Sentence:  Then the sails on the mainmast were backing and we started getting stern way. "Eagle" was caught <WSD>aback</WSD>. 
Meaning of the word marked with <WSD> tags:  Backward against the mast; said of the sails when pressed by the wind from the "wrong" (forward) side, or of a ship when its sails are set that way. (First attested in the late 17th century.)<ref name=SOED/>