Thanks guys, that makes sense, especially Karg's CA explanation. After all, the first cruisers were quite lightly armoured as I recall, built for speed rather than protection. Then you slap armour on them, and they become both heavier and "armoured" - hence CA.
Excellent work!
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)