Design of a small fisheries research ship with low level of

Design of a small fisheries research ship with low level of
underwater-radiated noise
Yasuo YOSHIMURA; Graduate School of Fisheries Sciences, Hokkaido University, Japan
Yasunari KOYANAGI; Kyokuyo Co., Ltd., Japan
SUMMARY
It is important for a fishery research ship to reduce the underwater-radiated noise. In 1995, ICES
recommended the noise level to ensure the acoustic research works and to prevent the fish reaction against
the noise1). For a small size ship, however, it is very difficult to design because of not enough space or
capacity to make sound insulations. Moreover, the Froude Number: Fn(=V/√Lg, V:ship’s speed, L:length
of ship, g: acceleration of gravity), that is the non-dimensional scale of ship's speed and determines the
ship’s propulsive power, becomes higher for a small ship. The higher Froude Number generally provides
the larger propeller load and cavitation, which introduces the significant level of underwater-radiated noise.
Since the construction and operation as well as maintenance cost is lower for a small ship, the research
cost becomes lower. It would be better and efficient if a small ship could comply with the ICES ’s
recommendation and take the place of a large research ship.
In this paper, the authors propose a successful design method for a small ship to reduce the
underwater-radiated noise level. Utilizing this design, a small fishery research ship whose capacity is
290GT and length is 33.5m has been designed 2). From the sea trials after the construction, it has been
proved that the underwater-radiated noise level of this ship is the world lowest class in 10 knots (Fn=0.28)
of researching speed and the ship has sufficiently complied with the ICES's recommendation even in 11.6
knots (Fn=0.33) of the high-speed region.
Fig.1
The small fisheries research ship “AL AMIR MOULAY ABDALLAH” with world
lowest level of underwater-radiated acoustic noise2)
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1. INTRODUCTION
It is expected that a small fishery research ship can provide lower construction and operation cost than
the large-sized research ships. However, since a small ship has a short length of ship, hull resistance
becomes relatively larger even in the same speed because of the high Froude number of ship. This large
amount of resistance requires the larger propulsive power, which often causes the propeller cavitation. The
noise of the propeller cavitation usually makes the high level of underwater-radiated noise. Moreover, for
the small research ships, there is little space for insulate the mechanical noises such as engines and another
machine plants. Therefore, it is generally disadvantage for the small research ships to reduce the
underwater- radiated noise.
a) 135-1.66 log(fHz )
: (1< fHz <1000)
b) 130-22 log(fHz /1000) : (1,000< fHz <100,000 )
Fig.2
Recommended underwater-radiated noise level by ICES at 11 knots free-running
for all vessels used in fisheries research1)
As for the underwater radiated-noise of fisheries research ships, the level is requested to be as much
as possible lower in order to keep the performance of acoustic survey such as an integrated fish finder.
Moreover, as many kinds of fish are sensitive to the noise in the low frequency region, the noise level is
also limited to prevent the fish avoidance by the noise. Otherwise, the investigated data may be affected by
the avoidance and become inaccurate. In 1995, for this purpose, International Council for the Exploration
of the Sea: ICES recommended the maximum level of the underwater-radiated noise of a fisheries research
ship as shown in Fig.21). The recommendation consists of two parts that are described as the followings.
Maximum level of the underwater-radiated noise
(dB re 1µPa@1m) =
135-1.66 log(fHz )
: (1< fHz <1000)
130-22 log(fHz /1000) : (1,000< fHz <100,000)
{
----------------------- (1)
The criterion in the low frequency region (1< fHz <1000) comes from the prevention of the fish avoidance,
and in the high frequency region (1,000< fHz <100,000) from the acoustic survey, respectively. The detail
introduction of this recommendation is described in the reference [1]. Resent most fisheries research ships
have began to reflect this recommendation in their construction specifications.
