ISO 20233-1:2018

INTERNATIONAL ISO
STANDARD 20233-1
First edition
2018-03
Ships and marine technology — Model
test method for propeller cavitation
noise evaluation in ship design —
Part 1:
Source level estimation
Navires et technologie maritime — Méthode d'essai sur modèle
pour évaluer le bruit de cavitation des hélices dans la conception des
navires —
Partie 1: Estimation du niveau d'émission de la source
Reference number
ISO 20233-1:2018(E)
©
ISO 2018

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ISO 20233-1:2018(E)

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ISO 20233-1:2018(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Model test setup and conditions . 3
4.1 Test setup . 3
4.1.1 Test facility . 3
4.1.2 Model propeller . 3
4.1.3 Wake generation . 3
4.2 Test conditions . 4
4.3 Calibration . 5
5 Noise measurement instrumentation . 5
5.1 Hydrophone and signal conditioning . 5
5.2 Data acquisition . 6
5.2.1 General. 6
5.2.2 Sampling frequency . 6
5.2.3 Resolution . 6
5.2.4 Synchronization for multiple channel sampling. 6
5.2.5 Filtering . 6
5.2.6 Acquisition time . 6
6 Noise measurement procedure . 6
6.1 Propeller cavitation noise measurement . 6
6.2 Background noise measurement . 6
6.3 Reference field measurement . 7
6.3.1 Objective . . 7
6.3.2 Virtual source and input signal . 7
6.3.3 Measurement condition . . 7
7 Post processing and scaling . 7
7.1 Sound pressure level . 8
7.2 Background noise adjustment . 8
7.3 Transmission loss . 8
7.4 Model scale source level . 9
7.5 Scaling to the full-scale noise levels . 9
7.6 Other option for full-scale noise prediction .10
8 Uncertainty .10
Annex A (informative) Wake extrapolation methods .11
Annex B (informative) Uncertainty assessments .12
Bibliography .13
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ISO 20233-1:2018(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
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URL: www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 8, Ships and marine technology, SC 8,
Ship design.
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ISO 20233-1:2018(E)

Introduction
In order to reduce shipping noise, the characteristics of ship noise should be understood. Propeller noise,
which is the major noise source in commercial ships, is mainly due to its turns as spectral harmonics
and to cavitation as broadband noise. Special ships such as fishery research vessels and military vessels
require quiet propellers with less or no cavitation in their operating conditions.
The propeller cavitation noise can be assessed by experimental and/or numerical methods in the
propeller design stage. The numerical method such as CFD or empirical formulae might be a good
alternative to propeller cavitation noise evaluations. However, the model tests are still used widely to
predict the full-scale acoustic source strength of the propeller cavitation for a wide range of frequencies.
This document was developed to provide a standardized model test method for propeller cavitation
noise evaluation. This document is aimed for appropriate evaluation of the propeller cavitation noise
characteristics at the early design phase via model tests.
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INTERNATIONAL STANDARD ISO 20233-1:2018(E)
Ships and marine technology — Model test method for
propeller cavitation noise evaluation in ship design —
Part 1:
Source level estimation
1 Scope
This document specifies a model test method for propeller cavitation noise evaluation in ship design.
The procedure comprises reproduction of noise source, noise measurements, post processing and
scaling. The target noise source is propeller cavitation. Thus, this document describes the test set-up
and conditions to reproduce the cavitation patterns of the ship based on the similarity laws between
the model and the ship. The propeller noise is measured at three stages. The measurement targets
for each stage are propeller cavitation noise, background noise, and transmission loss. For the source
level evaluations, corrections for the background noise and the transmission loss are applied to the
measured propeller cavitation noise. Finally, the full-scale source levels are estimated from the model
scale results using a scaling law.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 17208-1:2016, Underwater acoustics — Quantities and procedures for description and measurement
of underwater sound from ships — Part 1: Requirements for precision measurements in deep water used for
comparison purposes
IEC 61260, Electroacoustics — Octave-band and fractional-octave-band filters
ITTC — Recommended Procedures and Guidelines 7.5-02-01-05: Model scale propeller cavitation noise
measurements
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1
acoustic centre
position where all the noise sources are co-located as a single point source
Note 1 to entry: The acoustic centre is the centre of the expected cavitation extent.
3.2
background noise
noise from all sources other than the source under test
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ISO 20233-1:2018(E)

3.3
cavitation number
σ
n
1 
22
non-dimensional quantity defined as ()pp− / ρnD where
0 v  
2
 
p is the total static pressure;
0
p is the vapour pressure;
v
ρ is the density of the fluid;
n is the propeller rotational speed (rps);
D is the diameter of the propeller.
Note 1 to entry: The total static pressure (p ) consists of atmospheric pressure and submergence depth pressure
0
which is usually taken at a specific point approximating the centre of the expected cavitation extent in the upper
part of the disk, such as 0,7 R(R : radius of the propeller), 0,8 R or 0,9 R above the propeller centreline, although
the propeller centreline is also used.
3.4
noise source
noise generating mechanism or object
Note 1 to entry: For the purposes of this document, the main noise source is the propeller cavitation.
3.5
propeller plane
imaginary plane orthogonal to the shaft centre line and including the intersection (point) of the shaft
centre line and generator line
3.6
propeller thrust coefficient
K
T
2 4
non-dimensional quantity defined as T/(ρn D ), where T is the thrust of the propeller
3.7
propeller torque coefficient
K
Q
2 5
non-dimensional quantity defined as Q/(ρn D ), where Q is the torque of the propeller
3.8
reference distance
distance used for source level conversion and defined as 1 m apart from the acoustic centre
3.9
reference field
sound pressure field that is measured using a virtual source located at a given position, i.e. acoustic centre
Note 1 to entry: The reference field shall be used to calculate the source level.
3.10
sound pressure level
SPL
L
p
ten times the logarithm to the base 10 of the ratio of the time-mean-square pressure of the measured
sound pressure, in a stated frequency band, to the square of a reference value expressed in decibels by
2
 
