The next compartments are the cargo holds which may number from one to eight, ten or more depending on the size and type of ship. Holds are traditionally numbered for reference from bow to stern. Each hold is accessed through a hatch which is an opening in the main or weather deck. There may be intermediate decks (tween decks) in the holds. On deck there may be cranes or gantries for cargo handling. Between the floor of the hold (paradoxically sometimes referred to as the ‘ceiling’) and the outer hull of the ship is a space called the double bottom. This is used for the storage of fuel, water or ballast. Many modern ships also carry fuel and/or ballast in tanks between the hold and the side of the ship (wing tanks). Aft of the holds is the engine room which contains not only the main engines but auxiliary power sources. Above deck over the engine room is the accommodation for the vessels crew and storage spaces for ships stores. The propeller shaft runs from the engine room through the stern to the propeller which is in front of the rudder. Above the propeller shaft may be a further cargo hold or water tank. On larger container ships there is usually a deck area for container stowage. Reference has been made to loadlines. Each vessel is allocated, on construction, a loadline which is basically a mark on the ship’s side which indicates the maximum draft to which the vessel can be loaded. Because a vessel can be loaded to a deeper draft with safety in calmer weather, different marks are allocated to different seasons and also to allow for loading in fresh water which is less buoyant than salt water. To assist in establishing the vessel’s draft “draft marks” are painted on the vessel’s hull at the bow, stern and amidships by the loadline.