Hi Derek
About 45rpm is the normal cruising speed - if I get on board this year I'll come back with a confirmation, but you'll have to wait till May as its midwinter here.
A few years ago I was lucky enough to see the engines opened "flat out" - that was either 56rpm or 58rpm - looked very impressive.
In her last days of Cal Mac ownership (early 1970's), before preservation, 35rpm was more normal - on too many cruises "time ashore" at the destination was not possible as she was so late she had to come straight back.
Regards and belated happy New Year to all
David