FIELD OF THE INVENTION
[0001] The present invention relates to a propulsion unit according to the preamble of claim
1.
BACKGROUND ART
[0002] Prior art propulsion units normally comprise a first housing, a second housing, an
electric motor, a shaft, and a propeller. The first housing is a longitudinal housing
extending in a first direction downwards from a hull of a vessel and the second housing
is a longitudinal housing extending in a second direction perpendicular to the first
direction. The first housing is rotatably attached to the hull of the vessel. The
electric motor is positioned within the second housing. The shaft passes in the second
direction through the electric motor and is rotatable supported with bearings within
the second housing. The propeller is attached to an outer end of the shaft protruding
from one end of the second housing.
[0003] In propulsion units where a high thrust at low speed is needed, there is further
a nozzle surrounding the perimeter of the propeller. The nozzle is supported at the
second and/or the first housing. The nozzle forms a central duct with an axial flow
path for water from a first end to a second end of the nozzle. The thrust produced
by the propeller is amplified by the nozzle at low speeds. The nozzle may produce
up to 40% of the total thrust at low speeds, whereby the propeller produces 60% of
the total thrust. Nozzles are used in so called Dynamic Positioning (DP) vessels used
in oil drilling. There may be several struts in such vessels and the vessel is kept
steady in position by the struts. A big thrust is thus needed at low speed in order
to keep the vessel continuously in position in rough seas.
[0004] JP patent publication 2000142576 discloses a watercraft propulsion apparatus for luxurious passenger boats. The apparatus
has a casing arranged in a hollow portion of the hull. The casing comprises a vertically
positioned electric motor and a vertical shaft passing through the electric motor.
The lower end of the vertical shaft extends from the bottom of the casing into a strut
and is connected with a transmission to a horizontal shaft. A propeller is connected
to the outer end of the horizontal shaft protruding from one end of the strut. The
electric motor is completely within the casing and the casing is completely within
the hull of the vessel.
[0005] WO patent publication 2009/141254 discloses an azimuth propeller device for a vessel. The device has a longitudinal
housing placed in water below a hull of the vessel. The housing comprises a first
shaft having an outer end protruding from an end of the housing. A propeller is attached
to the outer end of the first shaft. The housing is supported with a hollow arm at
the hull of the vessel. The hollow arm is rotatable supported with bearings at a support
structure surrounding the hollow arm. A second shaft extends in the hollow arm from
the housing to the interior of the hull of the vessel. The lower end of the second
shaft is connected via a transmission to the first shaft. A first electric motor is
positioned within the hull of the vessel around the second shaft so that the rotor
of the first electric motor is connected to the second shaft. In order to keep the
mounting height of the azimuth propeller device low, the first electric motor is designed
as a ring motor which is annularly arranged around the second shaft. A second electric
motor is positioned within the hull of the vessel at the upper end of the hollow arm
so that the rotor of the second electric motor is connected to the hollow arm. The
second electric motor is also designed as a ring motor annularly arranged around the
hollow arm. The first electric motor drives the propeller and the second electric
motor turns the hollow arm and thereby also the housing in relation to the hull of
the vessel.
[0006] US patent 7,641,526 discloses a marine vessel and an azimuthing thruster assembly. The vessel has a hull
and a hull bottom, with a first well and a second well formed in the hull. The wells
extend in the vertical direction within the hull of the vessel and face downwards
toward a sea floor. The wells comprise a movable canister, which is integrally connected
to an azimuthing thruster. The canisters and thereby also the thrusters can be lowered
to a deployed position from a retracted position. The propeller in the azimuting thruster
is driven by a vertically in the canister positioned first electric motor. The first
electric motor is connected with a vertical shaft and a transmission to the propeller
shaft extending in the strut. The azimuting thruster has a propeller connected to
the propeller shaft and a nozzle surrounding the propeller. The thruster is positioned
completely within the canister within the hull of the ship in the retracted position
and in the water below the bottom of the hull in the deployed position. The first
electric motor will in all situations be positioned within the canister within the
hull of the vessel. The azimuting thruster is connected to a turning wheel within
the hull of the vessel. The turning wheel and thus also the azimuting thruster is
rotated with a second electric motor connected to the turning wheel.
