TEN years ago at a symposium in Gothenburg
on propeller technique,Mr. C-A Johnsson from
the Swedish State Shipbuilding Experimental Tank (SSPA), among others, pressed for an investigation into the pos-sibilities of using contra-rotating propellers for merchant ship propulsionw. The ideawas not new. As long ago
as 1824 Perkins in England had put forward the idea.
and in 1836 John Ericsson had produced an actual con-struction. In spite of this no modern merchant ship has yet been provided with this type of propulsion. The idea is, however, well-tried in the field of torpedoes
and it
should also be mentioned that the American submarine "Albacore" was, in one design, equipped with contra-rotating propellers.Unfortunately, Johnsson's viewpoints did not win any
support when they were put forward, as the mechanical difficulties were judged to be too great. It was not until 1965 that experimental work could be put in hand seriously
at SSPA after receipt of a grant from the Malmfonden
the Swedish Ore Foundation for General ScientificResearch
and Industrial Development. During this time work had started in other countries and gradually reports began to come in of large research programmes in this field from, among others, U.S.A., England, Holland and Japan..
Thanks to earlier naval activity, mainly concerned with torpedo development, SSPA had a flying start. Experience gained in the development of computer programmes and experimental equipment for their cavitation tank could be
put tc use, and some programmes could be used with only small additions and modifications. It should be pointed out, though, that the theories are particularly complicated and that many assumptions still remain to be verified experi-meritally. Similarly, further refinement of the theories
in-volved would be justified.
Vessel development in the last 10-15 years is illustrated in the accompanying table, Fig. 1. Tanker development has
been characterised chiefly by increase in size. The block
co-efficient has increased radically while the speed has essen-tially remained unaltered. The dry cargo ships' development
has, on the contrary, been characterised chiefly by a
considerable increase in speed. Sizes have not increased
Shipping World & Shipbuilder 1885
Lal).
V.----4 ;Hydrodynamic Aspects
of Contra-Rotating
Propellers
By Hans Lindgren*
Fig. 1 Ship development over past 10-15 years.
appreciably, and the block coefficient has tended to drop. For both types of vessels, therefore, the power requirements
have increased several times. From 1966, both these types
of ships have been included in SSPA's investigation into contra-rotating propulsion.
What these developments have implied on the propeller side is shown by Fig. 2, which illustrates propeller loading and cavitation number, 0, for various different projects. A high o--value and a low propeller loading mean little
cavitation risk. The cavitation-free zone (homogenous flow)
lies in the upper left side of the diagram. Increasing size for tankers, as well as increasing speed for cargo boats, means increased risk of cavitation. Dotted lines connect those alternatives which are included in the above-mentioned investigation into contra-rotating propellers. Contra-rotating propellers seems to be a more favourable alternative from the cavitation viewpoint.
*Chief Engineer, Swedish State Shipbuilding Experimental Tank
1955 DANAL AND 1966 1500001dw tanker project 1953 BONANZA 1966 12 000 tdw container-ship project Dve tons 118000 .150000j 10 000 12000 Displacement /7 m3 23 000 180400 14 940 19090 Length Lpp m 160 282.5 141,7 156,2 Breadth El m 21,3 44.6 19,4 23,5 Draught om 9,2 17,3 8.2 8,8 Block coefficient 4 PP 10,75 0,831 0,66 0,591 Length! Breadth Lpp/ D 7,5 6,3 7,3 6,6 Breadth/Draught B/D 2,3 2b 2,4 2,7 Trials speed V knots 15,6 . 167 J17,7 23,01
Shaft horsepower PD HP 6570 29200 31100 7950 25670
Speed N rpm III 91 113 112 128 .
Propeller clam. D m 58 8,3 7,5 5.5 6,0
Cavitation number a 8.5 18,1 19,9 7.3 3.6
Propeller Arrangements The Conventional System
The nearest alternative to the conventional high-speed diesel drive is a Single low-speed propeller or a conventional twin-screw arrangement. In both cases the surface loading on the propeller is reduced and there is less risk of cavitation problems. 'Both alternatives are accompanied by increased open propeller efficiency, no, but as a result of decreased wake and therefore hull efficiency the twin-screw arrange-ment does necessarily give a better overall propulsive
efficiency.
Cavitation number
0:121-9.. v2
lOr 2 A
1886 Shipping World & Shipbuilder
Cavitation - tree none.
Cargo carrier m10000 tdw 15 knots 15 o C oritturte Contra-rot. 92 rpm ___-to
Onset of tip vortices
SS LOR 76 rpm Contra-rot. St. 14 Propeller loading.VRT dw i9000 tdw J
Fig. 2. Cavitation curves.
