MEDDELANDEN FRAN
STATENS SKEPPSPROVNINGSANSTALT
(PUBLICATIONS OF THE SWEDISH MARITIME RESEARCH CENTRE, SSPA)Nr 89 GOTEBORG 1981
CAVITATION ON HIGH SPEED PROPELLERS
IN OBLIQUE FLOW
- INFLUENCE OF PROPELLER
DESIGN AND INTERACTION WITH SHIP HULL
by
OLLE RUTGERSSON
Paper presented at the
13th Symposium on Naval Hydrodynamics Tokyo, Oct 1980
- 'Distributed by:
Liber Distribution -S-162 89 VALLINGBY Sweden
ISBN 91-38-06124-4_ ,
CONTENTS PAGE SUMMARY 2 INTRODUCTION 3 NOTATION 3 WORKING CONDITIONS 5 3.1 WAKE FIELD 5 3.2 CAVITATION NUMBER 9 3.3 PROPELLER LOADING 10
SYSTEMATIC PROPELLER TESTS 14
4.1 PROPELLER GEOMETRY I 14
4.2 SHIP MODEL 16
4.3 TEST FACILITIES 17
4.4 PRIMARY RESULTS 18
4.5 INFLUENCE OF CAVITATION ON PRESSURE AMPLITUDES 24 4.6 INFLUENCE OF HULL ON PROPELLER CHARACTERISTICS 28 INTERACTION ON A TRIPLE-SCREW SHIP 33
5.1 INFLUENCE OF PROPELLER-HULL CLEARANCE 33 5.2 PROPELLER-PROPELLER INTERACTION
36
CONCLUSIONS 37
6.1 WORKING CONDITIONS 37
6.2 SYSTEMATIC PROPELLER TESTS 38
6.3 INTERACTION ON A TRIPLE-SCREW SHIP 38
ACKNOWLEDGEMENTS 39
REFERENCES 39
SUMMARY
Problems concerning the performance of propellers working behind high-speed displacement ships are discussed.
Results from measurements of the flow field behind multiple pro-peller hulls in towing tank are given and compared with theore-tical calculations. Mean wake and static pressure are shown to change with ship speed and trim. These variations are probably explained by the change of wave pattern with ship speed and trim.
The influence of propeller geometry on the propeller-hull inter-action is given by results from six propeller models beeing tes-ted behind a ship model in the cavitation tunnel. The hull in-fluence on the characteristics of propellers of supercavitating conditions is discussed. Some results showing the influence of propeller geometry and cavitation on the pressure fluctuations induced by high-speed propellers are given.
Finally some results showing the propeller-propeller influence on a triple-screw ship are given.
INTRODUCTION
When using water propellers for the propulsion of high speed vessels inclined propeller shafts are necessary in most cases. Other characteristic features of these arrangements are:
o propellers located far aft close to the stern
o propellers located close to ship bottom and rudders
For such propeller locations the flow field is rather sensitive for changes of ship speed and trim [2]. Interaction between propeller, rudder and hull is also important and its effect
should be included in the prediction of the prototype performance.
[2.9].
In the present paper the working conditions for propellers wor-king in oblique flow behind trimmed high speed displacement ships will be further discussed. The influence of cavitation, propel-ler geometry and wing propelpropel-lers on the propelpropel-ler-hull interac-tion will also be studied.
NOTATION
a vertical clearance between propeller and hull (m)
AD developed blade area (m2)
A0 11)2/4 (m2)
pressure fluctuation, single amplitude, peak value, (Pa)
Cp, CpT Ap/p/2V2 = pressure coefficient
propeller diameter (m)
maximum camber of propeller blade section (m)
FN V /1/4E = Froude number
0 draught of propeller centre at zero speed (m)
VA/nD = advance ratio of propeller
advance ratio at free running condition
0
2C/pn2D2 = amplitude coefficient, pressure fluctuations
KPnc amplitude coefficient at cavitation-free conditions
K Q/pn2D5 = propeller torque coefficient
KT T/pn2D4 = propeller thrust coefficient
KTB thrust coefficient at behind condition
KTO thrust coefficient at free-running condition 1 profile length (m)
ship length between perpendiculars (m)
number of revs (r/s)
atm atmospheric pressure (Pa)
P
PO static pressure at propeller centre (Pa)
vapour pressure (Pa)
Pv
Ap pressure difference between undisturbed flow and
pro-peller centre (Pa) pitch (m)
power (Mw)
propeller torque (Nm)
radius of propeller blade section (m)
D/2 (m)
propeller thrust (N)
ATA sinkage of ship stern (in)
VA advance speed of propeller (m/s)
V ship speed (m/s)
V0.7R VAVI + (0.7ff/J)2 = inflow velocity for blade section at
0.7R
wake fraction
r/R
nR nB/no = relative rotative efficiency B propeller efficiency at behind condition
0 propeller efficiency at free-running condition
density of water (kg/m2)
aVA
(po -
pv)/0.5pVA2 = cavitation numbera0 (130 - pv)/0.5pv0.7B = cavitation number
CpT
T aVA (1 - w)2 = cavitation number in tunnel
propeller loading coefficient
TC
pitch angle (degrees)
3. WORKING CONDITIONS
The environmental conditions influencing the work of the pro-pellers can be discussed under the following subheadings:
wake field
cavitation number propeller loading
3.1 WAKE FIELD
The relation between the water speed at the propeller disk and the ship speed is defined by the wake fraction in the following
way:
VA/VS = 1 - w (1)
When analysing the influence on propeller performance it is con-venient to split up the wake field into:
circumferential variations of axial wake mean value of axial wake
1.05 1.0 0.9 1). . N
,
N,
N,
Wing propeller IIIIIIIrpuu . I Cent N 0.8 Nr roeller
1.0 -Froude number FN -v g L.Fig 1. Influence on ship speed on axial wake for a
triple-screw ship
The viscosity has a dominant influende on the wake field of full ship forms. On these 'ships the propeller works partly inside the boundary layer, which causes two characteristic effects:
o large variations in the circumferential wake field o water speed always: considerably lower than ship speed'
On high speed vessels the propeller normally works outside the hull boundary layer [1]. Disturbances from inclined propeller shafts and struts will; however, create narrow peaks in the cir-cumferential wake distribution [1, 2]. As the peaks are narrow the mean wake is usually considered to be low on high speed ships
(1 - w 1). . The variations with -ship speed and trim can,
how-ever, be considerable [2, 3]. In Fig 1 results from measurements with Prandtl tubes in the towing basin are shown. The ship model represents the 3-propeller project shown in Chapter 5 of this
paper.
