the boatbuilding.community

1.1 Background

Powerboats rely completely on their propellers in order to achieve their performance. The reasons 'why propellers' work and the factors influencing propeller design and performance become much more meaningful when we understand the engineering development of the propeller throughout its history. It is indeed interesting that, after the propeller was conceived from the original discovery of screw propulsion, it saw relatively modest further innovation. It may be that the first designs were remarkably good!

What makes the propeller work? How do we choose the best propeller? and just how can we get the most performance from our propeller design? This is a multi-part article on the engineering basics of propellers. This week, we'll look at the history of propellers and some of the forces in play as a propeller does its work.

The concept of a screw propeller is not new. In 950 BC, the Egyptians used a screw-like device for irrigation purposes. Archimedes (287-212 BC), the first scientist whose work had a lasting effect on ship propulsion is credited with the invention of the screw. His screw pump, created to pump out flooded ships and for supplying water to irrigation ditches, was the forerunner of the screw propeller.

Drawings prepared by Leonardo da Vinci (1452-1519) contain pictures of water screws for pumping. However, his famous helicopter rotor more closely resembles a propeller screw. (see Figure 1, below)

Even with this developing knowledge, the application of screw propulsion to boats could not take place until the advent of steam power. Due to greater suitability with the slow-turning, early steam engines, the first powerboats used paddle wheels for a form of water propulsion.

In 1660, Toogood and Hays adopted the Archimedean screw as a ship propeller. Even by the 19th century, screw propulsion was still considered only a second-rate means of moving a ship through the water. However, it was during this century that screw propulsion development actually got underway. In 1801, John Stevens experimented with a single-screw and a twin-screw steam-driven boat. Unfortunately, due to a lack of interest, his ideas were not accepted in America.

The acknowledgment for the invention of the modern style propeller goes to Smith and Eriksson, who acquired patents in 1835 for screw propellers, marking the start of it's contemporary development. Eriksson's patent showed a rotating bladed wheel, as well as twin-screw and single-screw installations. Eriksson's propeller design took advantage of benefits of the bladed wheel.

Most of these Archimedean screw inventors came up with little to really improve the configuration of the screw as a propulsion device. Uninspired variations consisted of changing the number of convolutions or altering the diameter over the length of the screw. Francis Petit Smith accidentally discovered the advantages of a \\"shortened\\" Archimedean screw. Originally, his wooden propeller design had two complete turns (what we might call \\"double-pitch\\"). Nevertheless, following an accident in a canal, his boat immediately gained speed after half of his blade broke away.

Smith capitalized on the \\"lucky\\" event by increasing the number of blades and diminishing the blade width - and came up with a design comparable to modern propellers. Notwithstanding this success, it was still many years before propellers truly displaced paddle wheels in ships.

The final transition to what is now recognizable as a screw propeller was made by George Rennie's conoidal screw. Rennie combined the ideas of increased pitch, multiple threads, and minimum convolutions in what he called a Conoidal propeller, patented in 1840.

Despite the successes of Smith, Eriksson and Rennie, there remained many implementation problems to be solved for screw-propelled ships. Early wooden ships were subjected to heavy vibration, and iron hulls were needed to resist the vibratory forces. With shaft and machinery below the waterline, stuffing boxes and transmissions had to be developed to prevent leakage. Thrust bearings were required to transmit the force exerted by the propeller to the hull. Higher speed engines had to be developed in order to realize the inherent efficiency of the screw, and techniques were needed for casting and machining strong, tough metals. As the engineering problems were overcome and higher speed engines were developed, more and more screw propellers were installed to replace paddle wheels.

In 1870, C. Sharp, of Philadelphia, Penn., patented a partially submerged propeller for shallow-draft boat propulsion. Charles Parsons unintentionally discovered the phenomenon of propeller supercavitation when his first turbine ship, ('Turbinia') failed to achieve his predicted speed of 30 knots.

He fit three propellers to each shaft, and solved this problem. The invention of the marine reduction gear soon made \\"multiple propellers per shaft\\" unnecessary.

Screw propellers installed in the 1860's lacked sophistication, but their performance exceeded all other devices conceived up to that time.

The paddle wheel gradually became obsolete as the screw propeller became the only propulsive device installed in seagoing ships. During the twentieth century, marine propeller technology has made some advancements toward greater efficiency, more reliable design, better performance, improved materials, and cavitation resistance.

Let us have a look at how present day propellers work.

A propeller can be said to 'push' the hull through the water. To understand this concept, let us consider a propeller, with one of its blades projecting out of the page (see Figure 3), and rotating (from top to bottom). So the propeller is moving from left to right.

As this single blade rotates, it forces (pushes) water down and back. At the same time, (because every force has a reaction) water will move in behind the blade to fill the space (low pressure) left by the downward moving blade.

This results in a pressure differential ΔP between the two sides of the blade - a positive force (the pushing effect) on the underside; and a negative force (the pulling effect) on the topside. This is also just how an aircraft wing works. This same action occurs on each propeller blade as the propeller shaft rotates.

The pressure differential (ΔP) causes water to be drawn into the propeller from the front (due to the low pressure, behind) and accelerated out the aft (due to the higher pressure, ahead).

This is just like a household fan \\"pulls\\" air in from behind it and \\"blows\\" it out the front. A boat propeller pulls water in from the front. As the propeller turns, water accelerates through and around the propeller creating a stream of higher-velocity water behind the propeller. This \\"water jet\\" action of pulling water in and pushing it out at a higher velocity is called \\"adding momentum\\" to the water. This change in momentum or acceleration of the water results in a force called \\"thrust\\".

In the picture below (Figure 5) a propeller blade has been sectioned along its blade chord. Note the difference in shape between the top and the bottom of the section. The bottom side has a more prominent camber or curvature to its shape - just like a \\"wing\\".

It is this curvature that creates the low-pressure on the back of the blade, thus inducing lift, much like the wing on an airplane. of course with a propeller, this \\"lift\\" is translated into a horizontal movement component.

A propeller moves though the water in a similar manner as a mechanical screw moves forward through a piece of wood. The distance or forward motion depends mainly on the propeller pitch - defined as how far the propeller moves in one complete revolution.