They give a ship increased speed

Usage examples of stunsails.

Now the men were busy cutting away the torn stunsails and halyards and clearing the booms.

Mama mia, with stunsails set and running almost dead before the wind it was hard to see forward.

During the night he had expected the stunsails to carry away at times as occasional but brief gusts came up astern without warning.

Ramage had not set the topgallants or stunsails, but apart from that the Jocasta showed no sign of the assault by the caldereta.