Contra-rotating propellers have benefits when providing thrust in marine applications. Contra-rotating propellers are used on torpedoes due to the natural torque compensation and are also used in some motor boats. The cost of boring out the outer shafts and problems of mounting the inner shaft bearings are not worth pursuing in case of normal ships.
Torpedoes such as the Bliss-Leavitt torpedo have commonly used contra-rotating propellers to give the maximum possible speed within a limited diameter as well as counteracting the torque that would otherwise tend to cause the torpedo to rotate around its own longitudinal axis.
Recreational boating also found applications: in 1982 Volvo Penta introduced a contra-rotating boat propeller branded DuoProp. [1] The patented device has been marketed since. After the Volvo Penta patents ran out, Mercury Marine has also produced a corresponding product, MerCruiser Bravo 3.
However, in commercial ships and in traditional machinery arrangement, contra-rotating propellers are rare, due to cost and complexity.
ABB provided an azimuth thruster for ShinNihonkai Ferries in form of the CRP Azipod, [2] claiming efficiency gains from the propeller (about 10% increase [3] ) and a simpler hull design. Volvo Penta have launched the IPS (Inboard Performance System), [4] an integrated diesel, transmission and pulling contra-rotating propellers for motor yachts.
At lower power levels, contra-rotating mechanical azimuth thrusters are one possibility, convenient for CRP due to their inherent bevel gear construction. Rolls-Royce and Steerprop have offered CRP versions of their products. [5] [6]
A propeller is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working fluid such as water or air. Propellers are used to pump fluid through a pipe or duct, or to create thrust to propel a boat through water or an aircraft through air. The blades are shaped so that their rotational motion through the fluid causes a pressure difference between the two surfaces of the blade by Bernoulli's principle which exerts force on the fluid. Most marine propellers are screw propellers with helical blades rotating on a propeller shaft with an approximately horizontal axis.
An azimuth thruster is a configuration of marine propellers placed in pods that can be rotated to any horizontal angle (azimuth), making a rudder redundant. These give ships better maneuverability than a fixed propeller and rudder system.
A propulsor is a mechanical device that gives propulsion. The word is commonly used in the marine vernacular, and implies a mechanical assembly that is more complicated than a propeller. The Kort nozzle, pump-jet and rim-driven thruster are examples.
Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque. Examples include some aircraft propellers, resulting in the maximum power of a single piston or turboprop engine to drive two propellers in opposite rotation. Contra-rotating propellers are also common in some marine transmission systems, in particular for large speed boats with planing hulls. Two propellers are arranged one behind the other, and power is transferred from the engine via planetary gear transmission. The configuration can also be used in helicopter designs termed coaxial rotors, where similar issues and principles of torque apply.
Aircraft equipped with contra-rotating propellers (CRP) coaxial contra-rotating propellers, or high-speed propellers, apply the maximum power of usually a single piston engine or turboprop engine to drive a pair of coaxial propellers in contra-rotation. Two propellers are arranged one behind the other, and power is transferred from the engine via a planetary gear or spur gear transmission. Although contra-rotating propellers are also known as counter-rotating propellers, the term is much more widely used when referring to airscrews on separate non-coaxial shafts turning in opposite directions.
The Voith Schneider Propeller (VSP) is a specialized marine propulsion system (MPS) manufactured by the Voith Group based on a cyclorotor design. It is highly maneuverable, being able to change the direction of its thrust almost instantaneously. It is widely used on tugs and ferries.
A Z-drive is a type of marine propulsion unit. Specifically, it is an azimuth thruster. The pod can rotate 360 degrees allowing for rapid changes in thrust direction and thus vessel direction. This eliminates the need for a conventional rudder.
An impeller, or impellor, is a driven rotor used to increase the pressure and flow of a fluid. It is the opposite of a turbine, which extracts energy from, and reduces the pressure of, a flowing fluid.
Azipod is a trademarked azimuth thruster pod design, a marine propulsion unit consisting of a fixed pitch propeller mounted on a steerable gondola ("pod") containing the electric motor driving the propeller, allowing ships to be more maneuverable. They were developed in Finland in the late 1980s jointly by Wärtsilä Marine, Strömberg and the Finnish National Board of Navigation.
