The Dutch flyboats mainly operated in the shallow waters off Zeeland and Flanders where larger warships with a deeper draught, like the Spanish and English galleons, could not safely enter. The Dutch enjoyed an unchallenged naval advantage in these waters, even though their navy was inferior in naval armament. An essential element of the plan of invasion, as it was eventually implemented, was the transportation of a large part of Parma's army of Flanders as the main invasion force in unarmed barges across the English Channel. These barges would be protected by the large ships of the Armada. However, to get to the Armada, they would have to cross the zone dominated by the Dutch navy, where the Armada could not go. This problem seems to have been overlooked by the Spanish planners, but it was insurmountable. Because of this obstacle, England never was in any real danger, at least from the Duke of Parma and the Army of Flanders. Because of the eventual English victory at sea, the Army of Flanders escaped the drowning death van Nassau had in mind for them.