91 rpm
Single -rot.
Onset of
bubb-,... les
512000 Idw) _ Onset of PS
ngle roL140 rem
-Onset of $S OAR 150000tdw --°113 rpm Singte-rot. 150 000 tdw Tanker 115rpm Onset of SS 0.755
Fig. 3. Contra-rotating propellers for container ship on test in
the SSPA tank.
.0,7
Contra-rotating propellers, SSPA's experimprls American
--0 from von Manen
----Single propeller, SSPA 5,60
NOVEMBER 1967
0.3 i ,
1 0.5 1,5 20 2,5 3,0
ITT'
Propeller loading j4
Fig. 4. Propeller efficiencies according to test results by different associations.
Different alternative arrangements for placing two pro-pellers in the wake behind the vessel have been suggested.
For example, with two propellers over each other,
asmentioned by van Mame) or tandem propellers, i.e., two propellers mounted one behind the other on the same shaft. Grim's suggestion
of a free-running
turbine propeller("Propeller und Leitrad") is very interesting(5.6). On the same shaft behind a conventional high-speed propeller a free-running propeller of large diameter is mounted. The inner portion of this operates as a turbine and is driven by the main propeller's slipstream. The outer part acts as a low-speed propeller of large diameter and contributes to the forward thrust.
All these arrangements have the advantage that the wake behind the vessel can be effectively utilised. The
contra-rotating alternatives have the extra advantage that the
rotation in the propeller wake is eliminated. The normal contra-rotating alternative with two propellers on concentric
shafts would seem to be the most realistic, Fig. 3.
Information available from experiments with contra-rotating propellers is sparse. Propeller efficiencies, as
measured in American, Dutch and Swedish experiments have been collected in Fig. 4. For comparison, the results from SSPA's standard propeller family have also been
included.
The Dutch propeller testso) show somewhat better results
than those obtained by the
Swedish. This result can probably be attributed to the fact that the Dutch experi-ments used three-bladed propellers while the Swedish ex-periments used four-bladed propellers for the cargo ship and a special four-front/five-rear bladed propeller for thetanker. The agreement between the American and the
Swedish results for the cargo ship is good.
Ducted Propellers
Many other alternative propeller arrangements have been discussed. One of the most promising of these is the ducted propeller (propeller in nozzle), which can offer advantages in particular for heavily loaded propellers for large tankers. This design has been in used for a long time for tugs. Both theoretical and experimental development work for this type of propeller are being carried out at present by SSPA(7).
15
to
0
2- 0
1888 Shipping World & Shipbuilder Fundamental Problems and Criteria
When comparing different types of propellers there are three separate hydrodynamic properties which must be borne in mind; namely efficiency, cavitation and vibration. As there are no stringent criteria concerning acceptable cavitation and vibration levels from propellers, it is im-possible to make a completely fair comparison. Below are given some aspects of these problems with special reference to the results obtained in the Swedish experiments.
As known, propulsive efficiency is expressed by the relationship
EHP = SHP
The following terms are also normally accepted: = open propeller efficiency
= relative rotative efficiency = hull efficiency = (1
t)/(1 - w)
where
t = thrust deduction factor w = wake fraction
Thus: t) = nE nit
The propeller loading can be expressed by the dimension-less term: KT = thrust coefficient J = advance ratio KT Tn2 T = thrust
P
pV
p = mass density n = rate of revolution I VA = speed of advancewhich is independent of propeller diameter.
0.8r
flo as SSPA 50(opt.-5%)
5 2P
Propeller loading 4 KT
J4
Fig. S. Comparison of different propulsive arrangements for a tanker of 150,000 d.w.t.
The open water propeller efficiency increases with reduc-ing loadreduc-ing, Fig. 5. Upon changreduc-ing to a low-speed sreduc-ingle propeller, KT/J' drops with n and >10 increases. As /pi is practically the same in both cases and nn somewhat lower for the low-speed alternative, it is not possible to make use of the entire gain in no. However, the low-speed propeller does give a considerably higher propulsive efficiency than the high speed one.
A transfer to conventional twin-screw drive means a radical drop in KT/P. This depends partly on the driving force, T, being divided between two propellers and partly
c 0,7 74.; 065 UI 0,60 0,75 0,65 0,60 NOvEMBER 1967 o Single propeller Contra-rotating prop.
Fig. 6. Propulsive efficiency for different propeller speeds for a 150,000-d.w.t. tanker.
on VA increasing through a reduction in the wake factor w. Simultaneously, the hull efficiency Ix drops with w and the result is that the propulsive efficiency is no better than that for the high-speed single-screw alternative, Fig. 5.
