some new 'flowcoat' epoxy to cover the bracket is it best to primer the metal first or can it go straight on?

 

[vyv_cox]

Yes, the red is Hammerite non-ferrous primer. It seems to be a fairly effective antifouling.

Your epoxy will almost certainly not adhere without a primer, but there are epoxy versions. I suggest you need to roughen the metal quite vigorously before applying it. Adhesion of paint to brass and bronze is quite tricky.