Passenger ships constructed on or after 1 July 2002 must carry Voyage Data Recorders (VDRs). The same goes for other ships with a gross tonnage of 3000 and upwards. Existing cargo ships of 3000 gross tonnage and upwards must have a simplified VDR (S-VDR).*
This is to assist in accident investigations if anything goes wrong. VDRs enable accident investigator to review procedures and instructions in the moments before and incident and helps to identify the cause of any accident. Just like black boxes carried on aircrafts.
S-VDRs are not required to store the same level of detailed data as a standard VDR. Nonetheless they should maintain a store, in a secure and retrievable form, of information concerning the position, movement, physical status, command and control of a vessel over the period leading up to and following an incident.
*The mandatory regulations are contained in chapter V on Safety of Navigation of the International Convention for the Safety of Line at Sea, 1974 (SOLAS).