What is the term for the horizontal structure at the top of the ship's hull?

The term for the horizontal structure at the top of the ship's hull is the Main Deck. This deck serves as the primary working and operational platform for various activities on the vessel, as well as the main route for personnel and equipment movement. It typically spans the entire length of the ship and is often where important equipment, cargo, and crew space are located. The Main Deck also plays a significant role in structural integrity, providing a strong foundation that supports the weight of the vessel and its operational needs.

In contrast, the Weatherdeck refers specifically to those parts of the deck exposed to the weather, which may or may not align with the Main Deck depending on the ship's design. The Stem is the forward part of the ship's hull, while the Rudder is the structure used for steering. Both of these terms relate to specific components of the ship's design rather than describing a broad horizontal structure.