Journal of Ship Production and Design, Vol. 30, No. 1, February 2014, pp. 18 [622550]

Journal of Ship Production and Design, Vol. 30, No. 1, February 2014, pp. 1–8
http://dx.doi.org/10.5957/JSPD.30.1.120052
A Simple Model for the Underwater Noise Source Level of Ships
Dietrich Kurt Wittekind
DW-ShipConsult GmbH, Lise-Meitner-Str. 1-7, Schwentinental D-24223, Germany
Underwater noise becomes a field of growing concern because of the possible inter-
action with sound vocalization of marine mammals. Modeling the effect of shipping
noise being a predominant contribution worldwide requires more than statistics of
measured ships in the field. This article is an attempt to characterize the underwaterradiated noise level of a ship by relating spectral components of noise to naval
architectural features of the ship.
Keywords: underwater noise; propeller noise; low frequency shipping noise
1. Introduction
UNDERWATER IRRADIATED noise of commercial shipping becomes
a growing concern in view of its effects on marine life.
It is observed that in most regions of deep water oceans, the low
frequency range below approximately 300 Hz of the underwatersound spectrum is caused by ships, thus masking natural back-
ground noise and communication calls of marine mammals such as
the baleen whales. The level of this noise appears to increase withthe number of ships, size, and propulsive power, which are growing
with global trade (Andrew et al. 2002; McDonald et al. 2006).
Approaches to describing underwater acoustic levels radiated
from ships are traditionally generated from field measurements ofincidentally passing ships (Wales & Heitmeyer 2002; Hatch et al.
2008). Frequency ranges considered vary as do descriptions of
measuring conditions, measuring setup, and geometry. Operatingconditions of the ships are not described. Existing empirical
acoustic ship models (Ross 1976; Wales & Heitmeyer 2002) and
others are limited in frequency range and ship parameters considered.All do not model the familiar hump at approximately 50 Hz of the
ship underwater noise spectrum.
However, it is known from research around naval ships and work
done in full and model scales on merchant vessels that radiated
noise from ships shows great variations depending on certain design
parameters (Wittekind 2009). Systematic investigations into therelationship of design parameters and radiated noise are missing.It can even be assumed that these parameters are not yet known.
This article is an attempt to describe the underwater source
level based on observations in the field and rational relationshipsbetween mechanical and geometrical parameters. Findings are
related to ship design parameters familiar to naval architects.
Besides literature sources, this article is based on work funded
by the Okeanos Foundation, the German Federal Environment
Agency, and the German Federal Agency for Nature Conservation.
2. Contributions to radiated noise of ships
There are no field measurements made under controlled condi-
tions that relate naval architectural features to spectra and noise
levels. A speed dependence of radiated noise has been describedin detail by Arveson and Vendittis (2000) on which much of the
following work relies.
Full-scale and model tests reveal features of background noise
recognizable when measured in close vicinity of the propeller
(Bark 1985; Heinke 1991; Baiter 1992; Wittekind 2009). It could
also be shown that important features of the spectrum appearreliably in model tests with good match to the full-scale ship
(Bark 1985; Heinke 1991).
1. Propeller noise (fixed pitch propellers and controllable pitch
propellers at design pitch)
a. Tonals at blade rate
These are a consequence of the individual propeller blade
passing through the uneven wake field behind the ship.The areas with the highest wake (smallest axial flow veloc-ity into the propeller) in a single-screw ship are observed
in the upper region (around the 12 o’clock position) of the
propeller disk. Under these conditions, the angle of attackon the blade is highest and therefore the pressure on the
suction side (facing into forward direction) lowest. For all
propellers of seagoing ships at their service speed, this
Manuscript received by JSPD Committee July 9, 2013; accepted October
4, 2013.
FEBRUARY 2014 2158-2866/14/3001-0001$00.00/0 JOURNAL OF SHIP PRODUCTION AND DESIGN 1

pressure will be below evaporation pressure. The forming
of steam-filled bubbles is referred to as cavitation. The
cavitation bubbles form only around the upper position.