[0007] The problem in prior art propulsion units comprising a first housing, a second housing
and an electric motor within the second housing and a propeller outside the second
housing driven by the electric motor is lack of torque at low speed. The nozzle surrounding
the outer perimeter of the propeller will increase the thrust of the arrangement at
low speed. There is however a need for more torque at low speed and thus more thrust
at low speed in the previously mentioned DP vessels.
[0008] The problem in prior art propulsion units where the electric motor is positioned
within the hull of the vessel and the propeller is driven by two shafts connected
with an angle transmission is related to the service of the electric motor. The service
of the electric motor has to be done from inside of the vessel i.e. it is not possible
to change the whole strut with electric motor from the outside of the hull of the
vessel.
BRIEF DESCRIPTION OF THE INVENTION
[0009] An object of the present invention is to achieve an improved propulsion unit.
[0010] The propulsion unit according to the invention is characterized by what is stated
in the characterizing portion of claim 1.
[0011] The propulsion unit comprises:
a cylindrical first housing extending in a first direction downwards from a bottom
of the hull of the vessel, an upper end of said first housing extending through an
opening in the bottom of the hull into the interior of the hull, said first housing
being rotatable attached within the hull by means of a slewing bearing,
a second housing being attached to a lower end of the first housing,
a first shaft having a first end and a second opposite end, an axial centre line of
the first shaft extending in a first direction within the first housing, said first
shaft being rotatably supported with first radial and axial bearings,
a second shaft having a first end and a second opposite end, an axial centre line
of the second shaft extending in a second direction within the second housing, said
second shaft being rotatably supported with second radial and axial bearings, said
second end of the second shaft protruding from the second end of the second housing,
a transmission connecting the first end of the first shaft to the second shaft,
a propeller being attached to the second end of the second shaft outside the second
end of the second housing, said propeller rotating with the second shaft,
[0012] The propulsion unit comprises further:
a first electric motor being positioned within the first housing, said first electric
motor comprising a rotor and a stator surrounding the rotor, said first electric motor
being directly cooled to sea water surrounding the first housing through a shell of
the first housing, said rotor extending along the axial centre line of the first shaft
and being attached to the first shaft, whereby the first electric motor drives the
propeller via the first shaft, the transmission and the second shaft.
[0013] The position of the first electric motor within the upper portion of the strut makes
it possible to use a smaller and cheaper first electric motor compared to an electric
motor positioned within the lower portion of the strut.
[0014] The first electric motor can be of any technology that is available today. The rotation
speed of the first electric motor could be higher compared to the rotation speed of
an electric motor that drives the propeller directly. This is due to the transmission,
which makes it possible to lower the rotation speed of the first electric motor to
a rotation speed suitable for the propeller. The torque that can be produced by an
electric machine is proportional to the volume of the active parts in the electric
machine. By using e.g. a gear ratio of 4 in the transmission, one could reduce the
volume of the first electric motor by one fourth. The outer diameter of the first
electric motor could thus be halved compared to a first electric motor driving the
propeller directly if the length of the electric motors would be kept the same. The
power P of an electric machine is P=M*ω. The power of the electric machine will thus
increase when the rotation speed ω of the electric machine is increased in a situation
where the torque M of the electric machine is kept the same.
[0015] The first electric motor positioned in the first housing can be cooled directly to
the sea water surrounding the first housing. The stator of the first electric motor
can be arranged so that heat produced in the stator is transferred directly to the
shell of the first housing and further from the shell to the sea water surrounding
the first housing. This means that a separate cooling system, which is needed in case
the first electric motor is situated within the vessel or within a movable canister
can be eliminated in the invention.