In Fig 1 an example is given of the variations of the wake at the propeller plane with ship speed. The values given are mean values of ±900 blade position and are therefore approximately
valid for the propeller centre. The variations with ship speed are characteristic for this type of ship. 1 - w shows a rapid
increase at speeds just below FN = 0.5, a maximum at FN = 0.5 and then a decrease with increased speed so that 1 - w 1 at
FN = 1. The magnitude of these variations depends on the hull
form and can be considerably higher than shown in Fig 1. The variations with ship speed are shown to be more pronounced for the wing propellers. The reason for this is probably the loca-tion of the wing propellers being closer to the wave perturba-tions than that of the centre propeller.
The wake of a high-speed craft is sensitive not only to varia-tions in speed but also to trim changes, as shown in Fig 2.
The wake shown in Fig 2 was measured at FN = 0.55 in the towing basin on the twin-screw ship model described in Chapter 4. A larger trim by the stern gives higher speeds at the propeller disk and also a larger inflow angle relative to the propeller shaft. The inflow angle measured by a 5-hole Pitot tube is shown to be larger than eight degrees, which is the angle between ship bottom and propeller shaft. When the ship is trimmed by the
stern the inflow angle increases by about one third of the rate of the increase in trim angle.
The characteristic features of the wake field of a high-speed craft seem to be:
the mean wake is often negative (1 - w > 1)
the mean wake is sensitive to speed and trim variations the flow is oblique relative to the propeller shaft
As the wake changes with trim variations at constant speed one
could assume the wake to be rather much influenced by the poten-tial flow. The total wake is then the sum of the effect of
viscosity, wv, potential flow (without wave-making), w, and
P
wave making, ww:
1.10 1.05 1.0 11 7 .4.6/ %47 ,417.Shaft relative inclination .
----1.**"
to ship bottomcW
0 2 4 6Trim angle (degrees)
Fig 2. Influence of trim on axial wake and inflow angle on a
twin-screw ship. Reproduced from [2].
In order to obtain the magnitude of the different parts in Eq (2) some theoretical calculations have been carried out by the Hess & Smith method [4] and compared with measurements. This method does not include the influence of the free surface.
In the towing basin the local wake at the propeller centre was measured to w = -0.076. Measurements behind the same model moun-ted in the cavitation tunnel with the same trim as in the basin but with wooden plates representing the water surface gave w = -0.035. Theoretical calculations gave -0.074 for the tunnel
con-figuration and -0.033 without the tunnel (still flat plates as the water surface). These results lead to the conclusion:
Viscosity wake w +0.039
Potential wake w = -0.033 Wave-making wake w -0.083
Blockage in the tunnel wB = -0.041
Thus the wave-making part dominates in this case and useful results from theoretical calculations cannot be obtained unless the free surface is taken into account.
3.2
.CAVITATION NUMBERThe performance of the propulsion system of a high-speed craft is highly dependent of the development of cavitation on the pro-peller. The basis of a good correlation between model and proto-type must therefore be a realistic estimation of the cavitation
number:
PO - Pv
aVA =
17TRFIT
.,A .
The static pressure at the propeller centre,
po,
is usually calculated as the sum of the atmospheric pressure and the press-ure of the water column from the propeller centre up to the free surface. The wave pattern in the case of a high-speed craft, however, makes the estimation of the static pressure more com-plex. In the first place it is not obvious which surface shouldbe used for the calculations. Secondly, the potential flow in-duces pressures, which in some cases cause important changes of the cavitation number. Figs 3 and 4 show some results from measurements of the static pressure at the propeller centre on
the triple-screw model mentioned earlier. Fig 3 shows the press-ure difference between a Pitot tube ahead of the model and a Pitot tube at the propeller disk.
The absolute static pressure at the propeller centre is obtained by adding Ap of Fig 3 to the atmospheric pressure and the
ure of the water column from the propeller centre to the undis-turbed water surface:
Patm
-p + pg(H0 + ATA)
v
avA
-p/2Vs2(1 -
w)2 (1 - w)2In Fig 4 this method is compared with the method of using the water column up to the transom as an approximate value of the
static pressure.
At high speeds, FN 1.:$ 1.0, the agreement between the methods is
shown to be good. At lower speeds where the trim and sinkage at the stern are large the calculated static pressure is too high. For the wing propellers this difference
is
not negligible. The theoretical calculations described in 3.1 also included static pressure. The values obtained have in Fig 5 also been compared with measured static pressures.The measurements in the towing basin and in the cavitation tun-nel happened to give almost identical results for the propeller centre. This is not usually the case and correction for this difference is made when calculating the cavitation number for
the cavitation tunnel:
CpT
GT = aVA (1 - w) 2
In Fig 5 the theoretical calculations also agree very well with the measurements at the propeller centre in the tunnel. The differences for locations closer to the hull are probably due to disturbances from shaft and struts, which were not included in the calculations. The calculations for the towing basin show also here that useful results cannot be obtained unless the free surface is taken into account.
