In Byzantine times commercial vessels decreased in size, becoming much smaller than ancient Greek and Roman ones. Economy and flexibility were the reasons behind this change, as ships had to meet the needs of the time. Loaded with large quantities of cargo, they had to travel fast on long voyages over rough seas, in constant fear of pirates.  The decrease in size did not always mean they were smaller in capacity - text descriptions tell us there were ships that transported very large quantities of wheat to Constantinople.

In addition to written texts, shipwreck investigations are a valuable source of information on how ships were made. These include the Yassi Ada shipwreck off Ancient Halicarnassus, opposite Kos, which has been dated to the early 7^th century on the basis of coins found on it, and the Serçe Liman shipwreck on the peninsula opposite Symi, which dates to around 1025. Yassi Ada was a trading ship about 20 metres long and 5.3 metres wide, with a cargo capacity of around 60 tons. This wreck is extremely interesting, as it was built in two different ways. One part was made in the Greco-Roman way, where ​​the outer shell (the planking) was made first and then the frame fixed to the inside – this would be like us building house walls and putting in the supports afterwards. The rest of the ship was built using the medieval technique, where the frame was made first and then the external panels attached to it.

Another significant change in the field of shipbuilding was the invention of the single rudder, which steered the boat much more easily than the one or two large oars earlier used near the stern. This change must have taken place ​​before the 10th century, both in Western Europe and in the Arab controlled Red Sea area. The widespread use of triangular sails went hand in hand with the building of even lighter commercial vessels called lateens. By the 12th century lateens were well established in the Mediterranean, and from there they spread to northern Europe. These changes enabled ships to make best use of the wind, made rowers almost unnecessary and allowed longer voyages possible.

As far as navigation is concerned, we know that there must have been specialized sailors who calculated distances from points on land or based on the position of the sun and stars, using an astronomical instrument known as an astrolabe. Information on ports, coastlines, markets and the products of each region were contained in special books called periplous (circumnavigations); the only ones that have survived are the journey around the Red Sea (in the 3rd century) and that around the Black Sea (in the early 6th century). We are unsure whether there were maps giving details of set routes, reefs, distances between ports, etc., as the oldest known portolan charts (sailing maps) are from the 16th century.
 
Warships and warfare
Warships in Late Antiquity followed the shipbuilding tradition of the Greco-Roman world. Surviving written sources mention small, manoeuvrable ships called liburnians, with a row of fifteen rowers on each side.

In the 6th century the dromons (“runners”) made their appearance. According to the historian Procopius , these took their name from the speed that they could reach. Dromons were the main Byzantine warships in the 10^th century. They were 40 metres long, with two sets of oars and one or two masts: a central one with a rectangular sail and a secondary one bearing a triangular sail, which was used for fast sailing when not giving battle. They were the largest ships in the fleet and had a crew of up to three hundred men. Those manning the bottom row of oars were regarded as true oarsmen, whereas those above were more likely marines. The captain’s cabin (kravvatos) was located in the stern; from there, protected from the enemy arrows, he had a full picture of how events were unfolding. Shields fixed were fixed at particular points on the ship’s side to protect the marines. The dromons had a raised corridor with war machines such as catapults, crossbows and cranes for firing projectiles at enemy ships during battle. At the base of the main mast there was an enclosed wooden tower called the xylokastron, from which soldiers could hurl stones, iron rods and clay vessels full of lime, snakes, lizards and scorpions at the enemy. A catapult for launching containers of Greek fire was mounted in the prow. The ram on the front of ships was no longer used from the middle of the 7th century; it reappeared after the 14^th century, when warships were equipped with cannon. The dromonia were auxiliary ships used for reconnaissance and special missions. They were shaped like the dromons and had one or two masts with triangular sails, but were not armed. Other small boats were the chelandia, with two rows of oars and one or two masts with triangular sails for greater speed.

Information on Byzantine tactics in naval battles is given by Leo the Wise in his book Naumachika (The Naval Battles). The head of the fleet avoided confrontation with the enemy if weather conditions were bad, since his main concern was to protect the crew and ships.

Battle formation depended on circumstances – a straight line was used to attack head on, but otherwise ships formed a semicircle, with the admiral’s dromon in the centre and the larger dromons at the edges, to avoid being encircled by the enemy. When the signal to attack was given, each ship would speed up to draw close to the enemy fleet, having a particular ship in its sights. On approaching the soldiers would try to break down the enemy defences, while the captain would give orders to draw up alongside the enemy ship and tie up to it, to enable the soldiers to board and begin fighting man to man.

Neorion, the military port at Constantinople, was in use for as long as the Byzantine Empire existed. After the 7th or 8th century there were local naval bases and shipyards for building new ships and repairing old ones. Constantinople still had a shipyard in the 10^th and 11^th century, and small dockyards (shipyards) must have existed in regional ports.