Hallo again Vicky -
The records are all very confusing when you first look into them, so I'll try to give you a clearer explanation of what I suggest you look for.
Crew Articles of Agreement aka 'Crew Lists'
For people sailing as part of the crew (e.g. Buchanan), you need Crew Agreements. Those for UK registered ships are not all located in the same archive (or even in the same country!), nor is there a central index to where they are located. You will find them roughly broken up like this:
The PRO (National Archives) in Kew holds a random sample of 10% of Crew Agreements from 1861 -1938, and 1951 - 1989 in the BT99 class. The National Archives are also the home of the BT100 class - 'Celebrated Ships' (e.g.
Titanic,
Lusitania,
Olympic etc.)
The National Maritime Museum at Greenwich (James Caird Library) holds the remaining 90 percent for 1861, 1862 and all for years ending in 5 except 1945, up to 1972.
This is the one you're after for 1915. Kew does not hold any duplicates of these records. You'll need the Official Ship number for the
Suevic, as this is how they are filed. They can do a look-up for you in Lloyd's, but I've got it here handy somewhere and will find it for you. I suggest you contact them beforehand and let them know what you want to see - you may also want to read up on what information you'll need to get a Reader's Card. Here's a link to some online information about contacting them:
http://www.nmm.ac.uk/site/navId/005002002
Other archives, such as the Liverpool Records Office and some County Record Offices, hold crew lists for vessels registered in ports within their area, for the years from 1863-1912.
The Registry of Shipping and Seamen holds all crew lists from 1939-1950, and those from 1990 to the present day.
The rest for the period 1863-1972 are held at the Maritime History Archive at the Memorial University of Newfoundland.
BT350 class - The Fourth Register, (Central Index Register CR10) of Merchant Seamen: Sea Service Records
This was started in October 1913 and continued until 1941. The entries for 1913-1918 were destroyed, and so the register covers the actual period 1918-1940. Most cards are circa 1918-1921. Copies of the registry cards are held on microfiche within the National Archives; the original cards are held by Southampton City Archive. The records usually include a photo of the merchant seaman.
If you had time enough time, you might find your ancestor on BT26 - it would help to narrow it down as much as possible, though!
Hope this helps - I've spent many a happy hour in Kew and Greenwich, so I think you'll have a great time!