Before the beginning of the 12th century sea trade was important. So much so that N.J.G. Pounds feels that above all "seaborne shipping was the most important, and if it could be expressed in tons/miles it's importance would seen overwhelming."
In the Middle Ages things were pretty much the same. Lopez and Raymond note that "throughout the Middle Ages transportation by water was far more important than transporation and travel by land, especially in long-distance trade." They go on to make the point that shipping was cheaper, usually faster, and often safer. It is clear that harbours and entrepots represented crucial channels through which trade could pass.
It is useful to explain a little about the ships involved in sea trade. The two basic types of ships that were used Mediterranean trade were the rounded nef and the thin, swift galley. The nef was powered mainly by wind and carried bulky cargoes of low value (i.e. wheat, salt, wool, allum and timber). Nefs dominated the commerce of the Mediterranean sea. The thin, swift galley's main method of propulsion was oars.
The galley emerged later on. This had a supplementary sail which meant that it could maintain a certain speed, where as under the same conditions a sailing ship would remain virtually motionless. Galleys were used for valuable commodities like silk and spices. However, this type of ship was expensive to operate and only the larger trading cities (Venice, Genoa and Florence) were able to maintain a fleet of galleys.
Water-transport provided opportunity, especially where moving building materials great distances was concerned. Timber was floated down the rivers from Burgundy and even ashlar masonry was exported to southern England from Caen in Normandy. As a result of the seas becoming safer for peaceful shipping and an increase in the volume of trade - as was the case with bulky commodities - maritime trade took on a more important role in the commerce of Europe.
Sea loans helped encourage the growth of trading. The borrower would pledge the return of the loan only on the condition that the ship carrying the borrowed money and/or goods suceeded in completing it's voyage safely. Many sea loan contracts had a clause. It was commonplace for some of the ship's cago (generally goods purchased with the loan) to considered as security for the return of the loan.
Any objections that ecclesiastical authorities might have had were avoided by ensuring that no interest or premium was openly disclosed - hence the term 'rate of exchange' was adopted instead.
The mercantilist policy was an effective way of controlling trade. Pounds comments that "governments made great use of embargoes on trade and of the forced canalisation of trade through specific channels." England was better placed than most states to control foreign trade as her number of ports was limited. Evidently, it is easier to control trade via restrictions on access to fairs and markets (an internal diversion of trade in effect).
Accompanying the complications of mercantilist policy was the problem of navigation being a seasonal business. A third of the year saw the ships being laid up in the harbour and a further third it was considered too dangerous to sail. As a consequence some harbours were virtually redundant for the best part of a year.
A decline of one port can be linked to the development of another. Damne, which silted in the 13th century to be replaced by Sluys, is a good embodiment of this process.
On the other hand, some ports have benefited from the intervention of outside factors. In the first half of the 12th century Zwin waterway was opened up because floods and storm-surges made the sea level rise slightly. Subsequently more trade was diverted towards the Low Countries.
There are numerous examples where the fate of a port has depended upon the stability of having a healthy flow of trade. This can be applied to the decline of Flanders, the neighbouring areas of northern France and the Low Countries. Flanders was heavily dependent upon the income of the Champagne fairs but fell prey to a dry-up in the business.
Ports and entrepots were subject to constant change. Some were growing in wealth and inportance (Florence), some were in decline (Flanders) and some got lucky. The port of Sluys appears to fall into this third category. It was unfortunate for Damne when it was exposed to the cruel elements of nature, but advantageous in elevating Sluys.
Trade intensified along certain routes because traders gravitated to specific ports and harbours.