Four Days' Battle. The Four Days' Battle, also known as the Four Days' Fight in some English sources and as Vierdaagse Zeeslag in Dutch, was a naval battle of the Second Anglo-Dutch War. Fought from 1 June to 4 June 1666 in the Julian or Old Style calendar that was then used in England, in the southern North Sea, it began off the Flemish coast and ended near the English coast. It remains one of the longest naval engagements in history. Dutch accounts referred to its dates as 11 June to 14 June 1666 by using the New Style calendar. The Dutch inflicted significant damage on the English fleet, which lost ten ships in total, with over 1,000 men killed, including two vice-admirals, Sir Christopher Myngs and Sir William Berkeley, and almost 2,000 English were taken prisoner including a third vice-admiral, George Ayscue. Dutch losses were four ships destroyed by fire and over 1,550 men killed, including Lieutenant Admiral Cornelis Evertsen, Vice Admiral Abraham van der Hulst and Rear Admiral Frederik Stachouwer. Although the result was a clear Dutch victory, it did not render the English fleet incapable of further action, as it was able to prevent a Dutch attempt to attack and destroy it at anchor in the Thames estuary in early July. After quickly refitting, the English fleet defeated the Dutch fleet off the North Foreland on 25 July in the St. James's Day Battle. The introduction of sailing ships with a square rig, of a type later called the ship of the line, which were heavily armed with cannon, brought about a gradual change in naval tactics. Before and during the First Dutch War, fleet encounters were chaotic and consisted of individual ships or squadrons of one side attacking the other, firing from either side as opportunities arose but often relying on capturing enemy ships by boarding. Ships in each squadron were supposed to support those in the same squadron, particularly their flag officer, as their first priority. However, in the melee of battle, ships of the same squadron frequently blocked each other's fields of fire and collisions between them were not uncommon. Although Lieutenant-Admiral Maarten Tromp had formed a line against the Spanish fleet in 1639 in the action of 18 September 1639, this was not a planned formation but a desperate attempt to hold off a greatly superior, but badly organised, enemy. The initial sea battles of the First Dutch War were largely indecisive melees, but later in that war Robert Blake and George Monck issued instructions for each squadron to stay in line with its flag officer. At the Battle of Portland Tromp's attempt to overwhelm the English rear by concentrating his whole fleet against it and using his favourite tactic of boarding was frustrated by the English rear remaining in line ahead at the Battle of the Gabbard, the English fleet in line ahead forced the Dutch into an artillery duel that defeated their more lightly-armed ships with a loss of Dutch 17 ships sunk or captured. Between the first and second wars, the Dutch built some larger ships with heavier armament, although the shallow waters around the Netherlands prevented them building ships as big as the largest English ones: additionally, English ships of the same size tended to have more and larger guns than their Dutch equivalents. However, many of those the Dutch built were relatively small convoy escorts, frigates by English standards, and not all those planned had been completed or fitted out by the start of the war in 1665. At the time of the Second Dutch War, the English fleet also had a signalling system which, if still rudimentary, was better than the Dutch reliance on standing instructions to fight in line. In the Battle of Lowestoft and the St. James's Day Battle, the English fighting in line ahead defeated the Dutch who did not. De Ruyter favoured the tactic of concentrating his attack on a portion of the enemy's line, so achieving a breakthrough, and the capture of ships by boarding. However, in the Four Days' Battle the Dutch generally fought in line, and the English fleet did not do so, at important stages in the fighting. From early in the 17th century, the Dutch navy had used fireships extensively, and in the First Anglo-Dutch War at the Battle of Scheveningen, Dutch fireships burned two English warships and an English fireship burned a Dutch warship. The Dutch in particular increased the number of their fireships after the outbreak of the Second Anglo-Dutch War but, at the Battle of Lowestoft, it was two English fireships that burned six Dutch warships which had collided and become entangled with one another.