3.3
PROPELLER LOADINGThe propeller loading is also a fundamental parameter for the
Fig 3. Influence of ship speed on the static pressure CL 7 0 L. 0 Ors 0.10 0,4
as
as
to
- 0. 05Fig 4. Comparison of measured and calculated water column
pressure for triple-screw model
Center propeuer
if
\./
/
/
\
\I,/
V
/
/
/
/
/
Wing propeller 1e
o
[A
Wing prop. measured
Center
prop --.
Water column to
transom Froude number FN =vjr
0.4 0.6 08 1.Bottom plating
egl
-0.15
-0.2
Distance below bottom plating
0 Measured pressures
CI Calculated pressures
Filled points: towing tank Open . points: cavitation tunnel
Location of points where pressure have been measured and calculated UI 0. 0 0 0
Fig 5. Comparison of measured and calculated static pressures
for a twin-screw model at FN = 0.55 !I. 4. a
v
o Fa' C I-0 0 .r. ta a, 0 e.., m 0. m o2
Et .cs .., wv
0 05 --t e retleat C43.---.tOV----The0 ...,-- _...a.-;no WI*...-a--jr..---'.Measurements towing tank
-ments cav.tunnel
\
\
\
..93-Y
ale-\
-meoretic,-131 c.:---'-1.5'... ..otr ** -co* tunnel -...--..all the traditional towing tank problems:
towing tests for measurements of resistance of hull and appendages [5]
self propulsion tests for determination of the thrust deduc-tion factor and wake fracdeduc-tion [3, 5]
calculation of propeller loadings by the use of scale factors and correlation factors, empirically estimated on the basis of earlier experience:
J2 _
(6)KT/
pD2VA2
For most merchant ships the cavitation does not develop so far as to influence, the propeller characteristics. The self propul-sion tests in the towing tank can therefore form the basis of the prediction of power and number of revolutions at different speeds. The purpose of the cavitation tests is for most merchant ships to check the erosion properties of the propeller and to measure the vibration excitation forces and the noise generated by the propeller.
The characteristics of a high-speed propeller are, however, very much influenced by cavitation. Further, this influence is dif-ferent in constant flow and when the propeller is working behind the hull (as shown later on). Thus both propulsion tests and cavitation tests are necessary for a reliable power prediction. In the latter case the tests have to be carried out in behind
condition.
The purposes of the different tests are:
open water tests in towing tank give the relation between thrust and advance coefficients and form the basis of calcu-lation of effective wake fraction
propulsion tests in towing tank give propeller thrust and wake for different ship speeds
cavitation tests in behind condition give the relation be-tween thrust, torque, efficiency and number of revs at
cavi-tating conditions. Input parameters are the cavitation number according to Eq (5) and the propeller loading according to
Eq (6)
4.
SYSTEMATIC PROPELLER TESTSA number of systematic propeller series for propellers specially designed for high-speed ships have been presented in the litera-ture, for example [6, 7, 8, 91. These propellers have, however, usually been tested in uniform flow. Very little is known of the influence of propeller geometry on for example erosion, inter-action with hull and pressure fluctuations, when the propellers are working behind a hull. It is the purpose of the present investigation to improve the knowledge in this field somewhat.
4.1
PROPELLER GEOMETRYSix 3-bladed high-speed propeller models with the diameter 250 mm were chosen for the investigation. The main propeller para-meters are shown in Table 1.
The three first propellers (Cony 1.05, Cony 0.75 and Cony 0.50) represent a blade area variation of propellers -of rather conven-tional design, with symmetrical blade shape and NACA sections.
Propeller Warp 0.75 is a blade shape (warp 1200) variation of the conventional propeller with blade area ratio 0.75.
The two last propellers have two different supercavitating sec-tions. Propeller S.C. 0.50 has a face shape according to the
3-term distribution and a modified 2-term thickness distribution [9]. The last propeller model was designed to have improved cavitation properties in the partially cavitating (P.C.) region. The profile is a combination of 5-term face shape and an empiri-cally derived back shape.
Further information about the propellers, such as blade shape, pitch distribution, camber distribution and design method is given in the Appendix.
Propeller 1:16ti.gn point No .J I ,KT P1391 1416 0.19 0, 1.05 NACA 16 unload sym Cony 1.05 P1477 ' 1.16 0.19 0.75 NACA 16 unload sym
Cony 0.75 0
P1514 1.16 0.19 m 0.50 NACA 16 unload sym Cony 0.50A
-P1714 1.16 0.19 co 0.75 NACA 16 unload 120° warp Warp 0.75 --al--P1439 1.15 0.146 0.58 0.50 wedge opt sym S.C. 0.50 P1790 1.15 0.146 0.58 0.50 spec opt sYm P.C. 0.50 Table 1Main propeller parameters
K /J 2 -T' (knots) -. e
-,.
r0Pa11ar4Oad_niiA
and ,cavitation numbers tested
Length 010 2,5 Area B H (m2) diam Max speed
firIM
23' --11' caY number ' 6,.6641 . . High speedLow speed section
section original with insert, 9.6 2.6 x-1.5 2.6 x 1.15
145
050
*EmPtlitunriOI:"At::TroPs14si-;OSs
right-angle gear dynamometer Set.s.,cr'=45ahe-:WwWIlmit.-.
test sections of cavitation
Profiles Pitch Blade Designation Notation in distr 'shape figures
Fig 6. Propeller arrangement on ship model
4,2
SHIP MODELThe ship model used for the investigation is a twin-screw patrol craft in scale 1:5.0 with a propeller. arrangement according to
Fig 6.
The main interest in this investigation is devoted to the speed range 30-35 knots. In order to have a typical
low
speed point.20.knots was, however, also included in the test program. The pro-peller loadings used for this study were chosen to
fit
thepitch
and diameter of the propellers. .
