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Propellers are the most commonly used means of propulsion for vessels. Driven by an engine or a motor, propellers consist of a shaft ending in blades that rotate in the water, generating thrust to move the vessel. Propellers come in many different materials, sizes, and designs, each suited to different vessel designs and operating conditions. Propellers typically have between 3 and 6 blades that either have a fixed angle (known as fixed pitch) or can be adjusted according to operational demands (known as variable or controllable pitch). Historically, propellers were designed with speed and efficiency in mind. While these remain important factors, researchers and engineers – including those involved in Quiet Vessel Initiative-funded projects – are now also considering how to make propellers quieter.
Watch a video to learn how propellers work
Development of Noise Reduction Measures for Conventionally Propelled Whale Watching Vessels
JASCO ShipConsult partnered with whale-watching operator Eagle Wing Tours for the project Development of Noise Reduction Measures for Conventionally Propelled Whale Watching Vessels. The first part of the project explored how low-frequency noise and high-frequency noise are generated at slow and fast speeds. The project considered three of Eagle Wing Tours’ vessels: one catamaran equipped with two engines that drive five-blade propellers, and two speed boats, one with three outboard motors equipped with three-blade propellers, and one equipped with surface-piercing propellers.
Overall, the vessel with the surface-piercing propeller was the quietest despite often travelling much faster than the other vessels. Because the propeller is half in and half out of the water, it pulls air bubbles into the water column as it spins. These bubbles create an air blanket that dampens noise. In addition, because the propeller is located at the surface as opposed to a metre or more down as most conventional propellers are, air introduced by wind and waves surrounds the propeller, creating a further dampening effect.
The project determined that most of the low-frequency noise came directly from the engines. The outboard motors on the speedboats also proved to be a source of low-frequency noise, with noise from the exhaust gas outlet propagating into the water. Other low-frequency noises were what is known as airborne noise – essentially vibrations from the various machinery onboard that are transmitted through the vessel’s hull into the water column. At higher speeds, however, the vessels’ noise profiles were dominated by high-frequency noise from propeller cavitation.
Propeller Optimization for Noise Reduction
The navy has consistently excelled at building and operating quiet ships, with the design process prioritizing eliminating noise. Equipment is on resilient mounts and anything that vibrates has dampening tiles to absorb sound so the vessel can operate in silence when necessary, usually at lower speeds. Meanwhile, commercial ships have prioritized energy efficiency to reduce operating costs and aimed to maximize cargo capacity. Although many measures that increase energy efficiency can also reduce underwater noise, for commercial ships, noise reduction has not been part of the design criteria. To bring naval experience in quiet design to commercial vessels, Martec Limited and Defence Research and Development Canada (DRDC) Atlantic partnered to design a quieter propeller for the Orca class patrol vessel without significantly reducing the vessel’s operating efficiency. These vessels are non-combatant training vessels built to a commercial standard and so are well-suited to assess what noise reduction is feasible for a non-naval ship.
Working closely with the Maritime Research Institute Netherlands (MARIN), the Propeller Optimization for Noise Reduction project focused on designing a retrofit propeller for the Orca class vessels. The design was constrained to have the same propeller mass and turning resistance (moment of inertia) so the new propeller would work with existing elements of the ship, like the hull form and engine system. The goal was to test how much noise reduction could be achieved by only improving the propeller.
Initial phases of the project included trials to measure onboard vibrations and underwater radiated noise levels using the Underwater Listening Station in Boundary Pass. The measurements demonstrated that onboard vibration levels can be used to predict underwater radiated noise and to monitor propeller cavitation. Currently, cavitation on the Orca class vessel starts at around 8.5 knots. As the vessel spends most of its time travelling between 7 and 12 knots (with a maximum speed of 20 knots), it is cavitating most of the time. Increasing the cavitation “inception speed” (the speed at which cavitation starts) to 12 knots would remove a major noise source during a significant part of the vessel’s operation.
