In 1637, Tromp was promoted from captain to Lieutenant-Admiral of Holland and West Frisia in 1637, following the dismissal of Lieutenant-Admiral Philips van Dorp, Vice-Admiral Jasper Liefhebber, and other flag officers due to incompetence. Although formally ranking under the Admiral-General Frederick Henry of Orange, he was the "de facto" supreme commander of the Dutch fleet, as the stadtholders never fought at sea. Tromp was mostly occupied with blockading the privateer port of Dunkirk. Both Tromp and Witte de With, who was named as Vice-Admiral, had been born in Den Briel and served as flag captains of Piet Heyn.