Here is a repeat of a message I posted earlier. It should help:
You can write to the National Archives at:
National Archives and Records Administration
Modern Military Reference Branch
8601 Adelphi Road
College Park, MD 20704
Ask them for a copy of the Movement Card for the ships you are interested in. This will show you all the places the ship went, usually showing date and place of departure and date and place of arrival.
The movement card will indicate (sometimes) what convoy, if any the ship was part of. You can find the voyage details for the trip by asking the Archives for a copy of the Armed Guard Voyage Report for whatever voyage you are interested in (either tell them the convoy or the dates). The voyage report usually details events of the trip, and also frequently includes the crew list (both Armed Guard and Merchant seamen).
The Archives sometimes (usually these days) charges for the material. If you initially ask for just the movement card, you'll probably get that for free (I have been successful with that), but if the other material is more than a few pages they usually send you an estimate of the cost. Don't be surprised if it takes weeks to get a response. Sometimes I get replies in a few days, other times it takes many weeks.