Many late-medieval western European seaports showed a ship in the central image of their municipal seal (a ship seal). Municipal seals to identify and authorize documents ranging from important supra-regional charters to local ephemera were widely used at the time, and probably also on trade contracts and (predecessors to) maritime bills of lading. This study of 115 ship seals from 80 seaports between ad 1200 and ad 1450 reveals that the central image of all ship seals contained striking details, strongly reminiscent of semiotic pictorial signals that conveyed information. The supposed signals were assigned a probable meaning by linking them to nautical aspects of the approach to each seaport and to the state of the harbour at the time. The results strongly suggest that the signals provided information about the risks and hazards in the approach to port, the water depth, and the facilities at the port in question. Compelling evidence comes from ports issuing successive ship seals over an extended period of time: updated images with properly adjusted signals were shown on subsequent seals when significant changes in the approach or port facilities had occurred, but the signals remained the same as long as the local situation stayed unchanged. These findings provide a clear explanation for the divergent ship portraits; ship seals served a dual purpose: first, to confirm the identity of the seaport and the authority of the council in its various actions and tasks, and, second, to provide up-to-date insight into the nautical challenges when approaching the port and in the local port facilities, in a manner similar to the current ‘notices to mariners’ from hydrographic offices. The rapidly increasing number of ports using ship seals in the thirteenth century and their continued use for two centuries supports the success of this late-medieval nautical information system. The study results also provide up-to-date impressions of the 80 seaports involved and of various nautical aspects of the hazardous maritime traffic in the thirteenth and fourteenth centuries, about which written archival information is scarce. The results may help to put the rare yield of maritime archaeology of this period into perspective. The ship seals lost their appeal as buoys and beacons made approaches safer, harbours became deeper and better equipped, and written information (‘rutters’) became widely available.
