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Propellers are the primary means of vessel propulsion, efficiently converting engine power – or battery-electric power – into thrust to move ships through water. Alternative systems include waterjets, which use high-pressure streams of water to move vessels at high speed, air-propulsion fans used by hovercrafts, and sails to harness the power of the wind. Alternative propulsion systems may not have the same noise signatures as propeller-driven systems.
Quantifying Underwater Noise Reductions from Environmentally Friendly Tugs
Moving from diesel engines to battery-electric power has allowed for the creation of a quieter and less polluting tug. This change also provided an improved (and initially strange) experience for the crew – a tug that starts up and runs with no noise, no vibrations, and no belch of smoke. Robert Allan Ltd.’s project Quantifying Underwater Noise Reductions from Environmentally Friendly Tugs, in partnership with HaiSea Marine and researchers from the University of British Columbia, focused on understanding and quantifying the underwater noise generated by battery-electric tugboats.1,2
Tests with hydrophones in 2023 demonstrated that the electric tugboats are much quieter than their diesel counterparts, most of the time. The hydrophone tests compared two diesel powered tugs to the HaiSea Wamis in both battery mode and generator mode. On average, the diesel tugs were about three times (about 15 decibels) louder than the electric tug when transiting between 5 and 8 knots. When transiting at 10 knots, the electric tug was about half as loud as the diesel tugs (a noise difference of 10 decibels on average). These non-cavitation noise results drew attention to a knowledge gap – how is machinery noise from inside the vessel getting into the water column? Tracing the noise path combines real-world data from onboard measurements and sea trials with a computer model to track how noise moves from its starting point at vessel machinery to arriving in the water column.
Unfortunately, propeller cavitation is unaffected by a change in power source and was still an issue when the tug was operating under load, such as when pulling or pushing other vessels. The level of underwater noise generated by both types of tugs operating at full towing power was very similar. For the electric tug operating in battery mode, the noise generated by full-power pulling and pushing activity was about six times (30 decibels) louder than when the tug was transiting. An area for future design investigation is whether controllable pitch propellers can be effective for tug operations. Tug propellers are typically fixed pitch to provide maximum thrust, but a controllable pitch propeller can be adjusted to reduce the likelihood of cavitation during transit.
With battery-electric vessels producing considerably less greenhouse gas emissions than diesel-propelled vessels, and the need to rapidly reduce greenhouse gas emissions to meet national and international commitments to address climate change, electric tugs are expected to become more common. As a result, being able to predict the potential noise emissions of electric tugs (and, indeed, other electric vessels) is becoming more important during the vessel’s design phase. For this reason, Robert Allan Ltd. has also been refining a tool called TugEM (Tug Emissions) to predict the noise generated by different electric tug designs. The model is still in development, but the tool should allow vessel designs to be tested against different operational behaviours typical to tugs. These behaviours include idling, transiting, pushing and pulling vessels, and supporting vessel berthing/unberthing.
The TugEM tool requires more data for validation – and the designers have access to an excellent dataset as a result of this Quiet Vessel Initiative project. The level of effort required to test a vessel’s underwater noise profile using hydrophones is significant. This tool could potentially be applied in the future to show how quiet a design will be, without having to test the tug after construction.
Propeller cages: developing a quieter safety feature
It’s quite common for small fishing boats, recreational vessels, including dive boats, and lifeboats to be equipped with a propeller cage to keep lines (and indeed people and marine life) away from the propeller’s rapidly spinning blades. However, propeller cages, especially if ill-fitted, can increase drag and consequently vessel noise.
Quantifying Underwater Radiated Noise (URN) Effects of Propeller Cages
Lloyd’s Register and Martec Limited undertook Quantifying Underwater Radiated Noise (URN) Effects of Propeller Cages, the first project to quantify the impact of propeller cages on underwater radiated noise. This project used a combination of in-water trials and computational fluid dynamics to examine the effect of the cages. The work revealed that while vessels with propeller cages are noisier than their uncaged counterparts, the primary source of this increased noise was not the cages themselves but the propeller.
Vessels with propeller cages must operate at higher revolutions per minute (RPM) to maintain the same speed as those without a cage. With increased speed comes cavitation, and with cavitation, more noise.
Cages with biofouling – algae and other marine organisms growing on them – made more noise at lower speeds than clean cages. The reason is simply that vessel operators with fouled cages needed to increase their speed even further to compensate for the extra drag these organisms were causing compared to vessels with clean cages.
The project is still ongoing. Currently, the project team is working on designing propeller cages using computational fluid dynamics to develop designs that provide the necessary safety benefits while being quieter and more efficient, then testing and refining the designs using real vessels. This project started examining propeller cages for Cape Islander lobster boats, approximately 40 feet in length, and is now looking to address larger vessels like marine survey boats with intentionally engineered propeller cages to reduce underwater noise.
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.
Featured picture credit: TransOceanic Wind Transport
Biofouling: The growth of marine life on the hull and other underwater parts of a ship that results in increased “drag” or friction when the ship is underway. This drag increases the amount of noise and decreases the energy efficiency of the ship’s operation.
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.
Decibel: A unit used to measure the level of sound pressure (intensity of a sound) or the power level of an electrical signal. It is a relative unit, not an absolute one, and is used to express a relative change. Decibel is used to describe sounds in terms of their loudness. For underwater ocean sounds, a reference pressure of 1 microPascal (μPa) is used to describe sounds in terms of decibel.
Hydrophone: An underwater microphone that can be deployed individually or in groups. Groups of hydrophones can be arranged either horizontally on the seafloor or vertically at different depths in the water column. Hydrophones detect pressure changes in the water caused by sound waves. These sensors convert the underwater pressure fluctuations into electrical signals, which can then be analysed to determine the properties of sound, such as volume and frequency.
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, 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.
- Zhi Cheng, Brendan Smoker, Suraj Kashyap, Giorgio Burella, Rajeev Jaiman. (2025). Cavitating wake dynamics and hydroacoustics performance of marine propeller with a nozzle. Physics of Fluids 37, 015128. ↩︎
- Zhi Cheng, Suraj Kashyap, Brendan Smoker, Giorgio Burella, Rajeev Jaiman. (2025). Integrated Numerical and Experimental Study of Cavitating Flow and Underwater Noise in Tug Vessels with Ducted Propellers. Proceedings of the OMAE 2025 44 International Conference on Ocean, Offshore & Arctic Engineering. ↩︎
