Ropes on a sailboat are not called ropes, They are called lines. The lines that control sail are called sheets. The head sail (or the sail in the front of the boat) has two sheets attached to it. One for starboard side, one for port side. The main sail. ( the biggest sail attached to the mast) has one sheet

The sheet is the line that controls the sails on a ship. If the line is not secured, the sail flops in the wind, and the ship loses headway and control. If all three sails are loose, the ship is out of control.