 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 s.

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.Sources do not report when any of the "Havmanden" boats were commissioned, but all boats were active for neutral Denmark during World War I.