[0016] The removal of the electric motor from the second housing makes it possible to optimize
the second housing. The second housing can be shortened and the form of the second
housing can be improved in view of hydrodynamic properties. The axial length of the
second housing can be made much smaller e.g. 60% smaller compared to a situation where
the electric motor is situated within the second housing. This means that the propulsion
unit can be installed in a narrower vessel. A shorter second housing means a smaller
diameter rotating around the vertical shaft of the propulsion unit. The dimensions
of a possible service opening in the bottom of the vessel through which the strut
can be lifted into the vessel can also be made smaller in a corresponding way.
[0017] The production of heat in the second housing will decrease dramatically as only the
bearings and the transmission produce heat in the second housing. The cooling of the
second housing directly to the surrounding sea water will thus be sufficient.
[0018] The lower temperature in the second housing will also have a beneficial impact on
the life time of the sealing between the second shaft and the second housing. Said
sealing will only be subjected to the heat produced by the radial bearing near the
sealing. The operation temperature of the sealing will thus be lower, which will prolong
the life time of the sealing.
[0019] This arrangement is especially suitable to be used in the previously mentioned DP
vessels. The thrust needed in the DP vessels where the invention is especially suitable
is in the range of 50 to 150 tons. The power of the electric motor in these applications
is in the order of megawatts. The diameter of the outer periphery of the propeller
is in the order of several meters.
[0020] In order to exemplify the dimensions one could refer to a prior art Azipod® arrangement
for a DP vessel delivered by the applicant. The power of the electric motor within
the prior art propulsion unit is 20 MW, the torque of the electric motor is 1340 kNm,
the diameter of the outer periphery of the propeller is 5.5 m and the axial length
of the total entity is 11.4 m. A propulsion unit according to the invention having
a similar output would result in a reduction of the length of the total entity to
4 to 5 m. The propulsion unit according to the invention forms a compact integrated
package.
[0021] The position of the first electric motor within the first housing means that the
torque of the first electric motor does not affect the steering torque of the propulsion
unit. The stator of the first electric motor is within the first housing and the first
housing is rigidly connected to the second housing where the transmission between
the first shaft and the second shaft is positioned. The torque of the stator and the
torque of the first pinion are acting in the opposite direction within the housing
arrangement. The load acting on the steering arrangement including the slewing gear
ring, the pinion, the gear box and the second electric motor is lower compared to
a solution with a separate first electric motor. This leads to a lower weight and
lower costs for these components.
[0022] The propulsion unit can further in one advantageous embodiment comprise an annular
nozzle being fixedly supported on the first and/or on the second housing. The axial
centre line of the second shaft forms also an axial centre line of the annular nozzle.
The annular nozzle surrounds an outer perimeter of the propeller and forms a duct
for water flowing through the interior of the annular nozzle.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the following the invention will be described in greater detail by means of preferred
embodiments with reference to the attached drawings, in which:
Figure 1 shows a propulsion unit according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Figure 1 shows a propulsion unit according to the invention. The propeller is pushing
the vessel forwards in a first direction S1. The propulsion unit 20 comprises a first
housing 21, a second housing 22, a first shaft 31, a second shaft 41, a transmission
34, 44, a propeller 51, an annular nozzle 70 and a first electric motor 60.
[0025] The first housing 21 extends downwards from a hull 10 of a vessel. An upper end 21
B of the first housing 21 extends through an opening 11 in the bottom of the hull
10 of the vessel into the interior of the hull 10 of the vessel. The opening 11 in
the bottom of the vessel is sealed against sea water with a slewing seal 27. A second
housing 22 is rigidly attached to a lower end 21A of the first housing 21. The first
housing 21 forms a support member for the second housing 22. A part of the upper portion
21 of the strut 20 extends through the hull 10 of the vessel into the vessel into
an enclosed compartment 80 within the hull 10 of the vessel. The second housing 22
has the form of a torpedo and comprises a first end 22A and a second end 22B. The
first housing 21 and the second housing 22 could be formed as two separate entities
or as a single entity.
