Why Propellers and not Paddle Wheels

This small text is not technically pretentious, its only objective is to spotlight the ship propeller, its history, and manufacture process. Propellers are marvellous pieces of engineering, both in mechanical and hydrodynamic terms. Propeller design and its matching with propulsion machinery are also an art, as well as its construction. Let’s talk about propellers then.

In 1845 The British Admiralty decided to conduct an experiment to determine which was the superior form of ship propulsion—the paddle wheel or the screw propeller.

Accordingly, Their Lordships invoked the British love of a well-matched contest and staged a tug-o’-war between HMS Rattler, a screw propelled ship designed and built by Isambard Kingdom Brunel, and the Alecto, a merchant ship fitted with paddle wheels.

Both ships were the same size, around 800 tons, of about equal power, and the contest was staged in calm seas. It was not recorded the how the betting went, but the final result was that HMS Rattler towed the Alecto away at a steady two and a half knots. It was hailed as a victory for engineering progress by the protagonists of the screw propeller.

But in fact the paddle wheel is a very efficient propulsive device and is competitive with modern propellers. Where it falls down is in its vulnerability to damage. It also has an unfortunate tendency to leave water when the ship lists in heavy seas and it presents complicated steering problems. But the main reason why the Alecto lost the tug-o’-war against HMS Rattler was because a paddle wheel operates close to the water surface and it suffers excessive slippage under heavy load and slow speed and high thrust. HMS Rattler’s propeller, on the other hand, was well submerged with good low speed thrust.

After the contest the marine screw propeller found increasing favour among naval architects and today nearly all ships are fitted with propellers, except certain specialised river steamers operating in shoal waters and some ships plying on sheltered lakes.

In broad terms, when a propeller screws itself through the water, each revolving blade describes a helical curve as it moves forward in the direction of movement of the ship. The longitudinal distance corresponding to one revolution of the propeller is called the pitch, as in any screw device.

This screw motion produces the thrust to drive the ship by imparting momentum to the water the propeller displaces in an astern. In other words, it pushes the water backwards and this in turn develops a reactive force which thrusts the ship forward. It sounds like a straight forward exercise. But in practice the propeller is one of the most critical components on a ship.

It has to operate continuously from 250-300 days a year in a corrosive medium, working at or very close to the maximum power. Superimposed on a fairly high mean stress are strong cyclic stresses. In addition, transitional stress more than twice the mean stress can be imposed by crash reversal of the ship, and sometimes by the ship’s heaving and pitching motions.?Add to these factors the erosion problems caused by cavitation (formation of bubbles on the blade surfaces), and the possibility of mechanical damage to the blades, and it can be seen that there are few engineering components which have such severe performance levels imposed on them.

The adoption of the propeller for a ship propulsion led to the establishment of specialist manufacturers making the design of fixed pitch solid propellers, although in 1930, a competitor appeared, the controllable pitch propeller, which will be addressed in another text.

Some key stages in making a fixed pitch propeller. The art!