The Vikings also fought several sea battles among themselves. This was normally done by binding the ships on each side together, thus essentially fighting a land battle on the sea. However the fact that the losing side could not easily escape meant that battles tended to be hard and bloody. The Battle of Svolder is perhaps the most famous of these battles. As Arab power in the Mediterranean began to wane, the Italian trading towns of Genoa, Pisa, and Venice stepped in to seize the opportunity, setting up commercial networks and building navies to protect them. At first the navies fought with the Arabs , but then they found themselves contending with Normans moving into Sicily, and finally with each other. The Genoese and Venetians fought four naval wars, in 1253-1284, 1293-1299,1350-1355, and 1378-1381. The last ended with a decisive Venetian victory, giving it almost a century to enjoy Mediterranean trade domination before other European countries began expanding into the south and west. In the north of Europe, the near-continuous conflict between England and France was characterised by raids on coastal towns and ports along the coastlines and the securing of sea lanes to protect troop-carrying transports. The Battle of Dover in 1217, between a French fleet of 80 ships under Eustace the Monk and an English fleet of 40 under Hubert de Burgh, is notable as the first recorded battle using sailing ship tactics. The battle of Arnemuiden , which resulted in a French victory, marked the opening of the Hundred Years War and was the first battle involving artillery. However the battle of Sluys, fought two years later, saw the destruction of the French fleet in a decisive action which allowed the English effective control of the sea lanes and the strategic initiative for much of the war.
Question: During the Battle of Dover, how many more French ships were there than English ships?
Answer: 40