There are mechanical noises such as engines emitted into the water through the ship hull, propeller cavitation noise, and flow noise due to friction on the hull.
As the ship's speed increases, the weight of the cargo on board increases, or the resistance increases by waves, the sound generated by propeller cavitation becomes dominant.
Propeller cavitation (Figure 1) is induced by the increase and decrease in pressure as the propeller blades across the wake (slow-flow region) at the rear of the hull. Cavitation causes a broadband noise and a harmonic component peak at the blade frequency (blade number x rotation speed) in the underwater noise spectrum. Broadband noise is caused by the rebound of growth and contraction, and further collapse of vast amounts of cavitation bubbles in the water. The peak of the harmonic component is caused by the cavitation volume variation during one propeller rotation.
Propeller cavitation in model scale1 and full scale2.
Full scale measurement results of the underwater radiated noise (URN) from ship3 are introduced here. The recorded underwater noise sound, 1/3 octave spectrum (Figure 2) and narrowband spectrum in the 1Hz band (Figure 3) are shown here. As the engine output increases from HALF to FULL, the noise level suddenly increases due to cavitation occurrence from Figure 2. In addition, the peaks of harmonic components up to the fifth order due to cavitation volume changes are clearly shown in Figure 3. When you listen to underwater noise sound, you will be able to hear like a "flapping'' sound.
One-third octave spectrum of underwater radiated noise from a small cargo and passenger ship (CPA：Closest Point of Approach).
Narrowband (1Hz) spectrum at maximum output condition.
Why it's a problem?
In addition to impacts from the sound of piling when installing offshore structures, the sound of sonar transmissions (Figure 4), and the sound of airguns during seabed geological exploration, the URN from ships into the water is also a problem.
These sounds are identified possibly affect marine ecosystems such as whales, dolphins, and seals.
Whale stranding suspected to be caused by military sonar4
Since they are unable to communicate with humans, it is unable to clarify their effects directly, however a decrease in the number of individuals due to an increase in the number of ships traffic, changes in their behavior when ships approaching, and variations in physiological phenomena such as hormones, etc. have been pointed out. In addition to investigations in the ocean, the results of research by playing back the recorded URN from ships in the laboratory tank have also been reported.
Figure 5 shows a comparison of the sounds produced by marine organisms during their predation and communication, their audible frequency ranges, and the frequency range of human-made sounds produced by ships, etc. The frequency range of whales and other species is wide and overlaps with anthropogenic noise sources. Below are some examples of recent research on the effects of URN on marine ecosystems.
Sound generation and audible range in marine ecosystems, and frequency range of anthropogenic underwater noise sources5
As for the impact to mammals, the URN appears to impede dolphin’s communication during feeding, but not their social communication. Furthermore, no clear conclusion could be reached that it is having a negative impact on the ecology of cetaceans like humpback whales.
For crustaceans, behavioral changes in spiny lobsters and prawns exposed to regenerated URN in laboratory aquariums, physiological changes in blood and protein concentrations, as well as DNA, were investigated. As a result, it is concluded that URN is causing them stress. The effects on crabs are also being investigated.
For invertebrates, the ability of clams to reconstitute sediment, an important ingestion mechanism, was found to be affected by URN. The effects on Aplysia, cuttlefish and mussels are also being investigated.
Regarding fish, further research is needed to see if they exhibit similar behavior under the sea. By replaying URN in experimental tanks, it was found that small fishes behaved differently when defending against predators and guarding their eggs. Other studies are investigating the effects on reef fish, eel responses to predators, sea bass, sticklebacks, and cod.
An impact on humpback whales, which flock to the Ogasawara Islands in Japan to breed in the winter as shown in Figure 6, was investigated from 2016 to 2018. It was concluded that there was no clear evidence that URN is having a negative impact on the ecology of cetaceans like humpback whales.6
The International Maritime Organization (IMO) recently approved revised versions of its 2014 Ship URN guidelines, which had neither previously been recognized nor followed. There is now entering an Experience Building Phase (EBP) for the revised guidelines. The next step is also being planned, which could be the enforcement of URN level limits, although that is partial sea area.
With the opening of shipping routes in the Arctic Ocean, there is a movement to consider the impact on seals, etc. as an issue.
How can URN from ships be measured?
The measurement method7 shown in Figure 7 has been proposed by ISO and overseas classification societies guidelines, but it is time-consuming and costly, and there are problems with evaluating the dispersion of measured values.
It is necessary to pay attention to the measurement uncertainties8 caused by not only measurement equipment but also weather and sea conditions and seabed geology.
There is a need for the development of a simple measurement method, and then on-board monitoring methods9 are researching currently underway such that measure by fitting hydrophones (underwater microphones) directly to the ship's hull, and that predict underwater noise from vibration measurement results (Figure 8).
URN Measurement Configuration – Beam Aspect Test Course7
URN monitoring technology based on onboard measurement
How can URN from ships be predicted?
There are simple estimation methods based on measurement data analysis such as Brown's formula, cavitation tunnel test method, and theoretical calculation simulation method, however they are still at the stage where their accuracy is required to be improved.
