Why 21 inches became the effective torpedo limit

Is 21 inches the practical limit for ship-launched torpedoes, and would shortening a 24.5-inch weapon solve its problems?

Twenty-two inches is about the outer practical limit, but 21 inches is the effective one; shortening a 24.5-inch torpedo would not cure its wider installation problems.

The absolute practical ceiling was near 22 inches, but 21 inches became the effective limit adopted by most navies. Shortening a 24.5-inch weapon would not remove the broader weight, handling, and installation problems that blocked it. Larger sextuple launchers could appear if torpedo requirements continued growing, but they would follow a demonstrated operational need rather than size alone.