As 11 knots of the researching speed in the ICES ’s recommendation is comparatively higher, the
target ship of the recommendation may be the large fisheries research ship that surveys internationally. The
large ship can easily produce 11 knots by low propulsive power. For a small ship, however, relatively
higher engine power is required to achieve this speed. It is sometimes difficult to make it. Although the
basis is not clear that ICES requests this standard even for the small fisheries research ships with difficult
achievement technically, the researchers intend to comply with the ICES’s recommendation of
underwater-radiated noise level as possible. In this paper, the authors summarize the noise souses and
successful design method to reduce the noise particularly for a small ship, referring to an example of
fisheries research ship.
2. NOISE SOURCES OF AND THE TECHNICAL MEASURES
In order to design the ship of low underwater-radiated noise, the noise sources and their mechanisms
should be correctly clarified. Typical noise sources are summarized in Table 1 of the following. These
noises can be divided as the following three groups from the physical meaning.
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Noise sources
Propulsion Engine
Generator Engines
Gearbox, Pumps
Propeller surface force
Propeller cavitation
Table 1 Noise sources and their measures
Frequency domain
Measure to reduce noise
(1) Noise Absorbing suspension (Rubber, etc)
∼2kHz
(2) Low-noise type machine
(3) Hull damper
(4) Increase the thickness of the bottom hull
(5) Keep the Enough propeller tip clearance
∼1kHz
(6) Reduction of propeller load
1kHz∼100kHz
à Reduction of hull resistance (EHP)
1) Optimum ship hull dimensions
2) Optimum Cp curve
3) Bulbous bow
(7) Smoothing the wake flow around the propeller
àAdopt stern bulb & fine frame line
(8) Highly skewed propeller
(9) Low rpm & large diameter of propeller
(10) Deep propeller immersion
1) Mechanical acoustic noise
The first group is the mechanical
acoustic noise that comes from diesel
engine, gearbox and pumps. The frequency
domain of this noise is generally less than
2 or 3 kHz. For the noise of diesel engine,
rubber suspension: (1) as well as the lownoise type engine: (2) is well used. As the
gearbox also makes a significant noise
peak below 1kHz as shown in the example
of Fig.3, rubber suspension is also desired.
Fig.3 An example of underwater-radiated noise
In this case, however, both engine and
spectrum by a gearbox.1)
gearbox are suspended by elastic materials,
and then it is required to design the suitable mass-spring system avoiding an abnormal shaft vibration. As
the mechanical sound noise is mainly produced in engine room, it can be insulated by a hull damper: (3)
that is coated inside of the ship hull. This kind of hull damper is now already in market and easy to obtain.
Since the under-water radiated sound noise is spread by the vibration of ship hull, the increase of the
thickness of bottom hull as described as (4) in Table 1 is also important together with the above mentioned
hull damper.
2) Propeller surface noise
The next noise group is the sound noise induced by the propeller surface force. This force comes from
interactive force between propeller and ship hull, and the frequency domain is less than 1kHz in general. As
the magnitude of this force is increased when the propeller’s tip clearance (the closest distance between
propeller tip ship hull) becomes small, this clearance must be kept more than 25% of propeller diameter as
described as (5) in Table 1. However, it is noticed that the large clearance makes the propeller diameter
small, which is not advantage for the next cavitaion noise.
3) Propeller cavitation noise
The most important noise shall be the propeller cavitaion noise. It strongly influences the principal
design of the research ship. The frequency domain of this noise is over 1kHz to 100kHz that affects the
acoustic survey significantly. In order to reduce the propeller cavitaion, the following items that are
described in Table 1 are important. Those are reduction of propeller load: (6), smoothing the wake flow
around the propeller: (7), adopt a highly skewed propeller: (8), adopt low rpm & large diameter of
propeller: (9) and deep propeller immersion: (10).
