^ Large Ship Propeller (Note the Geometry) Image Courtesy of Eastimages at shutterstock.com
Why are Propellers so Important?
Propellers are the devices that power the global economy. This may sound gross exaggeration, but is, in fact, the truth. Why? Because, they drive most ships. And ships transport over 90% of the globally traded merchandise.
Not for nothing is the health of shipping a potent indicator of the state of the global economy. Anything and everything that affects the efficiency of the humble looking propeller therefore holds vast potential far beyond what many can possibly fathom.
Propellers made from composite materials are one such advancement. Not only do they offer better propulsive efficiency with lower noise and vibrations, but also cut costs and weight. Furthermore, they hold the potential to minimize maintenance and inspection.
Taking a broader view, advances in material technology have always stimulated the development of mankind. This is because material progress offers fresh opportunities and humans evolve to tap them.
The Need for Composite Propellers
Scientists and engineers have researched and fine tuned the propeller ever since we invented this instrument. Through these years, the propeller blade geometry and design have evolved into an intricate entity that includes numerous controlling parameters.
Its form, contours, and dimensions have evolved with the times but nickel aluminum bronze (NAB) maintains its position as the number one material for building propellers. As of today, engineers build as many as 80% propellers using NAB.
Designers prefer stainless steels for building ice-class propellers. And there are manganese bronzes (high tensile brass) and manganese aluminum bronzes that find application for making a small number of propellers.
But then, the future availability and rates of copper are precarious. Copper is not a renewable resource. As a booming construction industry in the developing world places greater demand on copper, we will witness an acute scarcity.
Not to mention the speculative futures trading and possible stockpiling of copper that will send the prices spiraling up out of control. The expected future cost alone is enough to justify investment into composite propellers.
Plus, ship owners and operators are looking to cut down on fuel consumption. This not only cuts down fuel expenditure, but also slashes gaseous emissions that are the subject of increasingly tightening emissions at present.
Lest you wonder what the fuss on fuel prices is, the current phase of splendidly low oil prices will not last forever. This is because the following five fundamental drivers of oil demand will continue to be robust in the long run. And prices do rise with demand. These five drivers are:
- Population Growth
- Economic Growth
- Consumption Patterns
- Efficiency of In-Use Technology
With over 7 billion human inhabitants, the population of the world is immense. And it continues to multiply. Then again, we live in an era of motorized transport and fossil fuel dependent economies. A large chunk of this energy comes from oil.
Middle East is among the largest exporters of oil. And conditions here continue to be volatile. So much so, the risk of discontinuation of supplies can materialize anytime. It only takes one flare to light a charged up situation.
Again, markets operate on sentiment. A slight possibility of a supply side shock will send oil prices jumping through the roof regardless of whether the possibility actually unfolds. Speculators jump on such possibilities and add to the inflationary trends.
Technological advances will cut the demand and prices of fuel. But only slightly. The remaining four drivers will more than offset this drop. Such progress will cut down emissions though and make compliance with emission norms easier.
Composites for Propellers
A combination of two or more materials with dissimilar physical or chemical properties, composite materials or simply composites possess features that are completely different from that of their constituent elements.
Common composites include:
- reinforced plastics
- ceramic composites
- metal composites
- construction composites such as cement and concrete
Researchers have started to incorporate actuation, sensing, communication, and computation elements into composites. This has created a fresh class of materials called robotic materials.
Typical applications for composites include aircraft and spacecraft bodies, ship hulls, bridges, buildings, automobile bodies, swimming pool panels and the like.
Designers opt for composite materials owing to their superior strength and corrosion resistance. As compared to NAB propellers, composite propellers offer better propulsion efficiency while generating low noise-vibrations and cutting costs, weight, inspection, and maintenance.
Two composites look particularly promising:
- Carbon Fiber Reinforced Plastic (CFRP)
- Glass Fiber Reinforced Plastic (GFRP)
Fiber reinforced plastics are lighter than metals and alloys. With these, we can build larger propeller blades that improve propulsive efficiency thereby saving fuel and lowering emissions. Plus, the transport and assembly of lighter materials is simple and inexpensive.
Japanese classification society Class NK has spearheaded efforts to examine the viability of using fiber reinforced plastic propellers for commercial shipping.
Nakashima Propeller Co. Ltd. led a research project for developing a propeller mainly made from CFRP. Other players included Japan’s National Maritime Research Institute (NMRI), NYK Line, the University of Tokyo, Imabari Shipbuilding, and Monohakobi Technology Institute.
In 2012, this consortium fitted such a propeller as the side thruster for the 499 GT chemical tanker Taiko Maru. The results were good. And in 2014, the owner of the ship viz. Sowa Kaiun YK went ahead and installed a CFRP propeller for its main propulsion system.
Being 60% lighter as compared to its predecessor – the 1.96 meter diameter NAB propeller – the CFRP propeller is 2.12 meter in diameter. As mentioned, lightweight material enables the use of larger propellers that offer better propulsive efficiency.
Improved efficiency transformed into a 9% reduction in the required shaft power for cruising at the same speed. Furthermore, the CFRP propeller generated cognizably low noise on board. This was because the flexible propeller distributed pressure more evenly and trimmed down cavitation.
Propeller cavitation is an important cause of onboard noise and vibration. In fact, propeller cavitation produces most of the broadband noise. Shafts and bearings transfer these disturbances to the structure of the vessel.
In practical terms, the vapor pressure of a liquid is the amount of pressure you have to exert on its surface in order to prevent it from vaporizing. Liquids with higher vapor pressures are more volatile.
When driving ships forward, propellers create high pressure on the face of their blades and low pressure on the rear. The reverse happens for backward motion. Sometimes this low pressure falls below the vapor pressure of water. Water evaporates and leaves behind voids or bubbles.
These bubbles flow with water and collapse when velocity falls or when pressure increases. The collapse causes water from the surrounding to gush in, something that creates a surge or a pressure wave that generates the noise. This collapse of bubbles is cavitation.
If the bubbles collapse near the propeller, the propeller is at the receiving end of this pressure wave. And, many bubbles do collapse around the propeller. Flexible CFRP propeller blades sidestep this pitfall through better pressure distribution in their vicinity.
Submarines and torpedoes also employ the propeller. For them, noise and vibration are a strict no-no because it interferes with their ability to move undetected.
Other, related developments include:
- In 2003, British defense technology major QinetiQ developed and fitted a 2.9 meter diameter composite propeller aboard the RV Triton, a research trimaran vessel of QinetiQ
The 2.9 meter, five blade propeller is the largest composite propeller in the world at present. QinetiQ had built the RV Triton to demonstrate the viability of the trimaran hull form for large warships
- Airborne Composites provided a 2.5 meter diameter propeller for the Alkmaar Class mine-hunter vessels of the Royal Netherlands Navy (RNLN) in 2010. In the 1970s, the navies of Netherlands, France, and Belgium developed the Alkmaar Class vessels
Noise and vibration are big issues for mine-hunter vessels as the acoustic interference can set off those very mines they seek to neutralize
While weighing 70% less than a conventional NAB would, this composite propeller offers least acoustic disturbances. It also promises to minimize inspection and maintenance
Designers are also considering hydroelastic tailoring i.e. appropriate design of the flexibility of the composite propeller to optimize its performance or the performance of the entire system of this composite propeller
- S. Navy’s composite rudder for the DDG-51 Class Destroyers twists in order to offer a different angle of attack at diverse water depths
Two heads are better than one. When it comes to alloys and composite materials, two elements get entangled in a positively synergic relationship. Such synergy is exactly what designers look to tap when building composite propellers.
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