The full scale ship correlating to these loadings could be a twin-screw patrol craft with a displacement of about 100 m3
20
15.3
*Riolip-t
Fig 7. The SSPA large cavitation tunnel with low-speed test
section in place. Dimensions in metres
operating at speeds of about 30 knots with propellers with a
diameter of 1.25 m absorbing about 1:5 Mw at about 700 r/m.
4,3
TEST FACILITIES
The cavitation tests were carried out in SSPA large cavitation tunnel. The tunnel is fitted with two interchangeable test
sec-tions, i e:
o one circular, high-speed test section
o one rectangular, low-speed test section large enough for
tests with combinations of propellers and complete ship models
A sketch of the tunnel with the large test section in place is
given in Fig 7. The most important data of the test sections are given in Table 3.
The low-speed test section is covered by a recess, in which the ship model was placed. The model was the one used for tests in the towing basin and was at this investigation made of fibre-glass. The model was placed in the tunnel with the correct draught at the stem and with the same trim as at the tests in the basin. Individually cut wooden plates were then fitted to simulate a flat free surface. At the tests in this study also a dummy simulating the wave behind the transom was mounted under the plates behind the model. With this mounting the static press-ure at the propeller centre was measpress-ured to be 0.16 p/2V2 lower than the free stream pressure, as is also shown in Fig 5. The test section and the recess were completely filled with water at
the tests.
To drive the propeller models one AC electric motor was used for each propeller. Strain gauge dynamometers for measuring thrust and torque were placed in the shafts close to the propellers.
The operating range of the test section with insert covers ship speeds up to about 45 knots at the water speed 8 m/s. At this investigation the water speed 7 m/s was used.
A more thorough description of the tunnel and its background is given in [10].
4.4
PRIMARY RESULTSThe primary results from the cavitation tests are:
cavitation patterns
thrust, torque and number of revs risk of erosion
pressure fluctuations induced on the hull
In Figs 8 and 9 photographs of the cavitation patterns with the blade in top position are compared for the six propellers. Due to the oblique flow the back cavitation has its maximum at blade
position +900 and its minimum at -900, the variations being more pronounced for the inner radii than for the outer ones. Thus the
Cony.' 1.05 Cony. 0.75 Cony. 0.50
Fig 8 Cavitation patterns
at tests with a complete ,ship, mOdel.
= 20 knots V5 30 knots Vs = 35 knots
Pro-peller
Ag
BACK FACE
Leading edge Leading edge
PROPELLER
, Root erosion on
Cony. -1.05
face and back.
Cony. 0.75 Warp 0.75 P.C. 0.50
auf
Cony. 0.50 No erosion 1994,9 rsRoot erosion on
face. Erosion on back No erosion No erosion on face. 'Slight root erosion on back. Fig 10Cavitation erosion obtained with the
paint test -technique
after 30 minutes test
atpropeller loading corresponding
to 30 knots.
ILS
No erosion on
face. Rootcavitation patterns in Figs 8 and 9 represent a kind Of mean cavitation extension.
One of the 'most serious problems with high-speed propellers, in
oblique flow is root erosion. At SSPA a paint test technique has
been 'developed, which shows the risk of erosion after, only. 30
minutes test in the cavitation tunnel [111. Fig 10 shows the, results of testing the propellers with this technique at the 30 knots propeller loading. The most severe erosion is found on the
conventional propellers with wider blades. The propellers of the sUpercavitatingtype show, however, very little. erosion.
Especi-ally'propel],er
p.c.
0,50 seems to be very successful'from"this
-point.
of vieW.
The propellers Cony 0.50 and Warp 0.75 werecOril-ple:EelYfree fkot,erosion, depending,on almost fully cavitating
conditiOnS2at.this -propeller loading,
When running the. tests' the number of revs is adjusted'.so that the propeller: loading according to,Eq (6) is obtained. 'At
loading the tbrqte coefficient and advance ratio are then
measured, forming the' basis for calculations of power 'demand and
number:9f revs for the prototype.
The POwer demands for the different propellers shown iiiFig 11 are fairly close
to
each other at 20 and 30 knots (except for propellers Cony.o.so
And Warp 0.75 at 30 knots), Note especially that the,propellers of the supercavitating type have only slight7 11flhigher power demands than the conventional propellers with blade arearatios 1,05 and 0.75. At 35, knots the differences between the propellers are, however, considerable.The prettu±efluctuations induced on the hull are important results from' theCavitation tests, as excited vibrations and noise emanate from them: Fig 12 shows the pressure fluctuations measured çn the
hull
just above the propellers. The values given are the mean blade frequency amplitudes analysed by the digitalmethod*scribed. in [12]. !:t
notice-2.5 2.0 1.0 0.5 0 Warp
0.75/
/
/
/
/Cow 0.50IF
P. C. 0.50i
Cony. 0.75 Cony. 1.05/I/
S.C. 0.50I,
Cs Vs =30 knotsA
/,ii
/*
1 7
/
tVs=20knots 500 700 900 1100Number of revs. Np (r/m)
Fig 11. Power and number of revs for the prototype predicted
from cavitation tunnel tests
propeller Cony 0.50 induces very large amplitudes propeller Warp 0.75 induces rather low amplitudes
the propellers of the supercavitating type do not induce larger amplitudes than the conventional propellers
4,5
INFLUENCE OF CAVITATION ON PRESSURE AMPLITUDESThe pressure amplitudes in Fig 12 have also been plotted in Fig 13 as the dimensionless amplitude coefficient K used for the scaling of pressure fluctuations. Using this coefficient the
levels are shown to be fairly constant at decreasing cavitation numbers in spite of the fact that the cavitation extensions are increasing. A probable explanation of this, also proposed in
[13], could be that the pressure amplitudes are increased at lower cavitation numbers due to the cavitation, but at the same time decreased because of reduced propeller loading.
