The principal function of a mast on a Liberty Ship is to support cargo booms for lifting cargo from the pier onto the ship and into its holds. They also support signal lights, wireless antennae, and the crow’s nest for lookouts. The three masts on Liberty Ships are the fore, main, and mizzen.
These terms date back to the age of sailing ships. Each mast has four booms hinged at their base which work together with block and tackle and steam winches to facilitate the loading and unloading of cargo.
The first mast you see here is the Main Mast, the second mast from the bow of a ship, which is used to load cargo into the No. 3 hold. The O’Brien’s Main Mast and its booms had a maximum lifting capacity of 35 tons making it capable of loading 30-ton M4 Sherman tanks.
The special “Jumbo Boom” forward of No. 2 Hold was rated at 50 tons and was the only set of gear capable of lifting the weight of heavier vehicles by using a larger, 9” diameter steam-powered piston.
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As you continue forward, observe the angled structure above you. It’s a Life Raft, which is one of four on the ship.