Here's the definition from a UK site:
"This document contains personal information and a record of each voyage undertaken. The certificate acted as both a passport and a reference. The captain of a ship would record the name of the ship, the dates of the voyage and the ability and conduct of the seaman on that voyage." (From Memorynet: http://www.twmuseums.org.uk/memorynet/
Below is a link to a Google Book called "Sessional Papers, Volume 77" By Great Britain. Parliament. House of Commons. http://goo.gl/CqP7Q
(It should take you directly to page 67 of the book.)
This link will take you to the Registry of Shipping and Seamen: http://www.dft.gov.uk/mca/mcga07-home/workingatsea/mcga-trai...
- at the bottom of the page, under the Historical Information heading, click the link for details of their historical records - it opens as a Word document but explains where certain documents are stored and how to make a request.
My dad was a US Merchant Marine in World War II, so I don't really have any suggestions in your case. I hope the links help! My dad took several trips to South America, and they were usually to pick up crude oil and bring it to a US refinery; however, I've seen a few that also exported "exotic" fruit like bananas. If you have the names of the ships, please post them - sometimes you have better luck searching the ships than searching the crew!