Which of the following contributes to the safety of a ship by being erected around the edge of decks?

Lifelines are essential safety features on a ship, designed to help prevent crew members and passengers from falling overboard. These ropes or cables are typically erected around the edge of the decks and can be tightened and adjusted to provide a secure barrier. When the ship is in motion or in rough seas, lifelines act as a physical deterrent, ensuring that individuals can stay safely on deck without the risk of accidental falls into the water.

In maritime safety, lifelines enhance situational awareness and serve as a clear indication of the ship's perimeter where movement should be carefully managed. This is particularly vital during tasks that may require leaning over edges or in conditions where the ship might roll or pitch.

The other options, while relevant to ship operations, do not specifically relate to enhancing safety through physical barriers around decks. Freeboard refers to the distance between the waterline and the upper deck, affecting how high a ship is above the water but not directly serving as a barrier. The rudder and propeller are components involved in steering and propulsion, respectively, and do not contribute to the safety function that lifelines provide on deck.