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H - 146 : Trade Routes and Thermal Winds

Updated: Mar 4

A Roman Cargo Ship leaves the Port with its Spritsail (Foresail) set to manoeuvre


One of the many things I enjoy is looking at maps. They always provide valuable context. Events do not happen somewhere but in a precise space. Location, geography and time matter. One of the best sites for contextual maps is where Simeon Netchev beautifully illustrates historical events. It is always worth a visit.


Trade Routes in the Roman Empire - Love this Map - so Accurate - by Simeon Netchev

For a while now I am looking for maps showing Mediterranean trade routes in ancient times. In general, the routes are drawn as straight lines as if mariners a few thousand years ago had motor boats. We know that the Phoenicians sailed from the Levant to Andalusia for its tin and silver. But why did they not follow the African coast line on the Way to Cadiz? Why did they take a northerly course via western Sicily, southern Sardinia, Mallorca and then Spain? The answer is simple. They mariners were exposed to the prevailing north-westerly winds in the Western Basin and the westerly sea current. On the African coast, the sea flows west to east to replenish the evaporating water in the Eastern Basin. Sailing east along northern Africa is easy. Sailing in the opposite direction requires the ability to sail windward.

Phoenician Trade Routes and Expansion from 11th to 6th Century BC


Did ancient mariners know how to tack against the wind? They were pretty skilled navigators already and built boats with a high degree of sophistication. They could sail for days on open water without seeing the coast. This year, we are going to visit two of the boats built several centuries before Roman time : the Kyrene ship in Girne and the Uluburun ship in Bodrum.


North-westerly winds dominate the Med - Mistral & Meltemi are 2 other important factors

Sailing windwards has a few basic but important requirements:


  • The ship must have a deep keel – the sea then provides the necessary resistance (force) to partially offset the force applied by the wind

  • It needs a Latin sail or at least a spritsail to make sure the ship does not turn automatically into the wind

  • The mariners need a map to plot a tackling course with regular and timed zig-zags

  • They also need a pretty precise tool to measure the time length of the lapses

  • Last but not least, they  need to be able to determine the general direction


Graffiti from Pompeii of a Roman Cargo Ship with a deep Keel and two Sails

Phoenician and Greek ships already had deep keels. When sailing at night, the mariners could navigate by the stars and had a good idea of their general direction. A few hundred years later, Roman merchant boats had a foresail and after the 4th century AD Latin sails which found their way from the Indian Ocean to the Mediterranean. We know from a sarcophagus from 350 AD that Romans had hour glasses to measure time. But no such instrument is ever shown on a vessel or was ever found on a ship wreck. They were introduced by the Byzantines in the 9th century AD though.  What nobody had were maps. They could not plot a course at sea.


Depiction of an Hour Glass (Green Highlights) on a Roman Sarcophagus from 350 AD

Getting from Rome to Alexandria was - and still is - easy. The prevailing northwesterly winds literally blew the Roman merchant ships from Italy to Egypt. We know that they could make the trip in 14 days. Get out of Ostia harbor, hoist the sail and off you go. They just had to keep the general direction and would hit the Egyptian coast. But how would they get back without the ability to sail windwards? The same way the Phoenician reached Cadiz – by using alternative wind patterns and selecting a different course.

I first had the idea in 2018 when we sailed along the Cilician coast (Alanya to Iskenderun) and found the big port of Kalenderis (Aydincik today), eternally memorized by the beautiful harbor mosaic found a good decade ago. Why was it there? The colorful mosaic we were shown by the local archeologist team is like a harbor map. Its simple existence  confirms the importance of the port. Right behind the port, the Taurus Mountains soar into the sky. There is no hinterland and no economic activity to speak of. The same question can be asked for Knidos further to the west. It was so important it even had two harbors. There is no economic activity in Knidos’ hinterland either. As a matter of fact there are so many of this ports you wonder why. The ancients never built big structures without a purpose. This applies to Egyptian obelisks which had a religious meaning as anything else.


The Local Archeologists show us the Kalenderis Mosaic on 7 August 2018

All these ports on the Levant, in Cilicia and in Lydia had an invisible asset which is rarely talked about but experienced every day. The changing land and sea breeze which makes the mornings fresh and the afternoons tolerable. The mountains behind the coast line create thermal winds. As the land heats up fast with the morning sun, air rises. Cool air from the sea is sucked inland. The land breeze starts around 9 am and 10 am. The completely flat sea from 4 am, when fishermen head out gives way to a wavy sea. Towards the evening, when the sun sets, the cycle reverses. The land mass cools faster than the sea. Wind from the mountains blows out on the sea and stops sometime between 9 pm and 10 pm.


On the 10 August 2018, 9 am, the Fishermen from Liz Kalesi returned and sold us some of their fresh Catch - the Sea Breeze was just setting in

We have no idea when the mariners discovered these thermal winds. But surely, they did. There would be no harbors on these coasts. The thermal winds were offsetting the prevailing north-westerlies in the Eastern Basin. By following the coast of Judea, Lebanon, Cilicia and Lydia, the Roman grain freighters could get to Knidos and catch there the northerlies (Meltemi) to travers to Crete and the Peloponnese. Thanks to the Boras in the northern Adria, the northerly winds continue for the crossing from Patras to Puglia where thermal winds can be used again. They get you around Calabria and push your boat through the Straits of Messina towards Naples and Ostia (Rome).


Roman Merchant Ship with Sprit Sail on a Sarcophagus from Ostia 3rd Century AD

Very interesting research was done recently by Christopher Davey on the capacity of Roman ships to sail windward. He concluded that we underestimated the ability of ancient mariners to sail windward. They could hold a windward course of 65 degrees. Quite a remarkable finding which I was not aware of. With this capacity, Roman captains could well sail against the wind as long as they had a point of reference on land. They were still missing a precise time measuring device (the Egyptian water clock was unsuitable for ships) and proper maps. But definitely had the capacity to use the thermal winds to sail up and down the coast and get into the harbors they choose.


A Roman Ship with Latin Sails entering the Kalenderis Harbor

My conclusion from all of this evidence is that the trip from Ostia to Alexandria took 14 days but the return trip along the Middle Eastern coast could take 6 – 8 weeks. Harbors like Kalenderis and Knidos must have been very busy. Given 8 weeks travelling time, only one round trip per year was possible from Ostia to Alexandria. The Roman sailing season lasted from June to September. My conclusion also ties in with the story of Saint Paul’s trip to Rome. His ship set sail from Knidos in October and got – as Paul predicted – into one of the first autumn storms. Blown of course they eventually shipwrecked in Malta. I hope that more research will be done and that one day the trade route maps will be adjusted.










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