In going through my father's collection, I came across 11 covers and postcards sent by a passenger aboard the Swedish American Line passenger ship M/S Gripsholm during February and March 1937. I enjoyed them so much I thought I'd share a few with the forum.
The Gripsholm, built in Newcastle, U.K. in 1925, was SAL's first transatlantic motor ship. On this particular voyage, the Gripsholm departed Goteborg, Sweden bound for Rio de Janeiro by way of New York.

Here is a sampling of the covers, cards, and markings:




There was nothing in the collection after the 27Feb37 posting from Praca Maua, Rio de Janeiro so I'm assuming the sender, whose name I've been unable to make out, left the ship there and flew back to the States. (EDIT: Just found nine more covers and cards from ports of call on the return trip - Martinique, Trinidad & Tobago, and Jamaica.)
Interestingly, about five years after this cruise, the Gripsholm was pressed into service as an exchange and repatriation ship during World War II. Chartered by the U.S. State department between 1942 and 1946, she departed from New York on her first voyage in that role on June 18, 1942 with 1,083 Japanese nationals, bound for Japan.
I'd be interested in seeing other examples of ship mail if any of you have them.
Steve