In [2] it was shown that Kp for a non-cavitating propeller divi-ded by the thrust coefficient KT is almost independent of the
advance ratio. Using this parameter the influence of clearance ratio on the pressure amplitudes for 3-bladed propellers is
shown in Fig 14, reproduced from [2]. The theoretical curve was calculated by a method outlined in [14] and the experimental values emanate from different investigations in the SSPA large cavitation tunnel. The points with flags are the propellers in the present investigation.
By assuming that also for a cavitating propeller the non-cavitat-ing contribution is proportional to the thrust coefficient the following expression is obtained:
KPnc =C nc-KT (7)
where Cnc is given by Fig 14.
The non-cavitating pressure amplitudes thus reflecting the thrust coefficient are given in Fig 15.
The relation between the values in Figs 13 and 15 gives the cavi-tation amplification of the pressure amplitudes in Fig 16. The amplification factors given in Fig 16 show magnitudes which seem to be well correlated to the cavitation pictures given in FigsS and 9. The photographs with the most extensive cavitation
corre-80 Jc 60 0. 40 0. 20 0 20 0.2 0.0 07 10 0 Cavitation number
Fig 13. Influence of cavitation number on the dimensionless
pressure amplitudes ..i .., Cony. 0.50 ./
le
, X .,"/
co),. ../`z./
.'
/.
Cony.1.05i
...:....--4-\ ___.0.50.--/.
,
...-- ...-...-...---'---- ...---- -..._..."----..-- - ...._.io._..____.--
\
S.C. 0.50 gr. "..., fr---.---. Cony. 1.05\
\* A--* Cony. 0.50 ..,,...' Cony. 0.75 .../ 1:1C 5-11).--; ---' .. ---;:cl..C.0.50r>.
/\*
Warp 0.75,/x.-\
'. .\____
" ----4 . ./
/
' 25 30 35Ship speed Vs (knots)
Fig 12. Comparison of blade frequency amplitudes induced on
1. 0.8 0.6 0.4 0.2 0 Kp 2,4172
,
7117'I
' 6 nc 0.7to
20 Cavitation number 0TFig 15. Variation of the non-cavitating contribution to the
pressure amplitudes Theoretical conventional o Measurements conventional Measurements calculations propeller simple arr. propeller simple arr. propeller on complet conventional ° supercavitating A 1Measurements A shipmodel propellers
i
Theoretical urve Cony. 0.75 Cony. 1.05 ' ,.=/
../ ... -,, .-
-S..C.0.50 P C.0.50 ...---4/ .----' ....-Cony. 0.50 .. _.. 0.75 6.---02 04 06Vertical clearance az/D
Fig 14. Influence of clearance on the blade frequency pressure
4.0
07 1.0
20
Cavitation number CT
Fig 16. Variation of the cavitation amplification with
cavitation number
spond to the largest values of the amplification factor. Two levels of this factor are also shown in Fig 16. The majority of the propellers end up at values around 2.0 at a = 0.7. For the propellers with complete thrust breakdown, however, the amplifications are about 3.5.
b..
-,:..-
./...)\\
fr''../.
-,...---4\
\.,cy/p
0 75_or 0.50\.
\\,
0....--e----
Cony. 0.75-,
A
-,-...--Cony.1.05\'
.N. _ ... ___... PC.0.50 , , ,SUM
NN.1.0
Cavitation number Cf-r
1.0 2.0
Fig 17. Hull influence on thrust and efficiency
4.6
INFLUENCE OF HULL ON PROPELLER CHARACTERISTICSIn [2, 9] the influence of hull on the characteristics of a cavitating propeller was reported to be a kind of wall effect reducing thrust and torque up to 10%. In Fig 17 the hull influ-ence obtained in this investigation is given as the influinflu-ence on thrust and efficiency. The thrust coefficients and efficiencies in behind condition (KTB, nB) were measured at cavitation tests with the complete ship model according to Fig 6 with the
cavi-tation number
aT in the tunnel. These values have been compared
with thrust coefficients and efficiencies (KT0, no) measured with the same propeller model working in homogeneous flow at zero shaft inclination in the high speed test section of the
Conv.1.05 SCa5
-7-41
-.11"--"Pcaso V*/
7/7
0.50y
,VCony.
7 --
---::; Warp 075
:'/'
Cony. 0.75\e/
\
Warp 0.75/
z
S.C.0.50 . . ar---.-\- --
...----../
\
Cony. 1.05,
--a/
7---cv 7.
., Cony. 0.50 Cony. 0.75 P...---- . Cavitation number CrT 1.020
1.0
0.7 / 1.0 2.0
Fig 18. Influence of shaft inclination 8 degree i on the
characteristics measured
in
the high-speed test sectioncavitation tunnel. In order to compare identical cavitation numbers KTO and n were measured at the cavitation number aVA =
0
aT - 0.16, which is the cavitation number at the propeller
centre when the ship model is mounted in the tunnel.
The influence shown in Fig 17 thus includes not only the influ-ence of the hull but also the influinflu-ence of rudders and the shaft inclination. The influence of rudders covering only half the propeller diameter has been shown to be very small [15]. The influence of the shaft inclination 8 degrees on the tested pro-pellers, except propeller Warp 0.75, is given in Fig 18. This
Conv. 0.50 ....::.--1k,::---- ....::.--1k,::----... ... ... 11..._...:=1_,..
"-AY'
N...;, .._...
...___ _ Conv.1.05/
1:1' ----Cl.""-..."" ...."*" Cony. lb/
Cony. 0.50 ./ ......
