The O 21 class sacrificed dedicated minelaying for a smaller, streamlined hull able to dive deeper and approach twenty knots on the surface. Four incomplete boats were towed to Britain, finished there, and served in the North Sea, Mediterranean, Indian Ocean, and Pacific. O 23 and O 24 completed numerous patrols and sank Italian and Japanese shipping. Three hulls that could not escape entered German service as UD-3, UD-4, and UD-5, where they achieved only limited results. UD-5 survived to return to the Netherlands and remained in service until 1959, giving the design a career that outlasted both wartime operators.
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O 21 class: advanced design divided between Allied and German service
The fast, deep-diving O 21 class entered the war among the world's most advanced submarines, but incomplete hulls split between Dutch and German service.