Development of Overlapping Propellers
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Development
of Overlapping
Propellers
Development
of Overlapping
Propellers＊
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Masaki AndaMasaki
Yasunori
Iwasaki
Kazuyuki
Ebira
Anda
**, Yasunori
Iwasaki***, Kazuyuki
Ebira
**
ABSTRACT
improving the propulsive performance.
The authors have developed an advanced propulsion system,
called the Overlapping Propellers System (OLP), which
utilizes bilge vortices to obtain high propulsive performance.
A pair of bilge vortices symmetric about the hull centerline
occurs around the propeller disk, which has an inward
rotational component.
OLP fully utilizes the rotational component of bilge vortices
because the center of each propeller is close to that of the
bilge vortex and the turning direction is outward, opposite to
the rotational direction of bilge vortices. In OLP, since the
center of each propeller is close to the hull centerline and
about half of each propeller closely overlaps, the added
resistance due to the shaft bracket and so on is negligibly
small. The hydrodynamic characteristics of OLP, in which
the propellers are in the vicinity of each other, are especially
important in designing a ship with OLP. The authors have
designed OLPs for various types of ships such as large LNG
carriers with various propeller inflow improving devices.
Energy savings by OLP were proven to be over 10 %,
compared with single screw ships.
From the standpoint of propeller design, it is difficult to
greatly improve the propulsive efficiency by increasing the
propeller diameter and or decreasing the propeller revolution
because it involves change the aft draft of ballast condition.
Under such circumstances, the authors have developed the
Kawasaki Overlapping Propeller System (Kawasaki OLP
System, Photo 1). This system takes advantage of the bilge
vortices to produce a highly efficient propulsive
performance.
This paper describes the Kawasaki OLP System, focusing
on its propulsive performance, cavitation behavior and
bearing force performance.
This paper describes the hydrodynamic characteristics of the
propulsive performance, the cavitation behavior, the
propeller excited hull pressure pulses and the shaft bearing
force variations of OLP.
1. INTRODUCTION
Growth in the world economy has led to growth in the world
marine transportation every year. In recent years, the social
demand for reduction of environmental impact has become
strong in the shipping field as well, and it is urgently
demanded to reduce the ship's fuel consumption by
*
Received February16, 2011
** Kawasaki Heavy Industries, LTD. ,Kobe, Japan
*** Akashi Ship Model Basin, Akashi, Japan
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2. Feature of Kawasaki Overlapping Propeller System
The OLP System consists of fore and aft two propellers;
their blades overlapping near the hull centerline to increase
propulsive performance.
2.1 Characteristics of Conventional Ship Design
On typical merchant ships, the pair of symmetrical bilge
vortices that rotate inward arise on both sides of the hull
centerline, inflowing to the propeller. To increase propeller
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efficiency, propellers should be rotated in the direction
opposite the rotational direction of the bilge vortices.
As shown in Fig.1, single-propeller vessels do not take full
advantage of bilge vortices, since the direction of bilge
vortex differs from the propeller rotation direction on either
side of the hull centerline.
In conventional twin-propeller vessels, the load of each
propeller is half of that for a comparable single-propeller
vessel, resulting in higher propeller efficiency than
single-propeller vessels. With conventional twin-propeller
configurations, either inward or outward propeller rotation is
selected to improve propulsion efficiency, since the
propeller centerlines lie some distance from the bilge vortex
centerlines. This condition prevents the use of the rotational
component of the bilge vortices.
In addition to the differences described above, conventional
twin-propeller vessels also require propeller support
components in the form of shaft brackets and bossing. The
components produce the added resistance and increase the
water resistance of the vessels.
centerline, rendering the additional resistance of the
propeller supports almost negligible as compared to
conventional twin-propeller vessels.
The OLP System ensures that the two propellers are situated
at the same position longitudinally to minimize lateral
dynamic unbalances. The propeller blades are also raked
(fore propeller raked forward, aft propeller raked back) to
minimize the distance between them.
The overlapping arrangement of the propellers in the OLP
System results in a complex wake flow environment. In
particular, the aft propeller is partially subject to the
periodically varying wake from the fore propeller in the
region where the two propellers overlap – a factor that also
increases the importance of flow studies involving the
propeller exciting force.
Subsequent chapters outline the design of the hull, supports,
and propellers for the OLP System, describing the respective
hydrodynamic characteristics (propulsive performance,
cavitation behavior and bearing force performance).