As the reduction of propeller load means the reduction of hull resistance or effective horsepower: EHP,
it requires the same process in the optimisation of the propulsive performance. This means that the ship
designer should take the consideration of the acoustic characteristic into the principal design. For this
purpose, the following processes are required.
l Optimisation of ship hull parameters to determine ship dimensions such as L/B, B/d, Cb, Cp
l Optimisation of Cp curve of ship
l Utilization of the appendages to reduce the EHP such as bulbous bow
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L/B, B/d, Cb, Cp are so important parameters to determine the hull resistance as well as the stability
and other performance of ship, that it is necessary to chose the optimum values as to be minimum in hull
resistance. Particularly for the small fisheries research ship where the Froude Number becomes higher,
these parameters shall be settled in order to minimize the wave-making resistance of the ship, since the
wave-making resistance depends on the Froude Number. For this purpose, the optimisation of Cp curve and
application of the appendages to reduce the EHP such as bulbous bow are requited by means of utilising
experimental tank tests, conventional linear potential theories or recent CFD (Computational Fluid
Dynamics) techniques .
Prediction of cavitation area
Meanwhile, it is important to estimate the propeller cavitation area from the degree of propeller load.
Although the precise propeller cavitation is not easy to predicted without the propeller cavitation test, the
cavitation scale can be roughly estimated by the following chart. Fig. 4 shows the actual relations between
degree of propeller load and propeller cavitation area. The vertical axis is the degree of propeller load
described by the eq.(2), and the horizontal axis is the cavitation number defined by eq. (3).
Degree of propeller load = T/ (ρ/2)Ap {Va2 +(0.7πn DP )2 }
------------------------------ (2)
σ0.7R= (p-e)/ (ρ/2) {Va2 +(0.7πnDP )2 }
------------------------------ (3)
,where
T
Ap
n
p-e
ρ
Va
Dp
: propeller thrust
: propeller disk area
: propeller revolution per sec.
: static pressure at the propeller
: density of water
: inflow velocity to propeller
: propeller diameter
30%Bk.Cav
0.4
Burrill's Chart
20%
Large Dia,
small rpm
DANGEROUS ZONE
10%
0.3
T/0.5ρAp[Va 2 +(0.7πDn)2]
5%
Lerbs
2.5%
13kts
0.2
12kts
Upper Limit for
heavy loaded prop.
Taka-maru
13kts
10kt
Ship-A
12kts
17kts
15kts
9kts
11kts
Kaiyo-maru
+10dB
10kts
10kts
0.1
ISEC+0dB
NSMB
small Dia,
large rpm
0.05
0.1
-10dB
Upper Limit for
Merchant ship prop.
0.2
0.3
0.4
SAFETY ZONE
0.5
0.6
0.7
0.8 0.9 1.0
( p-e)/0.5 ρ [Va2+(0.7 πDn)2]
Fig.4 Relations between propeller cavitation area and degree of propeller load together with
the safty design lines for the ICES’s noise recommendation (Modified Burrill’s chart)
The degree of propeller load becomes high and the propeller cavitation tends to occur strongly in the
upper left in this figure. This chart is well known as Burrill’s chart 6). In this figure, resent characteristics of
fisheries research ships3)-5) are estimated. It can be seen that the results at 10 knots free-running of
‘Kaiyo-maru’ that is the electric motor driven large fisheries research ship locates in the lower right in this
figure. At this 10 knots free-running, the Froude Number is lower and both ship resistance and the degree
of propeller load are small. The result from 8 knots free-running of ‘Taka-maru ’ that is a small fisheries
research ship shows the similar characteristics, too. According to these data of fisheries research ships, it is
found that less than 1% of propeller cavitation is required for the completion of the ICES ’s
recommendation. The corresponding degree of propeller load lines to the ICES ’s recommendation are
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proposed as dotted bold lines in this figure. It will be very useful to determine the propeller load or the
rough estimation of the nois e level.
The next important point to prevent the propeller cavitation is the smoothing the wake flow around the
propeller that is listed as item (7) in Table 1. For this purpose, a stern bulb and fine frame line are effective.
For a small fisheries research ship, however, since there is little space of an engine room and demand from
the stability of ship, the aft frame lines tend to grow fat, which can not makes the flow pattern smooth. In
such case, a high skew propeller: (8) will become one of the solutions.
2) Prediction of cavitation noise
After determining the propeller cavitation area, the underwater-radiated acoustic noise by a propeller
cavitation is predicted in turn. Although the accurate prediction of cavitation noise is very difficult even by
the cavitation tunnel test, Brown’s method7) is one of the simplest ways to predict it in the design stage. The
acoustic noise level is easily calculated as the following formula.
