Hi Ben,
The keel bolts (actually studs and nuts) on Moodys were always of mild steel rather than stainless because of fears regarding the phenomenon of crevice corrosion which have long since been dismissed by most other yacht builders, although there is still a difference of opinion in this community. A consequence of this material choice is that any salt water getting into the bilge (e.g. when withdrawing the log impellor for cleaning) tends to start rusting on the exposed parts of the studs, nuts and backing plates, which always looks quite ugly but mostly can be fixed with a wire brush and some paint. When owners have chosen to draw the studs, they almost always find that there is absolutely no corrosion on the studs below the top of the backing plates.
OK, so I've used the weasel words mostly and almost. If the keel to hull joint has deteriotaterd to the extent that seawater can reach the studs from ouutboard, rather than the bilge, and this condition goes untreated for some years, the loaded part of the keel studs can corrode. I have only read of one account of this being found on a Moody, and it was on the Information Exchange part of this site. The stud concerned (I only saw a photo of a single stud) had lost a significant proportion of its cross section, but had not failed. as a result of this, I regard thesignificant rusting around the hull / keel joint as more significant than the rusty studs and nuts. As the top face of the keel rusts, it admits sea water further into the joint (rust is porous) allowing the rust to propagate until it meets the studs. You can defer the day you need to drop the keel by digging out the sealant cleaning out the rust and refilling with new sealant. However, in my mind, this will not cure the problem, but simply defer the need to drop the keel in order to remake the joint.
we did have the keel dropped and the joint remade on Wild Thyme in 2014, not because it was desparately needed, but because we were replacing the engine and, one of the keel studs on the Moody 425 is almost inaccessible with the engine in place and the one small patch of rust appearing in our 2 year old Coppercoat was at the keel / hull joint. To do this job, you need to remember that, when sat in a cradle, most of the weight of a yacht sits on the keel - the props are just to stop her falling over. You also need to be aware that the keel weighs tons ( 2.04 in the case of a Moody 34) so you need a good means of supporting both boat and keel while they are separated.
It didn't take me long to decide this is a job for the yard. They supported the boat in a travel hoist and handled the keel in a purpose built jig on wheels built for just this sort of job. We chose 316 stainless for the new studs, nuts and backing plates, the old studs came out looking perfect below the exposed part and there was little migration of the rust on the top keel face inwards, so it would have lasted many years more. However, since then, no more rust has appeared at the keel / hull joint.
I attach a couple of small photos the yard took when her keel was off:
Hope this helps,
Peter.