Seacocks! (Stainless ones...)

Graham_Wright said:
Intheory, yes. In practice there is ample oxygen from turbulent water.
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Maybe. I admit I'm doubtful.

Like many, I have a stainless steel prop shaft. There's a small amount of pitting from before I bought her. None since, but I attribute that to the lumps of zinc I attach to the prop shaft.
 
We replaced our two heads outlet stop cocks with Stainless Steel. PERFECT! However if we could have got the PLASTIC one's we would have fitted them instead. The type used on swimming pools.

Peter
 
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There is a full explanation of crevice corrosion, with lots of examples, at http://coxengineering.sharepoint.com/Pages/Crevice.aspx. Briefly, a lump of stainless steel in seawater will last pretty much indefinitely without corrosion. As soon as threads, flanges, things bolted on, etc are added the likelihood of crevice corrosion increases. There is plenty of oxygen in seawater but not in the small gaps (= crevices!) created by these additions.

I recently had discussions with a manufacturer of stainless steel valves and fittings http://www.cspray.co.uk/. He tells me that his 316 and duplex stainless steel valves suffer from crevice corrosion, although some of these are in oilfield duties in very warm waters. The only grade he knows that does not is 904, which is the stuff Rolex use for their watch cases. I didn't bother to enquire the cost of these.
 
LittleSister said:
The Marelon (GRP) sea cocks also combine a skin fitting, seacock and hosetail, and aren't too bad a price compared to the cost of buying the separate bits in DZR or bronze, but they do need a larger hole in the hull as the wall of the skin fitting is thicker than usuall the case in metal. (Though I think I did read on this forum recently someone saying there was a new design/make of Marelon fitting which was the same outside diameter as the metal fittings, but haven't found them.)
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Just to update/clarify -

The 'Forespar' GRP seacocks (also sold as 'Seaflow', it seems) are sold as an integrated unit (skin fitting, seacock, hosetail), and needs a slightly larger hole in the hull than the standard brass/bronze fitting (external diameter for a 1.5" skin fitting is 52mm IIRC) but are overall more compact.

The 'Trudesign' GRP seacocks, skin fittings, and hosetail are sold separately, and use the standard BSP parallel thread and are therefore the same thru-hull diameter as the brass/bronze fittings(48mm for a 1.5" fitting).
 
I need to replace a Cockpit drain seacock on my Oceanlord - I turned it off to remove the pipe and when I replaced it it would not turn on again - so a replacement is necessary.

I measured the outside of the thread at 48mm, is this effectively the same as 2 inches described in the ASAp catelog?

Also the through hull part should not need changing should it??? ( no sign of pink on the metal when scraped.)
 
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