Towing Tank Facility - nptel - Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara
Towing Tank Facility
The Towing tank is housed in a separate building of length 90 m, width 10 m and height
5 m. The towing tank hall is well ventilated and is equipped with very effective lighting
system.
The towing tank is of length 82 m, width 3.2 m and depth 2.8 m. At the forward end of
this tank, a trimming tank is constructed to facilitate proper ballasting of the model to the
required draft. Arrangements are provided for filling and emptying the towing tank. A
weir has been provided at the other end.
Precise alignment of rails is made parallel to the water level. The tolerance observed is
± 0.1 mm. Such a precision is necessary to prevent undue accelerations or
decelerations of the carriage, which might be transmitted to the model.
The Towing Carriage
The carriage has the necessary provisions for conducting the rating of current meters
for flow velocity measurement. The attachment devices for the rods of water current
meters are arranged at the rear end of the carriage on a special frame. Three current
meters can be tested at a time (Plate).
Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara
The carriage has two speed ranges,
(i) 0.02 m s -1 to 0.50 m s -1
(ii) 0.50 m s -1 to 5.50 m s -1
In the low speed range it can be operated by a low duty D.C. motor (manual control
only), whereas in the higher speed range it can be operated by a heavy duty D.C. motor
fed by a Ward-Leonard set, (both by manual and electronic control) and the A.C./D.C.
power supplied via overhead bus bars. The approximate speed of the carriage is
indicated on a speedometer fixed in the panel on the carriage.
The towing carriage is provided with braking system for stopping at any position in the
towing tank. Also for safety purposes, emergency braking rails are provided at both the
ends of the towing tank, which retard the towing carriage at 10 m s -2 .
For Testing of ship models in shallow water, the observation bridge and the measuring
frame can be lowered in the centre of the towing carriage.
Calibration of Flow Meters
Flow meters such as pitot tubes, pilot cylinder, and current meters are rated before
being put to use. For example, in the case of current meters, it is necessary to
determine the constants which establish the relation between the number of revolutions
Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara
per second of the propeller and the water flow velocity rating is carried out in still water
of towing tank.
Test Installation for Rating of Current Meter
Measuring Equipment
The carriage is equipped for the concurrent connection of three current meters to the
sockets located on a small switch desk. The exact measurement of the speed of the
carriage is made with the use of a measuring wheel. The distance measuring wheel
provides for the three different distance sampling signals to suit different speed ranges.
These signals are routed through a crystal controlled timing mechanism.
The Data Acquisition System is mounted on the carriage for conducting the test. Also
the digital timing mechanism which receives and indicates on four individual displays,
the time intervals during each run of rating as follows:
1. The time that elapses for the carriage to traverse a distance of 10 m at the set
constant speed.
2. The time that elapses for the completion of 10/20 revolutions of each current meter.
Testing Procedure
The procedure for Rating of Current meter as in accordance with HIS: 3910 -1966 and
HIS: 3918- 1966
Spin Test
The current meter is made to rest on horizontal plane and a spin is induced to the
bucket wheel by means of an air jet from a solenoid valve/nozzle. The air jet is
controlled to be active for a duration till the equilibrium condition is reached, maintaining
an air pressure of 4 kg cm -2 . At the end of the solenoid valve action, a digital timer is
used for measuring the spinning time.
Current Meter Installation
Three current meters can be fixed to the special frame at the rear end of the carriage.
Two current meters are fixed at a distance of 50 cm from the sides of the tank and the
third one is fixed at the centre. The current meters are submerged to a depth of 50 cm.
Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara
They are held by vertical rigid rods fixed with the axis of the current meters parallel to
the tank wall.
Threshold Velocity
The current meter is towed at very low speed range from 0.02 ms -1 in steps of 0.010 ms -
1 till the current meter begins rotating after overcoming the inertia and the corresponding
velocity is taken as threshold velocity.
Rating Run
The rating of the current meter is carried out in still water. The rating run length at
constant velocities are
(i) 10 m for velocities less than 0.60 ms -1 and
(ii) 20 m to 30 m for velocities above 0.60 ms -1 .
The carriage runs are made in the forward direction at selected velocities. The velocity
of the carriage is changed from 0.20 to 3.60 ms -1 in steps of 0.2 ms -1 initially and then
0.3 ms -1 so as to obtain about 14 readings. The accuracy of rating is of the order of
±0.5%.
Evaluation of the Data
From the readings, the actual value of the velocity of the carriage, V (ms -1 ) and the
corresponding revolution per second of the cup/propeller 'N' are calculated and plotted.
One or more straight lines are manually fitted through the data points as necessitated
and the corresponding equation(s) are determined. The graphical plot will also be
provided with the report for reference.
Tabulation
Using the rating equation(s) determined as above, the current meter rating tables are
prepared on a digital computer. Special remarks if any are noted in the rating chart
(Table 1).
Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara
Typical Chart
Environmental and Water Resource Engineering Division,
Dept. of Civil Engineering, I.I.T. Madras, Chennai – 600036
Table 1: Ready Reckoner for Velocity, given for the number of revolution
Time in secs
40
41
42
43
44
45
65
66
67
68
Indian Institute of Technology Madras
Meter No.
Report No. CIE /EWRE/
Date of Calibration
Spin Test secs
V max ms -1
Threshold velocity ms -1
Velocity Equation(s) V = m*n+c
Velocity in m/s
Velocity in m/s
5 Revs 40 Revs 50 Revs 60 Revs 250 Revs 300 Revs
Salient Technical Data
Towing Tank
Length 82.0m
Width 3.2m
Maximum Water Depth 2.8m
Gauge of Rails
Towing Carriage
3.5m
Net Weight 3.9 tones
Length 5.3m
Speed Range 0.02 to 5.5 ms -1
Test length for 5.5 m/s 50.0m
Velocity Precision ± 0.1%
Maximum
Acceleration/Deceleration
3 ms -2
Drag Balance
Maximum drag 100 N
Least Count 0.01 N
Error range ± 1%
Ratio of Force Transmission 2 : 1 or 4 :1
Time in secs
40
41
42
43
44
45
65
66
67
68