Eventually I managed to get 6 or 7 quinquiremes stacked together. Every turn though, even when not in combat, I lose up to 3 ships from each unit. So my 6 x 50 goes down to 6 x 47, to 6 x 46, and so on. This happens every turn, without any fighting, and it happens even when I have the ships in a port retraining. I put a damaged fleet of 6 ships in, pay to get them all back up to 50 each, and one turn later there we have it: a retrained full stack of 6 full-strength...er....47-ship units.
It's a pain in the neck because if you use the M command to merge them again post-action, you steadily erode the number of units in the stack. Last night my quinquireme superstack destroyed three Greek fleets in succession but when I hit M there were only 4 full units left. Next turn after no fighting they're down to 47 ships each again. What's happening??