O 21-class submarine
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 . Design. One of the requirements  the Royal Netherlands Navy set for the "O 21"-class was that it had to be able to dive deeper than the previous class, which was the "O 19"-class submarine. Construction. The ships were built by three different shipyards. "O 21" and "O 22" were built by the "Koninklijke Maatschappij". "O 23", "O 24", "O 26" and "O 27" by "R.D.M, Rotterdam" and "O 25" also in Rotterdam at "Fijenoord" shipyard.