The Romans took great care in shipbuilding. In his chapter dedicated to  naval warfare, Flavius Renatus Vegetius (IV,34,1) says that the material for building a ship should be sought with great accuracy, in the same way as the material for building a house, ‘because a defective ship is much more dangerous than a defective house’. Vegetius (V,4) also gives us useful information about the material employed for the building of the liburnae: cypress, larch, fir or pine wood, and copper nails instead of iron, because ‘the iron nails, exposed to warmth or damp, are quickly corroded by the rust, while those of copper preserve their original consistency also among the waves’.

Without clear remains of warships, there is little that can be asserted with absolute certainty about their construction. However, it may be assumed with some confidence that elements of construction found in merchantmen, such as plank fastenings, internal support and order of construction, were paralleled in warships.

From the third century, Roman ship and boat construction techniques – especially for military use – were transformed, the product of cultural exchange with other shipbuilding peoples. At the end of this period, shipbuilders no longer used the traditional shell-first hull construction and had adopted the new method of building the hull’s frames first, which were covered by planks only at the end.

The new system brought a decrease in the use of mortise-and-tenon joints, and generally lighter hulls and thinner planking for warships. Decreasing the weight of the hull would have been most advantageous to warships when rowed, for they could be rowed at greater speeds with the same number of oarsmen or be rowed for longer periods at given speeds. More efficient use of the wind when under sail was also a potential advantage of lighter warships. Furthermore, the time and expense of repairs would be drastically reduced, time often being a crucial factor during war.

However, apart from this structural evolution, which was slow, another important development in nautical technology was the adoption of the Latin sail (which in later centuries became the ‘lateen’ sail, used worldwide). Roman warships had unquestionably utilized both sails and oar propulsion. A square sail with brails set on a single yard was the predominant configuration observed in iconography until the fourth century, and the squared sails of the Roman ships were flanked by the spritsail. The Latin sail was the result of the need to make better use of winds other than stern winds.

The Latin sail was introduced during the third century, influenced by the Levantine world. It is clearly visible on a squadron of merchant ships arriving in Ostia harbour in a third-century relief. By the fifth century the majority of the Imperial dhromones were fitted with them. But the square sail never disappeared completely from the Roman ships, and the process of adopting the Latin sail was slow. The process of the disappearance of the spur began in a parallel way, either in the bronze full form of antiquity or in the shape of a simple cutwater.

Weapons used on board during naval combat included the double axe, as explained by Vegetius (IV, 46), which was a large, pointed iron weapon with two cutting edges sharpened on both sides. Employed in battle by the most

expert sailors or soldiers, it could cut the cables on the sides of the rudder to make the enemy ship ungovernable. In addition the sailors used various kind of scythed weapons to cut ropes and sails, and of course missile weapons of all sorts.

Catapultae, skorpions, ballistae and, from the fourth century, onagers were the most common throwing weapons that could be found on the deck of a ship, or sometimes mounted on wooden towers with which the larger ships were often fitted.

The general tactical trend to avoid ramming attacks and instead fire missiles to burn the enemy’s ships brought the development of terrible sea weapons. Marine fire began to appear during the early sixth century, when the fleet of Marinus destroyed that of the usurper Vitalianus with a mixture of inflammable liquid – forerunner of the Greek fire of the following centuries. The key ingredient in almost all the formulas for ‘Greek fire’ since the Hellenistic Age was what the ancients referred to as naphtha or crude oil, which, throughout the oil-rich areas of the Near East, could be scooped up at dozens of points where it seeped out of the ground. Although it was inflammable enough in its simple state, the usual practice was to lace it with sulphur, pitch or quicklime.

You can read more in NVG 244 Imperial Roman Warships 193–565 AD.