Trans RINA, Vol 153, Part A1, Intl J Maritime Eng, Jan-Mar 2011
gap between the propeller tip and under-side of the hull along a vertical line through the propeller centre was 0.25 pD , in accordance with the classification society’s
recommendation.
The thruster strut was perpendicular to the hull bottom at the location of the thrusters, and therefore there was a tilt angle with respect to the water plane area both outwards (11° relative to the centre plane) and nose down (7.6°). There was no yaw angle (``toe out`` angle) from the centreline of the hull. The centre of the propeller was 1.306 pD below the waterline in order not to let the
propeller blade tips come out of the water during severe wave conditions. The centres of the two propellers in the twin configuration were 3.026 pD apart. The thrusters’
heading angle range was limited to -40° ≤ δ ≤ 40° in pulling mode, which was the same range as tested for thrusters without the ship hull. Torque and thrust were measured for both the port and starboard propeller, while the shaft side force and bending moment components were measured only for the port propeller.
Table 1: Model
parameters Length overall
Length betw. perp. Breadth waterline
hull main dimension and LOA
Length on designed waterline
LWL LPP BWL
TFP TAP
[m] [m]
[m] [m]
Draught at LPP/2 T [m] Draught at FP Draught at AP
[m] [m]
Volume displacement Prismatic coefficient Block coefficient
Midship section coefficient
Longitudinal C.B. from LPP/2
Wetted surface
CP CB CM
[m3] [-] [-] [-]
form
5.405 5.265
4.879 1.266 0.452 0.452 0.452
2.176
0.7846 0.7792 0.9931
LCB [m] -0.196 S [m2]
9.547
Table 2: Propeller main specifications Propeller diameter Hub diameter
Design pitch ratio P D p/ Skew
Expanded blade area ratio
250 mm 60 mm 1.1
25 deg 0.6
3. EXPERIMENTS
The shaft side force and bending moment components were measured in a reference frame rotating with the shaft, and then converted to a frame fixed with the
©2011: The Royal Institution of Naval Architects
Figure 2: Test set-up for the experiment using a system of cables and bar to keep the model fixed in yaw.
This connection system restrained the model in surge, and thereby controlled the velocity of the model. The loading on the propeller shaft might be different for the thrusters at negative and positive heading angles. Hence, data was measured over the range -40° ≤ δ ≤ 40°. The
A- 11
The tests were carried out with the two azimuth thrusters at the same azimuth angle. The tests were repeated for different conditions i.e. different
azimuth angles and
different advance velocities. In order to cover a range of advance coefficient the carriage velocity was varied in the towing tank. To avoid the complication of autopilot control, the ship was restrained from the swaying and yawing motion by using a system of cables and a
transverse beam, shown in Figure 2. thrusters by using the measured propeller angular
position in Figure 1. The fixed reference frame is aligned with the thrusters as indicated in Figure 1, so that it is oriented at angles with the hull as described above. The propeller shaft rotational speed and pod azimuth angle were also measured. The positive thruster heading angle was defined clockwise when the thruster is viewed from above (inward to the ship hull centreline for the thruster measuring propeller side forces).
Since the shaft side force and bending moment are converted in the coordinate system fixed with the shaft, the magnitude of the radial force and moment (resultant force and moment) could be obtained. In addition to the tests in calm-water, the propeller and shaft loads were also measured in regular waves in different sea states to investigate the effect of the hull motion (mostly pitching) on the periodic propeller loads. Measurements were made only in head and following sea conditions in a straight-line course.
3.1 EXPERIMENTAL PROCEDURE IN CALM WATER
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