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OCR for page 552
24th Symposium on Naval Hydrodynamics
Fukuoka, JAPAN, 8-13 July 2002
Propeller Inflow at Full Scale During a Manoeuvre
G.Kuiperi, M.Grimm2, B. McNeice2, D.Noble3, M.Krikke4
(~ Marlin, The Netheriands, 2Royal Australian Navy, 3DRDC Atiantic
Canada, 4Royal Netheriands Navy)
ABSTRACT
The propeller inflow of a patrol boat of the Australian
Navy was measured at full scale using a newly
developed LDV technique. The velocity distribution
in front of the propeller was measured, both on a
straight course and in a turn. Model tests that were
carried out in a towing tank with the model at a drift
angle were shown to adequately simulate the most
important effects of the wake distribution. The model
wake data were used to complete the full scale data
and an analysis was made of the power absorption of
the propellers in relation to the inflow velocities.
It was found that the transverse velocities in
the propeller plane were almost fully responsible for
changes in power absorption in a turn. The mean
axial velocity was about equal for the inner and the
outer propeller in a turn. Unexpectedly the transverse
velocities induced by a turn were small in the upper
half of the propeller plane. The effects of the inflow
measurements on the cavitation inception speed have
also been estimated analytically.
INTRODUCTION
Propeller design has traditionally been based on the
propeller inflow on a straight course under ideal
conditions. In operational conditions effects of
waves, ship motions and manoeuvres can however be
important. This is especially so for the cavitation
inception speed of a naval vessel because of the
increase of radiated noise when cavitation starts.
Optimization of the cavitation inception speed on a
straight course may lead to a propeller which has a
high inception speed, but which is very sensitive to
small disturbances in the propeller inflow. In practice
the inception speed will be much lower than in ideal
conditions on a straight course. The effects on the
propeller inflow of waves, ship motions and
manoeuvres are still not predictable.
The decrease of the cavitation inception
speed of Navy ships in a seaway is often obvious.
This is of course partly due to increases in the
propeller loading caused by added resistance in
waves and partly due to wave velocities and the
resulting ship motions. Observations of cavitation in
heavy seas also indicate that the effect of the rudder
in coursekeeping can be important. In some cases the
autopilot was switched off in heavy seas and the
fluctuations in cavitation decreased significantly. The
effect of the rudder can also be experienced when
monitoring vibrations due to cavitation in a seaway.
The vibrations come and go, apparently without any
relation with the ship motions. The rudder actions,
required for coursekeeping, are then the cause of the
variations in vibrations. In the present project the
focus was primarily on the effects of manoeuvres on
the inflow of the propeller, although some attention
was also paid to the ship motions. Since cavitation
inception is of primary importance for Navy ships,
the focus was also on frigate type, twin screw
configurations.
FULL SCALE MEASUREMENTS
Since very little is known about the effects of
manoeuvres on propeller inflow and cavitation, this
study can be characterized as exploratory.
Investigations performed at model scale would have
to be extrapolated to full scale. However, the
differences between model and full scale wake cannot
yet be fully described analytically, particularly when
subject to ship motions in a seaway or while
manocuvring. This means that, to arrive at
conclusions for full scale from model tests, empirical
corrections would have to be applied. Consequently it
was decided to focus on full scale measurements and
cavitation observations. Model tests were carried out
OCR for page 553
after the full scale tests in a preliminary effort to
simulate the full scale phenomena at model scale.
During the period 1991-1996 the Canadian
Navy, the Royal Australian Navy, the Dutch Navy
and Marin jointly collaborated in conducting full
scale sea trials to investigate the effects of operational
conditions on propeller cavitation inception. These
trials included wake field measurements and
cavitation observations on the Canadian Forces
Auxiliary Vessel CFA V QUEST and on the
Australian patrol boat HMAS TOVVNSVILLE. The
wake measurements on CFA V QUEST did not cover
the whole propeller disk reliably, but the
measurements on HMAS TOVVNSVILLE produced a
complete dataset over a significant portion of the
propeller disk. Therefore, in this paper the results of
the measurements on HMAS TOVVNSVILLE obtained
in July 1993 are reported and analyzed. These trials
consisted of Laser Doppler Velocimetry (LDV)
measurements of the inflow into one main propeller
for both straight line running and moderate turns to
both port and starboard. The LDV measurements
were complemented by cavitation observations and
video recording as well as shaft speed and torque
measurements.The intention was for these trials to be
undertaken in calm water such that only the effects of
steady manoeuvring were observed.
The triple screw patrol boat of the Royal
Australian Navy on which measurements were
performed is shown in Figure 1.
Figure 1: The Australian patrol boat HMAS
TOWNSVILLE
The main particulars of the ship and outer propellers
are given in Table 1. Two outboard fixed pitch
propellers are the main propulsion while the smaller
auxiliary, controllable pitch propeller at the centerline
is only intended for slow speed operations and as
such is feathered when the main propellers are in
operation.
The propeller arrangement is shown in
Figures 2 and 3. The outer propellers have their shafts
supported by A-brackets and turn outward over the
top. Figure 2 also shows the relative locations of the
LDV head, its measuring plane and video camera
used on the sea trial.
Waterline length
Beam
Draft
Displacement
Propeller diameter I
Propeller pitch at 0.7R
Number of blades
Expanded Blade Area
Ratio
38.5 m
6.54 m
2.145 m
25 1.3 tons
1.275 m
1.168m
3
1.221
Table 1: Main particulars of IMPS TOWNSVILLE.
ll ~~
1< 1 ~~
AUX~
~WE}[rSP4f:E
_~
~ - WE
Figure 2: Position of LDV head, measuring plane
and video camera housing relative to A-bracket struts
and main propeller
Figure 3: Shaft arrangement showing both main
propellers and the center propeller in the feathering
position.
