Marine propellers ended the era of sails and masts in maritime travel. Attached to the bottom of the stern, just behind the fin, the propeller provides thrust for the ship. It spins way slower than propellers on a prop plane since water is denser than air. In addition, ship propellers can spin in reverse, whereas propellers in planes—while having the capability—rarely do it.
A special type of industrial bronze has been developed to make marine propellers. Ordinary bronze just won’t do as these parts will spend its life for the most part, underwater, moving ships as long as the Empire State Building (on its side) and as heavy as a thousand blue whales. Not only must the propellers be corrosion-resistant, but they must also be strong and resilient, as well as highly efficient and easy to maintain.
Constant exposure to seawater would rob ordinary bronze of its zinc coating in a matter of days, if not weeks. Instead, complex alloys, such as nickel aluminum bronze (nibral) are used. According to manufacturers, nibral is strong, lightweight, and can be repaired with simple welding techniques. The resulting metal may not look like conventional bronze due to the aluminum and nickel.
However, the most popular is silicon bronze, which contains 4 percent silicon and other metals. Silicon is known to last longer in corrosive conditions than most metals.