Building ships in the ancient world relied mostly on rules of thumb and inherited techniques rather than science. Early shipbuilders built the outer hull first, then proceeded with the frame and the rest of the ship while the planks forming the outer hull were sewn together. Building a ship (that would not sink) starting from the outer hull was quite a difficult task and required a lot of experience.
From the sixth century BC onwards, the locked mortise and tenon method rather than the sewing method was used to join the planks together and starting from the first centuries AD, Mediterranean shipbuilders shifted to another shipbuilding method which consisted of building the ship starting from the frame and then proceeding with the hull and the rest of the ship. This shipbuilding method (frame first, hull, then rest of the ship) is still the method being used today to build modern ships. It is more systematic and allowed the Romans to build ships on an almost industrial scale.
Sensing the threat that Carthage posed, a committee was set up in 311 BC to plan for the development of the Roman navy. The Romans captured a Carthaginian quinquereme that had run aground as it tried to block the passage of Roman ships on their way to Sicily. The Romans reverse engineered the ship to build hundreds of large quinqueremes. The Roman copies were however far from perfect: Roman quinqueremes were much heavier and less manoeuvrable than their Carthaginian counterparts.
Warships were built to be lightweight, very fast and manoeuvrable. Because they were so lightweight, they would often lay crippled on the surface, and not sink, after a naval battle to be later towed back to shore. Warships also had to be able to go near the coast which is the reason why they were flat with no ballast. They had a heavy spike usually made of bronze that was used to pierce the hulls or break the oars of enemy ships. They used both wind and human power, and had a square sail and a large number of oars on each side.
The trireme was the dominant warship from the 7th to the 4th century BC. It had three rows (the word trireme us derived from the Latin word "triremis" meaning "with three banks of oars") with rowers in the top, middle and lower rows, with approximately 50 rowers in each row. Contrary to popular perception and what is shown in many movies, rowers on military Roman ships were not slaves but mostly freemen of the provinces called peregrines (peregrinus) and Roman citizens enrolled in the army. The quadriremes (four rows of oarsmen) and quinqueremes (five rows) were even larger than the triremes. According to Polybius, the Roman quinquireme was 45m long and 5m wide which was truly big for its time. It had 300 rowers with 90 oars on each side. Being heavier than the trireme (it would displace about 100 tons), it was also more stable in bad weather and faster. A 100 tons quinquireme ramming an enemy ship at high speed would totally pulverize it.
Rome tested and improved its warships during the First Punic War which lasted a good 23 years. The First Punic War started when Messana (today Messina) asked Rome to expel the Carthaginians from its territory. Sensing that Carthage was too close to its territory and could potentially pose a threat, Rome saw the opportunity to have a strategic presence in Sicily and to finally "deal" with the Carthaginian threat. Rome sent 230 warships and 100 freighters with an estimated 100,000 oarsmen and 40,000 soldiers! After over two decades of fighting and a number of epic naval battles, Rome managed to defeat the world's most powerful navy to become the dominant naval power in the Mediterranean or as the Romans called it Mare Nostrum ("our sea").
Another function of Roman military ships was to patrol the Mediterranean sea and to sometimes escort merchant ships. During the Empire it was quite common to see the huge galleys of the Roman navy patrolling the Mediterranean for any pirates and escorting other large merchant ships. The Mediterranean sea remained actually pretty safe up until the collapse of the Western Roman Empire in the 5th century.
The merchant ship's main function was to transport lots of cargo over long distances and at a reasonable cost. Merchant ships transported agricultural goods, for example olive oil from Greece, wine, grain from Egypt's Nile valley, and raw materials such as marble, granite, iron bars, copper, lead ingots, etc. Unlike warships, merchant ships did not have to be fast or very manoeuvrable. Since they anchored to ports, they also did not have to have a flat hull like warships and had instead a V-shaped hull and a ballast which rendered them more stable. They also had double planking which strengthened their hull thereby allowing them to transport heavy cargo.
The merchant ships' cargo capacity varied from 70 up to 600 tons for the largest Roman ships. Most ships had a cargo capacity of 100 to 150 tons, 150 tons being the capacity of a ship transporting 3,000 amphorae. The largest ships, with a capacity of 600 tons, were 150 ft (46m) long. Some ships like the Isis had a tonnage equal to 1,200 and were close to 180 ft (55m) long, with a beam of 45ft (14m) and 45 ft (14m) from the deck to the bottom of the hull. They could carry one thousand tons of grain or enough to feed an entire city for a year. If we compare to modern ships though, a Panamax container ship today has a length of 950 ft (290m) and a carrying capacity of 52,500 tons (DWT) or 44 times the capacity of the Isis ship. Nevertheless, Roman ships were huge for their time and after the collapse of the Roman Empire no ships of their cargo-carrying capacity were built until at least the 16th century AD.