2
OCR for page 554
A skeg at the center line of the boat extends
aft to within 20% of the waterline length forward of
the transom, which is slightly forward of the position
where the propeller shaft enters the hull. The
centerline auxiliary propeller was in the feathered
position during all measurements. The drift angles
involved in the present investigations were limited to
approximately 5 degrees, in order to minimize the
effect of the presence of the center propeller on the
propeller inflow of either main propeller.
MEASURING EQUIPMENT
A special LDV head was developed for this project to
be able to measure flow velocities up to a distance of
about 4 meters from the head. A cross section of the
head is given in Figure 4. After splitting the beam
both beams are led along the side of the head using
fixed mirrors. The last mirror is controllable, so that
the angle of the beam can be adjusted. This makes it
possible to locate the measuring volume from
between about 20cm from the head out to 4 meters.
Proper crossing of both beams is of course extremely
important.
Following the movement of air bubbles or
impurities in the water through the control volume,
the scattered light is transmitted through the window
and focussed by two mirrors, a large one and a small
one. It is led into a glass here in the center of the
large mirror for further processing. The proper
focussing of the light on the glass fibre is important
and requires a high accuracy of the mirrors. The
signal from the glass fibre is transformed into an
electrical signal and processed by Dantec Burst
Spectrum Analyzers (BSA).
~ WA - t
j .. ,
,
~1~1l~
_ \ ~ / _
I Hi
\ 1 /
I CO~Iq=LA~,t PI - *$
Figure 4: LDV head
For use in sea trials the head of the LDV
system was reduced to a tube of 45 cm in length and
25 cm in diameter. This made it possible to mount the
head above a window in the hull. The space between
the head and the window was filled with distilled
water to avoid breaking the path of the laser beams.
As indicated in Figure 5 the maximum angle between
the perpendicular to the window and the axis of the
head was 30 degrees. This allowed the major portion
of the inflow ahead of the propeller disk to be
measured, as illustrated in Figure 7.
Both the head of the LDV system and its
laser unit were located in the steering gear
compartment of the patrol boat. The 5 Watt laser,
required to obtain a proper signal up to 4 meters
outboard of the hull, was large (length about 1.2
meters), heavy and required a large external water
cooling unit, which was placed on the main deck.
This made it impossible to connect the laser directly
to the head. The laser was therefore installed
separately at a distance of about one meter from the
window with the head. Mirrors were used to bring the
laser beam into the head. In this set-up it was crucial
that the laser beam entered the head along the
centerline. This was done using a proper set-up of the
mirrors. However, vibrations of the ship made the
beam unsteady relative to the head. This was
overcome by actively controlling the mirrors on the
head in such a way that relative motions between the
laser and the head were compensated. The active
control was done by reading a part of the laser beam
through the mirror on a Charge Coupled Device
(CCD). The active mirror was then adjusted for
deviations of the beam from the required position. It
has to be mentioned that present day lasers with equal
power are small enough to be coupled directly to the
laser head.
,~4
/ I _OPTK.:~" I ~ lit
~ salts ~ 1 1 ' 1
1~
ifs
1
Figure 5: Position of optical head above hull
window
3
OCR for page 555
Due to the acute angle of the intersecting
pair of laser beams, the measuring volume at a
distance of 3.5 meters outboard of the hull was
approximately 10 cm long. Close to the hull this
length reduced to less than 1 cm. The width of the
measuring volume was always less than 5 mm.
Processing software analysed a burst from
particles or bubbles passing the measuring volume
and checked whether the shape of the burst was
acceptable. Only then was the burst stored. The ratio
between the accepted and the total burst rate is called
the acceptance ratio. In ideal conditions the
acceptance ratio can be as high as 70 percent. During
the sea trial the acceptance ratio varied from a few
percent up to 25 percent, depending on the distance to
the measuring volume. In a turn the acceptance ratio
was much lower than on a straight course, indicating
that increased turbulence in the flow can complicate
the LDV measurements.
Observations of cavitation on the port
propeller were made using the Osprey video camera
illustrated in Figure 6. A clear hemispherical dome at
one end of the camera contained the adjustable
optics. The camera dome was fitted inside a larger
acrylic dome that extended a short distance outside
the hull. For the sea trials on HMAS TOWNSVILLE,
the camera was located between the A-bracket arms
just forward of the LDV window. The camera was
remotely controlled from inside the ship to provide
significant pan, tilt and rotation of the view during
observations. To freeze the images of the individual
blades in successive revolutions, strobe light units
synchronized to the propeller rpm were placed on
adjacent windows.
FW4CIE ~ CAMERA
rem _ Be_
Figure 6: Video Camera Installation
The pulsed signals generated from a digital
magnetic sensor as it detected the teeth on a wheel
around the propeller shaft were processed through a
specially designed viewing controller to allow time-
lapsed observations of the blades at fixed angular
positions (for further details see Kennedy et al. 1989~.
Additionally the propeller torque and rpm and the
rudder angle were measured.
LDV MEASUREMENTS AT FULL SCALE
Measurements of the propeller inflow were carried
out on the port propeller. The measurement grid is
given in Figure 7. The measuring plane was upstream
of the propeller, as shown in Figure 2. The zero
degree radial is the vertical through the propeller
shaft centerline. The radials at which full scale
measurements were carried out are from -30
(inboard) to +30 (outboard) degrees, with steps of 5
degrees. Because of the shaft bracket bossing the
radials from -5 to +5 degrees could only be measured
above the shaft.