NL(dB re 1µPa@1m) = 163+10 log10 (z Dp 4 n 3/fHz2 )+10 log10 (Ac/Ad)
where,
Ac/Ad
z
fHz
------------------------ (4)
: propeller cavitation area ratio against the propeller disk area
: number of propeller blades
: frequency in Hz
In figure 5, the predicted noise levels are plotted for the fisheries research ship mentioned later. In this
figure, several calculated characteristics are shown for various cavitation area ratio together with ICES ’s
recommendation and the measured underwater-radiated noise of the large fisheries research ships “Dr. F.
Nansen”, “Corystes ” and “Kaiyo-maru”. From these comparisons, it is concluded that at least 10% of
cavitation area ratio is required when complying ICES ’s recommendation, and that less than 1% of
cavitation area ratio when aiming the world lowest class with the underwater-radiated acoustic noise.
160
Dr.F.Nansen
150
dB re 1μPa at 1m
140
130
Ship-A
V =10kts
Ac/Ap
Corystes
120
Kaiyo-maru 10 kt
110
100%
30%
10%
3%
100
Taka-maru 8 kt
90
1%
ICES Recommendation @11kt
80
70
10
100
Fig.5
1,000
10,000
Frequency(Hz)
100,000
Comparison of underwater-radiated noise between predicted by Brown’s
method7) and measured results and ICES ’s recommendation
3．DESIGN AND CONSTRUCTION OF THE SMALL FISHERIES RESEARCH SHIP COMPLY
WITH ICES ’S RECOMMENDATION
The designed ship (ship-A)2) is a stern trawler with single decker and long f’cle having
trawling-slipway-stern. In designing of the ship, the reduction of underwater-radiated noise is firstly aimed
in order to prevent any trouble in the new acoustic research equipment. For this purpose, the hull form
under the water is slimed and the hull damper is installed for the sound insulation of engines’ noise. In
addition, the hull structure is designed so that the center of gravity becomes lower in order to secure the
safety. The principal particulars are shown as the followings. The general arrangement is shown in Fig.6.
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Length between p.p:
33.50 m
Breadth (molded):
7.80 m
Depth (molded):
3.50 m
Designed draft (molded): 3.00 m
Gross tonnage:
293 t
Main engine output:
736 kW×1
Trial speed:
13.18 knots
Service speed:
12.2 knots
Cruising range:
6000 NM
Complement
21 persons
(Crew:14 persons, Scientists:7 persons)
1) Arrangements for the reduction of
underwater-radiated noise level
In order to minimize the underwaterradiated noise level, almost every
measures listed in Table 1 have done.
For the reduction of machinery plant
noise,
(1) Elastic rubber suspensions for main
engine, generator engines and
reduction gear
(2) Installation of low noise pumps
such as screw pump
(3) Application of hull damper to the
inside shell and wall of engine room
(4) Increment of the thickness of
bottom plate and bed of machines
Fig. 6 General arrangement of Ship-A
For preventing the propeller
“AL AMIR MOULAY ABDALLAH”
surface noise,
(5) 25% propeller diameter of the propeller tip clearance
For preventing the propeller cavitation noise,
(6) Optimum hull dimensions and hull form including bulbous bow and sonar dome have been obtained
by comp uter simulations and model tests in towing tank.
(7) Slimed fine open stern form with stern valve and hanging rudder without shoe piece for the purpose
of the smoothing the wake flow around the propeller position
(8) Design the optimum propeller dimensions including high skew and low rotation with large diameter.
2) Result of the measurement of underwater-radiated noise
The obtained spectra of underwater-radiated noise at 10 and 11.6 knots free-running speed of Ship-A
are shown in Fig.7, where the spectra of the fishery research ship “Corystes ”, “Dr.F.Nansen” and
“Kaiyo-maru” are compared. These three are the typical fisheries research ships whose underwater-radiated
noise level are lowest class in the world, and contributed to the evaluation of ICES ’s recommendation.