C0_
/1ar-Conv.1.05/,----",/
. -.sc.o.sor
If 4( Air ...---"... Conv. 0.75ce
Cavitation number ay 2.0 1. coinfluence is shown to be very different from the influence of the hull shown in Fig 17. The thrust coefficients and efficien-cies are generally increased a few per cent when inclining the
shaft, while the influence given in Fig 17 is a reduction of these coefficients. Thus if the obliqueness of the flow had been excluded the tendencies given in Fig 17 would probably have been even further emphasized.
When studying the influence of cavitation number on the curves for propellers Cony 0.75, Cony 0.50 and Warp 0.75 in Fig 17 it is somewhat surprising to see the relations between cavitation numbers 0.94 and 0.7. The hull influence seems to be larger at moderate cavitation extensions (a = 0.94) than at the extreme cavitation conditions at a = 0.7. This means that the limits for thrust breakdown should be very much influenced by the hull. In Fig 19 the loading conditions for propellers Cony 1.05, Cony 0.75 and Cony 0.50 have been plotted in a graph showing the limit for thrust breakdown for these propellers when tested in the
high-speed test section [16]. According to this limit the loadings 30 knots for Cony 0.75 and 35 knots for Cony 1.05 should not have thrust breakdown. However, when working behind the ship model they did have thrust breakdown, as shown in Fig 17. In Fig 19 some experimental values obtained in the high-speed test section with a propeller similar to Cony 1.05 are also given. At these
latter tests also a plate with clearance ratio 0.22 was mounted in the tunnel. The agreement between these plate tests and the tests with the complete ship model seems to be very good. The
limits for thrust breakdown obtained in unbounded flow apparent-ly give too small blade area ratios. This is also in accordance with the full scale experience given in [17].
The very small hull influence shown in Fig 17 at the extreme cavitation conditions (aT = 0.7 Warp 0.75, Cony 0.50) is in
agreement with studies of wall effects in [18] and [19]. When discussing wall effects on supercavitating and ventilated pro-pellers Morgan in the summary report [20] states that according
to the theories of Tulin [18] there should be no influence of wall effects. Results of measurements on geosim propellers in a
0.25
0.05
Vs= 20 knots
-/-,:i-d-g Limit for
breakdown propeller Incipient incipient thrust for SSPA series thrust measured 0.22 x D o . breakdown
with tip clearance
0:1(3 ..-T s,
../
iN 6, Vs =35 knots6 T
01 0.2 0.3Local cavitation number Go
Points within circles had thrust breakdown
in the present investigation.
Fig 19. Influence of hull on the limit for incipient thrust
breakdown T/coscP0.7 T C 7E12 AD/A0 1/2 pV0.72 PO - Pv a0 = 1/2 pVo.72
KT 1.1
Jo/Jo
1. 0. 0.8Measured in high speed test section
Measured behind ship model
Jo JB
Advance ratio J
07 1.0
20
Cavitation number GT
Fig 20. Hull influence treated as an influence on the advance
ratio Cony. 1.05 Cony. 0.75 '
4.,/
./ :
: Warp 0.75 Cony. 0.50'/
///-/
PC.0.50 ,...-1 .., .,..1.1
e...r \S.C.0.50cavitation tunnel given in [19] lead to the same conclusion. However, if the influence of the hull is considered to be a blockage effect, then correction of the advance ratios (which means correction of the wake) should be more appropriate than correction of the thrust coefficient. In Fig 20 this treatment has been used on the results of Fig 17.
The hull influence given in Fig 20 is shown to increase continu-ously with decreasing cavitation number. The points for a = 0.7 and propellers Cony 0.50 and Warp 0.75 have, however, been left out of Fig 20. The reason for this is that they are situated in the supercavitating region of the characteristics where the thrust coefficients are almost independent of the advance ratio. The ratio J0/JB could therefore be anything from 1.0 to 0.8. Treated in this way it is therefore very difficult to tell whether there is a large influence of wall effects at super-cavitating conditions (hidden in the fact that the thrust coeffi-cient is almost independentof advance ratio) or a negligible influence of wall effects.
5. INTERACTION ON A TRIPLE-SCREW SHIP
In this chapter some results from cavitation tests of a triple-screw ship according to Fig 21 will be given. First the hull influence at single-screw operation will be discussed and final-ly the propeller-propeller influence is shown.
5,1 INFLUENCE OF PROPELLER-HULL CLEARANCE
The influence of clearance on the propeller characteristics has been investigated at tests in the high-speed test section with a plate located above the propeller [2, 9]. Results from these tests are shown in Fig 22. One notable observation at these tests was the very slow decrease of the hull influence for the super-cavitating propeller at increasing clearances. These results have, however, now been confirmed at the clearance ratio 0.2 by tests with a complete ship model. At the tests with the
triple-Fig 21. Propeller arrangement on triple-screw model
Vertical clearance a/ D
=0.6
Fig 22. Influence of clearance on propeller thrust and
effi-ciency obtained by tests with a plate arrangement
1.0 1.-)c co
i-x
0.9 0.6 1.0 0.2 0.4 0.6 G=0.8 Supercavitoting propeller propeller =1.05 AD/A0=0.6 --Conventional AD/A0 0.2 0.6Model tests
Plate arrangement with rudders
--o
Complete shipmodelwith rudders
--a
1 only center propellerFullscale tests
Measurements on centerpropeaer
0.65 10 KQ 0.6 0.4 02 0 ....---O- -aa
6 atl 6 -0.709
1.1 1.3 JAdvance ratio.
Fig 23. Comparison of propeller characteristics obtained at
tests with plate arrangement and at tests with a com-plete ship model
screw ship also single-screw operation was tested as a compari-son with the earlier tested plate arrangement. These results are given in Fig 23. The agreement between the test arrangements is
shown to be very good at small and moderate advance ratios. At J = 1.1 the tests with the complete ship model, however, give
lower thrust and torque than the tests with the plate arrange-ment. The results from measurements on the prototype, also given in Fig 23, are shown to give the same tendency as the tests with the complete ship model.