The distance of the measuring points,
measured from the center of the optical head, was
from 0.25 m. to 1.75 m. in steps of 0.25 m. The
center ofthe optical head was 18.75 cm from the hull,
so the inner points of the grid were just outside the
hull. The measuring plane was upstream of the
propeller, as given in Figure 2.
Figure 7: The measuring grid. The open locations
were not measured at full scale, the black locations
were not measured at model scale .
The LDV head at full scale allows
measurements in one direction only. So only the
velocity components in the plane perpendicular to the
laser beams could be measured. The direction
perpendicular to the measuring plane will be called
4
OCR for page 556
the axial direction. It coincides approximately with
the axial direction of the ship. The direction along the
laser beam radials in Fig. 7 will be called the
"vertical" direction. The velocities in this direction
could not be measured at full scale. The direction
along the arcs in Fig. 7 will be called the "horizontal"
component. The vertical and horizontal components
of the transverse wake field can then be translated
into the common tangential and radial velocity
components in the cylindrical coordinate system of
the propeller.
Since the horizontal velocity components
could be negative, a Bragg cell was initially used in
the LDV head. The Bragg cell gives a phase shift to
the laser beam, causing the interference grid in the
measuring volume to move at a certain speed.
However, the Bragg cell caused a decrease in beam
intensity and was therefore omitted. To determine the
direction of the "horizontal" velocities in the
measuring plane, i.e. whether flowing inboard or
outboard, two measurements were taken at every
measuring point in directions 30 degrees on each side
of the axial direction. The horizontal and axial
velocity components were derived from these two
components. The vertical component could not be
measured with this system.
With the exception of measurement points
far from the hull each LDV measurement at full scale
had a maximum duration of one minute. The
measurement was completed in less time if 3000
bursts were recorded. The number of bursts during
one measurement varied significantly. The maximum
burst rate was found close to the hull. An example is
shown in Figure 8a.
There was considerable scatter in the
velocities due to turbulence in the ship's boundary
layer. This scatter decreased considerably at greater
distances from the hull, as shown in Figure 8b. At the
maximum distance of 1.75 meters the maximum
duration of the measurements was therefore set at 3
minutes instead of 1 minute for all other locations.
Even then only a limited number of bursts
(from 2 to 10) could be measured. This was
apparently due to a lack of seeding, although
occasionally large increases in the burst rate was
measured, probably caused by patches of bubble
clouds passing through the measurement volume. No
efforts were made to apply artificial seeding.
l
.
::
~ 0 ~ ~ I' ~ I'.$
velocity mlsec
a
L
count
...
..
;2- , . ,!
... , ,, Celtic ~~ . . '.'
b
Figure 8. Recording of axial velocities at 25 cm
distance (close to the hull, Figure a) and at 125 cm
distance from the hull (Figure b) on a straight course
TEST CONDITIONS AT FULL SCALE
Extreme clarity of the water is required for sufficient
backscatter signal strength at full scale. LDV
measurements were carried out in the waters near
Cairns (Australia) during winter, where the water
conditions are very good. The measurements were
done inside the Great Barrier Reefs. Moderate winds
of 15 knots with waves approximately one meter in
height were present during the measurements on a
straight course and at the end of the measurements in
a turn to port. The weather conditions were better
during the turns to starboard and during the cavitation
observations. There was considerable current in this
location.
To investigate the effects of a turn, the
propeller inflow velocities were measured in the axial
direction on a straight course and in two directions
(resulting in the axial and horizontal components of
the inflow) in a turn. The turn was set at a fixed
rudder angle of 8 degrees with both propellers
nominally at 520 rpm.
The position of the ship was measured using
a GPS reading at every 45 degrees of heading. The
result for a turn to starboard is given in Figure 9. An
average drift can be obtained from the shift of
locations at the same heading. By canceling out the
average drift the resulting path becomes as shown in
Figure 10. From Figure 10 an average turning
diameter was determined as well as an average drift
angle. The diameter was found from the distance
between opposite locations on the path and this
varied with the heading.
The average turning diameter in Figure 10 is
785 meters. The drift angle was found from the
deviation of the heading (0, 45, ...270 and 305
degrees) and from the tangent to the turning circle.
5
OCR for page 557
This tangent is perpendicular to the lines connecting
opposite points on the path in Figure 10.
In",
8;X ~ 9= ~ m~m
~ "W'
Figure 9: Path of the ship with rudder 8 degrees to
starboard.
~ _ I ~ ~ i_ ——-" - t a'/ I I I
-3~ ~ 1~ 1~ ~~ 1~ 1~ 1~ 1~ t~
320D ~ - i +2 /
E _ At=
Figure 10: Path corrected for drift with rudder 8
degrees to starboard.
The drift angles varied between 1 and 5
degrees. The path of the ship was slightly elliptical
instead of circular, due to the wind. The wind also
caused the variation in drift angle over the turn, that
increased with the wind strength.
It turned out that the turning diameter was
difficult to reproduce, even at nominally the same
rudder angle. Therefore the measurement of the
turning circle had to be repeated every time the
rudder was reset.
The time required to complete a full circle
was also measured. In combination with the turning
radius this gave the average ship speed in a turn. The
speed in a starboard turn was thus estimated at 12.4
knots. On a straight course, also with a shaft speed of
520 rpm, the speed was 13 knots.
The LDV measurements took several days
of running in circles. During the measurements it was
occasionally necessary to interrupt the turns and
continue measurements at another location. The
turning diameters and ship speeds varied during the
duration of the measurements, partly due to the lack
of repeatability of the rudder angle, partly due to
changes in weather conditions, especially the wind.
