New research by historian Andrei Mirea is shedding light on the challenges of winter navigation in the Black Sea during the Middle Ages. Published in the International Journal of Maritime History, his study reveals that this inland sea was largely impassable during the colder months.

A Sea Closed to the Outside World

For much of the Middle Ages, the Black Sea was effectively a Byzantine lake. The Byzantine Empire tightly controlled access through the Bosphorus, preventing foreign ships from entering. Until the thirteenth century, only Byzantine vessels could sail these waters, dictating the seasonal patterns of navigation. This meant that the empire’s own regulations, legal codes, and sailing traditions determined how and when ships moved within the region.

The Byzantines followed medieval maritime laws, such as those found in the Theodosian Code and Justinian’s Codex, which restricted seafaring to the warmer months. These laws generally defined the sailing season as lasting from April to October, with some extensions into early November if conditions allowed. The reasoning behind these restrictions became clear when examining the dangers of winter navigation in the Black Sea.

A Hostile Environment for Sailors

Mirea’s study highlights the Black Sea’s unique and hazardous climate, which made winter navigation far riskier than in the Mediterranean. Thick cloud cover, heavy rainfall, and strong winds were frequent, making it difficult for medieval sailors to navigate safely. The most dangerous element, however, was the cold. While the Mediterranean rarely saw temperatures drop low enough to cause serious maritime disruptions, the Black Sea regularly experienced freezing conditions, particularly along its northern and western shores.

Sea ice posed a major threat to medieval ships. In extreme cases, parts of the Black Sea could freeze over, effectively cutting off entire regions from maritime trade. Even when ice did not form, the combination of strong winter storms and treacherous waves made it highly impractical for ships to set sail. Without modern weather forecasting or reinforced ships, medieval sailors had little hope of surviving a winter voyage in these conditions.

Given these challenges, it is unsurprising that Byzantine and later Genoese and Venetian authorities discouraged winter navigation. Surviving documents from the late Middle Ages reinforce the idea that seafaring in the Black Sea was primarily a seasonal activity, with most voyages taking place during the safer months of spring, summer, and early autumn.

Genoese and Venetian Trade in the Black Sea

Although the Byzantines initially restricted access to the Black Sea, this changed with the rise of Italian maritime republics in the thirteenth century. The Genoese and Venetians, eager to expand their trade networks, gained control of key ports such as Caffa and Trebizond. However, even these experienced merchants struggled with the challenges of winter navigation.

Genoese statutes from the fourteenth century explicitly prohibited sailing in the Black Sea during winter months, with heavy fines imposed on those who attempted it. Venetian records similarly reflect a reluctance to allow ships to remain in the region beyond late autumn. These regulations were not merely bureaucratic restrictions but practical measures designed to prevent shipwrecks and economic losses.

However, there were occasional exceptions. Military campaigns, diplomatic missions, and emergencies sometimes forced ships to sail outside the usual season. One notable case occurred in 1365–1366, when Byzantine Emperor John V Palaiologos allegedly crossed the Black Sea in winter. Some sources claim he undertook this journey to reach Hungary, but Mirea’s study suggests that this account may have been exaggerated for rhetorical effect rather than a factual description of a successful winter voyage.

A Costly Lesson: The 1296 Venetian Expedition

One of the most striking examples of winter navigation gone wrong occurred in 1296, during a Venetian attack on the Genoese colony of Caffa. As part of the ongoing conflict between Venice and Genoa, a Venetian fleet led by Giovanni Soranzo launched an assault on the Black Sea port. The campaign was initially successful, with the Venetians raiding and burning Genoese ships. However, when winter set in, the fleet found itself trapped.

Unable to return home due to the worsening weather, the Venetian ships were forced to overwinter in Crimea. According to contemporary sources, this proved disastrous. Severe cold and a lack of provisions led to widespread suffering among the Venetian sailors, and by the time the fleet finally returned to Venice in the spring, it had lost nearly half of its ships. The expedition demonstrated the extreme risks of winter navigation and reinforced the necessity of adhering to the traditional sailing season.

The Role of Technological Advances

During the later Middle Ages, shipbuilding technology improved significantly, leading to larger, more durable vessels. The Genoese, in particular, pioneered advancements that allowed for greater success in winter sailing, at least in the Mediterranean. These included better sail designs, the widespread use of the sternpost rudder for increased maneuverability, and improvements in navigation techniques, such as the use of nautical charts and the compass.

Despite these advances, the Black Sea remained far more treacherous than the Mediterranean. The region’s extreme weather conditions meant that even the most technologically advanced ships of the period were not well-suited for winter voyages. While Italian merchants gradually adapted to winter sailing elsewhere, the Black Sea remained a place where caution prevailed.

The article, “The problem of winter navigation in the medieval Black Sea.” by Andrei Mirea is published in the International Journal of Maritime History. Click here to access it.

Andrei Mirea is a graduate student at Nicolae Iorga Institute of History in Romania. Click here to view his Academia.edu page.

Top Image: 15th-century map of the Black Sea – British Library Additional MS 15760, ff.74v-75r