Future Prospects for Partially Overlapping Ship Propellers
Overlapping propellers first appeared during the latter 19th century when navies sought to increase the speed of small watercraft. Sir Charles Parsons installed multiple small propellers on the same drive shaft on the world’s first steam turbine powered boat, the Turbinia. The sheer power and rotational speed of the steam turbine resulted in the destruction of propellers due to cavitation. The installation of multiple concentric propellers on the same drive shaft reduced cavitation and greatly increased propulsive thrust.
Beginning during the late 1930’s, designers of heavy aircraft installed concentrically mounted, counter-rotating propellers on the same axis to increase propulsive thrust. Some modern day helicopter designs utilize a pair of concentrically mounted counter-rotating rotors that provide the combination of lift and propulsion, while neutralizing the effect of torque reaction. Pairs of counter-rotating propellers on concentric drive shafts were subsequently adapted to maritime propulsion, as is the case of Volvo-Penta Marine that offers co-axially mounted contra-rotating propellers for high-speed maritime propulsion.
Most counter-rotating propeller designs require the use of gear trains or of electrical propulsion. Except that gear-driven propellers have limited application on large ocean-going ships where engine output exceeds 20,000-kW. Electric propulsion also raises capital cost and incurs a loss of efficiency when compared to a direct-drive mechanical propulsion system. However, a precedent in aeronautical propulsion involves a pair of closely mounted parallel output shafts that provide power to a pair of closely spaced and partially overlapping propellers.
The arrangement produces increased thrust in the area of the overlap, where the propeller blades move in opposite directions to each other, essentially duplicating the effect of counter-rotating propellers to boost propulsive thrust. Partially overlapping propellers offer the prospect of increased thrust with possible gains in propulsive efficiency. Researchers at Kawasaki Marine are adapting the partially overlapping propeller concept to maritime propulsion, for application on large bulk carriers.
A single engine driving through a gearless mechanical power transfer may operate a pair of partially overlapping propellers on mega-tonnage maritime bulk carriers. The Swiss-mechanism transfer drive mechanism uses crank throws, bearings and tension rods to transfer large amounts of power at high efficiency and may be installed at the front of the single engine. It could transfer some 50% of the engine output to a drive shaft installed alongside and parallel to the engine. A second drive shaft would carry power from the rear of the engine, with both shafts driving partially overlapping propellers.
Depending on the desired spacing between the 2-driveshafts, a second Swiss mechanism may be installed to the rear of the engine, to bring the output shafts and the propellers closer together. At the present day, ship propellers may have reached the maximum diameter that will be installed on freight ships. With many ports being located along the mouths of rivers or upstream along navigable rivers, restricted navigation draft at such ports and along such waterways ultimately restricts the diameter of the propellers installed on ships that sail to/from such ports.
Many commercial maritime freight operators prefer the combination of a single engine directly driving a single propeller. These operators may be willing to consider the option of single engine driving a pair of partially overlapping propellers, with some power flowing through a gearless mechanical power transfer case to drive a one of the propellers. The absence of sliding friction within the Swiss mechanism would greatly increase power transfer capacity while offering the prospect of a greatly extended service life.
A single engine, twin-propeller propulsion system may find application in an oceanic carrier or in an oceanic tug that may push an oceanic barge built to identical dimensions as a bulk carrier or container ship. The absence of an engine and fuel tanks aboard the main ship would increase payload capacity, perhaps sufficient to offset the operating cost of the oceanic super-tug. Conversely, single-engine and twin-propeller propulsion system installed aboard an oceanic ship would allow it to tow or to push a companion barge of almost equivalent size and payload capacity on extended voyages.
Harry Valentine can be reached at firstname.lastname@example.org for comments and questions.
The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.