During the last part of the measurements in a turn to
port the wind had increased considerably and it was
necessary to increase the rudder angle to 13 degrees
in order to turn the ship properly. The turning
diameter varied between 670 and 915 meters, the
estimated ship speed during the turns varied between
12.4 and 12.9 knots and the drift angles varied
between -1 and +5 degrees. These variations in
conditions are incorporated in the measured wake
field, as the time to measure the full wake field was
many hours and the trial spanned over several days
There is no reliable method to correct each inflow
reading for the effects of wind and wave induced ship
motions.
THE FULL SCALE WAKE FIELD
On a straight course only the axial wake component
was measured. The results are shown in Figure 11.
The interpolation between the measured points is
taken linearly, to avoid artificial peaks and valleys in
the velocity contours. It should be mentioned that in
this and the following wake distributions the
propeller disk was projected on the wake field along
the propeller shaft. This resulted in the propeller disk
being slightly further upwards in the wake field than
when it was projected along the baseline, as in Figure
7. The effect of the propeller position in the wake
field is considered small.
In Figure 11 the non-dimensional velocity
(l-w) of the outer flow is close to unity, indicating
that the ship speed of 13 knots, which was used to
derive the non-dimensional velocities, was correct.
Since the velocities are those of the total wake,
including the induced velocities of the propeller, the
occurrence of a velocity of 1.08 in the center of the
propeller is perfectly plausible.
In the top position two wake peaks showing
a minimum velocity (1-w) of 0.7 are present. The
ridge in between these peaks (1-w=0.94) is supported
only by one point. Although the scatter at this point
was considerable (it is the point given in Figure 8a)
it seems reliable.
6
OCR for page 558
indicating that in this turn the reference velocity was
higher than 12.4 knots, as it was in the turn to port.
..
Figure 11: Non-dimensional axial velocities on a
straight course.
In a turn two velocity components were
measured, which were decomposed into the axial and
the horizontal velocity components. The axial
velocity field in a turn to port is given in Figure 12.
Since the measurements were taken on the port
propeller, this is the inner propeller in a turn. The
velocity at the outer measurements arc is around 1.09,
indicating that the estimated ship speed of 12.4 knots,
based on the turning diameter and time to turn) was
underestimated. This is supported by the axial wake
of the propeller in a turn to starboard. Taking this into
account, the wake velocity in the top portion of the
disk again has two minima, providing wake peaks of
0.92 (1.01 from Figure 12 minus 0.09) and 0.94 (1.03
minus 0.09~. While the axial wake peak of the inner
propeller in the top position has widened and has
become less deep due to the turn it has remained in
the same general location. Low velocities of 0.94
(1.03 minus 0.09) are also seen at two locations along
the left (outboard) side of the propeller disk in Figure
12.
The horizontal velocity component in a turn
to port is given in Figure 13. The most significant
aspect of this result is that this velocity component is
much smaller in the upper half of the propeller disk
than in the lower half. The consequences of this
feature will be analyzed below.
The non-dimensional axial velocity
distribution for the total flow into the outer propeller
in a turn to starboard is given in Figure 14. Again the
outer flow has a non-dimensional velocity of 1.08,
7
i ..0. / '~
i/> l.
~-
.1
lit
me. !
- .' / 'i ~ ,'L/ `,T,~0
Figure 12: Non-dimensional axial velocities in a turn
to port (inner propeller)
EB to CL
Figure 13: Horizontal velocity component in a turn
to port (inner propeller, ship speed 6.39 m/see)
Taking this into account the two axial wake
peaks of the outer propeller are 0.89 (0.97 from
Figure 14 minus 0.08) and 0.77 (0.85 minus 0.08).
The minimum velocity in the upper part of the outer
propeller is also increased (from 0.7 on a straight
course), but not as much as in the wake peak of the
OCR for page 559
inner propeller. Figure 14 also shows some widening
of the wake peak relative to the straight course.
Figure 14: Non-dimensional axial velocity
distribution in a turn to starboard (outer propeller)
The horizontal velocity component in a turn
to starboard, so for the outer propeller, is given in
Figure 15. Again there is a stronger horizontal
velocity component in the lower half of the propeller
disk, as was the case for the inner propeller, but now
especially in the outer sector.
To analyze these measured propeller inflow
data further the powering data of the propellers at full
scale were measured. In combination with model test
results, as will be given below, these powering data
can be related to the propeller inflow.
POWER ABSORPTION AT FULL SCALE
Extensive torque/rpm measurements were done both
on a straight course and in a turn. On a straight course
the power absorption at 520 rpm was 400 kW per
shaft and the difference between the shafts was
negligible. In a turn the shaft torque and rpm
measurements were taken with the rudder at 8 and 12
degrees. The results are given in Table 2. Table 2
shows that the inner propeller was overloaded while
in a turn. The variation of the overload reflects the
lack of repeatability of the turning radius, which was
not measured during these tests.
Port
S3
~ 340
Figure 15: Horizontal velocity component in a turn
to starboard (outer propeller,ship speed 6.39 m/sec).
Trial
condition
straight
8 deg.
SB rudder
1 2 deg.
SB rudder
8 deg.
Port rudder
12 deg.
Port rudder
Outer propeller
_ shaft
_ =
Pow
Port
. SB
SB
pm
520
518
521
520
519
Inner propeller
KW
400
400
404
400
400
shaft .
SB
SB
Port
Port
rpm
520
519
520
521
522
Table 2: Shaft power measurements in a turn.
kW
400
422
460
453
481
For subsequent model tests with 5 degrees of
drift angle the average conditions from table 2 for
port and starboard turns at 8 degrees rudder angle
were taken: 520 rpm and an average power
absorption of 400 kW of the outer propeller and 436
kW of the inner propeller. It may be noted that the
rpm of both propellers in a turn remained very similar
to the straight run values. This is considered to be
related to the engine governing system on HM245
TOVVNSVILLE.