The project team used numerical model simulations to predict propeller performance and select an optimized design. The re-designed propeller was then tested and refined using a 12-foot scale model of the Orca hull and specialist tanks at MARIN, equipped with instruments to measure the propeller’s performance and cavitation inception speed. This scaled-down testing approach is similar to testing airplane prototypes in wind tunnels. The scale-model testing indicated the optimized propeller is free of cavitation up to a trial speed of around 14 knots and a service speed just above 12 knots. Service speed incorporates a 20% margin to reflect more “real-world” effects of wind, waves and fouling of the hull and propellers. These results mean no cavitation during typical operations. However, the optimized propeller has not yet been validated with real-world, full-scale conditions.
A follow-up scale-model study suggests that by removing the propeller mass and turning resistance constraints, a propeller could be designed for the Orca class vessel that pushes the cavitation inception speed closer to 16 knots. Such a propeller would weigh significantly more than permitted if built at full-scale. Further experimentation with new technology, such as composite materials, would be required to achieve a propeller that could function without the mass and inertia design constraints. Composite propellers can be half the weight of a bronze propeller and more flexible. The reduced weight and increased flexibility results in more efficient operation than the traditional bronze propeller.
Other retrofits that were not incorporated into the Orca class vessel that could contribute to quieter operations include vibration isolators, acoustic insulation and dampening tiles to reduce noise from machinery inside the vessel.
Composite Propeller Design for Noise Reduction
Related to the Orca class vessel propeller design project, Martec Limited and Lloyd’s Register undertook work on Composite Propeller Design for Noise Reduction, to refine and further develop modelling and software tools for improved analysis and design of composite propellers. The improvements provide greater flexibility in propeller modelling and enable the comparison of the modelling tool’s results to other studies for validation and verification of its capabilities. Another feature was incorporated to detect when composite propellers would deform to the point of failure and tested using the Orca class vessel composite propeller model, previously described.
An Innovative Physics-Based Machine Learning Framework for Near-Field Noise from Hull and Propeller (HARP)
When designing a composite propeller, researchers and engineers use a method called computational fluid dynamics to understand and predict how water would flow around the propellers. Researchers at the University of British Columbia began work on the Intelligent and Green Marine Vessel Program in 2017, developing in-house computational fluid dynamics and multiphysics framework for cavitation and underwater radiated noise tools in collaboration with Seaspan Shipyards Ltd. and VARD Marine Ltd. The result was a completely new propeller morphing toolbox.
The University of British Columbia’s project, An Innovative Physics-Based Machine Learning Framework for Near-Field Noise from Hull and Propeller (HARP), which is still in progress, builds on earlier work on in-house computational fluid dynamics and the multiphysics tools. The goal is to develop new tools to help vessel designers predict underwater radiated noise during the design stage and identify potential sources of vessel noise. Innovations under development include a novel propeller morphing concept and modelling hull-form vibrations due to machinery and wave interactions. The project is expected to be completed in 2025.
Computational Fluid Dynamics Tools for Prediction of Cavitation-Induced Underwater Radiated Noise
Computational fluid dynamics has been used by several Quiet Vessel Initiative projects, such as the HyPNoS project from Schottel (see Article 1: Technology for detecting and analysing underwater radiated noise). The University of Victoria project Computational Fluid Dynamics Tools for Prediction of Cavitation-Induced Underwater Radiated Noise looked at integrating this method into cost-effective and user-friendly tools based on open-source code to allow researchers, marine engineers, and naval architects to predict cavitation noise during the design phase.
Using computational fluid dynamics to understand and predict vessel noise can involve complex, data-intensive simulations. Although highly accurate, such tools are expensive and time-consuming to create and operate. Designing cost-effective tools means building a predictive model that is less complex and data-intensive without losing the detail needed by researchers, engineers, and naval architects for accurate results.
To meet this challenge, the researchers are focusing on refining the data needs and developing simplified models that are applicable to certain classes of vessels. The tools are already being put to the test. For example, one application of the tool provided insights into the noise generated by a marine propeller with a manoeuvrable rudder under different flow conditions.