[0026] The first shaft 31 has a first end 31A and a second opposite end 31 B. An axial centre
line Y of the first shaft 31 extends in a first direction within the first housing
21. The first shaft 31 is rotatably supported with first radial and axial bearings
32, 33 within the first housing 21. One of the radial bearings 32, 33 of the first
shaft 31 is advantageously a combined radial and axial bearing. A combined radial
axial bearing is needed in order to carry the load caused by the first shaft 31 and
the gear.
[0027] The second shaft 41 has a first end 41 A and a second opposite end 41 B. An axial
centre line X of the second shaft 41 extends in a second direction within the second
housing 22. The second shaft 41 is rotatably supported with second radial bearings
42, 43 within the second housing 22. The second end 41 B of the second shaft 41 protrudes
from the second end 22B of the second housing 22. The second direction X is essentially
perpendicular to the first direction Y. One of the radial bearings 42, 43 of the second
shaft 41 is advantageously a combined radial and axial bearing. A combined radial
axial bearing is needed in order to transfer the axial thrust produced by the propeller
51 from the second shaft 41 to the second housing 22 and the first housing 21 and
further to the hull 10 of the vessel.
[0028] The transmission 34, 44 connects the first end 31A of the first shaft 31 to the second
shaft 41. The transmission 34, 44 is formed of a first pinion 34 attached to the first
shaft 31 and a second pinion 44 attached to the second shaft 41. The cogs on the periphery
of the pinions 34, 44 are in contact with each other so that the rotation of the first
shaft 31 also rotates the second shaft 41. The first shaft 31 will rotate at a first
rotation speed determined by the first electric motor 60. The second shaft 41 will
on the other hand rotate with a second different rotation speed determined by the
transmission 34, 44 i.e. the pitch diameters of the pinions 34, 44. The second rotation
speed will be lower than the first rotation speed. The propeller 50, 51 is thus driven
by the first electric motor 60 at a speed which is lower than the rotation speed of
the first electric motor 60.
[0029] A propeller 50 comprising a hub 50 provided with propeller blades 51 is attached
to the second end 41 B of the second shaft 41. The propeller may also be a monoblock
type propeller in which the propeller hub 50 and the propeller blades 51 are integrated
into a monoblock construction. The propeller hub 50 is attached to the second end
41 B of the second shaft 41 outside the second end 22B of the lower portion 22 of
the strut 20. The propeller 50, 51 rotates with the second shaft 41.
[0030] The annular nozzle 70 is fixedly supported at the first housing 21 and/or at the
second housing 22. The axial centre line X of the second shaft 41 forms also an axial
centre line of the annular nozzle 70. The annular nozzle 70 surrounds an outer perimeter
of the propeller 50, 51 and forms a duct 75 with an axial flow path for water flowing
through the interior of the annular nozzle 70. The annular nozzle 70 is attached to
the first housing 21 with a first support part 73 and to the second housing 22 with
a second support part 74. The second support part 74 extends downwards from the second
housing 22. The rotating propeller 50, 51 causes water to flow through the central
duct 75 from the first end 71 of the central duct 75 to the second end 72 of the central
duct 75 in a second direction S2, which is opposed to the first direction S1. The
thrust produced by the propeller 50, 51 is amplified by the annular nozzle 70 at low
speeds. The situation can be such that the propeller 50, 51 produces 60% of the total
thrust and the annular nozzle 70 produces 40% of the total thrust at low speeds. A
nozzle 70 around the propeller 50, 51 is advantageously used in propulsion units in
so called Dynamic Positioning (DP) vessels used in oil drilling. There can be several
propulsion units in such vessels and the vessel is kept steady in position by the
propulsion units. A big thrust is thus needed at low speed in order to keep the vessel
continuously in position especially in rough seas.