They will appear and vanish with the rhythm of bladespassing. In a spectrum, these appear as tonals at multiplesof blade rate (shaft speed /C2number of propeller blades).
Depending on shaft speed (e.g., 90 rpm for a large ship,150 rpm for a small ship) and number of blades (typicallyfour, sometimes five, for high-power containershipssometimes six), the first harmonic is below 10 Hz for largeships and slightly above for small ships.
These tonals are often subject to investigation during ship
design because they influence comfort on board. Vibration
levels are limited by standards and are related to the pres-
sure acting on the hull above the propeller. Typical pres-sure maxima are 2 to 10 kPa for a first harmonic usually
(but not always) decreasing with the higher harmonics.
Values below 1 kPa can be assumed as a possible mini-mum even for a noncavitating propeller. These contribu-tions can be measured at the hull above the propeller if this
part of the ship is submerged. Model scale results are reli-
ably used to predict full-scale values. Accuracy decreaseswith the number of harmonics.
b. Broad band, low frequency
This contribution is the least understood. Background
noise attributed to shipping at long distances as well asmeasurements in close vicinity of the ship and the pro-peller show a broad band spectrum peaking at approxi-mately 40 to 50 Hz in most ships. It is unknown how thisspectrum is formed and why it seems that the peak isincreasing with speed but does not move in frequency(Arveson & Vendittis 2000).
The broad band, low-frequency contribution is not a sub-
ject to consider in ship and propeller design because itseffect on vibration is negligible compared with that of theblade rate tonals. On the other hand, it also does not con-tribute critically to the acoustic frequency range, partlybecause it fades away fast in the structure of the ship withdistance to the propeller and because it is already stronglyreduced by the A-weighting of the human ear such thatother noise sources (machinery, ventilation) mask thecontribution from the propeller.
c. Broad band, high frequency
This is the contribution that is often observed first when
a propeller exceeds cavitation inception speed. It first
appears as an increase of level at high frequencies,
which then dominates lower frequencies with increasing
ship speed. The reason is the high-frequency part of small
bubbles appearing and vanishing, often interacting with
the blade structure. It also contains continuous noise
such as hub vortex cavitation. The frequency where this
contribution appears first is not subject to consideration
here. If cavitation inception speed is considered, this is
defined as the speed where the tonals of blade rate har-monics start to increase with speed at a high rate. This
will occur later than the rise of the high-frequency part
of the spectrum.2. Machinery noise
a. Diesel generators
Large ocean-going ships typically have three generators,
either driven by dedicated med ium-speed four-stroke-cycle
diesel engines or by the propulsion diesel. Diesel generatorsare often resiliently mounted b ecause they ar e strong con-
tributors to noise after it propagated through the structureand being radiated into rooms aboard. The noise of these
diesels is characterized by tonals at multiples of half rota-
tional speed. Rotational speed is linked to electric mainsfrequency, which is typically 60 Hz. A typical diesel gener-
ator therefore turns at 720 rpm, but there are also aggregateswith 600 or 900 rpm. A diesel with 720 Hz has tonals at 720/
60/2 = 6 Hz and multiples up to kilohertz range.
b. Low-speed propulsion diesels
The low-speed two-stroke-cycle engine is the standard
engine type for ship propulsion. It drives the propellerdirectly without gear and therefore has the same rota-tional speed. As a result of low revolution, a high torqueis required, which leads to an impressive size of these
engines (up into the 1000 tons mass). They are always
rigidly mounted in the ship. As a result of their lowspeed, they are relatively quiet compared with a four-stroke-cycle engine but may contribute to radiated noiseresulting from their sheer size. The spectrum of these
engines has no clear signature; therefore, it is difficult to
identify a possible contribution to radiated noise.
c. Medium-speed propulsion diesels
These are used for propulsion of smaller ships. A typical
rotational speed is 514 rpm. This has to be reduced to
propeller shaft speed by means of a gear box. In a stan-dard design, the engine also drives a generator makinguse of the low fuel consumption of the engine comparedwith a diesel generator and saving one diesel as a gener-
ator drive. The generator requires constant speed. A stan-
dard feature for this propulsion system is therefore acontrollable pitch propeller to allow ship speed variationsby adjusting blade pitch independent of shaft speed.
