A Review and Meta-Analysis of Underwater Noise Radiated by Small (<25 m Length) Vessels
Round 1
Reviewer 1 Report
Second Review
I acknowledge that minor changes have been made to the text, however, the words used display a technical simplicity which causes me to consider the premise of the paper still to be flawed.
The authors appear not to have had intensive discussions with propeller experts. I am a propeller expert with 40 years of full scale, model test and computational experience.
From my 40 years of experience I can say that the fundamental phenomenon causing high levels of underwater noise is propeller cavitation, and this is dominated by propeller tip speed…….not boat speed. Tip speed is a shorthand for cavitation number.
Below a certain propeller speed, the propeller will not cavitate and hence URN is dominated by on-board machinery.
As any vessel increases in speed, the propeller will start to cavitate at the blade tip ( tip vortex cavitation); such cavitation is a dominant factor in URN broadband energy. At higher speeds, sheet cavitation comes in to play, but tip vortex cavitation remains dominant.
Taking the authors’ data for one vessel on which I performed sea trials (the NCL Princess Royal), the propellers were free of cavitation up to 600 engine rpm (23m/s tip speed). At 2000 engine rpm, the propellers cavitated substantially (79m/s).
Conclusion
I would like to see the paper re-worked using a cavitation-related parameter (tip speed), not ship/boat speed.
It is poor science to select parameters just because they are readily available in reported literature. It may have taken the authors longer to write to the vessel owners or the authors of their URN data in order to elicit meaningful (phenomenologically relevant) parameters.
Professor P A Fitzsimmons
Changed Text
48 Modelling of instantaneous and cumulative sound exposure levels at any given loca
49 tion requires knowledge of the number of vessels, the level of noise produced by each and
50 understanding their different operations. There are several potential sources of noise emit
51 ted by individual vessels, but oscillating bubbles produced by sheet and vortex cavitation
52 near the turning propellor dominate the acoustic output, emitting acoustic energy at fre
53 quencies related to diameter range of the bubbles [18-22]. These levels can be broadly es
54 timated from a selection of engineering coefficients including the cavitation number (a
55 function of size, speed, blade number, and depth of the propeller as well as environmental
56 factors), the block coefficient (a function of vessel length, breadth, and draft), and the Ad
57 miralty coefficient (a function of vessel power, speed, and tonnage) [18]. However, these
58 values are rarely, if ever, reported in bioacoustics literature and for broader application,
59 it may be more prudent to consider the individual contributing characteristics (e.g.,
60 power, tonnage and speed), rather than the coefficients themselves (e.g., Admiralty coef
61 ficient). In addition, although cavitation contributes significant energy to the broadband
62 level, it is not the sole source of noise, otherwise electric vessels would offer limited noise
63 reduction.
64 Significant effort has been invested in characterising the sounds of large ships and 65 understanding the potential drivers of source levels [see 6, 19-22], which have been prin
66 cipally related predominantly to the size and speed of the vessel in the bioacoustics liter
67 ature [19]. There is, however, little data publicly available on the source levels of small
68 (classed here as
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
This is a good overview of state of knowledge of underwater noise of ships.
I have the following remarks:
- There are displacement ships and planing ships. Correct all planning
- The normal name fro the propulsor is propeller rather than propellor. Should decide for one
- L 121 expression in brackets does not fit to what is said
- Table 1 category 3 ..displacement to planing hull… better displacement to planing condition
- Table 1 category 6 insert “slender” between separated and hulls
- Table 2 bow angle / hull angle. What is this? It could be waterline angle following the description
- Table 2 Propeller: propeller revolution rate rather than blade revolution rate
- Vessel load/tonnage better “displacement”
- Table 4 hard to read. There is sufficient room for larger letters
- Figure 3 and 4. Same remark as for table 4
- L 617 It is generally not correct that transition from displacement to planing conditions reduces power. Power just does not increase as rapidly with speed when planing. What happens is that the boat rises out of the water, i.e., all noise sources become more shallow and RNL reduces at low frequencies while MSL is not affected
- L 649 replace tonnage with displacement. Tonnage is associated with gross tons which is a measure of volume
- L 676 The variations of propeller depth has to be viewed in relative terms, i.e., if depth is half RNL reduces by 6 dB. A planing boat may have a propeller depth of 0.5 m at high speed and more than 1 m at displacement speed
- L677 Adjusting pitch is not a feature of modern vessels, only of those with controllable pitch propellers. And these are in general not acoustically favorable at low pitch
….but indeed blades themselves… What is meant? It suggests that there are ships which can adjust pitch of individual blades. If these exist, there should be a reference - L 748 This consideration is not entirely accurate. Due to Lloyd’s mirror effect, at low frequencies and moving away from the source parallel to the water surface the law is 40log r rather than 20logr. This together with the very shallow sound sources is the reason why small craft have much smaller footprint at low frequencies than large ships although MSL may sometimes come close to that of large ships (see figure 6)
- L 824 too shallow hydrophone depth carries the danger that the hydrophone is in the acoustic shadow because of downward refraction of sound waves in a negative sound speed gradient. Anything below a depression angle of 20° can have an issue there in case of a larger distance (>200 m) and at large negative gradient
- L 907 This is not generally true, see remark above for L748. It can be true for MSL and should be marked here
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The submitted paper is correctly performed and dealing with an interesting topic. However, the authors only focus on vessels’ noise on marine fauna, which implies to consider only noise produced underwater. Recently, lot of attention is being given by airborne noise from ship, which can also impact animals, but are mainly related to human health. I suggest the authors to mention those studies in order to enrich a bit the background information. Moreover, the title can be easily misunderstood and it should be specified that is underwater noise.
I have nothing more to add to a good paper, which only need to be proper put into the right places into the scientific panorama.
Suggestions of references to fix the issues are:
Bernardini, Marco, et al. "Noise assessment of small vessels for action planning in canal cities." Environments 6.3 (2019): 31.
Fredianelli, Luca, et al. "Pass-by characterization of noise emitted by different categories of seagoing ships in ports." Sustainability 12.5 (2020): 1740.
Nastasi, Marco, et al. "Parameters affecting noise emitted by ships moving in port areas." Sustainability 12.20 (2020): 8742.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
The paper was already good, plus the authors followed all my suggestions. The paper is ready for being published
