It is generally granted that the beam of the Greek ship propelled by oars is less than that of the galleys of the Middle Ages.
According to JAL (Archéologie Navale), the ratio of the beam to the length, in the middle ages, was for war vessels: 1 : 8; for merchant ships it was 1 : 7. Graser found that among the Greeks this ratio was 1 : 8¼, and according to SERRE (La Marine de guerre de l’Antiquité, p. 33) and LEMAÎTRE (Revue archéologique 1833, Vol. 8, pp. 149 et seq.) its value was 1 : 9.
Hence the vessels were narrow as compared with their length, which increased their degree of mobility.
Besides, the depth of the docks discovered shows that the boats drew but little and, consequently, that the ships slid along in a way, on the water.
In this respect too, the ancient boats did not differ much from those of the Middle Ages. Neglecting this detail, among others, Graser reaches a type of ship having far too great a draught.
Alongside of the war vessels, naves longæ, there were the merchant vessels, naves onerariæ. It was natural that great mobility should be sought for the former; it is this which explains the lengthened shape of these boats, whereas the merchant ships were shorter and had more beam.
Later, but still several centuries B. C., when the power of Rome was developed, when its population increased and when the importation of wheat and other provisions became more and more important and had to be made more and more quickly, the ship with oars was used as a merchant vessel in addition to the short, wide boat.
The Roman freight galleys seem, further on, to have been made broader, like those of the Middle Ages, with a view to increasing their capacity. But no new type followed from this; it was merely a new application of existing types.
It can be certified that no new type of boat was created later all at once, and that this change was due not to the shifting of lines of trade or to the construction of new ports. This latter condition could, at most, have modified the accepted dimensions.
The different types of antiquity remained in use for centuries and are still to be found to a great extent.