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Subject: Re: [worldcruising] Sea Cocks
From: Bruce A Bowman (bruceb@XXX.XXX)
Date: Thu Jan 11 2001 - 18:28:50 EST
Any Marelon underwater product will absorb water and swell, so sea cocks
made of Marelon are susceptible. A Marelon sea cock needs to be
operated frequently and lubricated regularly to keep it operating. No
idea what water absorption does to the strength characteristics, but I'm
not aware of Marelon sea cocks that have failed other than to freeze
(become immovable) because of swelling and not being well lubricated.
You can say the same things about bronze sea cocks: if they don't get
used and lubricated, they're going to freeze. Plus they're susceptible
to galvanic corrosion if not properly protected and (excluding the
Raritan bronze sea cocks) should probably be disassembled and lapped on
occasion to assure that they seal properly.
My personal preference is bronze (I have no experience with monel or
stainless) unless there's a weight consideration (e.g. a multihull),
then it's Marelon. I wouldn't choose plain nylon nor any of the other
plastic products I've seen below the waterline.
Bruce
===========
Jim Manzari wrote:
>
> Eric Lamoray wrote:
> >
> > >From a matalurgical standpoint, stainless steel should never, EVER, be
> > used as a seacock below the waterline on any hull material.
>
> Stainless steel sea-cocks are common on all types of steel boats built in
> Holland and Germany. For instance, our steel boat uses stainless steel for
> sea-cocks and some parts of the rudder fittings. As I understand it, there
> are two basic types of stainless steel, active and passive. Passive
> stainless (304 passive and 316 passive) is very close on the galvanic
> scale to Monel and can be used anywhere Monel can be used. Naturally, both
> Monel and passive stainless need to be protected by zinc(s) of the correct
> size and placement.
>
> I've been told, but have not yet experienced this, the failure of sea-cocks
> on many steel boats usually occurs (if at all) when the mild steel
> stand-pipe corrodes -- not at the stainless or Monel seacock itself.
> Modern coating systems have pretty much made this a non-issue, provided
> routine annual maintenance is carried out.
>
> > The best bet for a steel hull is marelon (so called plastic, actually
> > nylon reinforced with fiberglass strands) followed by marine grade bronze
> > isolated with a neoprene or similar gasket.
>
> I've heard this recommendation often. I've wondered, however, if anyone
> can provide official numbers for the amount of water absorption with age in
> marelon? Does marelon expand when water is absorbed into the plastic?
> Does marelon retain its strength with age?
>
> Jim Manzari
>
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