In order to confirm the results of Fig 22 also at large propel-ler-hull clearances some further tests with complete ship models
are needed.
5.2
PROPELLER-PROPELLER INTERACTIONAt tests with the ship model shown in Fig 21 identical wake fractions at atmospheric tests were measured at triple-screw operation and with each of the propellers alone on the ship model. With the same number of revs on all three propellers there was obviously no interaction between the propellers at non-cavitating conditions.
For the cavitating propellers different results were, however,
obtained. These results were obtained by comparing thrust and
efficiency in triple-screw operation (KTB3, nB3), with'the thrust and efficiency measured when each of the propellers was working alone on the ship model (KTB, nB). The propeller-propeller inter-action in Fig 24 is shown to imply a further reduction of thrust and efficiency, compared with the single propeller operation. The influence is larger for the wing propellers than for the
cen-tre propeller. It is also very much dependent of the advance
ratio. In fact the advance ratio seems to be a more important
parameter than the cavitation number. J = 0.8 with almost super-cavitating conditions gives less influence than J = 0.95.. This
is a similar tendency to that shown concerning propeller-hull influence in Fig 17, where the heavily cavitating propellers were less influenced by the hull than the partly cavitating
0.9 1.0
2
c,
r
W1.0 0 2 > .0 pr: w 0.95Port
propeller
Centrepropeller
Starboardpropeller
Filled points: advance coeft J=as
Open points:
J=0.95Fig 24. Interaction between propellers on a triple-screw ship
6.
CONCLUSIONSFrom the results presented in the present paper the following conclusions may be drawn:
6.1
WORKING CONDITIONSWake and static pressure at the propeller plane are sensitive to ship speed and trim. Accurate loading conditions should be based on measurements of these.
Useful results of theoretical calculations of wake and static pressure cannot be obtained unless the free surface is taken
into account. 0
Illiiir77
..--1111111111101Millapillil
<7141111111
,av number G=0.8
0 G=0.66.2
SYSTEMATIC PROPELLER TESTSPropellers of the supercavitating type give less risk of erosion, do not induce large pressure amplitudes on the hull and demand only slightly more power than propellers of the conventional type.
Propellers with highly warped blade shape induce considerably lower pressure amplitudes on the hull than propellers with symmetrical blade shape.
The cavitation amplification of the pressure amplitudes in-duced on the hull was about 2 for "moderate" cavitation con-ditions and about 3.5 when complete thrust breakdown occurred.
The hull influence on thrust and efficiency implies a reduc-tion of up to 15% of the thrust coefficient and a reducreduc-tion of up to 10% of the efficiency.
The hull influence on the limit for thrust breakdown implies that too small blade area ratios are chosen when using limits obtained in unbounded flow.
o The hull influence on the propeller characteristics may be a blockage effect. The influence at supercavitating conditions can then be considerably larger than it appears to be when thrust correction is used.
6,3
INTERACTION ON A TRIPLE-SCREW SHIPThe hull influence on propeller characteristics seems to be remarkably constant at increasing clearances.
Good agreement between measurements of thrust and torque with plate arrangement and with complete ship model.
Propeller-propeller interaction implies further reduction of thrust and efficiency.
'Propeller-propeller influence is larger on wing propellers than on centre propeller and is larger at "moderate" advance ratios than at small advance ratios.
ACKNOWLEDGEMENTS
The author wishes to express his gratitude to the Naval Materiel Department of the Defence Materiel Administration of Sweden for sponsoring parts of the present investigation and to Dr Hans Edstrand, Director General of SSPA, for the opportunity to carry out the study. Thanks are also due to those members of the staff of SSPA who took part in the investigation. Without their urgent work this paper would not have been possible.
REFERENCES
Reed, A, Day, W: Wake Scale Effects on a Twin-Screw Displacement Ship. Proceedings of the 12th ONR Symposium on Naval Hydrodynamics, 1978
Rutgersson, 0: On the Importance of Rudder and Hull Influence at Cavitation Tests of High Speed Propellers. Proceedings of the High-Speed Surface Craft Conference, Brighton, UK, 1980
Taniguchi, K, Chiba, N: Investigation into the Propeller Cavitation in Oblique Flow. Experimental Tank Laboratory, Mitsubishi Shipbuilding & Engineering Co Ltd, Report No 1800, 1964
Hess, J L, Smith, A M 0: Calculation of Non-Lifting Potential Flow about Arbitrary Three-Dimensional Bodies. Douglas Aircraft Company Report
No E.S.40622, 1962
Blount, D L, Fox, D L: Small Craft Power Prediction. Marine Technology, 13(1976):1Gawn, R W L, Burril, L C: The Effect of Cavitation on the Performance of a Series of 16-Inch Model Propellers. Trans INA, Vol 99, 1957
Newton, R N, Rader, H P: Performance Data of Propellers for High-Speed Craft. Trans RINA, Vol 103, 1961
yenning, E, Haberman, W: Supercavitating Propeller Performance. Trans SNAME, Vol 70, 1962
Rutgersson, 0: Supercavitating Propeller Performance. Influence of Propeller Geometry and Interaction between Propeller, Rudder and Hull. Proceedings of the Joint Symposium on Design and Operation of Fluid Machinery, Fort Collins, USA, 1978. See also SSPA Publ No 82, 1979
Johnsson, C-A: Some Experiences from Excitation Tests in the SSPA Large Cavitation Tunnel. Symposium on Propeller Induced Ship Vibration, London 1979, Proceedings.
