The O 21 class was a class of seven submarines, built for the Royal Netherlands Navy. The boats were still incomplete at the start of the German invasion of the Netherlands O 21, O 22, O 23 and O 24 were hastily launched and escaped to the United Kingdom. O 25, O 26 and O 27 were not able to escape and were captured by the German forces. The Kriegsmarine ordered the completion of the boats and they entered German service as UD-3, UD-4 and UD-5. The submarines diving depth was 100 meters (330 ft).
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