HNLMS O 21 was the lead vessel of her class of submarines constructed for the Royal Netherlands Navy during the 1930s, designed for European maritime operations. She was built at the Koninklijke Maatschappij De Schelde shipyard in Vlissingen, with her keel laid on November 20, 1937, and launched on October 21, 1939. The submarine was hastily commissioned on May 10, 1940, immediately following the German invasion of the Netherlands, despite being incomplete at that time. She was subsequently transported to Rosyth Dockyard in England for final outfitting. The O 21-class submarines measured approximately 255 feet (77.7 meters) in length, with a beam of 21 feet 6 inches (6.6 meters) and a draft of 13 feet (4.0 meters). They displaced 987 long tons (1,003 metric tons) on the surface and 1,488 long tons (1,512 metric tons) when submerged. The crew comprised 60 officers and enlisted men. Powered by two Sulzer 2,500-brake-horsepower diesel engines for surface travel and twin 500-horsepower electric motors for submerged operations, O 21 could reach speeds of 19.5 knots on the surface and 9 knots underwater. Her operational range was notable, capable of covering 10,000 nautical miles at 12 knots on the surface and 28 nautical miles submerged at 9 knots. She had a maximum diving depth of 330 feet (100 meters). Armament included eight 21-inch (533 mm) torpedo tubes—four in the bow, two in the stern, and two amidships on an external rotating mount—each with reloads. Defensive armament comprised two 40 mm Bofors anti-aircraft guns mounted on retractable, watertight mounts within the conning tower. During World War II, O 21 saw active service around England, the Mediterranean, the Indian Ocean, and off Fremantle in Australia. She achieved a significant victory by sinking the German U-95 in the western Mediterranean on November 28, 1941. The submarine survived the war, remaining in service until her decommissioning on November 2, 1957. She was sold for scrap in 1958 to V. Beekum in Alkmaar, marking the end of her notable maritime career.

This description has been generated using GPT-4.1-NANO based on the Vessel's wikidata information and then modified by ShipIndex.org staff.