[0031] The first electric motor 60 is positioned within the first housing 21. The first
electric motor 60 comprises a rotor 61 and a stator 62 surrounding the rotor 61. The
rotor 61 of the first electric motor 60 extends along the axial centre line Y of the
first shaft 31. The rotor 61 of the first electric motor 60 is attached to the first
shaft 31 so that the first shaft 31 rotates with the rotor 61 of the first electric
motor 60. The rotation of the rotor 61 of the first electric motor 60 is thus transferred
to rotation of the first shaft 31. The rotation of the rotor 61 of the first electric
motor 60 is thus transferred via the first shaft 31, the transmission 34, 44 and the
second shaft 41 to the propeller 51. The first electric motor 60 drives the propeller
51.
[0032] The first housing 21 is attached via a cylindrical part 27 to a slewing gear ring
24 within the hull 10 of the vessel. A second electric motor 65 is connected via a
gear box 66 and a third shaft 67 to a third pinion 68 being in contact with the slewing
gear ring 24. The cogs on the periphery of the third pinion 68 are in contact with
the cogs on the periphery of the slewing gear ring 24. The second electric motor 65
will thus turn the slewing gear ring 24 and thereby also the first housing 21 as well
as the second housing 22 fixedly attached to the first housing 21. The first housing
21 is thus rotatable supported on the hull 10 of the vessel by the slewing bearing
26 and can be rotated 360 degrees around a vertical centre axis Y in relation to the
hull 10 of the vessel. The figure shows only one second electric motor 65 connected
to the slewing gear ring 26, but there could naturally be two or more second electric
motors 65 driving the slewing gear ring 26.
[0033] The electric power needed in the first electric motor 60 is produced within the hull
10 of the ship. The electric power can be produced by a generator connected to a combustion
engine. The electric power to the stator 62 of the first electric motor 60 is supplied
by cables and/or bus bars running from the generator within the interior of the hull
10 of the vessel to the strut 20. A slip ring arrangement 90 is needed in connection
with the compartment 80 within the hull 10 in order to transfer electric power from
the stationary hull 10 to the rotatable strut 20.
[0034] The first electric motor 60 can be made rather long, which means that the torque
produced by the first electric motor 60 can be increased. The torque of an electric
motor is proportional to the volume of the electric motor. The volume of the first
electric motor 60 is increased by the length of the rotor 61 and the stator 62 of
the first electric motor 60. The first electric motor 60 can be designed so that sufficient
cooling of the stator 62 directly to the surrounding sea water through the shell of
the first housing 21 can be achieved. The sea water flowing on the outer surface of
the first housing 21 will cool the shell of the first housing 21. The stator 62 of
the first electric motor 60 can be arranged in contact with the shell of the first
housing 21. The cooling of the stator 62 can thus be arranged as a passive cooling
directly to the sea water.
[0035] The axial length X1 of the second housing 21 and the annular nozzle 70 is much smaller
than the corresponding length of a propulsion arrangement where the first electric
motor 60 is within the second housing 22. The axial length Y1 + Y2 of the first electric
motor 60 can be adapted according to the axial length of the first housing 21. At
least 90% of the axial length of the first electric motor 60 is outside the hull 10
of the vessel i.e. surrounded by water. The situation where at least 90% of the axial
length of the first electric motor 60 is outside the hull 10 of the vessel leads to
a situation where an efficient cooling of the first electric motor 60 directly to
the sea water surrounding the first electric motor 60 is possible. It would naturally
be more advantageous to have 100% of the axial length of the first electric motor
60 outside the hull 10 of the vessel, but the arrangement is often such that the first
housing 21 extends a little bit into the hull 10 of the vessel.