MS Arcadia is a cruise ship in the P&O Cruises fleet. The ship was built by Fincantieri at their shipyard in Marghera, Italy. At over 84,000 gross tonnage (GT), Arcadia is the second smallest of seven ships currently in service with P&O Cruises. The ship officially entered service with the company in April 2005 and was named by Dame Kelly Holmes.
Manoeuvering thrusters are transversal propulsion devices built into or mounted to either the bow or stern of a ship or boat to make it more manoeuvrable. Bow thrusters make docking easier, since they allow the captain to turn the vessel to port or starboard side, without using the main propulsion mechanism which requires some forward motion for turning; The effectiveness of a thruster is curtailed by any forward motion due to the Coandă effect. A stern thruster is of the same principle, fitted at the stern. Sufficiently large vessels often have multiple bow thrusters and stern thrusters.
The Finnish maritime cluster is a cluster of Finnish companies in maritime industries. In 2016 the total turnover was estimated at 13 billion euros and it employed 48,000 people.
The Kitchen rudder is the familiar name for "Kitchen's Patent Reversing Rudders", a combination rudder and directional propulsion delivery system for relatively slow speed displacement boats which was invented in the early 20th century by John G. A. Kitchen of Lancashire, England. It turns the rudder into a directional thruster, and allows the engine to maintain constant revolutions and direction of drive shaft rotation while altering thrust by use of a control which directs thrust forward or aft. Only the rudder pivots; the propeller itself is on a fixed shaft and does not.
An L-drive is a type of azimuth thruster where the electric motor is mounted vertically, removing the second bevel gear from the drivetrain. Azimuth thruster pods can be rotated through a full 360 degrees, allowing for rapid changes in thrust direction and eliminating the need for a conventional rudder. This form of power transmission is called a L-drive because the rotary motion has to make one right angle turn, thus looking a bit like the letter "L". This name is used to make clear the arrangement of drive is different from Z-drive.
A ducted propeller, also known as a Kort nozzle, is a marine propeller fitted with a non-rotating nozzle. It is used to improve the efficiency of the propeller and is especially used on heavily loaded propellers or propellers with limited diameter. It was developed first by Luigi Stipa (1931) and later by Ludwig Kort (1934). The Kort nozzle is a shrouded propeller assembly for marine propulsion. The cross-section of the shroud has the form of a foil, and the shroud can offer hydrodynamic advantages over bare propellers, under certain conditions.
The rim-driven thruster, also known as rim-driven propulsor/propeller is a novel type of electric propulsion unit for ships. The concept was proposed by Kort around 1940, but only became commercially practical in the early 21st century due to advances in DC motor controller technology. As of 2017, commercial models of between 500 kW and 3 MW are available from manufacturers such as Rolls-Royce, Schottel, Brunvoll, Baliño, Voith, Van der Velden, etc.
An underwater thruster is a configuration of marine propellers and hydraulic or electric motor built into or mounted to an underwater robot as a propulsion device. These give the robot movement and maneuverability against sea water resistance. The main difference between underwater thrusters and marine thrusters is the ability to work under heavy water pressure, sometime up to full ocean depth.
A marine thruster is a device for producing directed hydrodynamic thrust mounted on a marine vehicle, primarily for maneuvering or propulsion. There are a variety of different types of marine thrusters and each of them plays a role in the maritime industry. Marine thrusters come in many different shapes and sizes, for example screw propellers, Voith-Schneider propellers, waterjets, ducted propellers, tunnel bow thrusters, and stern thrusters, azimuth thrusters, rim-driven thrusters, ROV and submersible drive units. A marine thruster consists of a propeller or impeller which may be encased in some kind of tunnel or ducting that directs the flow of water to produce a resultant force intended to obtain movement in the desired direction or resist forces which would cause unwanted movement. The two subcategories of marine thrusters are for propulsion and maneuvering, the maneuvering thruster typically in the form of bow or stern thrusters and propulsion thrusters ranging from Azimuth thrusters to Rim Drive thrusters.
Steerprop Oy is a Finnish company that produces marine propulsion equipment such as azimuth thrusters and bow thrusters. The company was established in 2000 in Rauma by a group of people who had previously worked at Rolls-Royce Marine Division.
Propeller theory is the science governing the design of efficient propellers. A propeller is the most common propulsor on ships, and on small aircraft.