Among the many miscellaneous volumes in archive of the Livonian Governor General quite a few from the 1640's stand out. In one of them is preserved a letter from Carl Gustaf Wrangel (1613-1676), written shortly after the naval battle of Femarn on 13 October 1644. Wrangel's letter book from this period is available online (subscription needed), but this report is not included.
In the letter Wrangel describes how he after leaving Kalmar searched for the Danish fleet first near Bornholm and then near Mön, but without success. He then sailed to Wismar, where the fleet anchored on 8 October. In the evening of the 9th some of Wrangel's scout ships returned with the news that the Danish fleet was stationed between Langeland and Laaland. Due to unfavorable winds Wrangel was unable to sail until the 11th. He soon discovered the Danish fleet near Femern. It consisted, Wrangel writes, of 18 ships. As the hour was late he anchored. The following day there was a storm, so Wrangel was unable to attack. On the 13th the weather cleared up. Wrangel set sail and went away from shore, trying to gain the advantage. He then attacked the Danish Admiral (Pros Mund on Patientia). The battle was fierce, Wrangel writes, and lasted from10 a.m. to 4 p.m. Patientia and Oldenburg were captured by the Swedes and the rest of the Danish fleet fled and were pursued by the Dutch. They soon captured four ships. The Danes lost about 1,000 men, Wrangel writes, and the Swedes only 60. The Dutch had lost one ship.
Source: LVVA, fond 7349, op. 2, vol. 155