LDV MEASUREMENTS AT MODEL SCALE
For an analysis of the full scale data a comparison
with model test data is necessary. These model test
data will also be used to estimate the vertical velocity
component at full scale. This component is required
to be able to do propeller calculations for the inflow
at full scale. These propeller calculations will be used
8
OCR for page 560
to relate the measured powering characteristics with
the propeller inflow and to reach conclusions.
Proper simulation of turns at model scale
would best be accomplished using a rotating arm
facility, but no such facility was available. If it can be
assumed that the major effect of a turn is due to the
drift angle, model tests performed in a regular towing
tank can be used. This was done for a model of
HM24S TOWNSVILLE with scale ratio 8. As
mentioned above the drift angle was taken as 5
degrees, a representative drift angle for the condition
of 8 degrees rudder.
The model speed without drift was scaled
from 13 knots to 2.36 m/sec. The propeller rotation
rate of the model was chosen such that the torque
coefficient KQ both at model and full scale was 0.028.
This ignores the viscous correction on torque and
thrust due to the difference in Reynolds number
between model and full scale, but this effect is small.
The resulting model rpm was 1368 rpm. With S
degrees drift angle the ship speed of approximately
12.4 knots was scaled to a model speed of 2.24
m/sec. In this condition the rpm of the model was
proportionally reduced from the zero drift case to
1300 rpm. This means that the advance ratio of the
propeller was kept the same with and without drift
angle to simulate the same rpm in a turn and on a
straight course at full scale
At model scale 3-D LDV measurements
were made at the same grid locations as were used at
full scale. In the model tests the LDV head was on
one side of the model and the shadow of the shaft
bracket bossing now blocked measurements in the
inboard part of the grid. The density of the
measurements was less than at full scale. The
measured locations are indicated in Figure 17.
The axial velocity distribution of the model
with no drift and with the propeller working is given
in Figure 16. The transverse velocity distribution is
given in Figure 17. (For the model scale data the
propeller disk was projected on the measuring plane
along the baseline, resulting is a slightly lower
position of the indicated disk than at full scale. The
measured points are at the same locations, however).
The torque coefficient of both propellers
was 0.028, the same as at full scale. Comparison of
the axial wake peak with the one at full scale (Figure
11) shows that the double wake peak is also present
at model scale, but the peaks are less pronounced,
and wider apart. This is connected to the hull
boundary layer that at full scale is much closer to the
hull.
~ ~
"J _
-
~/ ,2;~ f~,'./,Jn.
~ __ ~ ~ ~
- ~ 3 \ ~ /1
~- \
I. ~
__ 3
f7
/
1 ,~'
`~',
f
Figure 16: Non-dimensional axial velocity
distribution on the model without drift.
1 imps]
/
1 ~
4~'
~ _
~ _
/ rat ~ 7` ~
~ 1.,
\\
/
/
/!
Figure 17: Transverse velocity distribution on the
model without drift (model scale velocities, model
speed 2.36 m/sec).
The velocities at model scale are more
uniform, especially above the propeller shaft.
Propeller calculations will be used later to assess the
effect of these differences on the inception speed.
The transverse velocities at model scale
cannot be compared to full scale because the latter
were not measured. Figure 17 indicates that the
9
OCR for page 561
"horizontal" velocity component is small over the
whole disk. The circulation in the inflow is very
small, as can be expected for this type of hullform.
The axial and transverse velocities with a
drift of 5 degrees are given in Figures 18 and 19 for
the inner propeller and in Figures 20 and 21 for the
outer propeller. The torque coefficient of the inner
propeller was 0.031 and that of the outer propeller
was 0.026. The drift angle therefore caused a
reduction of the loading of the outer propeller and an
increase of the loading of the inner propeller. In a
turn at full scale the propeller loading of the inner
propeller was increased to 0.031, but the loading of
the outer propeller was unaffected. The relation
between the wake distributions and the propeller
performance will be further analyzed below using
lifting surface calculations.
The axial velocity distribution in front of the
inner propeller with the model at 5 degrees drift is
shown in Figure 18. The velocities still have two
minima, as for the model at zero drift. The wake
peaks are shifted slightly inboard instead of outboard
and the inboard peak was not fully measured. The
depth of the wake peak was not much different from
zero drift.
~ ,
Jr) . ,
J I ~
f
Aim;
a' -'
. ,f
~ W~
. ~
Figure 18: Non dimensional axial velocity
distribution of the inner propeller behind the model
with 5 degrees drift angle (model speed2.24 m/sec).
~ ail
\\ \
_~ _
am_ - , ` - ~
\
\\
, .
1/ t~
~~ ~ ~ ~ / F\
~ $W Fs ~ ~ W f ~~W
Figure 19: Transverse velocity distribution of the
inner propeller behind the model with 5 degrees drift
angle (model speed 2.24 m/sec).
The transverse velocity of the inner
propeller at 5 degrees drift is given in Figure 19.
Similar to the observations in a turn at full scale
(Figure 13) the transverse velocities in the upper part
of the propeller disk are small. There is a clockwise
swirl in the transverse velocity. The magnitude of the
transverse velocity will be examined later.
, ~ ~
1 1
,tJ ~
it, ~
.~.- ~ _ ~.~1
Figure 20: Non dimensional axial velocity
distribution of the outer propeller behind the model
with 5 degrees drift angle (model speed 2.24 m/sec).
