 Jachmann was the senior-most officer in the Prussian Navy by the mid-1860s, second only to Prince Adalbert of Prussia.In 1867, Jachmann became the director of the Ministry of the Navy, and the following year, he was promoted to "Vizeadmiral".During the Franco-Prussian War of 1870–1871, he commanded the squadron of ironclad warships based in the North Sea, though his ships saw no action owing to engine difficulties.