Merchant ships mainly used mainly wind power. They had from one to three masts with large square sails and a small triangular sail called the supparum at the bow. They also had oarsmen (usually slaves).
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In a world where navigational instruments such as GPS's or even compasses did not exist, one can wonder how did the Romans manage to navigate the seas? We note that the compass was already in use in China from the second century BC but appeared in Europe only in the 14th century AD. Knowing the direction of north is one of the most basic conditions of navigating in open seas as it allows the mariner to know in which direction he is heading (north, south, east, west). The Romans' navigational skills were learned from the Phoenicians (the predecessors of the Carthaginians). The Phoenicians had learned astronomy from the Chaldeans from Chaldea, a semitic nation located in the far southeastern corner of Mesopotamia which existed from the late 10th century (or early 9th) to the mid-6th century BC. According to Pliny, they applied the Phoenicians' knowledge of astronomy to navigation at sea to become the best mariners of their time. For example, Phoenician mariners knew that the constellation Ursa Minor orbited the celestial North Pole in a tighter circle than Ursa Major and used Ursa Minor to give them a more precise direction of north. There were other, less accurate, ways of estimating direction. For example, Roman seamen would look at the sun at noon or they would estimate directions relative to the wind and swell.
Roman seamen navigated along the coasts whenever they could which greatly facilitated navigation. One of the advantages of the Mediterranean is the proximity of the mainland and the great number of islands (especially in places like Greece). Seamen sailed by noting their position relative to a succession of recognizable landmarks and used sailing directions which already existed in antiquity. The first sailing directions for coastal trips in the Mediterranean were written in Greek. They were called periploi in Greek and were introduced in the 4th century BC. By 50 AD, there were sailing directions written in Latin and other languages not only for the Mediterranean but also for routes along the Atlantic coast of France and Africa and for routes past the Persian Gulf to India and beyond.
Piloting these ancient Roman ships was far from easy. Ancient Roman ships did not have all the equipment of modern ships and used wind and muscle power alone. Just like with modern sailboats, seamen had to have a good understanding of the weather, of how to operate the three large sails in relation to the direction of the wind and in various weather conditions. Another challenge in both merchant and warships was coordinating the rowers. Rowers that were not well-coordinated were less efficient and could even hit each other's oars. In order to coordinate the sometimes over a hundred rowers, a wind instrument or sometimes a percussion instrument would be played. Another way to coordinate the rowers was to have a person make hand gestures kind of like a conductor conducting an orchestra.
Some Roman ships had cabins usually located at the stern that could accommodate only the most wealthy Romans. Rich Romans just did not sleep on deck.
Wealthy Romans would often own their own ships just like wealthy people today own yachts. It is worth noting that a 218 BC Roman law forbade Senators from owning ships with a capacity to carry more than 300 amphorae. The law was written so that Senators and the patrician class in general did not engage in trade and just stuck to generating wealth from agriculture from the large lands that they owned.
There were a large number of Roman ships constantly sailing the commercial shipping lanes of the Mediterranean on more or less regular schedules and routes, bringing supplies from the provinces (e.g. Egypt, Gaul, Greece, etc) to the ports of the Italian peninsula. Goods from all over the world would come to the city through Pozzuoli situated west of the bay of Naples or through the gigantic port of Ostia situated at the mouth of the Tiber river.
Large merchant ships approached the port everyday and were intercepted by a number of towboats to be dragged to the quay. It is estimated that 1,200 large merchant vessels reached the port of Ostia every year or about five per navigable day! We note that commercial navigation in the Mediterranean was suspended during the four winter months. The Romans called it mare clausum.
The time of travel along the many shipping lanes could vary widely. Roman ships would usually ply the waters of the Mediterranean at average speeds of 4 or 5 knots. The fastest Roman ships would reach average speeds of 6 knots. A trip from Ostia to Alexandria in Egypt would take about 6 to 8 days depending on the winds. Travel from south to north or from east to west would usually take more time due to the unfavourable winds.
Apart from military and merchant ships, there were smaller fishing ships, other auxiliary ships of various uses and private ships of wealthy Romans plying the waters of the Mediterranean.
Interesting facts about Roman ships