Two seas, two traditions
Medieval ships were not one design. The main traditions grew up around the northern seas, the Baltic and North Sea in particular, and around the Mediterranean, and each reflected local conditions, timber, trade and warfare. Records of ships are patchy, so much of what we know comes from archaeological wrecks, carvings, seals and manuscript pictures, and historians interpret them with caution.
Northern building: clinker hulls and cogs
In the north, boats were traditionally built shell-first. Builders laid down the keel, then attached overlapping planks one above the other, fastening them with iron rivets or nails. This is called clinker or lapstrake construction. Only after the outer shell was formed were internal frames added. The tradition is well known from Viking longships, which were long, light, shallow vessels suited to rowing and sailing in rivers and open water alike.
Later, the cog became the characteristic trading ship of northern Europe, especially from the twelfth to the fourteenth century. Typical features included:
- a broad, flat-bottomed hull with high sides, giving large cargo space
- a single mast with one large square sail
- planking that was often clinker-built on the sides
- a straight stern-post carrying a hinged rudder, which replaced the side-mounted steering oar of earlier ships
- raised platforms at bow and stern, which could serve as fighting positions
Cogs were roomy and relatively cheap to operate with a small crew, which suited the bulk cargoes of the Baltic and North Sea trade. Merchant towns associated with the Hanseatic League relied on such ships. A well-known cog wreck found in Bremen shows how the design looked in practice, though the details varied from ship to ship.
Southern building: galleys and round ships
In the Mediterranean, shipwrights often used a different method. Instead of building the shell first, they erected a skeleton of frames and then attached planks edge to edge so that they sat flush, called carvel construction. In the later Middle Ages this gradually became common in the north as well, since it allowed larger ships with smooth hulls, though the exact timing varied from place to place.
Two main types dominated Mediterranean waters.
Galleys were long, narrow vessels driven mainly by oars, with a sail as a backup, and they were fast and maneuverable in calm water. They served as warships and also carried valuable, lightweight cargo and passengers. Their drawback was that rowers needed food, water and space, so they could not carry much freight or stay at sea for long.
Round ships, with a rounder hull and mainly sail-driven, carried bulk goods. Many used lateen sails, triangular sails set along the mast at an angle, which handle winds from the side better than a square sail. Merchants from cities such as Genoa and Venice grew wealthy using both types, and the Venetian republic maintained a state-run shipyard to build and equip its fleet.
Steering and rigging
The change from steering oar to stern rudder was important. A steering oar hung over the side was difficult to use on larger ships, while a rudder fixed to the stern-post gave better control and could be handled with a tiller. Over the later Middle Ages, ships also added more masts and more sails, which made them easier to handle and more efficient. By the fifteenth century, multi-masted vessels were becoming more common, which prepared the way for the larger ships of the following era.
Finding the way
Navigation in the Middle Ages relied heavily on local knowledge and simple tools, not on precise measurement of position.
- Coastal pilotage. Most voyages kept land in sight or stayed close to known coasts. Pilots memorized headlands, harbor entrances, tides and currents, and sailing directions were sometimes written down in books describing coasts and distances.
- The lead line. A weight on a rope was dropped to measure water depth. The weight could have tallow on the bottom to pick up a sample of the seabed, which helped sailors know where they were from the type of sand, mud or shell.
- Sun and stars. Sailors judged direction from sunrise, sunset and the pole star, though this was harder in cloud or fog.
- The compass. A magnetized needle on a pivot appears in European sources from about the twelfth or thirteenth century. Its origins are debated, with Chinese sources describing magnetic compasses earlier, and the route of transmission is uncertain. Its great advantage was that it gave a direction in overcast weather. It supported the practice of dead reckoning, in which a pilot estimates position from the compass heading, speed and time elapsed.
- Sea charts. From the late thirteenth century, Mediterranean sailors used portolan charts, which show coastlines and ports with crisscrossing lines to help plot courses by compass. They are quite different from the symbolic world maps of the period, in that they were practical tools. Their accuracy for the Mediterranean was often impressive.
Measuring latitude with instruments such as the astrolabe or quadrant was more common toward the end of the Middle Ages and the early age of exploration, and was awkward on a rolling deck.
Risks and craft
Seafaring was dangerous. Storms, shoals, piracy and poor visibility were constant risks, and ships were often laid up in winter. Shipwrights learned their skills in family workshops and yards, and they used simple tools such as adzes, saws and augers, together with rules of thumb that fixed the proportions of a hull. They worked mostly in oak and, for masts, softwood such as pine, and they sealed seams with materials like pitch and caulking fibers.
The achievement of medieval shipwrights was less any single invention than the steady refinement of hull forms, rudders and rigs for particular trades. Together with growing practical navigation, this made long-distance sea trade across the North Sea, the Baltic and the Mediterranean regular and profitable, and laid a base for later voyages.