Sails. Looks like her owners were trying to reduce fuel consumption at a time of relatively high bunker prices [but nowhere near today's prices].
There were a number of such ideas around at that time - the Walker Wingsail was one, and Stephenson Clarke fitted a unit to one of their ships [Ashington?].
I also seem to recall that Contship had sails on one of their container ships, rigged to its cranes.

David Ford