From a post to ALT.BOOKS.DAVID-WEBER dated October 29, 1998:

Effectiveness of cruisers-vs-battlecruisers

    In a broadside-to-broadside duel, a BC will eat the lunch of anything smaller. The Alvarez-class cruisers of the GSN mount weapons of individually comparable power, but fewer of them, and their armor and sidewalls remain far inferior. Three Alvarezes could probably take on a single Sultan-class BC, but the probable outcome would be the mutual destruction of all four units; three Star Knights would normally be blown apart by the BC in return for serious but not fatal damage.

    Despite comments by some contributors to the armor thread, there have been no instances in which DDs have blown pieces off of BCs. There have been instances in which missiles fired by a DD (which would also be the missile fired by a CL) have damaged a BC, but missiles are not broadside energy weapons, and the DD firing them doesn't have to come into broadside energy weapon range of the BC it is engaging. Remember that a laser head usually has to get within about 30,000 km to burn through a sidewall. Against a BC, a DD's or CL's laser heads would have to get somewhat closer than that to do damage (which gives the BC more time and opportunity to pick them off short of target), but if they do get to that range, then they can inflict damage. But the damage they inflict will be relatively light thanks to the BCs armor and general toughness. Thus when Hawkwing engaged the Peep BC in Honor Among Enemies, she was never able to inflict major damage on the BC. She was able to damage sensors, perhaps get a few lucky hits in on the bigger ship's beta or alpha nodes, and knock out a few weapons; she was never able to seriously endanger the BC or threaten it with destruction. What made the damage "serious" in the opinion of the BC's skipper was the distance between her ship and any repair facility, which meant that fixing even minor damage was going to be a serious task.