The early U-boat arm depended on a tiny cadre

The early U-boat arm imposed severe physical and aptitude standards, and at first more than one tooth cavity could disqualify an applicant. From one cohort of 150 men, fifty passed the medical threshold and only twenty passed the later tests. This selectivity operated inside a force that was still very small at the beginning of the Second World War. Its experienced commanders and instructors were needed for patrols and for building the much larger wartime organization. Because one instructor might train twenty or thirty successors, an early casualty could remove an entire branch of future expertise. The comparison with German destroyer losses at Narvik shows how a few concentrated losses might have disrupted U-boat expansion before it gathered momentum.