Ship operators can implement several operational strategies while balancing these measures against navigational safety and vessel efficiency
|
Getting your Trinity Audio player ready...
|
Click to view glossary
Operational decisions that can make vessels quieter
Operational changes can be effective in reducing vessel noise. However, vessel owners must consider safety, maneuverability, and other navigational constraints when deciding which adjustments to adopt—if any. The following operational changes identified by Quiet Vessel Initiative projects are shared as general guidelines.
Slow down
As highlighted in the second article in this series, Quieter ship design: new-build and retrofit options, the propeller is a significant source of underwater noise. During normal operations, propeller-related noise falls into low-frequency bands. If a propeller begins cavitating, high-frequency noise is created. Reducing vessel speeds can reduce underwater noise—cutting low-frequency noise during normal, non-cavitating operations and, by slowing the propellers, reducing the potential for cavitation and its associated high-frequency noise.
Vard Marine conducted the Quiet Vessel Technologies Scan. For small vessels, the project found that, in general, travelling at speeds of 5 knots and slower generates significantly less noise than travelling at over 15 knots. Achieving this level of speed reduction may not be possible for many vessels as moving at very slow speeds can result in a loss of steering capability.
However, some vessels cavitate at very low speeds. In some situations, this can mean that the vessel is cavitating for the duration of its voyage and slowing down is not safe or possible. For example, JASCO Applied Sciences Ltd.’s Feasibility of Real-Time Shipboard Cavitation Monitoring and Management project noted that, for the bulk carrier MV Ferbec, cavitation starts at just 6 knots. With the vessel travelling through waterways like the St. Lawrence River, which has current speeds of up to 6 knots, slowing down to avoid cavitation would result in a loss of steering capability and safe navigation. For such vessels, retrofitting with a different propeller would be required to reduce high-frequency noise from cavitation.
Stay balanced
Staying balanced, or managing a vessel’s trim and draught, can also minimize noise by keeping the vessel operating as intended according to design criteria. Trim refers to the angle of a vessel’s orientation in the water, specifically the difference in depth between the bow (front) and stern (rear). If the stern sits deeper than the bow, the vessel is said to have a positive trim, while a bow-heavy vessel has a negative trim. Draught (pronounced “draft”) is the vertical distance between the waterline and the bottom of the vessel’s hull. It indicates how deep a vessel sits in the water and varies based on the vessel’s load. The more weight the vessel is carrying, the deeper its draught.
Optimizing trim and draught can improve vessel performance and fuel efficiency, and reduce underwater noise, as JASCO Applied Sciences Ltd. Feasibility of Real-Time Shipboard Cavitation Monitoring and Management project demonstrated. Adjusting trim and draught changes how water flows around the hull toward the propeller (the wake field). Properly balanced trim and draught allow for water to flow more consistently with less turbulence around the ship’s hull and propeller. When water flows evenly to the propeller, it can operate more efficiently with less chance of cavitation and vibrations.
Move over
Underwater noise from vessel operations creates noise and that noise moves outwards through the water from the point of origin. How far that noise travels depends on many different factors. As a general rule, low-frequency noise travels farther than high-frequency noise.
Learn more about how noise travels in the ocean in the first article in this series, Technology for detecting and analysing underwater radiated noise.
Several Quiet Vessel Initiative projects found that vessels could reduce the amount of underwater noise in an area by moving further away (called lateral displacement). This can be beneficial in areas of critical habitat for populations of marine life known to be sensitive to noise, such as Southern Resident killer whales.
As part of the Boundary Pass Underwater Listening Station project, JASCO Applied Sciences Ltd. analyzed Southern Resident killer whales’ listening range – how far away a whale can be from a noise source and still react to it. Since killer whales communicate with each other with whistles and clicks, being able to hear over greater distances improves their communication and well-being (imagine trying to have a conversation across a room while many other people are talking at the same time). When considering the noise created by whale-watching vessels, increasing the vessel’s distance from whales from 200 metres to 400 metres could improve the killer whale’s ability to hear in the area (their listening distance) between 55% and 140%. Even so, some of the communication frequencies the killer whales use could still be impacted by a noise occurring 400 metres away or more. JASCO notes that these estimates are based on data collected near the Haro Strait shipping lane, meaning there was background noise from many types of vessels. In quieter, shallow coastal areas with less ambient noise from other vessel types, whale-watching vessels could have a more significant impact on listening distances.
JASCO Applied Sciences’ project Summary of Modelling of Operational Measures to Reduce Underwater Noise highlighted the assessments they’ve performed over the past 20 years with specialized models to predict noise in a given location. JASCO’s modelling work was part of the evidence that inspired the Vancouver Fraser Port Authority’s ECHO Program’s voluntary slowdown measures. Other work by JASCO suggests that moving vessel transit routes into deeper waters, (where noise dissipates more rapidly than in shallow waters because it doesn’t reverberate off nearby underwater surfaces), could reduce noise in shallow water areas. Their recommendations included moving tug transit routes away from the coast and closer to shipping lanes. This was implemented as a voluntary effort in 2018 in the Strait of Juan de Fuca as part of the ECHO Program and has continued to be observed during the June to November season when Southern Resident killer whales are more likely to be present. With 99% participation by tugs, this measure achieved a 4-7 decibel noise reduction in the designated area of importance for the Southern Resident killer whales from each individual tug displacement, representing a 60-80% reduction in underwater sound intensity, according to the 2024 ECHO Program report.
