In the days before the Battle of the Downs, Maarten Harpertszoon Tromp had chosen a position off the coast near Dunkirk. De Broeck was one of his captains on this occasion, commanding the 67-man, 10-gun Frisian roeifregat "Rotterdam", and he was sent by Tromp to investigate the Spanish fleet reported at La Coruña. On receiving Broeck's report on it, Tromp decided to attack and in the ensuing battle Broeck was sent to Calais to bring back new supplies of gunpowder to the Dutch fleet. Having done so, Broeck rejoined the fleet and managed to capture a 140-man 18-gun Spanish galleon.