These diesel engines can be resiliently mounted quite easily. For
this, the shaft is decoupled by a rubber coupling making the contri-
bution of these diesels about equivalent to diesel generators. How-ever, still very often these engines are hard-mounted for cost reasonsand will therefore have a high noise output to the environment.
Controllable pitch propellers at normal service speed are acous-
tically equivalent to fixed-pitch propellers. However, at lowerspeed when pitch is reduced, they may appear considerably noisier,
because the inflow to the blades is far off the optimal performance
they were designed for. These ships become noisier at low speed.The exact relationship between noise output and speed still is to beinvestigated in the future.
3. Basic thoughts on an acoustic ship model
For simplicity we further look only at three components of ship-
ping noise, which are observed to dominate in almost all ships:
Low frequencies from propeller cavitation;
Medium to high frequencies from propeller cavitation; and
Medium frequencies from four-stroke diesel engines.
2 FEBRUARY 2014 JOURNAL OF SHIP PRODUCTION AND DESIGN

One of the prerequisites for a universal acoustic ship model is the
possibility to place it in an arbitrary environment and determine
received levels in a given environment. This would be ideally
possible if the model can be reduced to a monopole and be placed
in a known position relative to the water surface.
Noise of the propeller can be viewed as a monopole because the
cavitation bubbles are very limited in size and actually act as a
monopole as a result of their variation in volume during expansion
and contraction. However, their relative position to the surface
is difficult to assess. For determination of the exact position, we
would need the draft of the ship, the size of the propeller, theposition where cavitation occurs, and the height of the stern wave
created by the ship itself. The consequence of the presence of the
water surface as a pressure release surface manifests itself in the
Lloyd Mirror effect, essentially an interference effect of two sound
propagation paths, one directly from the source and the other
through reflection at the surface. At low frequencies, this behavior
merges into a dipole radiation with a pronounced directivity in the
vertically downward direction. The Lloyd Mirror effect for point
sources can easily be described analytically with great accuracy
for practical applications (Urick 1983).
For machinery noise, the source has a finite dimension consisting
of the hull shell in way of the source such as a diesel engine. This
cannot be modeled as a monopole but there is an integration effect
resulting from the distributed source with a large surface. In addi-
tion, because the contribution from machinery is not particularly
of very low frequency, it is acceptable that the Lloyd Mirror effect
does not affect radiation.
The numerical model presented here is primarily based on
Arveson and Vendittis (2000) and our own measurements on a
containervessel and field measurements. The containership was a
new 3400-TEU vessel built by the Nordseewerke yard in Germany.
One of the three ships built carried a set of acoustic sensors on
the hull above the propeller for several weeks and recorded low-frequency noise at various operating conditions (drafts and speeds).
Arveson reports on noise measurements of a small bulk carrier
in ballast at several speeds in well-defined conditions. Measure-ments of comparable quality for this kind of ship are not available
to the author.
4. The main ship parameters influencing noise
and their spectral behavior
The main parameters considered here are
Displacement;
Speed relative to cavitation inception speed;
Block coefficient as an indicator for wake field variations;
Mass of diesel engine(s); and
Diesel engine resiliently mounted yes or no.
Propulsive power is not explicitly addressed but it relates to
displacement, speed, and the block coefficient.
The block coefficient is the ratio of the displacement to
length /C2breadth /C2draft of the ship.
Considerations are valid for fixed-pitch propellers and control-
lable pitch propellers at design pitch.