10
OCR for page 562
between the horizontal velocities at model and full
scale are given in Figure 22. The grid was
interpolated to equal grid points both for the model
and the ship.
~ _
-
)
)
fly v~ l ~ \
t 2
~ t
f ~ _~.
/ ~ ~
~ ,- , ~ j
.,
W~ ',' ~~;
~ ~ ~ //
Figure 21: Transverse velocity distribution of the
outer propeller behind the model with 5 degrees drift
angle (model speed 2.24 m/see)
The velocity fields of the outer propeller
with the model at 5 degrees drift are shown in Figures
20 and 21. A comparison with the model results
without drift (Figures 16 and 17) shows that the wake
peak in the upper part of the propeller shifted further
inwards, which corresponds with higher horizontal
velocity components just above the shaft (Figure 21~.
The horizontal velocities closer to the hull become
small again.
COMPLETION OF THE FULL SCALE
VELOCITY DISTRIBUTION.
For an analysis of the full scale wake data it is
necessary to estimate the vertical velocity
component. For this the model data were used. It is
possible to derive the vertical velocity component
from the two other measured components using the
continuity equation in combination with the
assumption of zero axial gradient of the velocity.
However, this attempt failed due to the limited
accuracy of the measured data. A simpler approach
was therefore used. Assuming that the measured
wake distributions of the model at 5 degrees drift
angle are representative of the wake in a turn at full
scale, the vertical velocity component at model scale
can be used to complete the full scale data.
The validity of this assumption can be
assessed by comparing the horizontal velocity
components measured on the model and the ship. The
outer horizontal components of the inner propeller at
model scale (Figure 19), non-dimensionalized by the
model speed, are typically 0. 15. The difference
Figure 22: Difference in non-dimensional horizontal
velocity component of the inner propeller between
model tests at 5 degrees drift and the ship in a turn.
=
Y.l al ./
1
I'
43.H
Figure 23: Completed non-dimensional transverse
velocity field of the inner propeller of the ship in a
turn.
11
OCR for page 563
In the region where the data are measured
both at model and full scale the maximum differences
are approximately 0.1, but in the lower half of the
propeller disk these differences are generally smaller.
This illustrates the feasibility of using the model
values of the vertical velocity component to
complement the full scale wake. The inaccuracy of
the vertical velocity component is the more
acceptable here since this velocity component
generally plays only a minor role in regions where
cavitation inception is expected to occur (the top part
of the disk and, in a turn, the bottom part of the disk).
-
. //!
. :
—- 63.75 O. 1
Figure 24: Completed non-dimensional transverse
velocity field of the outer propeller of the ship in a
turn.
Adding in the vertical component from the
model wake, the completed transverse velocity fields
at full scale are given in Figures 23 and 24. These
velocity fields, in combination with the measured
axial velocity distributions, can now be used for
calculations of cavitation inception on the propeller
and for relating changes in the wake field to the
power data.
ESTIMATION OF THE TRANSVERSE
VELOCITIES
An important result from both the model and full
scale measurements was that the transverse velocities
above the propeller shaft were very small, despite the
fact that the skeg ended at 20% of the ship's length
upstream of the rudder. The third propeller,
especially in feathered position, may have been a
factor here. The small transverse velocities in the
upper half of the propeller were confirmed at model
scale, where the center propeller was also present.
This showed that the drift angle was a major factor
for the propeller inflow in a turn and not the rate of
rotation.
The undisturbed horizontal velocity component Vh
due to a drift angle at ship speed Vs is Vh/Vs =sin 6,
where ~ is the drift angle. With a drift angle of 5
degrees this component is VhtVs =0.087. For a ship in
a turn the rotation of the ship causes an additional
horizontal velocity component in the propeller disk.
Estimating the distance to the ship's center of rotation
from the propeller plane to be 3/4L, where L is the
ship length, and the time to complete a full circle as 7
minutes, the horizontal velocity component due to the
turn is Vh/Vs =0.068. The undisturbed horizontal
velocities due to a turn and due to a drift angle are
therefore of the same order of magnitude. This was
not always true for the measurements made at model
scale and full scale. Figures 25 and 26 show the
maximum transverse velocities divided by the speed
of the model or ship as a function of the non-
dimensional radius in the propeller disk.
0.3
no
VtransNs
AL,_,:
.. i_
. ~ ~ 1—
l ~ I ,
,' I ~ . ' 1 ~
0.20 0.50 0.80 1.0
RlRmax
'. ~ 1,
Figure 25: Maximum transverse velocities on the
inner propeller.
VtranstVs
00~7¢ `` ~ ~
0.10 ~ ~ ~
408 -- t I -- ~
0.20 0.50
RlRmax
0.80
1.0
Figure 26: Maximum transverse velocities on the
outer propeller.
12
OCR for page 564
At inner radii there is a discrepancy, but at
the outer radii the transverse velocities of the ship in
a turn and the model at a drift angle are almost equal.
For both the inner and outer propellers on the ship the
maximum transverse velocity occurs in the lower half
of the propeller disk.
Although the undisturbed horizontal
velocities are almost equally caused by drift and
rotation, Figures 25 and 26 show that the transverse
velocity in the propeller disk is dominated by the drift
angle. Model tests with a drift angle can therefore
simulate a ship in a turn. On the other hand the drift
angle during a turn is a major factor in changing the
propeller inflow.
The maximum transverse velocity was
between 1.5 and 2 times the horizontal velocity
induced by the drift angle. This is much higher than
would be found from potential flow calculations
around the hull. The viscous region above the
propeller increases the transverse velocities in the
lower half of the propeller disk and strongly
decreases the transverse velocities in the upper half of
the propeller disk. This has consequences for the
propeller loading, which will be analyzed next.
