In 1626 the Frisian Admiralty was exempted from its duty to deliver six warships, suggesting that not all was well in Dokkum financially and administratively. The admiralty, however, came into action under the command of Dutch supreme commander Lieutenant-Admiral Maarten Harpertszoon Tromp during the Thirty Years War to blockade the German Ems and Jade Rivers to prevent privateers working for the Habsburgs to leave port. During this blockade the Frisians even captured the freebooter "Du Mortier". In 1639 the admiralty equipped three ships and four yachts for the Republic. A few were sent out on scouting duties in the actions that would lead up to the Battle of the Downs, including a "roeifregat" (galley) from the Frisian Admiralty, the "Rotterdam", under command of Captain Joris Pieters van den Broeck. After it brought back its report, Admiral Tromp decided to give battle on 16 September, with a Dutch fleet of 29 ships against 67 Spanish ships. One of the largest ships in Tromp's fleet was a 70-man frigate with 22 guns under the command of the Frisian captain Tjaard de Groot. In the later Battle of the Downs Broeck defeated a 140-man, 18 gun Spanish galleon.