These parameters are now related to the three main contributors
to underwater irradiated noise low-frequency cavitation noise,
hereafter denoted F 1, high-frequency cavitation noise F 2anddiesel engine noise F 3. Each of these contributions shall be viewed
as an averaged sound pressure level L eqin third octaves. The
intensities of these contributions are then added to yield the
overall source level (SL).
SL¼10log 10F1
10ț10F2
10ț10F3
10/C18/C19
The low-frequency contribution F 1is formulated in a way that it
represents the monopole level, i.e., the level as it would appear inan unbounded medium. The monopole level is higher as the dipolelevel at low frequencies. At high frequencies, the dipole level is
higher (theoretically by 3 dB) because noise spreads hemispherically
as a result of the presence of the nonpenetrable surface.
The transition between high and low frequency is approximately
f¼
c/C1r
8zszr
with
c = speed of sound
r = slant distance between source and receiverz
s= source depth
zr= receiver depth
The 3-dB lower level at higher frequencies for the monopole is
ignored in the following avoiding too much complexity. Modelingthe low-frequency part as a monopole is very important because
there the propagation loss when moving away from the source
parallel to the surface follows 40log(distance) rather than 20log(distance). If the source level as presented in the following is basedon an input for a propagation loss calculation using an approxima-
tion of the Helmholtz equation, it will only yield correct results if
described as a monopole. For the higher frequency part, there is anerror of 3 dB but 20log(distance) remains a correct assumption.
5. Components of the acoustic model
5.1. Low-frequency propeller noise
In a noncavitation condition, the dominating noise is from blade
vibrations, which yield a very low noise contribution neglectedhere as compared with, e.g., machinery noise.
As soon as the propeller cavitates, its low-frequency contribution
quickly becomes dominant. Considering the cavitation bubble(s)developing on the blades predominantly around the 12 o’clockposition, these grow in size with decreasing pressure on the suc-
tion side and size of the propeller. The pressure on the suction side
will decrease with rising angle of attack on the blade, which inturn follows the wake. The higher the wake (i.e., the smaller theinflow velocity to the propeller), the higher the angle of attack andthe more pronounced is cavitation. Size of the propeller scales
with the length of the ship and area increases therefore with the
scale squared. However, our own measurements show that therelationship between ship size and noise level is not as strongas might be expected as follows from publications (Ross 1976;
Wales & Heitmeyer 2002) and our own measurements.
Furthermore, we have to consider that not in all cases the
propeller can be designed for highest efficiency, which relates torelative thrust loading of the propeller. For container vessels in
most cases, the highest efficiency propeller can be designed; how-
ever, for a slow big block ship, this may not be the case. It is
FEBRUARY 2014 JOURNAL OF SHIP PRODUCTION AND DESIGN 3

technically not feasible to accommodate a propeller of the diam-
eter required or buy an engine, which could operate with the
resulting low speed.
The quality of propeller/hull interaction can be judged by
looking at cavitation inception speed (CIS). This speed depends on
inflow speed variation, propeller loading, propeller submergence,and quality of the propeller design. This would suggest that big
block ships like a tanker have less favorable conditions compared
with a slender containership, i.e., CIS is higher in a containership.
However, looking at individual ships, CIS can vary greatly.
Table 1 shows the characteristics of five ships and Fig. 1 the
amplitude of the first harmonic of blade rate with increasing shaft
speed. The point where the level increases more rapidly indicatesthe shaft speed where cavitation has a measurable effect on pres-
sure fluctuations and supposedly radiated noise. Note that cavita-
tion may have started at considerably lower speed but may havefirst an effect only at higher frequencies, which may explain the
high CIS compared with traditional experience.
From Arveson and Vendittis (2000) it can be observed that an
increase of noise is following speed according to 80log v, whichis more than the classic relationship between flow speed and
increasing noise level. Figure 2 shows keel aspect spectra for a
bulk carrier at various speeds. Measured data are converted to 1 mlevels using 20logr as propagation law, therefore representing thedipole source level. This is the reason why there is a large devia-
tion at low frequencies, because the prediction is for the monopole
level (see also Fig. 4).
