According to Lionel Casson in Ships and Seafaring in Ancient Times, the size of commercial ships varied. Most of the information about commercial ship size was gathered from the excavation of ancient wrecks preserved over the years by clay. Examples of this type of evidence include the excavations of the Yassi Ada and Kyrenia ships, whose lengths were determined to be 21 meters and 13.7 meters, respectively. Casson refers to both of these boats as "small freighters," indicating that this the normal length of ancient commercial ships. The warships of ancient Greece tended to be a bit longer and narrower than the commercial ships, and according to Casson were usually about 30 meters in length and 6 meters in width.
Since the ships of the ancient world, both commercial and war, were not run by motor but rather relied on oarsmen and sails, nature played a major role in navigation. As far as the Greeks were concerned, the sea was navigable for about eight months of the year. This eight-month stretch was referred to as the sailing season and extended, depending on whose account, from last days of March until the early days of November. According to Hesiod’s Work of Days, the sailing year is broken down by the four seasons. In winter, the sea is closed (end of October – end of March). The spring and fall are times when the sea is navigable, but dangerous. And the summer is the preferred sailing season.
The general water circulation in the Mediterranean Sea is subject to numerous factors. Inflow from the Atlantic as well as surrounding river systems, evaporation in the atmosphere, seasonal change in wind and pressure systems over the sea, and the complex topography of the sea floor itself all contribute to the prevailing water circulation patterns in the Mediterranean. Specifically, the Aegean Sea is subject to all of the above factors as well as the inflow of surface waters from the Black Sea through the Dardenelles. Water flows in the Aegean is generally counter-clockwise, i.e. water travels sourthward down the western side of the Aegean, ten circirculates through the isalnds and meets with water from the Ionian Sea, to travel northward along the eastern coast to meet the waters of the Black Sea. Thus, it was relatively easy to travel to the east with the current. In the adjacent Mediterranean, water flow was west to east, where it hit land and circulated around Cyprus.
The Mediterranean lies in a belt of westerly winds with predominant high-pressure systems in the summer and low-pressure systems in the winter. In the summer, the high-presure systems coming off Africa were very large and often held off cyclonic storms1 to the north, allowing a nice westerly breeze to pass throughout the Aegean. By mid- to late summer these winds shifted to come predominantly from the north (the so-called "etesian" winds). Although the etesian winds were not particularly stormy, their direction made sailing toward the north of the Aegean difficult. As the seasons shifted, cooling set in. African high pressure retreated, and small low-pressure systems presiding over the Mediterranean allowed a more sourtherly course of the continental cyclones, thus creating turbulent water in the winter. Indeed, the prevailing winds in winter were those of a stormy northeaster in contrast to the pleasant sailing winds of the summer months. Thus, the Greeks chose to sail in the summer, when it was much easier and safer.
1Cyclones are weather systems with strong winds and rain surrounding low pressure.