Note: Descriptions are shown in the official language in which they were submitted.
CA 02509401 2010-11-29
ARRANGEMENT IN A PROPULSION SYSTEM
This application relates to an arrangement in a counter rotating propulsion
system (CRP).
The propulsion system is normally located in after part of the marine vessel.
The propellers
are equipped with hubcap, which is normally covering the propeller fastening
.bolt. The
circulation of water around each forward propeller blade forms a vortex near
the hub
before they joint to one hub vortex. This hub vortex cavitation is known to be
very harmful
to the propulsion unit or the rudder behind the main propeller. The hub vortex
is itself
erosive but it can also induce other harmful forms of cavitation on
construction such as
propulsion unit or after propeller blade.
Especially in the CRP propulsion concept whereby another propeller is arranged
close to
the main propeller this may cause extensive damages. The steering of the
propulsion unit is
not fully competent because of erosive hub vortices. This will shorten the
maintenance
interval of the propulsion system and thereby the overall costs increase.
Conventional way to avoid hub vortex cavitation is application of blunt cap
after the
propeller, which destroy hub vortex due to vast separation after hubcap. But
in subject case
of counter rotating propeller such way is unacceptable because separation
provoke
cavitation on the blades of aft propeller especially when aft thruster
operates in steering
mode so that thruster is turned by some angle, and so fore and aft propellers
are not co-
axial and blades of aft propeller during its rotation cross the separation
zone.
Accordingly, it is an object of the present invention to provide a new
arrangement, which
solves the problems caused by the vortices.
According to an aspect, of the present invention there is provided arrangement
in a
counter rotating propulsion system, which propulsion system comprises an aft
propeller
or steering propeller installed on a rotatable thruster and a forward
propeller installed on
a shaft or on a thruster, which propellers are arranged on substantially same
axial line,
whereby the aft propeller and the forward propeller have opposite directions
of rotating
and the aft and forward propellers each have a hub with a cap, the aft
propeller hub cap
and the forward propeller hub cap being opposite each other, wherein at least
two equally
distributed flow plates are arranged on the cap of the forward propeller and
wherein the
flow plates are radially projecting from the cap of the forward propeller.
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According to another aspect of the present invention there is provided
arrangement in a
counter rotating propulsion system, comprising an aft propeller or steering
propeller
installed on a rotatable thruster and a forward propeller installed on a shaft
or on a
thruster, whereby the aft propeller and forward propeller are arranged on
substantially
same axial line, the aft propeller and the forward propeller have opposite
directions of
rotating and the aft and forward propellers each have a hub with a cap, the
aft propeller
hub cap and the forward propeller hub cap being opposite each other, whereby
at least
two equally distributed flow plates are arranged on the cap of the forward
propeller and
the flow plates are radially projecting from the cap of the forward propeller.
This invention will reduce or eliminate the above mentioned problem in CRP
propulsion
concept and thus protect the after propeller and entire propulsion unit itself
from damage.
The invention will increase the capability to steer the propulsion unit behind
forward
propeller without danger of erosive hub vortex cavitation and without
dangerous cavitation
on the blades after hub of forward cavitation. This will lead to longer
lifetime and reduce
repairing costs of the propulsion unit.
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The invention is based to an idea to break the flow of the vortex caused by
the blades of
the forward propeller. The hub cab of the propeller, in detail the external
appearance of the
hubcap is formed so that it consists of at least two, equally distributed flow
plates
projecting from the outer surface of the hubcap. The number of the flow plates
is in
practice not higher than eight while four flow plates gives the most efficient
result.
On the other hand the cap itself should be well-streamlined, with relation of
cap diameter
and cap length not more than 2. It provides the absence of developed
separation after hub
cap with plates and so make it possible to eliminate blade cavitation in
separation zone
after fore hub when thruster is not co-axial with fore propeller.
The absence of the hub vortex allows the safe operation of after propeller and
steerable
propulsion unit. The invention is advantageous to implement, as it requires no
other
modifications to the structure or to operation of the propulsion system or its
peripheral
devices. Further as the invention mainly is realized with a special forming of
a separate
component, the invention is adaptable also to the propulsion devices in use.
According to one feature of the invention there is installed a well-
streamlined hubcap of
the fore propeller between two propellers in order to avoid separation after
the fore hub.
According to another feature of the invention the hubcap of the fore propeller
will have the
flow plates which are straight and similar to each other. This gives an
optimal effect and a
balanced structure.
According to another feature of the invention the number of the flow plates is
independent
of the number of the blades of the forward propeller and the position of the
flow plates is
independent of the position of the blades of the forward propeller. This
feature facilitates
the planning and installation of the hubcap, as there is no need to align the
flow plates with
the blades of the propeller. The same structure of the hubcap can be used in
the different
propeller configurations.
It is another further feature of the invention that the diameter of the tip
edges of the plates
is in the range of 0,4 -2 times the maximum hub diameter. Within this range
the efficiency
of the invented appliance is especially advantageous.
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The flow plates are fastened to the hub cab either by fixed to the cap by
welding or by with
bolts. The size and shape of the flow plates is easy to change and vary if
necessary
depending of the respective requirements. If the flow plates are used with the
vessels in
use, this possibility might be desirable. Alternatively the hubcap is moulded
as one piece
with flow plates whereby the hubcap can be handled as an integrated piece.
