Seacocks! (Stainless ones...)

langstonelayabout

langstonelayabout

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Well, the survey has recommended loads for the new (to me) boat, including needing new seacocks, one fairly soon.

Brass ones are cheap and last 5+ years
Bronze/DZR brass last forever and are £130+
316 stainless are cheaper, and are the reason for this posting...

Does anyone here use stainless ball seacocks on their boat? (Sea water) are they OK? How long would they last in such an environment?

Your thoughts would be appreciated - Many thanks in advance
 
All my below waterline fittings are 316 stainless and no problem so far but my boat has a steel hull so I did not want any brass/bronze below the water line.

I even replaced my prop with a fabricated stainless last year to reduce the anode usage even more.
 
langstonelayabout said:
Well, the survey has recommended loads for the new (to me) boat, including needing new seacocks, one fairly soon.

Brass ones are cheap and last 5+ years
Bronze/DZR brass last forever and are £130+
316 stainless are cheaper, and are the reason for this posting...

Does anyone here use stainless ball seacocks on their boat? (Sea water) are they OK? How long would they last in such an environment?

Your thoughts would be appreciated - Many thanks in advance
Click to expand...

Trouble is that stainless steel needs a supply of oxygen to remain stainless. it is a layer of Chromium Oxide on the surface that protects the steel. In the absence of oxygen it is likely to corrode - the usual term for it is crevice corrosion,

You shouldn't pay anything like that for a DZR sea cock. Make sure it has CR moulded into it.
 
lpdsn said:
Trouble is that stainless steel needs a supply of oxygen to remain stainless. it is a layer of Chromium Oxide on the surface that protects the steel. In the absence of oxygen it is likely to corrode - the usual term for it is crevice corrosion,

You shouldn't pay anything like that for a DZR sea cock. Make sure it has CR moulded into it.
Click to expand...

Once the Chromium Oxide is created does that Oxide corrode. My understanding is that if the Chromium Oxide is damaged it needs oxygen to reform and as Graham_Wright says there is oxygen from turbulent water.

If the threads are sealed properly as I have done there should be no crevice corrosion.

Do you get crevice corrosion on the thread of a stainless steel prop shaft which is the material used on most boats and to make it worse a bronze propeller is also fitted in direct contact. Why not use a stainless steel prop?
 
Stainless valves are not necessary on a GRP (or wooden) boat. DZR is fine. Downside with SS is they can suffer from crevice corrosion hidden away in the threads. It is nothing to do with galvanic action and difficult to predict or see where it might happen. As you will see from the ASAP prices, both SS and Marelon are substantially more expensive than DZR. - hence the popularity of DZR.
 
Rogershaw said:
Do you get crevice corrosion on the thread of a stainless steel prop shaft which is the material used on most boats
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Yes. Very common in the threads for the locknut and in the bearing surface of the stern bearing, particularly if the boat is left unused in the water. Not a pretty sight when it does happen.
 
Rogershaw said:
Do you get crevice corrosion on the thread of a stainless steel prop shaft
Click to expand...

No, but I assume that's due to the anode.

I did get severe corrosion of the stainless bolts holding Kindred Spirit's rudder on, though. 10mm bolt, looking more or less normal at the exposed ends, but when removed turned out to have eroded to a 2mm string in the middle. Replaced them, with lots of sikaflex to try to keep the water out, but a year later the new ones were rapidly corroding too (though they hadn't had time to reach the same extreme state).

We had a new rudder fitting and set of bolts custom-made in bronze by Classic Marine.

Pete
 
Tranona said:
As you will see from the ASAP prices, both SS and Marelon are substantially more expensive than DZR. - hence the popularity of DZR.
Click to expand...

Mackengineering's (no connection, despite similarity in name) prices for complete Marellon fittings aren't a great deal more than ASAP's for DZR, and much cheaper than ASAP's for Marellon.
Incidentally the link I posted (#2) earlier, when it worked, seems currently to be on the blink.
 
langstonelayabout said:
Bronze/DZR brass last forever and are £130+
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I think this price will be for a large (38mm) 'Blakes' seacock (and one other expensive make I've forgotten name of) which combine a seacock, skin fitting and hose tail.

The cheaper way to do it is to buy separate bronze or DZR skin fitting, seacock and hose tail with matching threads and bolt them together.

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.)
 
LittleSister said:
I think this price will be for a large (38mm) 'Blakes' seacock (and one other expensive make I've forgotten name of) which combine a seacock, skin fitting and hose tail.

The cheaper way to do it is to buy separate bronze or DZR skin fitting, seacock and hose tail with matching threads and bolt them together.

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.)
Click to expand...



True Design, perhaps.
 
macd said:
Perko?
Click to expand...

I think that's the one.

A higher quality approach than the standard skin fitting and ball valve combo, as it has a big wide flange that's bolted to the hull and also held in place by the skin fitting. It also has a bit less leverage for wrenching things off.

Pete
 
Stainless skin fittings and valves causes more problems than it solves especially if you connect say a stainless ball valve to a bronze or brass tail or skin fittings of dissimilar metals, then stainless to stainless and you still get crevice corrosion. stick to DZR if you like your boat.
 
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