Besides, the propulsion system of “Corystes ” and “Kaiyo-maru” are electric motor driven and are called the
ships with the world lowest noise level. From this figure, the level of measured noise spectrum of ship-A at
10 knots (Fn=0.28) indicates lower than that of “Corystes ” in the wide frequency range of over 500Hz.
Even in 11.6 knots (Fn=0.33), the measured noise level is lower than that of “Dr.F.Nansen”, whereby
ship-A has complied with the ICES ’s recommendation of noise level.
Table 2 Comp arison of researching speed and ship size that comply with the ICES ’s recommendation
Ship Size
Speed (kts)
Fn
Propulsion Unit
Ship-A
Designed Ship
Taka-maru
Corystes
Dr. F. Nansen
Kaiyo-maru
Lpp=33.5m
Lpp=25.0m
Lpp=48.992m
Loa=56.75m
Lpp=83.0m
10
11.6
8
11
11
10
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0.28
0.33
0.26
0.26
0.26
0.18
Diesel, CPP
Diesel with CPP
Electric Motor, FPP
Diesel with FPP
Electric Motor, CPP
160
fish reaction resion
150
acoustic survey
Dr.F.Nansen
dB re 1μPa at 1m
140
130
ICES Recommendation
120
Ship-A 11.6 kt
Corystes(electlic motor driven)
110
100
Taka-maru 8 kt
90
Kaiyo-maru 10kt
80
Ship-A 10 kt
70
10
100
1,000
10,000
Frequency(Hz)
100,000
Fig.8 Comparisons of measured underwater-radiated noise spectrum
5．CONCLUSIONS
In the construction of the fishery research ship, it is important to reduce the underwater-radiated noise.
In order to ensure the acoustic research works and preventing the fish avoidance to the noise, the maximum
underwater-radiated noise level is recommended by ICES that is coming the world standard for fisheries
research ships. For the design of such low noise ship, the followings are proposed.
1) For the reduction of machinery plant noise, it is necessary to adopt elastic rubber suspensions and hull
damper as well as the increment of the thickness of bottom plate.
2) For preventing the propeller surface noise, more than 25% propeller diameter of the tip clearance is
required.
3) The optimum hull dimensions and hull form are necessary to reduce the propeller load. This reduced
propeller load and the slimed fine open stern and the optimum propeller dimensions including high
skew make the propeller cavitation smaller.
4) Although the accurate prediction is difficult, Brown’s method and the extended Burrill’s chart that is
proposed here are very useful and effective for the prediction of the complicated cavitation noise.
5) Since the construction and operation as well as maintenance cost are lower for a small ship, the
fisheries research cost becomes lower. It would be better and efficient if a small research ship could
comply with the ICES ’s recommendation.
REFERENCES
[1] ICES: “Underwater Noise of Research Vessels Review and Recommendations”, Cooperative Research
Report, No.209 (1995).
[2] Koyanagi, Y.: “Design and Construction of the Fishery Research Ship “AL AMIR MOULAY
ABDALLAH”, Journal of Fishing Boat and System Engineering Association of Japan 2002-2, (in
Japanese), (2002)
[3] Knudsen, H.P. et al.: ”Hydro acoustic Performance of New Fisheries Research Vessels”, Symposium on
Fisheries and Plankton Acoustics (1995)
[4] Hatayama, Y.: “Fisheries Research Ship of National Research Institute of Fisheries Engineering
Taka-maru” Journal of Fishing Boat Association of Japan Vol.318, (In Japanese), (1995)
[5] Fishery Agency of Japan: “Fisheries Research Ship Wakatka-maru” Journal of Fishing Boat
Association of Japan Vol.319, (In Japanese), (1995)
[6] Steering Group for Propeller Excited Vibration Project in BSRA ,” Design Guidance Note on the
Avoidance of Propeller Excited Vibration Problems ” (1977)
[7] Brown, N.A.,: ”Cavitation Noise Problems and Solutions”, International Symposium on Shipboard
Acoustics (1976)
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