The tools could also provide insights into how ship design could be modified to be quieter and potentially guide operational procedures, such as how berthing manoeuvres could minimize the noise generated. Once the project is complete, the researchers plan to maintain an open-source website to share the tools and guidance on how to use them.
This article is part of a five-article series on ship design to limit underwater vessel noise.
Continue learning about the new discoveries and challenges in making vessels quieter with the other topics in this series here
The Quiet Vessel Initiative is a federally funded program through Transport Canada. Industry partners and researchers interested in potential research and development collaborations to advance innovative solutions in marine technology are invited to contact the Quiet Vessel Initiative team at Marine-RDD-maritime@tc.gc.ca.
Cavitation: Propeller cavitation is created by rapid changes in water pressure around the propeller. When a propeller turns, it creates a low-pressure area on one side of the blade and a high-pressure area on the other. When the propeller turns quickly, or the vessel and propeller are under a heavy load, the rapid pressure drop causes the water to evaporate and form vapour bubbles that move over the blades. As the bubbles reach the high-pressure area, they collapse, making noise.
Measurement efforts have shown that propeller cavitation is more common at higher speeds due to greater loads on propeller blades or when propeller blades are misaligned or damaged. Other vessel factors, such as the hull design, also influence cavitation. Propeller cavitation isn’t just a noise issue. It also erodes the propeller surfaces, reducing their performance and efficiency.
Computational fluid dynamics: Computational fluid dynamics is a method used to study how fluids, like water and air, move. It involves using computers to solve mathematical equations that describe fluid motion. With computational fluid dynamics, engineers and researchers can create virtual simulations to see how fluids flow around objects, such as ships, or specific components, like the hull or the propeller. Understanding how water moves around and interacts with vessels allows designers to see where drag or turbulence might occur and focus on optimizing vessel design. We can also use computational fluid dynamics to create simulations of different design concepts. This way, we can test and refine components without needing to physically build them, allowing for a more efficient design process.
Propeller, composite: Propeller built using fibre composites, such as glass or carbon fibres, mixed with resins like epoxy or polyimide—a type of high-performance and heat-resistant resin. Composite propellers are typically more corrosion and fatigue-resistant than conventional propellers. In addition, they are lighter than their traditional counterpart, offer greater manoeuvrability and faster speed change, and can adapt their shape slightly in response to water flow and pressure (adaptive deformation). As a result, composite propellers can be both more fuel-efficient and quieter.
Propeller, controllable (variable) pitch: A controllable pitch propeller, also called a variable pitch propeller, can be efficient for the full range of rotational speeds and load conditions, since its pitch can be varied to absorb the maximum power that the engine is capable of producing. When fully loaded, a vessel will need more propulsion power than when empty. By varying the propeller blades to the optimal pitch, higher efficiency can be obtained, thus saving fuel. A vessel with a controllable pitch propeller can accelerate faster from a standstill and can decelerate much more effectively, making stopping quicker and safer. A controllable pitch propeller can also improve vessel manoeuvrability by directing a stronger flow of water onto the rudder.
Propeller, fixed pitch: A fixed pitch propeller’s blades are set at a predetermined angle. Fixed pitch propellers are both cheaper and more robust than controllable pitch propellers and usually a good choice for deep-sea vessels as the propeller pitch can be set at the optimum angle for the desired operating speed and load condition.
Propeller, surface-piercing: Unlike most propellers that sit below the ocean surface, surface-piercing propellers are designed to have half the propeller in the water and half out. This positioning reduces drag and allows the propeller to create more thrust with less water resistance. They are most commonly used for high-speed vessels.
Knot: A unit of speed equal to one nautical mile per hour, exactly 1.852 km/h. A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.
Multiphysics: a coupled modelling approach of studies that demand simultaneous addressing of physical disciplines that were previously considered separately and combining them to generate relational mathematical models that are validated with controlled experiments to enhance the understanding of natural behaviour.
