Nelson and Rodney were powerful, useful battleships, so F3 is an alternative opportunity rather than the obviously correct choice. F3 offered roughly 28 knots, nine 15-inch guns, and a 10-inch belt on a standard displacement of about 34,000 tons, placing it close to a fast battleship. The cited sources differ over whether the proposed weapon was a 15-inch/48 or 15-inch/50, and the counterfactual uses the latter. The immediate choice was therefore a different mix of speed, protection, and gun development rather than a weaker Nelson.
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The F3 option and a 15-inch gun development path
F3 would trade some protection for about 28 knots, retain nine 15-inch guns, and introduce a new gun-development path without making Nelson and Rodney bad choices.