Seacocks and fittings.

[boatmike]

For those of you who are interested in a post before Christmas I challenged ASAP to publish a list of materials used in their suppliers skin fittings. They have done so. Well done ASAP Supplies! to access the info go to their website www.asap-supplies.com and look in FAQ.
It's interesting that even the fittings previously listed as gunmetal appear to contain 5% zinc where in my book gunmetal should be a maximum of 0.5% zinc but at least we have some information to base our choice on now.
There are also gaps. Surprisingly Blakes among others who I have assumed are Kosher in the past for one. PERKO look to be OK being a recognised alloy with a minimum of zinc although I see they have plastic ball valves incorporated.
Personally I will probably stay with Blakes for all critical valves below waterline as I have never had one fail on me although they are expensive....... I would rather have known the alloy used though......

 

[tillergirl]

Well done, very useful. I see Nigel Warren's book on Metal Corrosion in boats describes Gunmetal - at least "G bronze Admiralty bronze" as 2% zinc even though it is supposed to be a 'zince' free alloy.

 

[Graham_Wright]

What is wrong with stainless teel?

My local pipeline supplier sells 316 ball valves starting at around £30.

Got to be my choice or is there something I don't understand?

 

[AndrewB]

Gunmetal

Comes in various grades, which may contain up to 5% zinc, and also up to 5% lead. To be on the safe side, specify Admiralty Gunmetal, which is 88% copper, 10% tin, 2% zinc.

 

[Ships_Cat]

Blakes seacocks are claimed by them to be manganese bronze (ie a brass but as has been previously discussed is quite ok).

John

 

[boatmike]

Glad to hear so John. As I said I have put my trust in them for years. Apart from the alloy used the design is fairly bomb proof. Think I will stick to them.

 

[boatmike]

There are several problems with 316 stainless below the WL but to simplify the most common problem is crevice corrosion and a type of corrosion caused when deprived of oxygen. Would stick to gunmetal or an approved de-zincificaton resistant brass if I were you.

 

[AndrewB]

I thought Blakes seacocks used DZR brass (CZ132) rather than manganese bronze. Depending on grade both typically contain around 36% zinc, but manganese bronze has a small amount of manganese and in high-tensile grades, aluminium and iron: while DZR is a straight brass with a trace of arsenic.

Although DZR is resistant, it would be foolish to regard it as wholely bomb-proof and like all underwater brass fittings, needs proper inspection if perhaps at longer intervals. With the increase in on-board electrics in the last few years, yachts kept in marinas seem to be suffering more than formerly from electrolytic problems, and the fact that fittings made of DZR and other good quality brasses have had impressive longevity in the past is no guarantee they will continue to do so.

 

[Graham_Wright]

But my rudder stock, skeg bearing and prop shaft are all 316.

Had I better check if they're still there?

 

[boatmike]

No. but you should check periodically at the inner bearing point of the propshaft to see if there is any pitting there as there is a tendancy for the lack of oxygen there to promote a problem. The rudder stock can also suffer in time around the bearings but is unlikely to cause a serious problem. The skeg bearing is inopen water and unlikely to suffer.

 

[srm]

Had an interesting problem with a waverider buoy made of marine grade stainless steel. One season a number of barnacles settled on the bare ss. On removing the barnacles the ss had severe pitting corrosion along the boundary between the area open to the water and that covered by the barnacle. As said above the problem is oxygen deficiency which sets up an electrolytic cell that corrodes under the covered area.
Discovered similar problems on the ss rudder heel fittings on my new (second hand) catamaran. Looked OK from the outside and passed by surveyor but on dismantling some of the stainless steel looked like honeycomb. Have had new fittings made which I encapsulated in epoxy cloth, except for the bearing surfaces. Added zinc anodes to try and protect the bearing surfaces as can not wire heel fittings into the boat's main anode. Hopefully, it will be a few years before I need to dismantle again and check.

 

[Ships_Cat]

SS valves are used on aluminium hulled commercial and larger aluminium pleasure vessels (probably most of the time). But as has been said you have to be careful and should only use valves of known performance eg those with a classification society approval or known service use.

While it has been mentioned that care has to be taken in stagnant water due to low oxygen it is also so that problems can rapidly develop in highly oxygenated water where the metallugy of the ss has been changed through heat or welding. Examples I have come across have been failure of spot welds of ss seawater strainer baskets and when ss stand pipes are welded to mild steel hulls (resulting in contamination of the ss from the mild steel close to the weld). In those types of cases the ss in the affected areas needs to be epoxy coated (paint or resin).

John