Note: Descriptions are shown in the official language in which they were submitted.
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PROPULSION SYSTEM AND METHOD
This invention relates to vessel propulsion arrangements,
and, in particular but not exclusively, to propulsion
systems intended for operation in ice-covered waters and/or
in ice conditions.
Conventionally the movement of a vessel, such as a ship or a
ferry, has been provided by a propeller attached to a drive
shaft. The drive shaft is rotated by a drive apparatus
positioned within the hull of the vessel, and the drive
shaft is then lead through the hull such that the propeller
extends to the water. The vessels are maneuvered by separate
steering gears, such as by rudder gears.
At present much attention is being paid to the application
of so called azimuth thruster units or azimuthing propulsion
units which provide both the vessel propulsion and also the
maneuvering. These atzimuthing propulsion units are gaining
increasing popularity, and they are applied for many type of
vessels, as they have proven to provide many benefits when
compared to conventional solutions. They have proven to be
especially advantageous when using the vessels in ice
conditions.
One widely known azimuth thruster unit for ship propulsion
and maneuvering in ice is offered by ABB Azipod Oy, the
tradename for these being Azipod. These azimuthing units
operate in a pulling mode and consist of a streamlined strut
and a torpedo-shaped pod containing drive elements and a
propeller shaft with a screw propeller mounted on the
overhanging part of the shaft (for more details, see e.g.
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Azipod, Project Guide, Sept. 1995 or FI patent No. 76977 in
the name of ABB Azipod Oy).
A shortcoming of the azimuthing unit of the above type is
that the screw propeller is not protected against possible
damages caused by the ice while the propulsive efficiency of
the fixed-pitch propeller is not sufficient in all
conditions.
A Norwegian company Upland Offshore provides azimuth
thruster unit which operate in a pulling mode and consist of
a streamlined strut and a torpedo-shaped pod containing
drive elements and a propeller shaft with a controllable-
pitch ducted propeller mounted on the overhanging part of
the shaft (for more details, see e.g. Brochures on the
Fennica and Nordica Icebreakers published by Upland
Offshore, Norway).
The drawback of the above unit is also that the propeller
blades are unprotected against the destructive effect of
ice. The performance of a vessel operating in heavy ice is
also unsatisfactory as it is not advantageous to use a
nozzle arrangement surrounding the propeller owing to the
tendency of the nozzle inlet to clog with ice blocks which
are drawn in to the nozzle by the propeller. This results in
a sharp reduction of propeller thrust and an increase in
hull vibration. In case of clogging the ship often comes to
a standstill state which, among other disadvantages affects
of stopping the ship, increases the danger of collision with
the following ship moving in the convoy. If the ice is
seized between the blades and the nozzle when the ship is
moving through hammocky ice, the removal of this by
reversing the propeller has proven to be difficult and in
many instances impossible.
One known improvement is an azimuth thruster for ship
propulsion and maneuvering in ice conditions, which has a
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streamlined strut and a torpedo-shaped pod containing drive
elements and propeller shaft with the ducted propeller and
particular ice-breaking elements mounted on the overhanging
part of the shaft, thus making it possible to break and
crush the ice before entering into the nozzle (see Finnish
patent No. 91513 A, int. class B63H 5/16).
The drawback of such a unit is that the nozzle inlet is
still unprotected against clogging with ice fragments. It is
also impossible to throw ice fragments away from the nozzle
owing to the relatively small size of the ice-breaking
elements when compared to the propeller, and thus to the
nozzle, diameter. The unit disclosed by the FI patent 91513
is intended for breaking (crushing) of ice and admitting it
through the nozzle, but this operation can be accomplished
only for a substantially thin ice in conditions in which
comparatively small propellers are used, for instance in
propulsive systems used in harbor icebreakers. In heavy ice
conditions, such as in the Arctic, this unit is ineffective
and unable to throw the larger size ice fragments away from
the nozzle, while the smaller size fragments entrained into
the nozzle deteriorate the propeller performance.
The general problem lies on the fact that the prior art
proposals have not been able to satisfactorily to solve the
problem caused by iced conditions. What is needed is a
solution for propulsion units which improves the
characteristics of a vessel moving in iced conditions.
