Freya was the first privately built Danish submarine. It was built by Madsen in 2001-2002, as a demonstrator to try submarine technology. Having made over 500 dives, it was decommissioned, as it was not designed for a long service life. She was decommissioned in 2006 after UC2 Kraka had been worked up. She then docked and allowed to decay. Final decontamination was done, and she was towed out to sea by UC3 Nautilus, in August 2008, and sunk as an artificial reef near Copenhagen.
Madsen worked on the UC3 Nautilus, a privately built Danish submarine. It was launched on 3 May 2008 in Copenhagen, Denmark. It was constructed over a three-year period and it cost approximately US$200,000 to build (1.5 million in DKK).