Another anode question

[Pauly]

I've had a search on the forum here and found no relevant postings so hopefully I'm not going over well trodden ground. So here goes. My boat is a 21' grp hulled sailer and has no anodes fitted and doesn't appear to have had any at any time. Some of the seacocks appear to have suffered as a consequence. There is an anode on the prop shaft. So whilst it is in the yard I intend to fit some protection. The question is, how many and what size do I need? I guess I have 7 seacocks and an engine to consider.

 

[VicS]

Seacocks and other skin fittings should be a corrosion resistant material such as bronze or dezincification resistant (DZR) brass, or a plastic such as Marelon, none of which require the protection of an anode.

Some boat builders are now tending to fit ordinary brass, I believe, on the basis that it can be expected to have a life of several years under normal circumstances.
If you think you have ordinary brass fittings inspect them regularly and replace with one of the above materials if you find any signs of dezincification more than surface deep.

Cathodic protection of skin fittings is frowned upon these days .... See the MAIB report on the near loss of the FV Random Harvest

The engine may be protected by its own internal anode(s) if sea water cooled. If freshwater cooled there may be an anode in the heat exchanger. Refer to your engine manual for details.
It cannot be protected by external anodes

See the metallurgy section of Vyv Cox's website for some good info on dezincification etc

 

[Pauly]

Thanks for that explanation Vic. Not yet sure what material mine are made from, but I suspect the cockpit drains are plain brass as they are both shagged out. The ones on the dunny seem to have fared better, and are probably original fit and possibly bronze. If the engine is relying on its own internal protection the likelihood is that has expired as it is looking decidedly worse for wear,as I suspect are the taps. The one on the sink is seized, but exits above the waterline. It can be changed at any time.
I'll check out the maib report, but it sounds like I have one less job to do.
DZR and plastic fittings are at the top of my preferred list of materials.

 

[Ripster]

VicS has just about summed it up. Been through this a few times on boats and just changing out my current lot of std brass ones for DZR. Start by going here: http://coxengineering.sharepoint.com/Pages/BrassandBronze.aspx (as VixS says) and if you need to replace them I always use http://www.asap-supplies.com/ - no connection, just a happy punter. They are very helpful and know their stuff.

 

[vyv_cox]

Thanks for that explanation Vic. Not yet sure what material mine are made from, but I suspect the cockpit drains are plain brass as they are both shagged out. The ones on the dunny seem to have fared better, and are probably original fit and possibly bronze.

Identification info is on my website, kindly suggested by VicS and Ripster. The simplest guide is that if they are chromium plated they are almost certainly brass. If so you can reckon that the skin fittings and hose tails will be the same. Fortunately it is now possible to buy these fittings in DZR quite easily and not at great expense. ASAP do them, for example.

 

[Pauly]

I've had a look at all the suggested matter, all very informative. I still have some questions though. I read the report regarding Random Harvest and maybe I missed it, but there didn't seem to be any mention of it and I'm still none the wiser as to why linking all the cocks to an anode is a bad thing.
Also, just how good is the plastic stuff? There isn't much difference price wise between that and DZR and of course zero risk of electrolysis. So why aren't boat builders fitting this stuff from new?

 

[Tranona]

I've had a look at all the suggested matter, all very informative. I still have some questions though. I read the report regarding Random Harvest and maybe I missed it, but there didn't seem to be any mention of it and I'm still none the wiser as to why linking all the cocks to an anode is a bad thing.
Also, just how good is the plastic stuff? There isn't much difference price wise between that and DZR and of course zero risk of electrolysis. So why aren't boat builders fitting this stuff from new?

The report (which is on the MAIB site) will not help you much. The general issue with connecting anodes to skin fittings is that if you do have any electrical problems then it can cause dezincification of the fittings rather than prevent it. If you buy either DZR or Bronze fittings they do not suffer from dezincification on their own unlike brass which has a high zinc content therefore the zinc can go just leaving copper - hence the pink colour.

Non metallic valves and fittings are common in US and Australia/New Zealand, primarily because bronze is extremely expensive and DZR (a modified brass) is not generally available. Local suppliers have therefore developed non metallic alternatives. The problem in the UK is that the products have not been widely available until recently and the main US type (Forespar) is dimensionally different from metal fittings so not an easy replacement. The Tru Design valves from NZ are broadly the same size as metal but do not have any price advantage over DZR.

So metal fittings are almost universal in Europe, many of them plain brass. The biggest weak point with brass is dezincification of threaded components like skin fittings which are in seawater - valves themselves are less affected. The safest course of action is to replace both valves and fittings with DZR and forget about them. Just operate the ball valves regularly to stop them sticking up with deposits and they will give many years of trouble free service.

 

[VicS]

The MAIB considered that doubtful that linking the skin fittings to the anodes provided any protection from the type of failure that occurred and may have contributed to it.
They recommend disconnecting the through hulls from the vessels cathodic protection system

I'd suggest that if you really insist on cathodic protection of skin fittings and seacocks they should be linked to their own anodes completely independent of any others and not connected to the DC negative or other structures. If your seacocks are not all close together then two or more anodes will be required