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