Characteristics of the spectrum in Fig. 2 are also observed in
direct vicinity of the propeller (Fig. 3) in this case at the hull
directly above the propeller. The third octave spectrum level willbe made up by contributions from tonals of blade rate harmonics
and broad band noise.
From Arveson and Vendittis (2000) we now curve fit the low-
frequency contribution with the function and account for speed,block coefficient, and size:
F
1¼2:2/C110/C010/C1f5/C02:10/C07/C1f4ț6/C110/C05f3/C08/C110/C03/C1f2
ț0:35/C1fț125țAțB
A¼80/C1logv
vCIS/C18/C19
/C14/C1cB/C18/C19
B¼10/C1logD
Dref/C18/C192
3
with
f = frequency in Hz
A = factor modeling speed and block coefficientTable 1 Data are based on measurements by the Hamburg Ship Model Basin (HSVA) taken at the hull above the propeller
on modern ships in full scale
No. TypePropeller Diameter Number of Blades Rotational Speed Ship Speed Delivered Power
D (m) Z (1) N (RPM) V (knots) PD (kW)
1 Research 3.5 5 150 13.8 3.300
2 RoRo-Ferry 6.1 4 123 22.4 20.0003 Tanker 1 5.8 4 130 15.8 7.8604 Tanker 2 5.8 4 134 16.0 7.930
5 Tanker 3 5.7 4 130 15.3 7.062
Fig. 1 Effect of shaft speed (about linear with ship speed) on level of first harmonic of blade rate for five ships. CIS is approximately 21 knots for
ship 2, 13 knots for ship 3, 13 knots for ship 4, and 12.5 for ship 5. Ship 1 does not show cavitation. Dots are measured points, lines are interpolation.
CIS, cavitation inception speed
4 FEBRUARY 2014 JOURNAL OF SHIP PRODUCTION AND DESIGN

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66.6.6666.6
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e – – '01e;2:Iteeer=ousliZ etees :crrrwwir 111:121gt
'ICIL17770M: EE
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14-OCTAVE BAND CENTER FREOUENCY (Nt) 1 00 0
0
0
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4-4
190
185
(NJ 180 1-1
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LW 165
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t.ft
150
et:
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140 175
160
155 0 _
ÜB IN
81141111114111111411WAL_ nr4if-ieffseiB_WIEWIINfrellie7 -0- 1B15 E BHW .3:

wee eper .
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Fig. 2 Radiated noise of a bulk carrier in ballast at different speeds as received by a hydrophone vertically down and converted to source level
using 20log(distance) from Arveson and Vendittis (2000) compared with prediction for 14 knots. White line represents formula from Wales
and Heitmeyer (2002)
B = factor modeling displacement
v = speed through water in knots
vcis = cavitation inception speed in knots
cB = block coefficient
= displacement in t
Aref = reference displacement in t = 10,000
The resulting curve is displayed in a graph of source level in
third octaves in dB re 1 laPa against frequency and will peak at 40 Hz.
5.2. High -frequency propeller noise
The high-frequency part is known to rise from the high-frequency
end of the spectrum and covering the spectrum of machinery noise
down to lower frequencies with increasing speed. The physical
law to describe this behavior is not known; therefore, this behavior
is just curve fit to Arveson and Vendittis (2000). A stronger
dependence an ship shape is assumed for this contribution
representing the observation that it is observable in the radiated
spectrum before the low-frequency contribution
1000
100,0 F2 =-5  ln(f) , + 10 +B + C
– 86 rpm 101 rpm
Fig. 3 The sound spectrum measured above the propeller of a 3400-
TEU containership for two different shaft speeds. 101 rpm is full speed of
approximately 23 knots. lt can be observed that both tonals and broad
band contributions reduce with speed. Note the broad band maximum at
approximately 40 Hz. The ship has a five-bladed propeller C = 60  log ( 1000  cB)
vcts
with
C = modeling effect of speed and block coefficient
The diesel engine is taken from own observations and measure-
ments. It is assumed to be a medium-speed four-stroke engine
FEBRUARY 2014 JOURNAL OF SHIP PRODUCTION AND DESIGN 5

resiliently mounted. Its noise output will increase with power and
hence with weight assuming a roughly constant power-to-weightrelationship. This would mean an increase in noise with 20log
(power); however, as correlation of forces transferred to the ship
decreases with the size of the engine, we assume a 15log law. Inaddition, we include the number of engines in operation. A rigidly
mounted engine will have a 15-dB higher level describing the
insertion loss of a resilient foundation of moderate quality.
