The name "flyboat" is derived from Dutch "vlieboot", a boat with a shallow enough draught to be able to navigate a shallow "vlie" or river estuary, such as the Vlie. Armed flyboats were used by the naval forces of the Dutch rebels, the Watergeuzen, in the beginning of the Eighty Years' War, and comprised the Dutch contribution to the English Armada. The type resembled a small carrack and had two or at most three masts, a high board, and a dozen iron cannons. Small, inexpensive, and manoeuvrable, it was ideal for privateering activities in the European coastal waters, and was soon imitated by privateers or pirates of other nations. The Dutch navy, and their enemies, the Dunkirkers, at first extensively employed flyboats. In 1588, the army of Alexander Farnese was blocked in Dunkirk by a fleet of 30 Dutch flyboats commanded by Lieutenant Admiral Justin of Nassau, preventing him from joining the Spanish Armada to invade England.