What the third turret gave Renown's gunners

Would twelve proven 15-inch guns have been more plausible for Agincourt, and why did the extra turret matter on Renown?

An 18-inch ship could answer American gun development, while triple 15-inch mountings should have been useful on Courageous if available. Renown's third turret supported a more effective ranging and firing sequence.

More experience with the 15-inch gun makes a twelve-gun Agincourt an understandable proposal, but the counterargument connects an 18-inch ship to competition with American 16-inch development. Existing triple 15-inch mountings would also have been valuable on Courageous and Glorious, giving them six guns instead of four. Their absence is an indirect clue against that turret programme. Six guns are treated as the practical minimum for effective long-range firing. Renown's forward guns could establish the range before the after turret joined the engagement, helping explain the advantage of one extra mounting.