F
3¼10/C07/C1f2/C00:01/C1fț140țDțE
D¼15/C1logmðȚ ț 10/C1lognðȚ
with
D = factor modeling engine mass and number
E = 0 engine resiliently mounted, = 15 engine rigidly mounted
m = engine mass in tn = number of engines operating at the same time
6. Other parameters influencing radiated noise
It is observed in various publications (Ross 1976; Wales &
Heitmeyer 2002) that noise from ships seems not to vary beyonda band of 10 to 15 dB, at least if looking at frequencies above
100 Hz. However, their design characteristics can be very differ-
ent and would imply a larger variation.
Several reasons could be suggested for this observation. All
ships are designed according to certain standards defined by classi-
fication societies. Propulsive efficiency may vary a lot dependingon ship type and design effort. Fast ships are slender and have agood wake field; slow ships have a blunter shape and a worse wake
field. Almost all ships operate at their design speed when in open
water. It could be expected that the amount of cavitation in eachship differs not so much from most of the others so that a 10-dB
difference would result (note that if noise is related to area of noise
generation 10 dB mean, a difference in area by factor would be 10).
Of course, this would assume that in all these ships, propeller
cavitation dominates the spectrum.
In case of machinery noise, the contribution would very much
depend on source level and quality of the resilient foundation, if any.
For low frequencies dominated by broad band cavitation noise,
the possible spread is not well investigated because there are toofew reliable measurements. It must be observed that the LloydMirror effect has a strong influence on received levels and would
need to be considered when deriving a source level. To do so, one
would need to know source depth, receiver depth, and distance.These are not indicated in almost all reports.
From cavitation tunnel observations, however, it is observed that
levels of tonals at multiples of blade rate can be very muchinfluenced by stern and propeller design and application of append-
ages improving the wake field. It may be acceptable to assume that
the broad band level is related to the narrow band levels at har-monics of blade rate, i.e., tonals and broad band level decrease inunison. This needs to be confirmed by further research.
The model described here leads to a monopole source spectrum
at low frequencies in third octaves re 1 mPa. Figure 4 collects all
possible representations of the ship source spectrum. Full line is
third octaves. The dotted line will be observed when the Lloyd
Mirror effect is ignored. It leads to partial cancellation of the lowfrequencies as a result of destructive interference of the direct
propagation path and the one reflected at the surface. The radia-
tion pattern is then equivalent to a dipole. In this case, we assumeda source depth of 2.5 m.
It can also be seen that the typical 50-Hz maximum of radiated
noise from ships is becoming more pronounced as a result of thesurface influence.
We have not considered the difference between ships with
small (ballast condition) and full draft (laden condition). In the
few unconfirmed observations, the difference is reported as small(approximately 3 dB). Several effects lead to changes in radiation:
Resistance is lower, propeller is unloaded, and it becomes
quieter. There is a danger of face-side cavitation, which wouldmake the propeller noisier.
Static pressure is lower, which shifts CIS to lower speeds.
The propeller is closer to the surface so the Lloyd Mirror
effect leads to lower radiation.
The distance between the propeller and the surface
depends on sinkage, trim, and height of stern wave, all not
known for a particular operating condition. For a fast contain-ership, the water level above the propeller at full speed is
several meters higher compared with the still water line.