3. Kawasaki Overlapping System Design
2.2 Feature of Kawasaki OLP System
As shown in Fig.2, the OLP System ensures that the
centerlines of both propellers coincide with the approximate
centerlines of the bilge vortices, with propellers rotating in
the direction opposite to that of the vortices. This action
makes it possible to take full advantage of the bilge vortex
rotational components. Moreover, arranging the propellers
so that their centerlines coincide with the approximate
centerlines of the bilge vortices means the propellers overlap
by nearly half their diameters. In addition the shaft supports
are located in the low flow speed region close to the hull
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3.1 Hull Configuration
The primary characteristic of the OLP System is the
arrangement of two propellers in an overlapping
configuration. This configuration makes it possible to reduce
the length of the propeller shaft between the stern tube and
the propellers, eliminating the need for the bulky supports
(i.e., shaft brackets and bossing) used on conventional
twin-propeller vessels. It also eliminates the need for the
complex hull design seen with twin-skeg vessels.
Apart from the stern boss, the hull configuration used with
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the Kawasaki OLP System is identical to a conventional
single-propeller hull. The short distance of the plane
between the propeller shafts and the hull allows the use of a
horizontal bracket fin, a very simple form.
The OLP System implies the risk to generate harmful
cavitation, since the propellers generate high thrust when
they cross the slow flow region close to the shaft centerline
height of the hull centerline. For this reason, the water line
form of the hull was made as sharp as possible near the
plane of the propeller shafts to increase the flow speed near
the hull centerline.
3.2 Bracket Fin (BF)
As shown in Photo 2, the bracket fin (BF) is located in the
space between the propeller shaft and the hull. The BF can
be tilted up or down longitudinally to control the propeller
inflow.
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the wake field generated by the fore propeller, and axial
flow speed fluctuations are greatly increased near the hull
centerline above the propeller shaft when the fore propeller
is operating, as shown in Fig.3. This phenomenon may
result in increasing fluctuating pressures due to propeller
cavitation and increasing bearing forces.
To resolve this issue, a wake-improved fin (WIF) was added
at the stern hull part above the propeller shaft, as shown in
Photo 3, to moderate speed fluctuations in the wake. Fig. 4
shows the wake distribution measured with the WIF.
Compared to the wake distribution generated in Fig. 3 by the
fore propeller but without a WIF, the configuration with the
WIF installed features lower speed fluctuations near the hull
centerline and above the propeller shaft. The WIF is
expected to help smaller bearing forces and smaller
fluctuating pressures generated by propeller cavitation.
The wake distribution without propellers (Fig. 2) shows the
rotational flow in the same direction as the propellers of the
OLP System below the propeller shafts and close to the hull
centerline. However, the wake distribution with the fore
propeller operating (Fig. 3) shows faster axial flow close to
the hull centerline, due to it being affected by the fore
propeller. Both factors reduce the incident flow angle on the
propeller in the region in which the propellers overlap,
which results in generating a larger bearing force than that
for single-propeller configurations. To resolve this issue, the
BF was swept downward, as shown in Photo 2, increasing
the rotational flow component in the direction opposite
propeller rotation of the OLP System near the hull
centerline.
3.3 Wake Improved Fin (WIF)
The aft propeller in the OLP System is partially subject to
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ship model.
Moreover, to investigate the influence of the fact on the
cavitation behavior, the propeller induced hull pressure
pulses as well as shaft bearing force, these tests were carried
out in HYCAT, the large Hydrodynamics and cavitation
tunnel of Hamburg Ship Model Basin (HSVA) in Germany,
using a 8.3m long ship model and 250 mm diameter model
propeller.
The ship model was equipped with the OLP System,
essential to model the complexity of the flow conditions
properly. Various configurations comprising of different
stern shapes and bracket fins were tested. Tests with and
without the above mentioned WIF were carried out as well
as variations of the phase angle between fore and aft
propeller.
3.4 Propeller Design
It is important to obtain wake distributions when designing
propellers, since this data makes it possible to estimate
cavitation and bearing forces. Particularly with the OLP
System, the action of the fore propeller means that the aft
propeller operates under extremely complex flow conditions
to which the periodically fluctuating flows from the fore
propeller operation.
The normal wake distribution without the use of propellers
was used to design the fore propeller. Wake distributions
with the fore propeller operating and affecting the aft
propeller were used to design the aft propeller.
While cavitation behavior and bearing forces must
ultimately be confirmed by tank tests for propellers rotating
in a flow environment as complex as that for the OLP
System, wake distributions calculated from previous
empirical measurements were applied at the design stage
and incorporated into the theoretical studies of propeller
performance.
4.1 Propulsive Performance
Fig.5 shows the results of resistance and self-propulsion tests.
