geo, you were right. It works if my ship has shield depleaters and it does not - without any weapon assuming there is no SDD.
-BUT-
If enemy has no shields and my ships have no shield depleaters - it also works. My ships capture as many ships as they can and then ram others if there are any left
I've noticed an interesting thing: let's say I capture enemy ship that has some beam weapons but there are still enemy ships left and attack my rammers. The captured enemy ship runs away and waits until the ship that captured it is destroyed and only then gets involved into the battle. Maybe this is not a strict rule and happens not always but it really has some influence upon combat end IMHO.