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Hitherto the main area of maritime activity had been the Cinque Ports of Sandwich, Dover, Romney, Hythe, and Hastings, to which were added the 'ancient towns' of Rye and Winchelsea. In return for privileges granted by the Crown, the Portsmen undertook to provide fifty-seven ships for fifteen days, free of charge. These formed the nucleus of any fleet that put to sea for warlike purposes, as at the time of the loss of Normandy by King John in 1204, or during the earlier part of the Hundred Years War (1337-1453).
Such fleets were improvised and manned by impressed men as a feudal obligation, because there was no standing navy. Nor were there any royal warships, as distinct from armed merchant vessels. Most of the fighting was done by soldiers because tactics were primitive -- ramming, grappling, and boarding -- while the sailors (hence the word) managed the sails and the rigging. In wartime, in order to obtain the advantage of height, temporary 'castles' were erected at bow and stern, the latter becoming the poop which was built into the hull when the rudder was invented. When a crisis was over, the castles were dismantled and the fleet disbanded.
The broad-beamed, single-masted cog or balinger used by northern mariners underwent a transformation in the fifteen century, when a two-masted and then a three-masted vessel, sometimes as much as 1,000 tons, appeared. Few details are known about this critical stage in the evolution of what in time became the nef, 'great ship,' and later the galleon. As time went on the sail-plan was improved by the addition of a topmast, a foremast, a mizzen mast (rigged with a triangular lateen sail) and a spritsail below the bowsprit. The clumsy medieval round ship was thus slowly transformed into a capacious and manageable vessel which retained the high castles at either end as integral parts of the hull, making her capable of oceanic voyages.
Navigation remained largely a matter of using the sounding-line and lead, until the compass was introduced from the Mediterranean at the close of the twelve century. From the fifteenth century may be dated charts and sailing instructions, variously called rutters, routiers, or pilots. A simplified version of the astrolabe was occasionally used for taking sights.