I think Jim Conlin has the right take: If it aint broke, it may not need
fixing. If the boat was sailed about the same way you will so (or
harder) and the screws worked well, they'll work fine again, assuming
the materials haven't deteriorated. You can fill the existing holes
slightly with wood slivers and epoxy, drill them (correct size for
screws) again if needed, and re-install as the builder did it (using new
fasteners compatible with the chainplates).

Using a hole-saw from the inside to access the bolts might work OK, but:

1) Before you use the hole-saw, find if there is any stuff behind the
ceiling that you might not want to cut through (wires, hoses, frames).
Bend a coat hanger at 90* 3-6" from one end and stick it into the 1/4"
hole and rotate it in the cavity to see what it encounters.

2) If you put any 5200 on the threads, you might as well WELD the nut
onto the bolt - you won't be moving it again without lots of special
effort. This is not a good thing, since bolts have been known to need
re-tightening. Also, note that bolts can crush wood fiber and greatly
reduce it's strength. Get some advice before you crank down with a 3/4"
drive breaker bar.

Rufus

Neil wrote:

I am renovating a 28 foot wooden double ended sailing boat.
The chain plates are screwed to the outside of the hull (stripped planked).
I would like to fasten the plates with bolts though the hull but this would
mean removing tidy looking ceiling and that in turn mean removing the
bulkheads, furniture etc.
There does not seem to be any problems with the screw fittings at the chain
plates but my question is, would i be wasting alot of time fastening the
plates with bolts instead?
thanks
neil