An object of the invention is to provide an improvement to a
performance and characteristics of a vessel used in ice
conditions by providing a reliable protection of nozzle
inlet against clogging of the same with ice fragments and by
raising the effectiveness of propulsion in general in ice
conditions. A further object is to provide a corresponding
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improvement for vessels using azimuthing propulsion units or
thrusters in heavy ice conditions.
This object is attained..by specially designed propulsion
system comprising ice-breaking elements which are in form of
rotatable blades or vanes and attached to a portion of the
drive shaft projecting outside the water inlet of a nozzle
for breaking and/or crushing ice before the ice enters into
the nozzle are designed. The design is such that the point
of maximum diameter of the blades or vanes is having an
axial distance from the plane of the water inlet which is
0.02 to 0.25 times the diameter of the propeller and the
rotatable blades or vanes are having a diameter which is 0.6
to 0.8 times the diameter of the propeller. The inventive
method utilizes the above design.
According to a preferred solution the blades or vanes are
uniformly placed in a circle on the plane perpendicular to
the propeller shaft. According to a further embodiment the
propulsion unit is formed by an azimuthing propulsion unit.
In the following the present invention and the objects and
advantages thereof will be described by way of an example
with reference to the annexed drawings.
For a better understanding of the present invention and in
order to show how the same may be carried into effect
reference will now be made, by way of example, to the
accompanying drawings, in which:
Figure 1 shows, partially in section, an azimuth thruster
with ice-breaking elements,
Figure 2 shows the results obtained from model tests of the
propulsive unit fitted with ice-breaking elements.
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The azimuth thruster disclosed by Fig. 1 comprises a
streamlined strut or support 1 rotatably mounted relative to
the hull of the vessel. A torpedo-shaped pod 2 is attached
to the strut 1 and contains drive elements (not shown in the
figure). A propeller drive shaft 3 is connected to the drive
elements, and project outside from the pod 2. A screw
propeller 4 is mounted on the overhanging part of the shaft
3 and inside a nozzle 5. The nozzle 5 is a hollow, tube like
element (the nozzle is sectioned in figure 1) attached to
the pod 2 by means of support arms or mounting brackets 7
and has an inlet 10 for the infiowing water and
correspondingly an outlet for the outflowing water. The
azimuth thruster as a whole is usually fitted in the rear
end 8 of a vessel, but the thruster may also be fitted
otherwise, such as in the forward end of the vessel. The
skilled person is familiar with the above described basic
members of an azimuthing propulsion system provided with a
nozzle and the possible modifications and variations thereof
as well, and these are thus not explained in more detail
herein.
According to the present invention the ice-breaking elements
6 are in the form of blades or vanes which are fitted on the
propeller shaft 3 fore of the screw propeller and the nozzle
inlet 10 at a distance of O = 0.02-0.25 Dp, where Dp is the
diameter of the propeller 4. The blades or vanes 6 are
robustly constructed, i.e. they are made more solid than it
is actually necessary for guiding the flow of water, so that
they can effectively fulfill also the other basic functions
thereof, namely breaking and/or throwing away the ice in
front of the nozzle inlet.
The inventors discovered that the diameter of the ice-
breaking blades and vanes has to be chosen so that they can
effectively perform their basic functions: throwing away and
breaking/crushing of ice and formation of flow before the
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nozzle. For this purpose the blade diameter must be 1.5-2
times larger than that of the propeller hub 9. The upper
limit of the blade (vane) diameter is, in turn, dictated by
the need to avoid much heavier ice loads on the propeller
shaft than what is the case when using an open screw
propeller (i.e. no nozzle). In the course of thruster
operation the blades (vanes) will have to frequently mill
the ice. In this case, ice anti-torque moment will be
proportional to the blade diameter to the power 2-2.5 (see
e.g. 5"' Lips Propeller Symposium, Drunen, the Netherlands,
19-20 May, 1983). Therefore, the selection of the size of
the ice-breaking elements was considered to be a subject for
study which should be conducted by taking into account both
characteristics of the propulsion unit and the ship aft
lines, and, further, ice navigation conditions. The
inventors found that by selecting a blade (vane) diameter
(at the maximum diameter point) which is within the range of
0.6-0.8 times the propeller diameter optimal properties can
achieved in this sense. Accomplished model test confirmed
this discovery.