[0036] The swivel axis of the propulsion unit 20 coincides with the swivel axis Y of the
first shaft 31 in the embodiment shown in the figures. The first housing 21 could,
however, extend in an inclined direction downwards from the hull 10. The propulsion
unit 20 would thus be tilted in relation to the hull 10. This might in some circumstances
result in hydrodynamic advantages. The swivel axis of the propulsion unit 20 could
still be vertical, but the centre axis Y of the first shaft 31 would be inclined in
relation to the vertical direction in the same way as the first housing 21.
[0037] The centre axis X of the second shaft 41 is directed in the horizontal direction
in the embodiment shown in the figures. The centre axis X of the second shaft 41 could,
however, be inclined in relation to the horizontal direction. The second housing 22
would thus be inclined in relation to the horizontal direction. This might in some
circumstances result in hydrodynamic advantages.
[0038] The angle α between the swivel axis of the first shaft 31 i.e. the first axial centre
line Y and the swivel axis of the second shaft 41 i.e. the second axial centre line
X is advantageously 90 degrees. The angle α between the first swivel axis and the
second swivel axis could on the other hand be less than 90 degrees or more than 90
degrees. The transmission 34, 44 has to be adapted to the angle α between the first
and the second axial centre lines Y, X.
[0039] The invention and its embodiments are not limited to the examples described above
but may vary within the scope of the claims.
1. A propulsion unit comprising:
a cylindrical first housing (21) extending in a first direction downwards from a bottom
of the hull (10) of the vessel, an upper end (21 B) of said first housing (21) extending
through an opening (11) in the bottom of the hull (10) into the interior of the hull
(10), said first housing (21) being rotatable attached within the hull (10) by means
of a slewing bearing (26),
a second housing (22) being attached to a lower end (21 A) of the first housing (21),
a first shaft (31) having a first end (31A) and a second opposite end (31 B), an axial
centre line (Y) of the first shaft (31) extending in a first direction within the
first housing (21), said first shaft (31) being rotatably supported with first radial
and axial bearings (32, 33),
a second shaft (41) having a first end (41 A) and a second opposite end (41 B), an
axial centre line (X) of the second shaft (41) extending in a second direction within
the second housing (22), said second shaft (41) being rotatably supported with second
radial and axial bearings (42, 43), said second end (41 B) of the second shaft (41)
protruding from the second end (22B) of the second housing (22),
a transmission (34, 44) connecting the first end (31 A) of the first shaft (31) to
the second shaft (41),
a propeller (50, 51) being attached to the second end (41 B) of the second shaft (41)
outside the second end (22B) of the second housing (22), said propeller (50, 51) rotating
with the second shaft (41),
characterized in that the propulsion unit further comprises:
a first electric motor (60) being positioned within the upper portion (21) of the
strut (20), said first electric motor (60) comprising a rotor (61) and a stator (62)
surrounding the rotor (61), said first electric motor (60) being directly cooled to
sea water surrounding the first housing (21) through a shell of the first housing
(21), said rotor (61) extending along the axial centre line (Y) of the first shaft
(31) and being attached to the first shaft (31), whereby the first electric motor
(60) drives the propeller (50, 51) via the first shaft (31), the transmission (34,
44) and the second shaft (41).
2. A propulsion unit according to claim 1, characterized in that more than 90% of an axial length (Y1) of the first electric motor (60) is situated
within the first housing (21) below the bottom of the hull (10) of the vessel.
3. A propulsion unit according to claim 1 or 2, characterized in that the second direction (X) is essentially perpendicular in relation to the first direction
(Y).
4. A propulsion unit according to any one of claims 1 to 3, characterized in that there is an annular nozzle (70) fixedly supported at the propulsion unit, said axial
centre line (X) of the second shaft (41) also forming an axial centre line of the
annular nozzle (70), said annular nozzle (70) surrounding an outer perimeter of the
propeller (50, 51) and forming a duct (75) for water flowing through the interior
of the annular nozzle (70).