If a contra-rotating low-speed propeller is considered, the propeller loading becomes lower than for the single-screw alternative. Because of interference between the propellers v becomes lower than for a single propeller with the same loading, KT/J.4. As nit is relatively high, the product no na
becomes about the same for contra-rotating as
for asingle-rotating low-speed propeller. The hull efficiency in
is normally greater for contra-rotating propellers than for a single-rotating propeller of the same rev/min, chiefly depending on the fact that the propeller diameter is less. The overall result is thus that the propulsive efficiency is normally higher for contra-rotating propellers. Fig. 5.
The reasoning above is schematic and simplified. The whole principle of dividing up the propulsive efficiency into components is open to discussion. A summing-up of the results obtained for the tanker and container ship project (Table, Fig.
1), shows that for both types of vessels a
radical improvement in propulsive efficiency has been demonstrated when going over to contra-rotating propellers, Figs. 6 and 7.
In the case of the tanker it ought to be possible to take the propeller speed down to about 80 rev/min without the propeller diameter becoming unreasonable. For the con-tainer vessel, however, the limit lies at about 140 rev/min for the speed concerned. Any further drop of propeller
Contra-rotating propellers.
Constant (max) Curve for optimum
propeller diem. propeller diem.
50
m121.
Single propeller
100 150
rpm
Fig. 7. Propulsive efficiency for different propeller speeds for container ship project.
80 100 110 120
Cavitation number, cr 30 Pressure side cavitation. IN;Skl% vortex. Bubble coy. Thrust coeff. v2 A
Fig. 8. Limiting curves for the onset of cavitation.
.revolutions would therefore produce a worse result, Fig. 7.
However, care must be exercised not to draw too
far-reaching conclusions from these results: it might, for example, be possible to obtain a more favourable single-screw alternative for the comparison. In the case of the container ship a propeller was used with adjustable blades. Furthermore, the whole ship was unconventionally designed, which among other things meant that efficiencieswere low.
Cavitation
The cavitation problem has come increasingly to the fore with the increases in propeller loading and vessel speed. For single-screw designs various practices have grown up and cavitation criteria have been developed on the basis of both propeller erosion damage and other undesirable effects on full-scale vessels, as well as on cavitation data obtained
from model tests. Even if these criteria are, in some cases,
doubtful, there is much more knowledge available than for contra-rotating propellers. For these latter there are only model tests and theoretical anaylses at present. In general, the outlook ought to be better with contra-rotating propel-lers, because the propulsion power is distributed over more blades, and these blades, for various reasons, cannot be made too narrow. The situation is shown in principle in Fig. 8. The diagram shows the limiting curves for incipient cavitation, which have been found by model tests on tanker propellers in homogeneous flow. Even if the situation is changed somewhat in irregular flow, the mutual relationship still applies. Data for the propellers tested is shown in the table below:
No. of Blade
blades area ratio
Single-rotating propeller, high speed Single-rotating propeller, low speed Single-rotating propeller, forward
Contra-rotating propeller, aft
Diameter
7.5 m 5 0.60
8.3m 5 0.60
7.9 m 4 0.47
7.3m 5 0.47
Considering the contra-rotating propeller's superior cavi-tation performance, thought can be given to the possibility
of reducing the total blade area and thus the friction losses.
This could be done, for instance, by reducing the number
of blades and by using a combination of four on the
forward and three on the aft-propeller.
As the propellers operate in the irregular flow behind the vessel, it cannot be avoided that cavitation occurs when the blades pass the stem-post. A problem which can have special significance for the contra-rotating arrangement is that the cavitation vortices generated by the forward pro-peller may impinge on the aft propro-peller and cause damage there. On many vessels considerable damage has been found
on the rudder where it
has been subjectedto the tip
vortices from the propeller. The problem ought to be avoid-able either by reducing the loading on the blade tips of the forward propeller and thereby reducing the intensity of the vortices, or by reducing the diameter of the aft pro-peller. Although both alternatives would reduce the effici-ency, the second choice seems to be more realistic.Experiments in successively trimming off. the tips of the aft propeller have been carried out inone instance by SSPA, Fig. 9. This investigation showed that the forward propeller's
eddy system remained essentially unchanged irrespective of * to page 1919
Fig. 9. Contra-rotating
propellers for a
con-tainer ship in the
SSPA tank. (Left)
original propellers (right)
after propeller trimmed.
NOVEMBER 1967 Shipping World & Shipbuilder
1889
Contra-rotating propeller, low-speed.
