Pete,
Out of all 3 alternatives, I would use wrought iron. There are iron plated
ships still in service in excess of 100 years old. Bronze is OK if you do
not have other metals in use below the waterline. Iron is non-homogeneous
(it is not an alloy) (no internal batteries) and is the least reactive
electrolytically. In all cases I would increase the rod size to 30mm and not
worry about them forever. Use plenty, it's cheap insurance.
Steve

"pete" wrote in message
...
Thanks for your replies, I did think that A4 was equivalent to 316
but was nver sure and am pleased some people confirmed it.

Steve, thanks for your input, you did press this point on to me (and a
few others I believe) before, and I hear what you are saying,, I am
going to see if I can find suppliers of the alternatives, but how
would this affect the size of the bolts? For example, for my keel, the
architecht has specified 20mm diameter steel, need this be increased
if using bronze?

I do vividly remember Drum, plus a few others, the most recent being
Hooligan V a few months ago in the UK with the loss of a crewman, but
that, in fairness, was a case of the keel itself snapping in two at
the point of entry into the "letterbox" slot in her hull.


Ta very much, Pete


On Mon, 7 May 2007 19:12:56 +0200, "Steve Lusardi"
wrote:

Pete,
The DIN designation A2 is about the same as AISI 304 and DIN A4 is
equivalent to AISI 316. There are also variants 316 L (2% Molybdenum) and
316 TI (Titanium). Both A4 and 316 have superior anti corrosion properties
over A2 and 304. The 316 variants have even better anti corrosion
properties
and are primarily directed at the chemical industry. Type 304 / A2 are
used
in fresh water environments and A4 / 316 in salt water. As far as
strength,
all stainless bolts have less strength than a Grade 8 high strength bolt,
but slightly better than a Grade 5. Most stainless fasteners are made from
8-18. This variant has the corrosion resistance of A2/304, but has
superior
strength.

In application, stainless bolts used in threaded stainless holes(nuts)
have
a high tendency to gall and seize under load. Try not to do this. If the
threat of corrosion is significant, use a different alloy nut than the
bolt,
never use 304 with 304 or 316 with 316 and always use an anti-seize
grease.
Understanding the corrosion failure mode of stainless is very important as
well, before deciding on its use. Stainless steel relies on absorbing free
oxygen on its exposed surface not to corrode. If a stainless bolt is
exposed
to stagnant water AND is subject to even the slightest working motion
where
the surface is rubbed, the surface oxygen will be removed. The surface
will
then attempt to re-absorb free oxygen from its environment. As long as its
environment has free oxygen, its corrosion resistance will continue.
However, if the environment now has a depleted supply of free oxygen, then
crevice corrosion starts and it is very rapid.

So the gamble is the bolt never gets wet or is continually flushed with
new
water or the bolt never moves after installation. In the case of keel
bolts,
your odds are not good for a favorable environment. Remember "Drum" in the
Solent in the '80s or the British gentleman in the BOAC off the coast of
Australia in the '90s. These are the ones we hear about. It's very
difficult
to report a failure when you're dead and the boat is sunk. I love
stainless,
but not for keel bolts, Van der Stadt or otherwise. Think wrought iron,
manganese Bronze or even Silicon Bronze, but not stainless.
Steve