Aft. It stands for after. Because the stern of a ship is 'after' the forward part of the ship (bow). The bow of ships in this century have bow thrusters that provide transverse thrust to the vessel and are equipped with propellers and rudders at the stern that are connected to the engine. That's why you don't see any ships having holes for oars or masts with huge billowing sails anymore. Ships of old have a rounded hull shape that is called tumblehome. Now it's best to avoid this since it'll exaggerate the heel of a vessel (when it rolls in the sea) A raised and enclosed superstructure section of the hull which resembles a castle is known as the forecastle. The term is now reduced to focsle. The captain will steer the vessel at the bridge which is the main part of a ship's superstructure. A superstructure is everything that is above the hull. The hull is the main body of the ship. Before a ship is built, the hull is divided into its respective hydrostatics station that usually range from 0-12 from the aft to forward. Bulkheads are the vertical partitions that separate these stations from each other. The strongest bulkhead is called the collision bulkhead. It is built at the forward most part of the ship in case of collision with other ships. Cofferdams are void spaces between bulkheads to saparate fuel and water tanks from each other. What then follows is the longest part of the birth of a vessel. The planning part. The general arrangement of a vessel is thought out. Usually the bridge is built towards the stern, giving rise to the term aft superstructure. This is so that the engine room is directly below the bridge and is near to the propeller and rudder. The shape of a conventional hull gradually becomes fuller as it nears the bow. This is where careful planning is needed to avoid putting too much auxillary (machinery other than the engines) in the forward section since this can cause the vessel to trim. At worst, the ship will nose-dive into the ocean. In the olden days, ships were made out of timber. Hundreds of trees were felled just to build one ship. Nowadays, steel has overtaken timber as the material of choice due to its higher structural strength and durability. Shipmakers of old followed a certain way of making the ships just right so that it won't sink. These were derived from trial and error. In these modern times, naval architects rely on modelling and calculations of strucutural analysis of each member that makes the ship what it is. The stress concentrations, amount of torsion the ship will undergo once it is set to sail. Because if you think of it, a ship is essentially a long, big beam. If you're on a huge tanker that can span 100s of metres in length, you will see the deck twist and turn in high seas. Shear force, normal force, strain, stress. How much is it subjected to? On top of that, the vessel is never stationary, the weight it displaces in the water provides and upward buoyant force that makes the ship float. Contributing to these factors are the Longitudinal and vertical Centre of Buoyancy, centre of gravity, (LCB, LCG, VCB) and these values will have to be calculated to ensure that the ship will not sink.
So you see, designing a ship does not look so simple once you put it that way. But it sure makes for a very interesting process. Definitely!