In the post World War 2 world, a rebuilding Denmark required a modern undersea fighting force on par with its regional neighbors with the primary enemy to the West being the Soviet Union. This led to the design, development, and construction of four new submarines under the "Delfinen-class" name. The boats were Delfinen, Spaekhuggeren, Tumleren, and Springeren and these units were built from the period spanning 1954 to 1964. Commissioned service spanned from 1961 to 1990. Of the four, three were financed locally by the Danish government with the fourth made possible by a contribution from the United States (via "Cost-Share").
The class displaced 580 tons when surfaced and 660 tons when submerged. Overall length reached 176.9 feet with a beam of 15.4 feet and a draught of 13 feet - giving it good qualities to operate in the shallower waters off the Danish coast. Power was from 2 x B&W diesel engines outputting 1,200 horsepower for surface travel and 2 x BBC electric motors of 2,100 horsepower for undersea work. Maximum surfaced speeds reached 13.5 knots while undersea speeds maxed at 16 knots. Range was a useable 4,000 nautical miles.
Aboard was a crew of 33 men made up of seven officers and twenty-six enlisted. Armament became 4 x 533mm (21") bow-facing torpedo tubes with 8 x Reloads carried.
The profile of the boats was traditional, with a sail set at midships, a contoured bow, bulged hull sides, and tapered stern. A cruciform tailplane pattern was used. The dive planes were positioned at the upper sections of the forward hull area.
Lead-boat Delfinen was launched on May 4th, 1956 and entered commissioned service on August 1st, 1961. She managed a healthy service life until August 2nd, 1963, at which point she was decommissioned. Her stripped hulk was then sold for scrapping in September of 1984.
Spaekhuggeren was launched on February 20th, 1957, commissioned on August 1st, 1961, and served until decommissioned on July 31st, 1989. She was sold for scrap on December 3rd, 1991.
Tumleren followed on May 22nd, 1958, was commissioned on August 1st, 1961, and decommissioned on August 6th, 1981. She was sold off on September 7th, 1982.
Springeren was launched on April 22nd, 1963, commissioned on October 22nd, 1964, and decommissioned on March 31st, 1990. She became the only boat of the class to be saved from the scrapman's torch for she was given over to the Aalborg Maritime Museum to be preserved.
All of the boats were built by the Naval Dockyard of Copenhagen under "Project UB51". They represent the last indigenous submarines to emerge from Denmark. The modern Royal Danish Navy fields no submarines (2019).