Inox seacock suitable?

[Amari]

Just arrived Kusadasi, Turkey, for autumn liveaboard and find heads outlet seacock(replaced only 3 years ago) seized (lever won't move). Need to close it because need to investigate why holding tank filling up full despite Jabsco Y-valve set to ''pee to sea''. (?siphoning from seacock via Y-valve). Think I'll have to lift out to prob change seacock. Marina swindlery have 2 types 11/2'' :'brass' @ 12 GDP and Inox @ 50 GDP. Yard tell me Inox(stainless steel) no good in this situation because of electrolysis, experienced boat guardian says Inox OK. Recent PBO article nor Pat Manley's book mention Inox as used for seacocks (I'm assuming swindlery correct in labelling). Grateful for any thoughts.

 

[2Tizwoz]

Brass is not to be recommended due to de-zincification rapidly weakening it and resulting in possible total failure (it can snap like a carrot).

If this is a ball valve rather than some sort of combined valve and skin fitting, depending upon the nature of the skin fitting, electrolysis could be a problem.

If you prefer metal bronze is best.

Many people like the Blakes one. Even that model requires periodic maintenance, greasing to operate easily.

Nickel plated brass is still brass and when it still appears fine externally sudden failure remains a problem.

Marelon glass reinforced plastic seacocks are approved by the Underwriters Laboatory and don't suffer corrosion. The biggest problem with plastic over metal is mechanical damage, if they can be walked on or abused by strong hands.

You may find this site,which describes causes of sinkings, helpful

Presumably you left the boat afloat with the seacocks open. /forums/images/graemlins/shocked.gif

A little exercise helps to keep them in condition.

 

[Salty John]

I wouldn't use brass under any circumstances.
There are a couple of things to watch: The through hull to which the seacock is attached must match the thread and, prefereably, the material of the seacock. Even though the threads per inch is the same, the angles of the thread profile can be different between British and US threads.
The safest choice is a good quality bronze ball valve which has a chromed brass ball and neoprene seal. Also good is a Marelon seacock, made from Zytel.
Groco in the USA makes bronze and also stainless steel ball valves - some suppliers of these valves specify that the s/s valve and s/s through hulls should only be used above the waterline. This is likely to be because of fears of galvanic corrosion and crevice corrosion.
I understand that this leaves you with the problem of choosing between two unsuitable valves, but that's life! You're best bet may be to try and free up the existing valve if it is bronze. You could put a wooden bung in the through hull from the outside and then remove the existing valve to see if it repairable.

 

[Amari]

Many thanks and sorry for delay - lost wireless contact. Thanks for link to excellent website, Ball valve, lift out mane, will have to decide then. Yes, mea culpa, left seacockopen on leaving boat in June - lever stuck and decided to fix it at lay-up. But now SWIMBO cannot use heads - all goes to tank and pump out seems knackered as well, A right cock-up, to coin a phrase. /forums/images/graemlins/frown.gif /forums/images/graemlins/blush.gif

 

[Strathglass]

Do not use Inox (stainless) for an outlet seacock because of electrolysis. Your yard is correct.
As has been stated the best is the Blakes but it is very expensive.
Some people swear by the plastic ones, but I only use them above the waterline.
I tend to use ball valves.
The main thing not to use is plumbers type brass gate valves.

You can get all of those things Here

Cheers

Iain

 

[Graham_Wright]

[ QUOTE ]
Do not use Inox (stainless)………………………… because of electrolysis.

Iain

[/ QUOTE ]

Bit worrying for those of us with stainless prop shafts (and bronze props)?

What is the galvanic partner for the elctrolysis?

 

[2Tizwoz]

This table gives the probability of galvanic corrosion occurring with various marine materials

The bronze propeller stainless shaft combination being protected with a zinc sacrificial anode.

 

[Graham_Wright]

So I guess that would work with skin fittings also?

The table doesn't volunteer the electropotentials which you really need to establish the risk.

There is a lot in the forum about galvanic corrosion and it never seems to come to a satisfactory conclusion. Perhaps there are too many variables dependent upon local conditions.

One of the aspects that could be discussed more is the benefit or otherwise of connection electrically of skin fittings.

 

[2Tizwoz]

Further reading from the University of Plymouth site with electrical potentials