The sail is just flapping and there's no drive in it.

Out to the sides like wings, sails flapped into place.

Then we could leave the sail flapping for our signal all the time.

The sail creaked again, and flapped as the wind grew.

They could not talk, the old sail flapped so furiously, even if the other noises would have allowed them.

Tonight, his sails are flapping in a stiff and unpredictable breeze.

The sail flapped and filled with wind, tugging their boat forward.

The sails flapped three or four times and then slipped comfortably into curves.

The sail flapped, and spray flew up from under the bows of the boat.

The wind filled it gleefully, and the sail flapped eagerly.