As regards rivets, I posted this on the THS group recently:

'Some anecdotal evidence as regards rivets comes from the annual
surveys and inspections that were carried out, by law, by the Board of
Trade when these liners' passenger certificates were issued. When we
are dealing with any vessel of this sort, over a long period -- as you
say -- there is inevitable wear and tear in terms of routine
maintainence. This involves the occasional caulking or renewal of
rivets and Olympic was no exception. Looking at these inspections for
Aquitania, Berengaria and Olympic in the early to mid-1930s, I don't
recollect any occasion where Olympic needed more than two hundred
rivets requiring attention; whereas with the two slightly younger
Cunarders -- and I am thinking here of Berengaria -- sometimes over
5,000 rivets required renewal or caulking. As I recall, the two
hundred rivets in Olympic's case were near the central propeller
aperture -- an area exposed to a great deal of stress in terms of the
hard service over twenty-four years.'

There's a lot of confusion about doubling. If you imagine the ship's hull (up to the underside of B-deck) as a structural girder, to obtain the necessary strength you need to mass material at the lower part of the girder (i.e. the keel, double bottom and turn of the bilge), and the upper part of the girder (i.e. the sheerstrake and strength deck). Amidships, Olympic and Titanic's plating at the sheerstrake level was doubled and generally two inches thick. The structure amidships is the strongest as the hull has to cope with sagging and hogging stresses, all while being driven through the most severe storms. Open spaces such as the reciprocating engine room have compensating strength in terms of additional and stronger pillars within the hull structure.

In terms of the double skin installed on Olympic and Britannic, in structural terms it wasn't really a continuation of the double bottom -- it did not add strength to the hull as the double bottom did. However, the double skin did add additional watertight protection and was probably the best that could be done with the existing hull.

Best wishes,

Mark.
http://www.markchirnside.co.uk