The propeller: A brief guide to its history, design and maintenance
9th June 2016
An essential component of most vessels the boat propeller is a humble and elementary piece of technology that has evolved over time to deliver thrust to mechanised boating. From its origins in sculling to revolutionising the boating world, the propeller is often overlooked due its simple functionality but, with careful selection and diligent maintenance, can ensure your vessel performs at its optimum capabilities.
Fundamentally propellers create both momentum and lift. The former stating that accelerating a substance in one direction creates a force in the opposite direction and the latter (Bernoulli’s principal) creating a forward lift, essentially being a circular wing.
History of the propeller
The founding science behind the propeller is attributed to Archytas, disciple of Pythagoras, in 400BC who put an inclined plane on a cylinder but it was a century later when famous mathematician, Archimedes, developed the screw propeller. Known, aptly, as Archimedes screw this precursor of the modern day prop was first used or irrigation purposes and to bail water from boats. Well known for his interest in spirals in space, Archimedes is supposed to have invented the device after a trip to Egypt. It has been suggested that the contraption (consisting of a spiral within a cylinder) was actually in use up to 350 years prior to his introducing it to Ancient Greece.
Famous artist and inventor, Leonardo Da Vinci, is known to have incorporated an adaptation of the screw propeller within his designs for a theoretical aircraft published some 400 years before the invention of flight.
The incorporation of mechanised propulsion is but one element of the propellers evolution which also has a long history in the design of blades used for sculling. Manual rowing of a vessel largely depends on the style of the craft, the type of water being navigated and the position of the rower; as a result various blade designs evolved. Sculling is a particular form for rowing which has a technique upon which the propeller was innovated. The form involves the rower moving a single blade from side to side in an arc motion whilst maintaining the blade at a distinct angle for the most effective transfer of energy.
The combination of the Archimedes screw and the knowledge gained from centuries of sculling culminated in several breakthroughs in technology during the late 17th -18th centuries. In 1661, Toogood and Hays patented a design for marine propulsion which seems to have involved a sort of water jet propulsion. A few years later, English physicist, Robert Hooke, is credited with the idea of a propulsion system not unlike a windmill. Still far from the modern day propeller but growing closer.
However, by attaching a blade to a rotating shaft such that the arc could extend through 360 degrees, James Watt expanded on the design of his steamers (which used large paddle wheels to provide the thrust). His sketch of four angled blades attached to a screw, or a ‘spiral oar’ as he put it, is about as close to modern day designs as history can get but was never brought to fruition in Watt’s own vessels. Several patents followed, each expanding on the sketch that Watt made and, by 1804, Colonel Stevens had conducted successful experiments aboard a 25’ long 4’ beam vessel with a screw propeller of his own design achieving speeds of 4 mph. Though Stevens work was highly successful, and was one of the first engineers to correctly pay attention to the blades curvature (camber), placement of the propulsor and angle of the blades, his designs were not accepted or taken up.
It wasn’t until 1827 when Josef Ressel invented, patented and successfully trialed a ‘never-ending screw’ propulsion system. After manual trials on a smaller vessel his bronze screw propeller was installed on the ‘Civetta’, a 48 tonne steamboat, and reached speeds of approximately 6 knots. Ressel’s designs caught the imagination of other engineers and a whole host of similar trials were undertaken across Europe during the 19th century including John Patch in 1832, Francis Petit Smith and J Ericsson in 1836 and Isambard Kingdom Brunel in 1845.
As the teething problems associated with each design were ironed out the shipping world began to view the propeller as less preposterous and began to embrace the idea.
By 1844 the Admiralty, impressed by trials on the HMS Rattler, finally accepted the screw propeller and paddlewheel vessels began to decline in popularity.
It was Brunel’s designs, as a respected engineer, which finally led to mass take-up of the designs with his own being fitted to the first screw prop vessel to cross the Atlantic and that same machinery (being tested in 2004) having an efficiency of 65% by modern day standards.
Modern evolution of the propeller
The design of the propeller remained reasonably static from Brunel’s efforts until about 1970 when the fuel crisis at the start of this decade forced engineers to seriously consider the efficiency of the propeller. Combined with a need to optimise environmental outputs designs grew to be more complex with adaptations made to the shape of the stern and incorporating new, unconventional, shapes in blade design.
Propeller design continues to evolve, with modern technology able to hone and fine-tune efficiency up to approximately 70% and come in various designs, namely modular propeller, controllable pitch propellers and skewback propellers.
Routine care and maintenance of your boat propeller is just as critical as engine care. A prop that is not kept good working condition can cause a variety of issues which not only affect the performance and efficiency of your vessel which can in turn increase fuel costs but can also result in costly damage. Props are subject to corrosion, cavitation and surface damage and, ignoring the correct maintenance procedures could even result in your vessel being stranded.
Firstly, if you have a damaged propeller then you should not run the boat for long periods before you get it fixed or replaced. Doing so can result in bush failure or ‘cavitation burns’. Prolonged use of a damaged propeller can even damage the stern drive and engine.
Always be alert to signs of propeller damage including visual inspections (when possible), loss of speed, poor fuel economy, slipping and vibration.
It’s important to annually service your propeller and ensure that the prop shaft is greased using a water-proof grease as part of your regular maintenance.
Some stainless steel props are prone to corrosion and discolouration so should be treated to a protective coating.
Finally, always ensure that repairs are carried out by professionals in order to avoid additional damage.
For details of accredited propeller maintenance companies and all your other prop needs, be sure to check out our list of suppliers in our directory
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