Exactly how the propeller works is widely misunderstood amongst the general public.
The source of the ambiguity may have arisen from the earliest beliefs held by an
industry that forged the modern water transportation revolution. Unfortunately most
of the initial ideas were conjecture and it was not until much later that the scientific
framework was constructed. By then, long since embedded myths had become de facto
law in the minds of many.
One of these erroneous myths, viewed the propeller as a “screw”. This is understandable,
since a propeller can easily be visualized as operating in a similar way to a wood
screw or an auger. These two devices are based on the “Archimedes screw”, which
either winds its way through material (wood screw) or alternatively, winds material
through itself (grain auger). The principle is based around a structure called a
helix. Once entrained in a revolving helix, the relative rate of motion between
the material and helix will be constant.
Early inventors postulated that an Archimedes screw could be used to displace water
to propel a boat forwards. This principle is defined by Newton’s third law, stating
that “for every action, there is an equal and opposite reaction”, therefore it made
sense that a screw could be used to propel boats effectively. Many speculative patents
were drawn up during this early stage of development. At that time an inventor called
John Ericsson (1803-1889) was working for the British Admiralty testing the proposition
that an Archimedes screw could outperform a paddle wheel as a boat propulsor. Experiments
were not going well when a fortuitous mishap caused the screw to break, leaving only
a very small leading section of the helix intact. Surprisingly the boat’s performance
improved dramatically. It was not exactly clear why, however by the end of the nineteenth
century propeller designers had discovered that creating a “rounded” shape on the
upstream side of the propeller blade raised performance still further.
Without getting into too much scientific detail, what Ericsson had observed quite
by accident, was the principle of lift. This same principle was subsequently applied
to propeller and wing design in aeroplanes, leading to powered flight. Lift was
already utilised in nature by certain birds to allow them to hover in the wind without
flapping their wings. Humans had also made use of the principle as a way to make
sailing vessels go faster or even move forward against the wind, in a manoeuvre called
The principle of lift requires that fluid or gaseous particles passing over one surface
of the lift generating device (For example the “upstream” side, in the case of a
propeller blade) to speed up relative to the velocity of the particles passing over
the opposite surface of the same device. An aerofoil is a shape that forces fluid
to travel farther thus increasing particle velocity. The raised velocity causes
a drop in pressure on the foil side compared with the other, which in turn generates
lift. In the case of the propeller, the blades advance forwards towards the zone
of lower pressure while water is accelerated both axially (backwards) and radially
(spinning). As the water accelerates towards maximum velocity, the water pressure
from fore to aft through the propeller drops, causing the surrounding water to “suck”
inwards to fill the void that is created. The aerofoil ensures that the pressure
difference across the blades and the consequential lift is maintained.
Entire books have been written on propeller design and function. For the purposes
of this discussion however, too much detail would introduce unnecessary complication.
Simply put, theory and design considerations regarding the propeller are all aimed
at achieving one goal; acceleration of water over the blade surfaces of the propeller.
If you wanted to explore this in greater depth, a book by David Gerr called “Propeller
Handbook” would be a good start.
The motion of the water once it leaves the propeller is determined by the interaction
between the water plume accelerating off the blade and the surrounding water. This
is known as wash. Relative to the initial velocity of the water forward of the propeller,
the speed of the wash is high. Relative to the pressure ahead of the propeller,
the pressure of the wash is low. As the wash dissipates and slows, the pressure
gradually re-equalises with the surrounding water. To achieve all this, the propeller
must undergo something called slippage which causes the aforementioned pressure differential
and resultant lift to occur.
The story of the invention of the propeller clearly demonstrates that a helix structure
can be configured for different functions. On the one hand it can be optimised to
facilitate the generation of lift and acceleration of water across the structure
using slippage, that is, as a propeller. On the other hand it can be set up to allow
for sure and powerful transport of substances along the helix as in an Archimedes
screw. This is enabled by the entrainment and partitioning of the material within
the helix. Essentially this prevents any significant axial acceleration of the water
relative to this structure. Although both are based on the structure of a helix
their individual functions and characteristics are quite different. In this sense
the two designs have elements of mutual exclusivity between them. This was reflected
historically in the “accidental” invention of the propeller via the destruction of
a screw structure.
Propellers are actually complex devices that operate successfully in a very demanding
environment. Water is about 1,000 times more dense than air and moving through it
at speed is challenging, requiring vast amounts of energy. It was Ericsson’s fortuitous
accident that created the first efficient propeller, making high speed propulsion
possible and it would take a further 113 years for the Archimedean screw principle
to once again resurface as an alternative. Working in isolation on a New Zealand
high country sheep station during the 1950’s, a farmer trying to find a way of negotiating
the shallow water-ways, invented the world’s first commercially viable water-jet
propulsion system. The company that bears his name, CWF Hamilton, still make water-jets
today. Although initially using a centrifugal pump, Hamilton abandoned this for
an axial flow, pressure inducing impeller. Although not obvious on cursory inspection,
the jet impeller is effectively based on the Archimedes screw.