According to further advantageous feature the aft propeller is turnable and
the aft propeller
is used to propel and steer the vessel.
In one aspect, the invention provides a counter rotating propulsion system
(CRP),
l0 comprising an aft propeller installed on a rotatable thruster and a forward
propeller
installed on a shaft or on a thruster, which propellers are arranged on the
substantially
same axial line, wherein the aft propeller and the forward propeller have
opposite
directions of rotating and the aft and forward propellers are arranged against
each other,
the propellers having a hub with a cap, wherein at least two flow plates are
arranged on
the cap of the forward propeller radially projecting from the cap and that the
flow plates
are equally distributed on the cap.
The details of one preferred embodiment as well as advantages and further
features of the
invention will be apparent from the following specification and from the
drawings. In the
drawings:
Figure 1 is a schematic view of a CRP arrangement according to the invention,
Figure 2a is side view of a hub cap according to the invention and
- Figure 2b is front view of a hub cab of Figure 2a.
Figure 1 shows a propulsion arrangement 2 which is realized with counter
rotating
propellers (CRP), which is placed under the hull 4 of the vessel. The main
propulsion
propeller, so called forward propeller 6 is arranged onto the main driving
axis 8, which is
supported via bearings to the hull 4 of the ship. The forward propeller 6 is
driven e.g. by
the drive unit, like a diesel engine, directly or via a electric drive that is
supplied by a
diesel-generator unit by means of a frequency converter which is well-known in
the art.
The drive unit and the bearing and other features of the power transmission is
utilizing
conventional technique well-known in the art and there in no need to explain
in detail in
CA 02509401 2010-11-29
3a
order to understand the. invention. The forward propeller 6 comprises a hub 10
arranged to
the driving axis 8 and propeller blades 12 fixed to the hub 10. The number of
blades, the
inclination of the blades and the size of the blades will be defined when
dimensioning the
propulsion system of ship and they may vary case by case. The inclination of
the blades
may also be adjustable.
The hull 4 of the ship is designed so that the bottom of the hull curves over
the forward
propeller 6 ,and the bottom extends at higher level after the forward
propeller. Thus the
bottom 14 at the rearmost portion of the ship is higher than the bottom 16 of
the ship in
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front of the forward propeller. Under the bottom 14 there is arranged a
turnable steering
device 18, which consists of an azimuth propeller unit, like an AZIPOD unit,
which
steers and propels the vessel. The steering device 18 comprises a shaft 20,
which is
pillowed to be turnable 360 degrees around its vertical axis. A streamlined
casing 21,
which covers the driving motor of the steering propeller 22, is attached under
the shaft 20.
The steering propeller 22 is driven by the driving axis 24, which is
positioned on the same
level as the axis 8 of the forward propeller.
According to the CRP concept the well-streamlined hub cap 30 of the forward
propeller 6
and the hub cap 26 of the steering propeller 22 are against each other and the
forward
propeller 6 and the steering propeller 22 are rotating in the opposite
direction when the
ship is moving forwards. When turning the direction of the ship the steering
device 18 is
turned around the vertical axis of the shaft 20 in order to effect the desired
steering action.
The rotating planes of the forward and steering propellers are farther from
each other than
the double diameter of the forward propeller when the propellers are against
each other.
The hub cap 30 of the forward propeller 6 and the hub cap 26 of the steering
propeller are
thus positioned much closer. The vortex and the stress caused by it effects
thus heavily to
the hubcap of the forward propeller and also to the steering propeller.
According to the
teaching of the present invention the hub vortex cavitation and the aft
propeller blade
cavitation with its harmful effects are minimized by arranging a well-
streamlined hub 30
after the forward propeller and flow plates 28 onto the hubcap 30.
According to a favourable embodiment of the invention four flow plates 28 are
mounted
symmetrically or with equal distance to each flow plate on the outer surface
of the hubcap
as shown in the figures 2a and 2b. The figure 2a is the side view and the
figure 2b is
the front view when seen from the rear of the vessel. The hubcap has length L
and diameter
25 D, whereby the ratio D/L is not higher than 2. The flow plates are straight
plates that are
welded or fixed by bolts to the surface of the main propeller hubcap 2. The
flow plates 2
can also be cast together with the whole propeller hubcap. The flow plate has
been
- installed on the whole length of the cap surface and the flow plates link up
to each other
outside the cap surface extending a little over the top edge of the cap. In
this example the
30 height of the flow plate does not exceed the radial dimensions of the cap.
Thus the flow
plate does not extend over the diameter of the hubcap. The flow plates are
projected in the
radial direction from the surface of the hubcap and they are installed in the
direction of the
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propeller axis with no inclination. It has been shown that the tip edges of
the flow plates
may vary in the range of 0,4 to 2 times the maximum hub diameter D.
Accordingly this
range corresponds about 0,12 to 0,4 times the diameter of the propeller.
The number of the flow plates is not tied to the number of the propeller
blades and it may
5 vary from two to eight while four flow plates has been found to be
advantageous. Neither
the positioning of the flow plates is not bound to the position of the
propeller blades but
they can coincide or be aside.
The invention has been above described using its one modification as an
example. The
invention may have a lot of different embodiments, the scope of the invention
being
defined in the claims.