It requires more research to find and explain the influence of
draft. For this model discussed here, a standard source depth of2.5 m is the recommended compromise for all unknown conditions.
Fig. 4 The source spectrum of the bulk carrier from Arveson in
1/3 octaves. The dotted line is the so-called dipole level, i.e., the source
level is not corrected for the effect of the water surface. The full linesis the level corrected for the presence of the surface, i.e., it constitutes
the monopole level
6 FEBRUARY 2014 JOURNAL OF SHIP PRODUCTION AND DESIGN

7. Results of modeling
Figures 5 and 6 show modeled results for two ships. Any vari-
ations of parameters will lead to continuous changes of the spec-
trum. Sudden changes with speed as observed in the data of Fig. 2
cannot be modeled.
Considerable deviations might be expected when looking at
individual ships. As an example, there is still a large number of
general cargo ships underway, which may be very slender butslow and have a low-quality wake field and propeller. Also, their
machinery may be outdated and without secondary measures
such as resilient foundations.
Also ships with controllable pitch propellers at low speed are
not represented by the model.
The model can be applied to get an impression of what the
received level of ship is in an arbitrary environment.
During a background noise survey, we measured a tanker in a
shallow water environment. Conditions were:
Distance to ship at closest point of approach 0.4 nm;
Water depth 28 m;Hydrophone depth 26 m;
Ship displacement 50,000 t;
Speed 14.7 knots;Assumed CIS 11 knots;
Diesel generator 1 /C220 tons; and
Bottom sand.
From these data, the source level of the ship was derived and
fed into a sound propagation model based on the parabolic equa-
tion approximation of the Helmholtz equation. The calculation
was made in 1-Hz spacing, which was then averaged for each
third octave in the frequency range considered.
Figure 7 shows results. Noise propagation loss at frequencies
below 50 Hz could not be calculated as a result of too low wavelength/depth ratio. At frequencies below 150 Hz, the prediction of
received level compared with measured level is very good. There
is a band of strong tones measured at approximately 250 Hzcaused by propeller singing from a nearby ferry, which are, of
course, not shown in the model. Above this frequency range pre-
diction yields 4 dB higher values on the average than measured.
Besides the inaccuracies in the model, which are expected in
the range of ± 5 dB, there is, of course in addition, the inaccuracy
of the propagation loss prediction, which is also expected to be in
the range of ± 5 dB as a result of unknown properties of thebottom, which would be required down to a depth of several tens
of meters for an accurate prediction.
8. Outlook
With the numerical model presented, a reasonable prediction of
radiated noise of common merchant ships is possible based on
engineering parameters of a ship. It delivers similar variations as
observed in the field.
However, much more research is needed to find more influenc-
ing parameters, find the reason why not all ships would fit into
the model, and cover ships with controllable pitch propeller in
off-design pitch.
Another step would be required to relate the parameters dis-
cussed to data from AIS (Automatic Identification System), which
do not allow identifying the parameters discussed here directly.
Fig. 5 Modeled source level of a ship with 33,000-t displacement,
cB= 0.78, CIS = 9 kn, speed 14 knots, two diesel generators of 30 t,
and individual contributions. CIS, cavitation inception speed
Fig. 6 Modeled source level of a ship with 135,000 t displacement,
cB= 0.65, CIS = 14 kn, speed 25, 20 and 15 knots, two diesel gener-
ators of 50 t. CIS, cavitation inception speed
FEBRUARY 2014 JOURNAL OF SHIP PRODUCTION AND DESIGN 7

They could possibly be inferred from significant naval architectural
parameters such as ship speed, L/B, and B/T, which are part of theAIS protocol.
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Fig. 7 Source level from model, predicted received level using the
source level from model and a sound propagation code for propagation
loss and measured level at closest point of approach (CPA)
8 FEBRUARY 2014 JOURNAL OF SHIP PRODUCTION AND DESIGN