Lindgren, H, Bjarne, E: Studies of Propeller Cavitation Erosion. Proceedings of Conference on Cavitation, I Mech E, Edinburgh, 1974
Johnsson, C-A, Rutgersson, 0, et al: Vibration Excitation Forces from a Cavitating Propeller Model and Full Scale Tests on a High Speed Container Ship. Proceedings of the 11th ONR Symposium on Naval Hydrodynamics, London, 1976. See also SSPA Publ No 78, 1976
Weitendorf, E-A: Kavitationseinfllisse auf die vom Propeller induzierten Druchschwankungen. Institut flir Schiffbau der Universitat Hamburg, Bericht Nr 338, 1976
Johnsson, C-A: Pressure Fluctuations around a Marine Propeller. Results of Calculations and Comparison with Experiment. SSPA Publ No 69, 1971
(15] Suhrbier, K: An Experimental Investigation on the Pro-pulsive Effect of a Rudder in the Propeller Slipstream.
International Shipbuilding Progress, 21(1974):234, Feb
Rutgersson, 0: Propellers SSPA-FMV Series K131. Summary of Cavitation Properties for Propellers 3.105, 3.075 and
3.050. SSPA Report K131-24, 1974 (in Swedish)
Blount, D L, Fox, D L: Design Considerations for Propel-lers in a Cavitating Environment. Marine Technology
15(1978):2
Tulin, M P: Supercavitating Propeller Momentum Theory. Hydronautics Technical Report 121-4, 1964
van de Voorde, C B, Esveldt, J: Tunnel Tests on Super-cavitating Propellers. Proceedings of the 4th ONR Symposium on Naval Hydrodynamics, 1962
Morgan, W B: The Testing of Hydrofoils and Propellers for Fully-Cavitating or Ventilated Operation. Proceedings of the 11th International Towing Tank Conference, Tokyo, 1966
Johnsson, C-A: Comparison of Propeller Design Techniques. Proceedings of the 4th ONR Symposium on Naval Hydrodyna-mics, 1962. See also SSPA Publ No 52, 1963
Pien, P C: The Calculations of Marine Propellers Based on Lifting-Surface Theory. Journal of Ship Research 5(1961):2
Nelka, J: Experimental Evaluation of a Series of Skewed Propellers with Forward Rake. NSRDC Report 4113, 1974
APPENDIX
GEOMETRY OF TESTED PROPELLERS
0.9
3.1ki
0.8\
Conv.1.05 asA1111111111E1
o/
AIIIINIM
111101
111=111
aa/
/1111Mdd
IIW
0.L 03 02 01 0 01 02 ita OA 1.3 1A 15 16 0 001 002 x.0.9 .4l.IIIIIIMIWIMIII
'41MillMEMII.
'° '1111:111111=11.
0.5 0.4 0.3 02 0.1 0 0.1 0.2 0.3 0.4 0.5 15 1.7 1.9 2.1 0 0.01 If /D UDFig 25. Blade shape and distributions of pitch and camber for
propellers Cony 1.05, Cony 0.75 and Cony 0.50
lo/D P/D f/D
Fig 26. Blade shape and distributions of pitch and camber for
X=1D 5.C.0
/
,0 -...,,,,\ a0.98,
P. C. aso\
Q7\
0.6 1 0.5\\
\
\
\
/
iii
Q3\
.. # . I a 4 0.2 Q1 0 0.1 0.2 14 15 1.6 0 0.01 0.02 la/D If ID P/D YTOX DFig 27. Blade shape and distributions of pitch and face camber
for propellers S.C. 0.50 and P.C. 0.50
The propellers with NACA 16 profile shape have all been designed by the same procedure and with the same rather unloaded circula-tion distribucircula-tion. The lifting line calculacircula-tions were carried out according to a modified procedure for calculation of induc-tion factors described in [21]. The lifting surface correcinduc-tions were then calculated according to Pien's method [22] programmed
at SSPA. For propellers with symmetrical blades and no rake this design procedure gives very good estimates of pitch and camber.
The propeller warp 0.75 has the same chord lengths and circula-tion distribucircula-tion as propeller Cony 0.75, the blades, however, being skewed 120 degrees. Further the skew-induced rake has been compensated by raking the blades forward about 60 degrees. In
this way the clearance curve for propellers Cony 0.75 and Warp 0.75 are almost identical. When propeller Warp 0.75 was designed, however, very little was known about the effect of extreme for-ward rake on the propeller characteristics. The design was there-fore carried out by the same procedure as for unraked propellers. Later it has been shown that the effect of rake is to increase thrust and torque [23], which is not predicted by the design method. The propeller Warp 0.75 is consequently considerably overpitched at the inner radii.
The propellers S.C. 0.50 and P.C. 0.50 have been designed accord-ing to the procedure given in (8] with empirically derived pitch corrections according to [9]. The radial circulation distribution is optimum. Propeller S.C. 0.50 has wedge-shaped sections accord-ing to [9] with a 3-term face combined with a 2-term modified thickness distribution. Propeller P.C. 0.50 represents a first attempt to improve the performance of the supercavitating sec-tions at partially cavitating condisec-tions. In this case a 5-term face is combined with an empirically derived shape of the back.
In Fig 28 the free-stream propeller characteristics are plotted in the following way.
Propeller thrust and efficiency at cavitating conditions at the advance ratio JB used in the behind condition have been related to thrust and efficiency at non-cavitating conditions at the same advance ratio.
The reason for giving Fig 28 in this paper is to show the propor-tions between the influence of
cavitation (Fig 28)
propeller-hull interaction (Fig 17) shaft inclination (Fig 18)
propeller-propeller interaction (Fig 24)
on the propeller characteristics.
The influence of cavitation is obviously the most important effect of these. The other influences are more like second
order effects. Never the less it is important to take also
these second order effects into account when making predictions of prototype performance.
E 0.9 0.6 0.7 0.6 1. 0.9 0 2 0.9 T-08 Cavitation number a 10
Fig 28. Effect of cavitation on the free stream
propeller characteristics Conv. 1.05/ ../1/Y Cony. 0.75