It was found that the ice-breaking blades or vanes 6 must be
mounted fore of the nozzle inlet 10 and spaced from the fore
edge i.e. the inlet 10 of the nozzle 5. However, with the
blades positioned in too close proximity to the nozzle inlet
opening l0, ice casting away by the blades will be hindered
by drawing in forces of the nozzle. In this case, all ice
pieces in way of the nozzle inlet opening will be destroyed
by milling which will, in turn, result to an undesired
wasting of the shaft rotation energy and excessive loading
of the shaft line. However, the blades cannot be mounted at
a too great distance in front of the nozzle either since
they will then loose their screw/nozzle protection
capability. What was discovered in this sense is that the
optimum spacing 0 between the blades (vanes) at the point of
their maximum diameter and the plane of the nozzle opening
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is 0.02-0.25 times the diameter of the screw propeller in
the shroud. This was also confirmed by the model test.
The inventors also found that in most cases it is preferred
to position the ice-breaking blades or vanes uniformly in
the plane perpendicular to that of the propeller shaft in
order to eliminate inertial loads on the shaft line.
The final diameter of the ice-breaking blades (vanes), their
number and spacing from the nozzle fore edge for each
particular vessel and navigation conditions should be
selected on the basis of data obtained from tests in
hydrodynamic and ice model basins.
Mounting of ice-breaking blades fore of the nozzle leads to
a reduction of hydrodynamic efficiency of the propulsion
unit. Hence, it was necessary to estimate the degree of the
blades (vanes) effect on the hydrodynamic efficiency of the
propulsion unit proposed herein. The inventors carried out
special comparative hydrodynamic tests of the proposed
propeller and of an isolated " screw-nozzle " combination. In
both cases, the same " screw-nozzle " set was used, and the
blades (vanes) were modelled by mounting, at various
distances fore of the nozzle of an additional four-blade
propeller model having a diameter equal to 0.7 times the
diameter of the screw propeller in the nozzle. Using
dynamometers, hydrodynamic thrust Te on the shaft, torque Q~,
nozzle thrust TH were measured, as well as shaft rotation r~
and propeller speed V. Values of the following dimensionless
coefficients were calculated:
TB + TH
- total thrust K~
Pn
- propeller torque K
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where p is water density, and
D is ducted propeller diameter
relative advance is ~,= v , and
nD
S propeller efficiency is rip = K'~ ~'
K~ 2~c
Results of the accomplished model tests are shown in Fig. 2.
The values of ~, are presented on the x-axis and values of
K~, KQE and rip on the y-axis .
The curves (1), (2), (3) in this plot correspond to the
values of KTE, ICQE and r)p for the standard "screw-nozzle"
propulsion unit. The curves (4), (5) and (6) show the values
of KT~, KQE and r)p, respectively, for the proposed propulsive
unit .
Thus, it can be seen that the rotating blades/vanes mounted
fore of the nozzle do not impair significantly the
hydrodynamic efficiency of the propeller as defined in the
appended claims when compared to the traditional " screw-
nozzle " combination.
The operation of an azimuth thruster can be described
shortly in the following manner. A rotating screw propeller
develops a thrust that drives the vessel. Owing to the
nozzle the thrust is additionally increased by 20-25%.
Blades and/or vanes dimensioned as stated above and which
rotate together with the screw propeller cast away and/or
destroy ice and prevent blocking of the nozzle inlet
opening.
Thus, the invention provides apparatus and a method by which
a significant improvement is achieved in the area of
propulsion systems. It should, however, be understood that
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the above description of'an example of the invention is not
meant to restrict the invention to the specific forms
presented in this connection but rather the present
invention is meant to cover all modifications, similarities
and alternatives which are included in the spirit and scope
of the present invention, as defined by the appended claims.
For instance, upon reading the above description together
with the annexed drawing it will be obvious to the skilled
person to use this invention in connection with conventional
propulsion units.
