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Conventional Ship Testing
Experimental Methods in Marine Hydrodynamics
Lecture in week 37
Chapter 6 in the lecture notes
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Resistance tests
Propeller open water tests
Propulsion tests
Cavitation tunnel tests
Cavitation observation
Pressure pulses
Noise measurements
Cavitation erosion
Maneuvering tests
Free running maneuvering tests
Planar Motion Mechanism (PMM) tests
Conventional Ship Testing
- Topics:
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Resistance tests
Test procedure:
The model is accelerated to wanted speed
speed is kept constant for at least 10 seconds (or at least 10 load cycles)
Average values of the measurements for the period of constant speed is
calculated
Towing Carriage
Ship model
Measurement of:
Model resistance RTmModel speed
Sinkage fore and aft
Resistance
Dynamometer
Flexible connection
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Required length of measurement
The tow force might fluctuate considerably, especially for models with
low Drag/Displacement ratio and large displacement
In such cases, one needs at least ten oscillations in the time series
One must make sure to leave out the transient part of the time series,which is due to the acceleration
70
80
90
100
110120
130
140
20 25 30 35 40Speed [m/s]
ResistanceR
Tm[
N]
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Example time series entire run
-40
-20
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60
Time [seconds]
ModelResistanceRTm
[N]
-0.5
0
0.5
1
1.5
2
2.5
Carriagespeed[m/s]
RTm
Speed
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Seiching standing waves in the tank
Length Ltank
Depth h
Amplitude a
Horisontal velocity Vx
)sin()cos( kxta =
)sin()sin( kxtgk
V ax
=
hg
LT Tank
2
Wave elevation:
Horizontal velocity:
Wave period:
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Error from seishing on total resistance
-Example from the large towing tankWave amplitude a = 1 cm
Horizontal max velocity Vx = 0.03 m/s
Carriage speed Vm = 1.5 m/s
Total resistance: V2
Induced max. Error: 4%
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Waiting time between runs
Surface waves must calm down
Waiting time can be reduced by conventional wave dampers
Takes longer time in larger tanks
Seishing must calm down Might be difficult to see
Might be damped by special arrangements
Takes much longer time in larger tanks
Waiting time will be a trade-off between:
Accuracy
Efficiency
Typical waiting time between runs in large tanks: 15 minutes
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Testing resistance of high-speed models
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Propeller Open Water Tests
The propeller (or propulsor) is tested in open water
In case of ducted propeller, the duct is included
In case of pods and thrusters the pod/thruster body is usually included
One might argue that the rudder behind the propeller should be included Measurements
Propeller thrust, torque and revs
Water (or carriage) speed
Duct thrust (if relevant)
For pods and thrusters:
Propulsor total thrust is always measured
Propeller thrust is measured if the thruster is equipped for such
measurement
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Propeller Open Water Tests
- Measurement equipment
Water speed V
Measurement of:Torque QThrust TRate of revolutions n
Speed V
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Propeller Open Water Tests
- Measurement procedure Propeller revs are kept constant
Carriage speed is varied in steps from
zero speed to zero propeller thrust
Tests are performed at same revs as
expected for design speed in
propulsion tests
Tests might be repeated at higher
propeller revs (attempted full scale
condition)
Results are presented in non-dimensional form
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Propulsion tests
Test procedure (Continental Method):
The model is accelerated to wanted speed
Propeller revs are adjusted so that the model is getting the same speed as
the carriage, and then the model is released Measurement is made with found revs for at least 10 seconds
Average values of the measurements for the period of constant speed is
calculated
Dynamometer
Tow rope FD
Towing Carriage Measurement of:Torque QThrust T
RPM
Model speed
Sinkage fore and aft
el. motor
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Propulsion tests British Method
Measurements are as for continental method except that:
Tow rope force must be measured
Test procedure (British method):
The model is accelerated to wanted speed
Propeller revs is set to constant value
Applied tow rope force is measured.
The test is repeated with other values of propeller revs (at least three
values)
Values of thrust, torque and revs for correct tow rope force is found by
interpolation
Benefits of this method:
Re-analyses with other tow rope force values are possible
Availability of propeller over- and under-load results
Drawback: More time consuming!
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Cavitation testing of propellers
Purpose: investigation of:
Cavitation induced erosion of propeller blades
Effect of cavitation on propulsion efficiency
Vibrations and noise Test types:
Cavitation observation
Pressure pulses
Noise measurements
Cavitation erosion
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Cavitation test procedure
1. Choose flow velocity in test section based on actual advance ratio, J.
2. Install aft-body model and adjust wake field by mesh screens
3. Install propeller model
4. With atmospheric pressure in the tunnel, adjust propeller rate ofrevolution (and/or flow velocity) until the propeller torque is correct
according to the propulsion test in the towing tank (equal KQ). This is
called the torque identity principle.
