PhD defence: Blind spot in maritime history - Cogs, Small Cogs and Boats

Date of news: 30 September 2021

Among hundreds of shipwrecks excavated in the Flevoland polders (reclaimed land of the former Zuiderzee inlet) are more than twenty from the Hanseatic period (ca. 1250-1500). For his research, Karel Vlierman (79) reconstructed these medieval trading vessels - called cogs - piece by piece to contribute to maritime history. He will defend his thesis on 4 October .

With the construction of the polders in the former Zuiderzee area, a shipyard ofCover_uk more than 150,000 hectares fell dry. In total, the Netherlands Institute for Ship and Underwater Archaeology (NISA) and its predecessors have so far registered about 450 shipwrecks in this area. Over 350 of these have been excavated over the years. Maritime archaeologist Karel Vlierman was employed by NISA between September 1969 and July 2002, and was in charge of the daily archaeological supervision of various ship excavations. In 1999 he began his research on cogs, ships that were developed from the mid-12th century to transport bulk goods and that brought Hanseatic cities such as Kampen, Zwolle and Deventer great wealth in the 14th and 15th centuries.

Cog finds

Over seventy of the wrecks found in the former Zuiderzee area date from before 1600. Among those vessels are sixteen or seventeen large and small vessels which can be named cogs. Until Vlierman began his research, the details of most excavated cogs were still stored in files, and it was hardly known what a cog had actually looked like and how the ship was built. Vlierman decided to elaborate and analyse all the cog finds from the Netherlands. His research over the past twenty years makes clear not only that different types have sailed around - the famed seagoing cogs, small cogs for inland navigation and boats for transport over smaller waterways - but also that they were built and caulked according to a characteristic and unique method.


Vlierman's detailed analysis of the wrecks made it possible in most cases to work out a complete reconstruction of the hull (1:10 scale). Vlierman began by reconstructing the mostly deformed wreck into its original shape, into which he then placed the separately found parts and finally, based on his own knowledge, drew in the missing pieces.

Reconstruction Nijkerk II
Reconstruction Nijkerk II, Karel Vlierman

From his research Vlierman concludes that between approximately 1200 and 1500 ships were only built according to a specific method and in a specific building sequence in the core of the Hanseatic area (along the Overijssel IJssel/the eastern coastal area of the Zuiderzee, the coastal area of the Wadden Sea and North Sea up to and including the lower reaches of the Weser and the Elbe). So there was a cog building tradition. The large cogs were probably hardly part of the sea fleets anymore around the beginning of the 16th century. They were gradually replaced by other, larger ship types.

Complete overview

Vlierman's research has filled a blind spot in maritime history. Thanks to his research it has become clear how the various cogs were built and in what order. The research has led to a bulky and richly illustrated publication in two parts with a total of 1000 pages to which seventy appendices with Vlierman's handmade reconstruction drawings have been added. It is the first complete overview of cog finds in the Netherlands and Flanders, which amounts to about two thirds of all European finds. An English-language trade edition has been published by Spa publishers in Zwolle.

Karel Vlierman's defence will take place on 4 October, starting at 2:30 p.m. in the Academy Room of the Aula.