In November 1758 to April 1759, he was charged with a timber inspection cruise along the coasts from Turku up to the Gulf of Bothnia. Later, Chapman moved first to Stralsund (then a Swedish possession) where he stayed until 1762, and later to Sveaborg where he stayed until 1764. His first major assignment came in 1760. The recently-formed archipelago fleet ("skärgårdsflottan"), an inshore fleet independent of the navy under the command of the army, was in need of new vessels to replace the galleys which had proved problematic in the war against Prussia that had broken out in 1757. Augustin Ehrensvärd, the commander of the archipelago fleet and the man in charge of the construction of the new naval base and fortress of Sveaborg, began a successful collaboration with Chapman in designing new types of what would later be called "archipelago frigates" ("skärgårdsfregatter"). Inspired by Russian "chebecks" (variants of Mediterranean xebecs, hybrid sailing ships that incorporated features of galleys), the two created craft that could be rowed, but with heavier armament and additional protection for the crew, a necessity in the cold Baltic climate. The cooperation resulted in four new types of archipelago frigates: , , and hemmema, all named after the Finnish names of Swedish provinces in Finland, the coasts of which they were intended to protect.