From 1633 to 1637 Collaart served as the Vice-Admiral of the Royal Spanish privateer fleet of Duinkerken.
On 14 August 1635 Collaart was able to sail out of Duinkerken with a fleet of 21 vessels past the weakened Dutch fleet that tried to block this port. The Dutch fleet was weakened because Lieutenant-Admiral Filips van Dorp was using part of the fleet to assist a French fleet in th Gulf of Biscaye. Three days after they left Duinkerken the privateers met with a fleet of 160 herring-boats sailing from Enkhuizen. This fleet was protected by only one men-of-war. This vessel (39 cannon and a crew of 85) was quickly dispatched. 74 herring-boats were set on fire or sunk. The rest managed to escape to the Isle of Vlie.
On August 19 the privateers chased a squadron of herring-boats near the Doggersbank (= sand-bank). The 6 men-of-war that were meant to protect this fleet fled without a fight. Fifty of the herring-boats were set on fire or sunk. Most of the rest could leave after giving up their fishing-tackle. About 150 seamen (the elderly, boys or wounded) were moved to a merchant vessel from Hamburg and sent back to the Dutch Republic. Most of the fishermen (about 775) were put into iron on board the privateer vessels.
Shortly after this the Staten-Generaal of the Dutch Republic ordered all available vessels to set out after the privateers. The vessels set sail from the port of Rotterdam under the command of Willem Codde van der Burch. He was ordered to sail to the Isle of Texel to join the fleet of Filips van Dorp which was returning from La Rochelle and consisted of 12 vessels. A fleet of 15 vessels under the command of Vice-Admiral Quast would meet them there also.
Three days later the squadron of Collaart met with the combined fleets of Van der Burch and Van Dorp which comprised 20 vessels. Four vessels of the fleet of Van der Burch were heavily damaged during the engagement. The arrival of the fleet of Vice-Admiral Quast made Collaart decide to flee from the battle, because he was heavily outnumbered. Bad weather combined with a sudden course change were sufficient to enable the privateers to return to Duinkerken on September 8 of 1635 with 975 fishermen on board. Many of the fishermen at home were outraged at the abominable performance of the Vice-Amiral Quast and the commander Van der Burch. The amount of damage by Collaart done to the Dutch ship-owners was considerable: 2 million Guilders.
Jacob Collaart was less succesfull on 29 February 1636 when he met a Squadron under the command of Jan Evertsen near Dieppe. Collaart was captain on board one of three privateers from Duinkerken. After a battle of 5 hours the ships of Collaart and Mathieu Romboutsen were taken. The third privateer managed to escape to an English port.
In 1638 Collaart became Admiral of the Spanish privateer fleet and submitted designs to the Spanish king to reinforce the harbour of Duinkerken. Because of a lack of funds these, and other plans, were not realized.