Casson, “Speed Under Sail of Ancient Ships” (1951)
[Bibliography]

Casson, “Speed Under Sail of Ancient Ships” (1951)

Sailing ships were the backbone of ancient commerce and travel. They carried freight, passengers, news. How fast could they move from port to port? The question, an old one, has been dealt with numbers of times. The handbooks and the economic histories have taken it up in detail and even the general histories have given it a passing word. 
Despite all this attention, the subject requires a complete re-examination.⁠ One reason is that the scholars who have concerned themselves with it have committed more than their fair share of blunders. Another is that nobody has ever assembled all the available evidence.⁠ Passages from ancient authors cited by earlier writers have been over¬looked by those who came later while some excellent evidence offered in an ancient work entitled, of all things, "The Ship" was not used until 1941.⁠ The most important reason, however, is a far more basic one, namely, the approach to the problem. 
The traditional approach has been to collect all available records of voyages, compute the time required for each, and from these figures strike an average. Such a method, however, is fundamentally unreliable. The data provided by ancient authors are rarely a collection of random, normal samples. Writers of antiquity did not waste time recording the duration of an ordinary, uneventful trip; they noted a voyage either because it was a spectacular run made in record time, or because it was a disastrously slow one plagued by foul weather. To average a record-breaking run made with a following wind alongside a voyage delayed by calms and head winds does not yield a "normal" speed, but a statistical fiction that represents nothing real. 
Furthermore, any sound approach must make a sharp distinction between two entirely different sets of conditions: sailing with a fair wind and sailing against it. An ancient ship, with its square rig and lack of a deep keel, had totally different capabilities depending on whether the wind was dead astern or blowing from the beam or bow. To lumping all these voyages together under a single average speed obscures the true capabilities of ancient seamanship and the actual conditions under which ancient commerce operated.
https://penelope.uchicago.edu/Thayer/E/Journals/TAPA/82/Speed_under_Sail_of_Ancient_Ships*.html

See also: Lionel Casson, Ships and Seamanship in the Ancient World (1971) (revised ed. Johns Hopkins University Press 1995).