Chris, you might be referring to the old wood shaft bearings. They employed staves of a very hard wood called lignum vitae which supported the tail shaft between the propeller and the engine room. Sea water, as a lubricant, was allowed to flow past the gland into a bilge-well in the engine room, from where it was pumped overboard. Lignum vitae bearings were largely replaced by oil-filled white metal stern bearings in the 1970s and 80s. The seal at each end of the tail shaft was effected by sets of O-rings, with a small oil header tank in the engine room which maintained the oil pressure and so prevented the ingress of seawater. I was an engineer on a ship that had this conversion work done in dry-dock in Cardiff in 1976, and it was interesting to be involved in it.
Just lately, I see that a return to seawater lubricated bearings is contemplated, though it is unclear to me whether lignum vitae would once again be utilised. I'd like to see details of HMS Queen Elizabeth's arrangements.
And you're right...it's HMS, not the HMS :O)
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