These results indicate dramatic power savings (12 to 13%)
with the OLP System over a single-propeller vessel. While
most of this is attributed to propeller efficiency, the results
also indicate reductions in viscous drag attributable to the
effects of the Bracket Fin.
The single-propeller vessel was equipped with Kawasaki
Rudder Bulb System with Fins (RBS-F) energy-saving
devices, which reduce energy consumption by
approximately 4%. The OLP System achieves power
savings of 16 to 17% compared to vessels not fitted
withenergy-saving devices.
Six-bladed propellers with a skew angle of 24 degrees were
ultimately selected for use on the large LNG carrier fitted
with the OLP System. Differing pitches were used for fore
and aft propellers to ensure equal thrust, since the wake
distributions differ for the two propellers.
4.4 Model Tests and Results
Resistance and self-propulsion tests of the large LNG carrier
equipped with the OLP System were carried out at the
Akashi Ship Model Basin (ASMB), using a 7.64 m long
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4.2 Cavitation Behavior
Propeller cavitation was expected to be a critical issue with
the OLP System, since the aft propeller encountered the
cavitating tip vortex structure of the fore propeller. The
different phase relations between the propellers were
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investigated to deal with this problem. As shown in Fig. 7, a
very smooth cavitation behavior was finally achieved, which
is comparable to a single screw arrangement with respect to
aggressiveness of the cavitation phenomena as well as
cavitation extent (Fig. 6). With optimized phase relation the
fore propeller vortex structure did not negatively affect the
aft propeller cavitation behavior. It is evident that this kind
of investigation would be impossible in a conventional
cavitation tunnel, where the nonuniform propeller inflow is
generated by a wire screen or a dummy model.
4.3 Propeller Induced Hull Pressure Pulses
With the optimized OLP configuration the peak fluctuating
pressure measured on stern surface was approximately 30%
of that for single-propeller vessels for both the 1st blade
frequency in Fig. 8 and the 2nd blade frequency in Fig. 9
reflecting the fact that cavitation with the OLP System is
similar to that seen in single-propeller vessels, while the
thrust per propeller is approximately halved. Fluctuation
pressures on the stern surface should present no issues for
installing the OLP System on large LNG carriers.
fluctuations in the highly variable wake from the fore
propeller, which affects the aft propeller, together with the
significant advantages provided by the skewed propellers.
Comparison tests were carried out to confirm the effect ofthe
Wake Improved Fin. As a result, installing the Wake
Improved Fin approximately cuts in half the moment
component of the bearing forces.
4.4
4.3 Bearing Forces
Fig. 10 shows the measured bearing forces. These results
indicate that the force components imposed on each
propeller of OLP System are similar to those seen with
single-propeller installations.
Despite operating in a more complex flow environment, the
OLP System vessels show lower moment components than
do single-propeller vessels. This fact is attributable to the
effects of the Wake Improved Fin in moderating flow rate
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5.5 Conclusions
1) Ideal for large LNG carriers, the Kawasaki Overlapping
Propeller System was developed as a high-efficiency
propulsion system that takes advantage of bilge vortices.
2) Resistance and self-propulsion tests confirmed dramatic
power savings of 12 to 13% with the Kawasaki OLP System
compared to single-propeller vessels.
3) Cavitation tests confirmed extremely low fluctuating
pressures for the OLP System – approximately 30% of that
for single-propeller vessels – although the OLP System
operates in a complex flow environment.
4) Bearing force measurements confirmed a force
component comparable to that for single-propeller vessels,
as well as significantly smaller moment component.
5) The results of these propeller vibratory force
measurements are believed to be attributable to the
combination of the propeller, Bracket Fin, Wake Improved
Fin, and stern design developed during this research.
6) The nonstationary and complex flow conditions
especially encountered by the aft propeller required highly
sophisticated cavitation testing capabilities, comprised of
tests with the whole ship model with its OLP arrangement in
the test section.
This study confirmed the Kawasaki OLP System as a
propulsion system capable of resolving hydrodynamic
problems and increasing the propulsive efficiency of LNG
carriers. As an additional note, we hereby state that a patent
has been granted for this OLP system.
6.6 Acknowledgment
We would like to express our sincere gratitude and
appreciation to Dr. S. Yamasaki of NAKASHIMA Propeller
CO., LTD. for the invaluable advice of the propeller design.
7.7 References
1) J. Friesch, “Ten years of Research in the Hydrodynamics
and Cavitation Tunnel HYKAT of HSVA”, 50 International
Conference on Propeller Cavitation, April 2000.
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