Although there was at least one close call for the class leader Havmanden during World War I, none of the six boats was sunk in service.

The Havmanden class served as the basis for the Austro-Hungarian Navy's U-20-class submarines built during World War I.

[1] In May 1911,[1] while the first two were still under construction,[4] the Navy ordered two more boats from Whitehead, Thetis and Triton, and plans for a final two, Najaden and Nymfen.

diesel engine for surface running, and twin electric motors for submerged travel.

[1] The boats were equipped with two bow torpedo tubes of 457 mm (18.0 in), but had no deck guns.

[1] After the outbreak of World War I, the Austro-Hungarian Navy seized plans for the Havmanden boats from Whitehead & Co. and used them as the basis for its four U-20-class submarines.

[5] Initially Havfruen, Havmanden, and Thetis received pennant numbers of H 1, H 2, and T 1, respectively, but were changed to 2, 3, and 4 in April 1913.

The other three submarines—2den April (the newly renamed Triton), Najaden, and Nymfen—received pennant numbers 5, 6, and 7 at the same time.

[6] Havmanden had a narrow escape in September 1914 when a British submarine mistook her for a German U-boat and launched an unsuccessful torpedo attack on her.

The plans for Havfruen were purchased in 1910 and the boat was built at the Copenhagen Navy Yard.

In September 1914, Havmanden, displaying her pennant number of 3 on her conning tower, was mistaken for the German U-boat SM U-3 by the British submarine HMS E11, but Havmanden escaped without damage when E11's torpedo missed its mark.

[1] The plans for Nymfen were purchased in May 1911 and the boat was built at the Copenhagen Navy Yard.