---- Single propeller, low-speed.
SI , high- speed.
20
10
GM lo Kate KG Lpp N4. Ro SAO S(w) Sk(E0)
T.
TaniNOVEMBER 1967 Shipping World & Shipbuilder
1919 27k,
=
V g.GM List of Symbols Breadth of ship Depth of shipShip's centre of gravity Metacentric height
Virtual mass moment of inertia with respect to the rolling axis
General moment producing roll
Amplitude of rolling moment produced by an oscillator
Rolling moment due to waves
Height of ship's centre of gravity above the
keel
Length between perpendiculars Damping coefficient against rolling
Probability that exceeds a given value 0.
Restoring coefficient
Spectral density of rolling motion Spectral density of wave amplitude Spectral density of wave slope Draught of ship
Maximum draught of ship Natural rolling period Wind speed
Breadth of
free surface tank measuredathwartships
Probability distribution Acceleration of gravity
Height of connecting duct of U-tank Water depth in free surface tank at rest Wave number
Significant wave slope
Transverse radius of gyration Variance of the function 0(0
* from page 1889
the trimming. Further experimental data on cavitation, with special reference to tip vortices, are required.
When it is considered that the loading is distributed over considerably more blades with a pair of contra-rotating propellers than with a conventional propeller, it
is to be
expected that the vibration level should decrease. By choos-ing a different number of blades for the forward- and aft-propellers not more than two blades can pass each other
simultaneously.
The influence of the number of blades on the pressure
impulses induced on the hull by the propeller has been investigated for a conventional single-screw design, Fig. 10.
An increase in the number of blades from four to six
reduces the amplitude to less than one-half. The investi-gation was carried out during self-propulsion tests in the towing tank, which means that the effect of cavitation was
ignored.
The work summarised above covers only random
investi-gations and has chiefly been involved in efficiency
considera-tions. Before final conclusions can be drawn it is essential that more systematic investigation be carried out. The in-fluence of various factors, such as:
optimum radial circulation distribution of the
propellers;
optimum blade number for the fore and aft
propellers;
blade area ratio of the propellers; diameter relation between the propellers; speed and torque relation between the propellers; ought to be examined not only with regard to their effects on efficiency but also on cavitation and vibration.
30
121
0 No
Distance between the centre of gravity of the ship and the bottom of the tank
Length of connecting duct of the U-tank Width at bottom of a reservoir of the U-tank Height of the reservoirs of the U-tank Average fluid depth in the reservoir of the
U-tank
Model scale
Maximum wave slope at the surface
Block coefficient
Weight of displacement Volume of displacement
Maximum volume of displacement
Phase angle between the wave moment and the rolling motion
Phase angle between the tank moment and the rolling motion
Wave amplitude
Significant wave amplitude
Wave length
Non-dimensional roll damping coefficient
Frequency dependent part of vo
Mass density
Roll angle Roll amplitude
Significant roll amplitude
Limiting roll amplitude Static angle of keel
Angle between bottom of U-tank and a
plane through the middle of the fluid
sur-faces in both reservoirs
(1)Circular frequency
=
V g.GM Natural roll frequency of ship"
kowt = 7r/b V gh Theoretical natural frequency of water
transfer in the free surface tank
4blades
-.5 blades
6blades
Cl 131 Al
Measuring point
Fig. 10. Pressure impulses measured in the stern plane for
different numbers of blades.
References
JOHNSSON. C.-A.: "Motroterande propellrar", SSPA General Report No. 9, Goteborg, 1965.
WILLIAMS, Ake: "Jimforande undersokningar betraffande en- och tva-propellerdrift for handelsfartyg", SSPA General Report No. 21, 1967.
VAN MANEN, J. D., ICAmPs, J.: "The Effect of Shape of
Afterbody on Propulsion", Trans. Soc. of Nay. Arch. and
Marine Eng., New York, 1959.
NADLER. J. B., Moanniv, W. B., MEYERS, K. A.: "Advanced Propeller Propulsion for High-Powered Single-Screw Ships",
Trans. Soc. of Nay. Arch. and Marine Eng., New York,
1964.
GRIM, 0.: "Propeller und Leitrad", Inst. filr Schifftau der
Universitat Hamburg, Bericht Nr. 164, 1966.
Gam, 0.:
"Propeller und Leitrad, weitere Ergebnisse", Schniffstechnik, Heft 70, Hamburg, 1967, sid. 28.DYNE, G.: "Dyspropellrar", SSPA Report No. 16, 1966.
Ca 7.1 .= 20 a. 77. 110
