The Type XXI program promised modular submarine mass production but never achieved reliable interchangeability. More than one hundred major design changes made individual boats differ, while prefabricated sections could be 32 millimetres out against a required tolerance of two millimetres. Subcontractors lacked alloy steel, precision tools, and specialist skills, and late procurement left steering gear, snorkels, valves, and pipes unfinished or missing. Final assembly yards spent an average of 120 days correcting production mistakes, three weeks longer than assembly itself. Of the hundred boats completed by 1945 in the book's account, only two reached the Atlantic and neither scored a hit.
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Why Type XXI modular production failed
Design churn, bad tolerances, late components, and weak quality control turned the Type XXI program's promised mass production into prolonged rework.