p
 
L =10log where p =1μPa
ref
p 10
2
 
p
 ref 
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ISO 20233-1:2018(E)

3.11
source level
SL
converted quantity of the measured sound pressure at a reference distance 1 m apart from the
acoustic centre
3.12
virtual source
artificial sound source of which transmitting power is known a priori
3.13
wake
simulated ship wake at the propeller plane
Note 1 to entry: For the model test, ship wake is simulated using a wake screen or a ship model.
4 Model test setup and conditions
4.1 Test setup
In order to evaluate the propeller cavitation noise performance via model tests, it is important to
reproduce noise sources accurately, i.e. the cavitation patterns, based on the similarity laws between
the model and the ship. Test setup for the purpose comprises test facilities, model propellers, and wake
fields generation.
4.1.1 Test facility
Test facilities might vary between variable pressure water tunnels, circulating water channels with a
free surface in the test section to a depressurized towing tank. The variable pressure water tunnels,
which are called cavitation tunnels, are widely used for the model tests. Depending on their test section
sizes, suitable devices to generate wake fields should be utilized.
4.1.2 Model propeller
The size of a model propeller depends on the capacity constraint of the test facilities and on the
acceptable range of test section blockage. The size of the model propeller should be determined to
achieve the highest Reynolds number within the capacity constraints of the test facility. A typical
propeller diameter for a model scale propeller is in the range between 180 mm to 300 mm. The
accuracy of the model propeller geometry should be according to ITTC — Recommended Procedures
[1]
and Guidelines 7.5-01-02-02 which specifies that the offsets of the blade sections should be in the
range ±0,05 mm.
4.1.3 Wake generation
For the propeller cavitation model tests, the wake fields are to be generated by the wake screen or the
model ship. In general, the former is used in small-sized and medium-sized cavitation tunnels, while
the latter is used in the large cavitation tunnel. The important scaling parameter for the cavitation test
is the Reynolds number but its similarity cannot be achieved for practical reasons. In order to reduce
scale effect, the Reynolds number should be determined as high as possible within the capacity of the
test facilities.
For the medium-sized cavitation tunnels, the wake distributions are to be generated inside the
cavitation tunnel by using a wake screen composed of wire meshes. When a full-scale ship wake is
required, it is to be obtained by extrapolating the model scale wake field or by using computational
fluid dynamics (CFD). A dummy model in combination with wake screens can be applied in the medium-
sized tunnel as well. For twin screw ships, the inclined shaft, brackets and bossing can be mounted in
small- to medium-sized test sections.
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ISO 20233-1:2018(E)

For the large-sized cavitation tunnels, the wake can be generated typically from a ship model installed
in the test section. In some cases, the ship model with grids or the shortened model can be used as
well. The model ship is manufactured of various materials with the scale ratio that is dependent on the
dimensions of the ship and the tunnel. The model ship is installed inside the tunnel corresponding to
the full-scale draft. The free surface is covered by plates to suppress the wave interference to the model.
The model ship draft in the tunnel is increased within the capacity constraint of the test facilities to
compensate for the deceleration of the flow due to the boundary layer below these wave suppressing
plates. The detailed configurations of the model are strictly based on the drawings of the full-scale ship.
The accuracy of the model hull should be in accordance with ITTC — Recommended Procedures and
[2]
Guidelines 7.5-01-01-01 which specifies a tolerance of ±1 mm. The maximum blockage of the ship
model in the test section is in the order of 10 % to 20 %. A watertight dynamometer is to be installed
together with an underwater motor aligned precisely to the propeller shaft inside the model ship.
Thrust, torque and rotational speed of the model propeller are measured through the dynamometer.
The quality of the generated wake with respect to the target wake (measured wake in the towing tank
or estimated full scale ship wake) should be assessed by wake field measurements using velocimetries,
e.g. particle image velocimetry (PIV), laser Doppler velocimetry (LDV) or pitot tubes. Depending on
the configuration one may measure the axial velocity component only, the axial and tangential velocity
component or all three velocity components.
4.2 Test conditions
The cavitation test conditions are determined by the thrust identity method (or torque identity
method) at discussed (or specified) self-propulsion point. In cavitation tests, the propeller operating
condition is defined by the non-dimensional coefficients, propeller thrust coefficient K (or propeller
T
torque coefficient K ) and cavitation number σ .
Q n
During the propeller cavitation observations and noise measurements the pressure in the cavitation
tunnel is adjusted according to the local cavitation number at a specific point approximating the centre
of the expected cavitation extent in the upper part of the disk, such as
...