August 1851. In 1855, Jachmann was promoted to the rank of Korvettenkapitän (Corvette Captain). On 3 December 1856, he was appointed as the Director of the Technical Department of the Admiralty. Jachmann was promoted again in 1859, to the rank of Kapitän zur See (KzS—Captain at Sea). By this time, Jachmann was the third highest-ranking officer in the Prussian Navy, behind only Prince Adalbert of Prussia and the Swedish captain Henrik Sundewall. From 1859 to 1862, he commanded the frigate during the Eulenburg expedition to East Asia. Sundewall, the squadron commander, flew his flag aboard the frigate ; in addition to Thetis, his command also included the schooner and the clipper ship Elbe. The ships, without Elbe as she sailed to Singapore directly, sailed to Rio de Janeiro and then to Cape Town before meeting with Elbe in Singapore. There, the squadron embarked the diplomat Friedrich Albrecht zu Eulenburg. On arriving in China, Eulenberg concluded a trade treaty with the Qing government. On the way back to Germany, Jachmann had been instructed to investigate Patagonia as the site of a possible German colony, but he ignored the command, citing the poor condition of his ship after the long voyage to East Asia, and in particular after a severe storm had badly damaged her in South American waters. Indeed, Jachmann had to put into Salvador, Bahia in Brazil for repairs. Wars of German unification At the start of the Second Schleswig War in 1864, the bulk of the Prussian fleet was concentrated in the Baltic Sea. Prince Adalbert was the senior navy commander, and Jachmann, as the senior-most captain, became the squadron commander, with Arcona as his flagship. The much larger Danish fleet blockaded Prussia's main ports, trapping Jachmann's ships—Arcona and —in Swinemünde. On 17 March, Jachmann took his two ships and the armed paddle steamer out to challenge the Danish blockade force. In the ensuing Battle of Jasmund off the island of Rügen, Jachmann's squadron was defeated by the more powerful Danish squadron and forced to retreat. This was to be the only major naval battle fought in the Baltic during the war. Nevertheless, he was appointed Konteradmiral (Rear Admiral) for his aggressive handling of the fleet at Jasmund. The arrival of a Danish ironclad warship precluded any further attacks on the Danish fleet, as the traditional guns that armed Jachmann's ships were entirely useless against the armored vessel. On