Jonathan Dimbleby's "The Battle of the Atlantic" and the Battle of the River Plate

I am currently reading Jonathan Dimbleby's "The Battle of the Atlantic", and right from the start I had an uneasy feeling about his interpretation of events. Nothing definite, it would have taken days of research to confirm of refute the statements I thought looked suspect.

Anyway, I read away, until this morning (2016-08-31) I came to the part where he discusses the Battle of the River Plate. I stopped in my tracks at the statement:

Harwood's decision to attack the German battleship was audacious*: a broadside from the Graf Spee outweighed the combined fire-power of the three British cruises by a factor of more than two to one.                                                                                                         

The wording of this was something I did not like, first the ambiguity of language. The weight of a broadside is relatively unambiguous, but we are comparing it with the "combined fire-power" of Harwood's ships. I suppose he means the combined weight of their broadsides? If so the comparison is at least consistent (comparing like with sort of like), but is it true? What about the more appropriate weight of ordnance potentially deliverable per unit time? (I will pass over in silence -or not- my dislike of his using the term Battleship to describe GS, Panzerschiff, Pocket Battleship or Heavy Cruiser would have been acceptable, but not Battleship)

At this point I decided to do some calculations, as I was not at home with access to reference books I decided to use the NavWeaps site as source for the needed data on the weapons (and memory for numbers of barrels of each). The spreadsheet with the relevant calculations is shown below:

                                                                                                                                  

 Here we see that the total mass of ordnance potentially delivered per minute by the RN cruisers is substantially greater than that from GS (7663.2 kg/min as opposed to 5587.2 kg/min). Also the vanilla weight of broadsides does not favour GS by the alleged factor of two.

A word of caution is needed with respect to the rate of fire figures used, NavWeaps gives a range for those other than the German 11", I have chosen to use values towards the bottom of the range, if not the figure they quoted as achieved.

I would not claim this analysis proves the superiority of one side or the other, but does raise the question of the adequacy of the research behind the book.

*Maybe Harwood had consulted the shade of John Byng on the wisdom of caution?