TOTAL WAKE DISTRIBUTION AND
PROPELLER LOADING
The measured model and full scale wake distributions
can be used to calculate the performance of the
propeller in terms of propulsive performance (thrust
and torque) and minimum pressure. The latter is of
course important for cavitation inception.
The important role of the transverse velocity
field made it impossible to analyze the propeller
performance in terms of Taylor wake fractions, using
the open water characteristics of the propeller.
Instead lifting surface calculations were carried out
with the Marin program ANPRO. The velocity
distribution of the measured total wake, made non-
dimensional with the ship or model speed, was used
as input for the wake distribution. The mean axial
velocity was adjusted to obtain the measured torque
coefficient. The result was a pressure distribution on
the propeller (including the minimum pressure) and
an effective mean axial velocity Ve. This effective
axial velocity, derived from the required torque
coefficient, can be compared with the LDV
measurements.
Since the ship or model speed was known,
this mean axial velocity was written as a wake
fraction 1-w~VJVs. This is a calculated wake
fraction which includes the effects of the transverse
velocity. For the transverse wake distribution of the
ship, the model data used for the straight course and
for only the vertical component in turns. The result of
these calculations is summarized in Table 3.
It is noteworthy that the presently calculated
wake fractions are equal at full scale both on a
straight course and in a turn and, for the model, with
and without drift. This leads to the conclusion that the
axial inflow velocity of the propeller is not changed
by the turn or drift angle. The difference in power
absorption between port and starboard propellers is
therefore fully due to the transverse velocity field.
Condition KQ 1-w 1-w
calculated measured
LDV
Ship straight 0.028 1.03 1.03
Ship in turn 0.031 1.07~1.01) 1.11~1.05)
to port
Ship in turn 0.028 1.05 (0.99) 1.11 (1.05)
to starboard
Model 0.028 0.96 1.02
straight
Model drift 0.031 0.94 1.04
to port
Model drift 0.026 0.96 1.05
to starboard
Table 3: Calculated wake fractions using lifting
surface calculations (the data in brackets are for the
ship speed in a turn of 13 knots instead of 12.4 knots)
In Table 3 the calculated wake fractions of
the ship are greater than one, which is not plausible.
As indicated by the velocities in the outer flow,
obtained from the LDV measurements, the average
ship speed in a turn was probably higher than was
calculated from the transit time of the turn. Setting
the ship speed in a turn equal to the ship speed on a
straight course (13 knots) lowers the calculated wake
fractions of the ship in a turn to values close to one
and the measured wake fractions of the LDV data in
Table 3 to around 1.05. An estimate of the induced
velocities at the distance of the measuring plane
upstream of the propeller indicates that the propeller
induced velocities there are between 5 and 10
percent. Therefore, except for the ship wake on a
straight course, the results in Table 6 are consistent.
The calculated wake fraction of 1.03 for the ship on a
straight course seems approximately 5 percent too
high. This might be caused by the fact that the
transverse velocities of the model were used for this
calculation.
13
OCR for page 565
CAVITATION OBSERVATIONS
The location of the camera was between the shaft
brackets, as shown in Figure 2.0nly the upper half of
the propeller blades could be observed from that
position, so the observations were limited to the top
half of the propeller disk. From the calculations it
was found that the cavitation extent did not change
very much from 20 degrees before until 20 degrees
beyond the top position of the blades. This was also
the range where the largest effect on cavitation was
expected.
The cavitation pattern on a straight course
with 520 rpm is shown in Figure 27. The front blade
is near the top position. A small amount of sheet
cavitation is visible. A cavitating tip vortex is still
visible at about 120 degrees on the blade just ahead.
In a turn to starboard the outer propeller is observed
and the cavitation pattern does not change
significantly (Figure 28~. In a turn to port (Figure 29)
the propeller is overloaded, but this leads to only to a
slight increase of the cavitation, particularly further
inward from the tip. The overloading is mainly in the
lower half of the propeller disk and thus does not
affect the cavitation pattern in the top position of the
blades.
These observations were confirmed by
calculations of the cavity extent at the full scale
torque coefficient. On a straight course the calculated
cavitation was incipient near 0.95R, but hardly
visible. In a turn, both to port and to starboard, a
slight amount of sheet cavitation was calculated
(Figure 30~. So the effect of a turn on the cavity
extent was small.
14
Figure 27: Cavitation on the ship's port propeller at
520 rpm on a straight course
Figure 28: Cavitation on the ship's port propeller at
520 rpm in a turn to starboard (outer propeller)
Figure 29: Cavitation on the ship's port propeller at
520 rpm in a turn to port (inner propeller)
s.T
\ ~ /
, i.
NS 1 ,/
~ ~ . ''
_ _
I I I I I I I I ~ I r ~ I
. -. -~.28 B.~8 8.08 0.49
Figure 30: Calculated cavitation on the propeller in
the ship's wake in a turn to port..
OCR for page 566
The model tests showed hardly any
cavitation near the propeller tip, indicating that part
of the full scale cavitation was tip vortex cavitation,
which is delayed at model scale. In order to observe
the differences between the cavitation patterns in a
drift to port and a drift to starboard, the propeller was
overloaded by increasing the rpm by 10%. (Figures
Stand 32~. These predictions showed that the main
difference in the cavitation pattern occurred in the
downgoing blade (9Odegrees and further), where the
transverse flow began to influence the blade loading.
The differences in the top position of the blade were
small.
Figure 31: Observed cavitation on the model with 5
degrees drift to port (Vm=2.234 m/sec., nm-1426
rpm).