Brown's formula, introduced in the prediction method on this website, is an empirical formula derived from URN measurement results on full scale ships with side thruster, and can estimate the upper limit level of URN in a wide band range of approximately from 100Hz to 10kHz. It has been confirmed that it agrees well the full-scale ship measurement result, and that the estimation accuracy is good for the URN reduction effect due to slow steaming10。
There are also methods to predict from the length and speed of the ship, such as Ross's equation11.
Canada's ECHO program12 and the Japan Ship Technology Research Association (JSTRA) have proposed regression models from measurement databases of full-scale ships13.
How to reduce URN from ships?
It is introduced that the URN level from five container ships has been lowered by 6 to 8 dB through modernization improvements such as replacing propeller boss cap fins, bulbus bow, and engine derating, as shown in Figure 914.
The Vard report17 reviews currently available technological solutions to reduce URN from commercial vessels, including how and why they are reduced, how much they can be reduced, and for which frequencies. Details such as cost estimations, immediate applicability, pros and cons of other than noise reduction, and the types of vessels to which they are applicable are listed. It is an excellent, cutting-edge technology information book that is easy to reference for stakeholders such as ship owners, operators, and shipyards.
What are the trends domestically and internationally?
In order to protect killer whales, Canada provides discount on port dues for ships entering and departing from the Port of Vancouver that have taken measures to reduce URN or have received a classification society notation (an addition of a classification code)18.
Since 2014, a cash reward and incentive program in the Santa Barbara Channel near the Port of Los Angeles in the United States to reduce vessel speeds to less than 10 knots have resulted in a significant reduction in sound exposure levels of approximately 5 dB from participating vessels.
How have classification societies responded?
Overseas classification societies issued their own URN guidelines, with DNV recently issuing the first notation for merchant ships. Nippon Kaiji Kyokai (NK) also issued guidelines in October 2023.
Standard noise levels differ depending on each society, with the French BV regulations being the strictest (Figure 10).
One-third octave threshold level for each classification society.
ABS, DNV, RINA are radiated sound pressure noise levels (RNL). BV, LR are monopole sound source levels (MSL)19.
What's next?
Since many theories regarding the effects of underwater ship noise on marine life are difficult to prove quantitatively, measures to reduce the noise level of ships themselves are necessary from the perspective of preserving biodiversity and engineers. In order to protect the marine ecosystem, for example to prevent it from becoming like the animation below, the noise management plan consisting of the following items is required.
Support on URN measurement methods and implementation plans
Providing URN prediction tools, analysis and recommendations
Proposing of various URN reduction measures
Ocean Noise is a Japanese distributor of SHORTCUt CFD and can support CFD analysis of GHG reduction measures, such as propulsion performance prediction and energy-saving devices.
Hiroi, T. et. al. (2019), PS-2 Full-scale ship flow field measurement, underwater noise, and stern pressure fluctuation measurement on an ocean-going bulk carrier PDF
Japan Ship Technology Research Association (2017), Research on the effects of underwater ship radiated noise on marine life (Underwater Noise Project), June 2017
NOAA (2020), Beaked Whale Strandings in the Mariana Archipelago May Be Associated with Sonar, Article,(Feb. 19,2020)
Duarte, C.M et. al. (2021), The soundscape of the Anthropocene ocean, Science 371(6529), URL
Tsuji, K. et. al. (2018), Change in singing behavior of humpback whales caused by shipping noise, PLoS ONE 13(10): e0204112, URL
ABS (2024), Guide for the Classification Notation Underwater Noise and External Airborne Noise, Jun 2024 pp.32, PDF
ITTC (2017), Recommended Procedures and Guideline Underwater Noise from Ships, Full Scale Measurements, ITTC Recommended
Procedures and Guidelines 7.5–04,04-01, PDF
Republic of Korea (2022), Monitoring technology of underwater radiated noise from ships using onboard noise measurement, SDC 9/INF.9, 18 November 2022
Shiraishi, K. et. al.(2023), Verification of simplified underwater radiated noise estimation tool using Brown’s formula, inter-noise 2023.
Ross, D. (1976), Mechanics of Underwater Noise,Pergamon Press Inc.
MacGillivray, A. et. al. (2020), ECHO Vessel Noise Correlations Study, Final Report, Submitted to the Port of Vancouver, PDF
Sakai, M. et. al. (2023), Statistical analysis of measured underwater radiated noise from merchant ships using ship operational and design parameters, J. Acoust. Soc. Am. 154, p.1095-1105, URL
Gassmann, M. et al. (2017), Underwater noise comparison of pre and post retrofitted MAERSK G class container vessels, Scripps Institution of Oceanography, MPL TM 616
MOL (2017), PBCF Receives 2017 Nikkei Global Environmental Technology Award - PBCF Reduces 3-5% in Fuel Consumption; Underscores MOL Group's Commitment to Environmental Protection with Sales of Over 3,200 Units - Article
Kendrick, A. et al. (2019), Ship Underwater Radiated Noise, Report prepared for Innovation Centre of Transport Canada by Vard Marine Inc., Report 368-000-01, Rev. 5, PDF
Port of Vancouver (2023), Receive up to 75% off harbour due rates at the Port of Vancouver through the EcoAction Program, PDF
Hannay, D. et al (2021), Study of Quiet Ship Certifications, Analysis using ECHO Ship Noise Database, Transport Canada Pub. No. TP 15478E, p.34. PDF
