Merchant Shipping Movement Cards
If you are researching a particular vessel then it is vital that you obtain the "Merchant Shipping Movement Card" for the vessel from the National Archives (example here). This document lists all of the the ports of call, the dates of arrival and departure, damage, repairs, convoy designations etc., that the vessel made during the period January 1, 1939 - December 31, 1946. If the record exists then it may be obtained over the Internet world-wide from the National Archives for a modest sum.
Note that merchant vessels used as troopships, hospital ships etc. are not normally included.
The archive is divided into two sections - British and Allied ships. If you are having problems locating the vessel (exact spelling etc) then use the reference finder below to determine the alphabetical file reference that relates to the vessel and enter this in the 'Quick Search' in the National Archive link above.