Normally the Frisian fleet was rather small, but in view of the emergency the province made a strong war effort, building 28 new vessels, Tjerk supervising the formation of the strongest naval force Frisia would ever send out.
In the Four Days Battle of 1666, Tjerk, now calling himself De Vries ("The Frisian"), was second in command in the squadron of the Zealandic Lieutenant-Admiral Cornelis Evertsen the Elder.