The basic idea of the propeller is the same as that of the turbine. The difference arises from the fact that instead of converting a flow of fluid into energy, it acts as a means of propulsion. It consists of two or more blades with spiral surfaces arranged radically round an axis. A controversy arose in the propeller's early days as to whether it was superior to paddles as a means of propulsion. The question was answered in 1845 when a propeller constructed by an Englishman called Smith was fitted for the first time on to a British ship, the HMS Rattler (888 tonnes), and this ship was attached stern to stern to the Alecto (800 tonnes), which was driven by paddles. The two ships were equal in power, with 200hp engines, and floated at the same waterline. It was clear from the start that the propeller driven ship had the best of it, and the Rattler was easily able to tow its rival. The success of the propeller was guaranteed from then on.

Five successive stages in the evolution of the propeller (Shown above).

1. Early propeller by shorter, 1800.
2 Smith's screw-propeller, 1836. It proved fragile and was therefore shortened-3,
4. Steven's double contra rotation propeller.
5. Ericson's propeller of 1839, almost identical to a modern propeller.

A giant propeller as fitted on large ships compared with the size of a human figure. In 1858 the Great Eastern was launched. It was the largest ship ever built at that time and its propeller was 7.31 m in diameter.

The first attempt to use the propeller to drive lighter-than-air craft was probably made by Gifford, who used a steam engine. The propeller became more and more popular and played an important part in the development of aviation.

An up-to-date model of a ship's propeller developed by the United States Navy. It is called a 'supercavitating' propeller and makes use of cavitation (the formation of a space filled with water vapour round the turning blades of the propeller) to gain increased speed. In earlier models speed had been cut down by the pocket of bubbles caused by the propeller's eddy.