Interpreting Ship Logs

After compiling data on how long each ship's journey lasted, the researchers first asked whether other factors besides wind and weather - such as changes in how ships were built, what sort of cargo they carried, or what route they traveled - could have made a difference in how quickly the ships sailed. A lengthy investigation of other historical documents, including letters and journals, showed that these factors stayed mostly the same over the period of time the researchers were examining.

Confident that changes in the length of each journey would be mostly influenced by climatic conditions, the team then looked at how voyage length changed with time. They found a revealing pattern of change in voyage length, with voyages during the 30-year period of 1640 to 1670 lasting substantially longer than voyages before or after this period. From 1640 to 1670, voyages were around 123 days long, compared to a length of 79 days for voyages taken from 1590 to 1620.