Reducing tugs’ noise footprint with electrification
Tugs are small but extremely powerful vessels that can escort and even tow large vessels. They typically create underwater noise from engine and hull vibrations while transiting, and from propeller cavitation when towing. The amount of underwater noise created by a tug during high load conditions can be similar to much larger vessels. Robert Allan Ltd., a tug naval architecture firm, undertook sea trials with different types of tugs to assess the difference in underwater noise levels between battery-electric and traditional diesel-powered tugs. The results demonstrated that underwater noise levels during transit conditions were much lower for the battery-electric tugs as the vibrations and engine noise were eliminated. However, the noise created while the different tugs were towing was nearly identical due to propeller cavitation.
Learn more in a previous article on Alternative Propulsion Systems.
Other than shifting traffic lanes, one of the recommendations from Vard Marine’s Quiet Vessel Technologies Scan is to create areas where vessels would either be prevented from entering at certain times, or where their operations would be limited to control the amount of underwater noise created. Such exclusion and control areas would be targeted to reduce noise in critical habitats or near sensitive populations of marine life. These areas could be particularly beneficial during certain times, such as during breeding and calf-rearing or key foraging seasons and could reduce the amount of underwater noise coming from small and large vessels alike. An example of this approach is the vessel restricted zones implemented in critical habitat for Southern Resident killer whales on the West Coast of British Columbia, which aim to reduce acoustic and physical disturbances from vessels. Additionally, a restricted area has been established in the Gulf of St. Lawrence during the summer season when North Atlantic Right Whales congregate to feed in the waters of the Shediac Valley, north of Prince Edward Island. To reduce the risk of a vessel striking and injuring or killing a whale, vessels greater than 13 metres in length are asked to avoid the area entirely or, if they are part of the exceptions, slow down to less than 8 knots during the time the restricted zone is in effect. While these restrictions are established to reduce direct harm to whales from strikes, they can also reduce indirect harm from underwater noise.
Stay together
JASCO Applied Sciences’ Summary of Modelling of Operational Measures to Reduce Underwater Noise suggests that clustering vessel transits into convoys could create longer quiet periods between groups of vessels. For example, if four vessels were to travel through an area in close succession (e.g., four vessels in one hour rather than one vessel every three hours over a twelve-hour period), the area would be quieter more often. However, the modelling also shows that the benefits of this approach are highly dependent on local traffic conditions. In areas where many other vessels continue operating on their usual schedules, the potential quiet periods between convoys can be filled by non-convoyed traffic, limiting overall noise reductions. The study notes that in some locations convoying may provide meaningful benefits, but in others—with dense or diverse vessel activity—the improvements may be small or inconsistent. While convoying remains a potentially useful tool, its effectiveness depends on how busy the area is and the degree of coordination possible across vessel classes. Implementing convoys would also require higher levels of coordination and active traffic management to avoid introducing safety hazards from clustering vessels more closely together. Additionally, convoying could increase vessel dwell times in anchorage areas and contribute to higher levels of ambient noise pollution, depending on how the system is implemented.
Stay clean
Finally, regularly cleaning a vessel’s hull offers several benefits. First, a clean hull creates less drag in the water and requires less energy to maintain its speed, reducing greenhouse gas emissions. The more drag in the water, the more power is needed to move the vessel at the same speed. A hull that is not clean – with plants and marine animals attached to the vessel’s underside, a condition called biofouling – can also spread marine species from one coastal environment to another, where they could become invasive. In terms of underwater vessel noise, it’s suggested that propellers may need to work harder and spin faster to maintain the same speed, which could increase the likelihood of cavitation. The associated drag might also contribute to more turbulence in the water column. Anecdotal evidence indicates that biofouled or damaged propellers could cause cavitation to occur at lower speeds, though more research is needed to confirm these effects.
This article was prepared by Clear Seas on behalf of Transport Canada as part of the Quiet Vessel Initiative and is part of a four-article series on operational changes 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.
Frequency: sound moves through a medium like water as a wave, thus the term sound wave. Frequency, also known as pitch, indicates how often a sound wave repeats within a single second. Measured in Hertz (Hz), or cycles per second, higher numbers signify higher-pitched sounds, and lower numbers signify lower-pitched sounds.
Marine life: Mammals, fish, and invertebrates living in the ocean environment.
Propeller, cavitation: When a propeller turns, it creates a low-pressure area on one side of the blade and a high-pressure area on the other. In many cases, the pressure in the low-pressure area is low enough to cause cavitation, a change in phase from liquid to vapour, like boiling, but caused by a change in pressure rather than a change in temperature. When areas of sufficiently low pressure are generated in water, vapour bubbles form. As the vapour bubbles leave the area of low pressure, those bubbles collapse (implode).
Because the pressure differences are usually large, the collapse of cavitation bubbles is very powerful and loud. Other vessel factors, such as the hull design, also influence cavitation. Measurement efforts have shown that propeller cavitation is more common at higher speeds due to greater loads on propeller blades. Cavitation can also occur when propeller blades are misaligned or damaged. In addition to creating a great deal of sound, cavitation bubbles can damage or degrade metallic surfaces like propellers, reducing their performance and efficiency.
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.
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.