5. Keeping flow velocity and rate of revolution constant, reduce the
tunnel pressure until the specified cavitation number is achieved.
6. Do necessary cavitation observation and measurements.
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Afterbody model
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Mesh screen
0.0
0.2
0.4
0.6
0.8
1.0
0 15 30 45 60 75 90 105 120 135 150 165 180
Angular position [degrees]
w
Measured in towing tank Obtained in cavitation tunnel
r/R=0.36
0.0
0.2
0.4
0.6
0.8
1.0
0 15 30 45 60 75 90 105 120 135 150 165 180
r/R=0.568
0 715
30
45
60
75
90
105
120
135
143
1501
571
6518019
5
225
240
255
270
285
300
315
330
345
352
0.414
0.621
0.828
1.035
Axial wak0.500.450.400.350.300.25
0.200.150.100.050.00
Axial wake shown as color contoursPropeller disk indicated by dashed line
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Cavitation tests - Measurements
Propeller rate of revolution
tachometer
Thrust
Torque
Static tunnel pressure
Pressure tapping in tunnel wall
Tunnel water speed
Prandtl tube 5 cm from tunnel wall in test section
For measurement of pressure pulses: Pressure on the aft body hull surface at a number of locations
(typically 6-18 positions)
For measurement of propeller noise:
One hydrophone
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Measurements
Prandtl tube Pressure tapping
Drum membrane
Strain gaugesfor torque measurement
Prop. shaft
Inductive transducerfor thrust measurement
Thrust
Torque
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Maneuvering tests
Two alternative purposes:
1. Direct verification of maneuverability fulfillment of IMO criteria
2. Establishment of hydrodynamic coefficients for the maneuvering
equations Usually followed by calculation of maneuverability in a maneuvering
prediction program
Two alternative test schemes:
1. Testing of free-running model
Gives direct assessment of maneuverability
Hydrodynamic coefficients for maneuvering equations can be derived
2. Testing of captive model
Measurement of forces for establishment of hydrodynamic coefficients for
the maneuvering equations
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Types of Ship Maneuvers
IMO standard maneuvers:
Zig-zag tests
10/ 10 to both sides
20/ 20 to both sides
Turning circle test
35 rudder angle
Full astern stopping test
Additional maneuvers:
Spiral test Reverse spiral test
Pull-out maneuver
Very small zig-zag maneuver
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Zig-zag maneuver
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Turning Circle Maneuver
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Stopping test
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Free-running manoeuvring tests
Full geometrical similarity
Speeds are Froude scaled Hull friction scale effect (tow rope) can be corrected by use of air fan
Electric motor shall ideally be controlled to emulate ship engine
characteristics
Constant motor power is a simpler alternative
Constant propeller speed (what you get with an electric motor without some kind of
automatic control) give much too high thrust during the manoeuver
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Free-running maneuvering tests
- measurements
Propeller revs
Rudder angle
Speed
Heading
Position
Alternatively: 6 DoF position measurement
Rate of turn (for instance by use of gyro)
Important for fast models and when using auto-pilot
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Maneuvering tests with fixed model
Planar Motion Mechanism (PMM)
Rotating arm Yawed model tests
Measurement of:
Speed
Position
Forces
The model is subject to forced motions, and the applied
forces are measured
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Planar Motion Mechanism (PMM)
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Pure yaw test
Yaw and drift angle test
Yaw and rudder angle test
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Rotating arm tests
Set parameters:
Arm rotation speed
Model position (radius)
Model yaw angle
Gives complete control of:
Surge speed
Yaw rate
Sway speed
Measurement of:
Forces (in 6 DoF)
Speed
Radius, yaw angle
Rudder
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Yawed model test
The model is towed with a fixed yaw angle
Fixed or free in roll
Surge, sway and yaw forces/moments are measured
Repeated for several yaw angles (and surge speeds)
Variation:
Repeat at several rudder angles
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ITTC: International Towing Tank Conference
The ultimate source of accumulated knowledge on model testing
Work is performed in groups of 6-10 technical experts
Work is presented every third year in a common conference
Proceedings from the ITTC conferences are valuable references ITTC maintains standards of model testing and analysis techniques
ITTC Permanent web-site contains standards for model testing:
http://ittc.sname.org/
http://ittc.sname.org/http://ittc.sname.org/7/30/2019 Ship Model Testing
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Ship model testing - Summary
Resistance, propulsion and propeller open water tests are performed to
determine accurately the speed-power performance of the ship in full
scale
Cavitation tests are done in order to ensure that the ship propeller will
not get cavitation problems
Typical cavitation problems are:
erosion damage to propeller and rudder
Noise and pressure pulses induced on the hull from the propeller cavitation
Manoeuvring tests are performed to verify the manoeuvrability of the
ship
Compliance with IMO criteria for manoeuvrability
Detect and repair directional instability