Steamship
Screw-propeller steamers

The key innovation that made ocean-going steamers viable was the change from the paddle-wheel to the screw-propeller as the mechanism of propulsion. These steamships quickly became more popular, because the propeller's efficiency was consistent regardless of the depth at which it operated. Being smaller in size and mass and being completely submerged, it was also far less prone to damage.

James Watt of Scotland is widely given credit for applying the first screw propeller to an engine at his Birmingham works, an early steam engine, beginning the use of a hydrodynamic screw for propulsion.

The development of screw propulsion relied on the following technological innovations.

Steam engines had to be designed with the power delivered at the bottom of the machinery, to give direct drive to the propeller shaft. A paddle steamer's engines drive a shaft that is positioned above the waterline, with the cylinders positioned below the shaft. SS Great Britain used chain drive to transmit power from a paddler's engine to the propeller shaft – the result of a late design change to propeller propulsion.

An effective stern tube and associated bearings were required. The stern tube contains the propeller shaft where it passes through the hull structure. It should provide an unrestricted delivery of power by the propeller shaft. The combination of hull and stern tube must avoid any flexing that will bend the shaft or cause uneven wear. The inboard end has a stuffing box that prevents water from entering the hull along the tube. Some early stern tubes were made of brass and operated as a water lubricated bearing along the entire length. In other instances a long bush of soft metal was fitted in the after end of the stern tube. SS Great Eastern had this arrangement fail on her first transatlantic voyage, with very large amounts of uneven wear. The problem was solved with a lignum vitae water-lubricated bearing, patented in 1858. This became standard practice and is in use today.

Since the motive power of screw propulsion is delivered along the shaft, a thrust bearing is needed to transfer that load to the hull without excessive friction. SS Great Britain had a 2 ft diameter gunmetal plate on the forward end of the shaft which bore against a steel plate attached to the engine beds. Water at 200 psi was injected between these two surfaces to lubricate and separate them. This arrangement was not sufficient for higher engine powers and oil lubricated "collar" thrust bearings became standard from the early 1850s. This was superseded at the beginning of the 20th century by floating pad bearing which automatically built up wedges of oil which could withstand bearing pressures of 500 psi or more.
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