Figure 32: Observed cavitation on the model with 5
degrees drift to starboard (Vm-2.234 m/sec., nm-1426
rpm).
Therefore, contrary to what was expected,
cavitation in the upper half of the propeller disk was
rather insensitive to manoeuvres. This can be
explained by the small effect of manoeuvres on the
propeller inflow in the top position. The main effect
of a manouevre, at least in this case, is in the lower
half of the propeller disk. This insight can be used to
assess the effects of higher rudder angles on
cavitation inception.
EFFECTS OF A TURN ON THE INCEPTION
SPEED
Assuming a linear relation between the axial inflow
in a turn and on a straight course, the inflow
distribution at various turning angles can be found
from a linear interpolation or extrapolation of the
measured wakefields. A similar linear relationship is
assumed between the torque coefficient on a straight
course and in a turn.
With these assumptions the wake fields have
been calculated from a straight course up to 1.5 times
the turning rate used on HMAS TOWNSVILLE in
steps of 0.25. The turning rate is thus expressed as a
fraction of the full scale experiment. These
wakefields have been used as input for lifting surface
calculations to find the minimum pressure on the
blades over one revolution.
When the minimum pressure is below the
vapor pressure, cavitation is expected. Assuming that
the propeller advance ratio, and thus the pressure
distribution, does not change when the rpm is
reduced, we can find the reduction of the rpm
required to make the minimum pressure equal to the
vapor pressure. The advance ratio then gives the
inception speed at the reduced rpm.
In Figure 33 the inception speed of both the
inner and outer propeller is given as a function of the
turning rate. Initially, at lower rates of turn, the
inception speed increases. This is due to the reduction
of the wake peak behind the shaft and brackets in a
turn. However, as the turning rate increases the
transverse velocity becomes increasingly important,
and above 0.5 times the turning rate of the full scale
test, the inception speed continuously decreases. The
minimum pressure under these conditions does not
occur when the blade is in the top position, but when
it is in the lower half of the propeller disk, where the
larger transverse velocities have increased the blade
loading and decreased the pressure. At 1.5 times the
15
OCR for page 567
turning rate of the full scale experiment, the inception
speed has dropped more than 1.2 mYsec on the inner
propeller.
Inception Speed versus turn strength
5.0
1 1
4.0 ~1
Mater
ropeler
.
. ~
inrurprup
1 1 !
000 02S 050 075
IM. Or
Figure 33: Inception speed of the inner and outer
propeller as a function of the turning rate. (A turning
rate of 1 is the turning rate of the full scale trial. The
vertical grid distance is 0.2 m/sec).
Figure 33 shows a rather complicated
behaviour for the changes to the inception speed.
Initially the outer propeller is the critical propeller.
With increasing rate of turn, however, the inner
propeller takes over. These predictions were not
verified by inception tests at full scale.
CONCLUSIONS
Contrary to what is generally expected the full scale
experiment on HMAS TOVVNSVILLE shows that the
main effect of a turn on the propeller inflow is in the
transverse velocities! The axial inflow of both
propellers in a turn remained the same. The
differences in propeller loading could be explained
fully by the transverse velocities. This explains why
on frigates with inward turning propellers the inner
propeller is unloaded in a turn.
According to the full scale measurements,
the transverse velocities in the upper half of the
propeller disk were small. The presence of the center
propeller may have blocked the transverse flow. This
blocking was not unfavourable, because in a turn the
wake peak due to the shaft and the struts was less
concentrated and thus less deep. The combination of
a small transverse velocity and a wake peak reduction
caused the inception speed to increase initially at
small rates of turn.
For fuller ships the transverse velocity in the
upper part of the propeller disk may not be negligible.
The velocity will then be inwards, towards the ships'
centerline. For cavitation inception it is then
favourable to have the propellers turning inward over
the top, and use the unloading effect of the transverse
velocities to offset the loading effect of the axial
wake peak.
It is remarkable that the properties of the
inflow in a turn could be simulated with a model at a
drift angle. The results show that the drift angle, and
not the rate of rotation in a turn, is important for the
crossflow in the propeller plane. This is important
also when the ship is on a straight course in seaway,
where the mean rate of turn is zero, but where
significant drift angles can occur due to
coursekeeping by the rudder. The effect of rudder
actions on the inception speed requires further
investigation.
The crossflow in the top position in a turn
was also investigated on CFAV QUEST. This is a
different type of ship, with bossings instead of
brackets. Here the effects of a turn on the inception
speed were stronger. The data are, however, less
complete than on HM245 TOVVNSVILLE.
The collaboration between the original
participating organizations in this project is
continuing. The conduct of full-scale trials on CFAV
QUEST, HMAS TOWNSVILLE and the Canadian
frigate HMCS NIPIGON and associated model tests
have been completed. The focus of the project has
now turned to the development of analytical tools and
methodologies for the prediction of inflow and
propeller cavitation performance. In parallel, the
feasibility of implementing alternative operating
strategies for a naval ship to increase its cavitation
inception speed is to be examined as an analytical
exercise. Such measures include for example an
adjustment of propeller pitch setting to suit the
prevailing operating conditions in the case of frigates
equipped with controllable pitch propellers. Design
guidance for consideration of propeller inflow
conditions under realistic operational conditions are
also expected to flow from the experience gained
from this project.
REFERENCES
Kennedy, J.L., Sponagle, N.C., Wheaton, D.W.,
MacDonald, P. and Creaser, R.W., "Video Systems
for Propeller Viewing Trials", Proceedings of the
22n~ American Towing Tank Conference St. John's
, ,
Newfoundland, 1989, pp. 197-202
16
Representative terms from entire chapter:
inner propeller
