Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02237067 1998-05-27
WO 97/20730 PCT/NL96/00473
Title: Tugboat having azimuthal propelling units
= The invention relates to a tugboat having azimuthal
propelling units.
Within the framework of the present invention, by the
expression "azimuthal propelling unit" is meant a propelling
unit whose propelling direction in horizontal direction can be
varied through 360 . Such azimuthal propelling units are
already known per se, for instance in the form of a nozzle
having a screw arranged therein.
Specific requirements are imposed on a tugboat with
regard to thrust and maneuverability. For instance, it is
desired that a tugboat cannot only produce hauling power in
forward direction, but also in rearward direction, and even in
lateral direction, although the hauling power producible in
lateral direction will be less than the hauling power
producible in longitudinal direction.
For instance from the article "Schottel tugs" in Small
Ships, Vol.99, No.1204, December 1976, page 95, it is already
known to fit a tugboat with azimuthal propelling units because
of the maneuverability provided thereby. Such tugboats, also
known by the name of "tractor tug", have two azimuthal
propelling units which are juxtaposed in transverse direction
and, viewed in the longitudinal direction of the tugboat, in a
.central position. However, some drawbacks are attached to
this. For instance, it is not properly possible to continue
using the tugboat if one of the propelling units has been
damaged.
Generally, the object of the invention is to provide a
tugboat which, in respect of the above-mentioned aspects,
performs better than the tugs known thus far.
= 30 In particular, the object of the invention is to provide
~ a tugboat which is less susceptible to damage, or at least has
still good utility in the event of failure of one of the
propelling units. A further object of the invention is to
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provide a tugboat which can travel economically, in particular
by utilizing not all propelling units present.
A further object of the invention is to provide a tugboat
which can. produce more power than the "tractor" tugs known
thus far, without the draft being increased. In known
"tractor" tugs, an increase of the power producible can be
realized by using two stronger propelling units, but a
consequence thereof is that the dimensions of the propelling
units are increased as well, which has an adverse effect on
the draft of the ship.
Another problem concerns the fire extinguishing
installations on board a tugboat, intended for extinguishing a
fire on shore or on board other ships. In order to meet
specific requirements (Fifi 1), a tugboat has two fire
extinguishing pumps, driven by the propelling engines. During
the use of the fire extinguishing pumps, those engines run at
full power. Consequently, without countermeasures, the
propelling units would produce thrust at full power, which is
of course undesired. On the other hand, in conventionai tugs,
it is not possible to switch off the propelling units
completely by uncoupling them from the associated engines,
because the propelling units have to produce a counterforce to
the force exterted on the boat by the fire extinguishing
water, to enable the boat to be kept in position and/or
displaced in a desired direction. The desired power of each
propelling unit should be steplessly settable over a wide
range, with the associated engine in each case running at full
power, for which purpose a slip coupling should be included
between each propelling unit and the associated engine. On
account of the above-mentioned power requirements, such
couplings are fairly expensive.
A further object of the present invention is to alleviate
this problem as well.
in order to realize the above objectives, a tugboat
according to the present invention has three azimuthal
propelling units whose centers, viewed from the top, lie on
the angular points of an isosceles triangle.
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According to the present invention there is provided a
tugboat having opposite sides, a front extreme point, a rear
extreme point and a longitudinally extending main plane of
symmetry extending between said front extreme point and said
rear extreme point, said tugboat comprising first, second
and third azimuthal propelling units, said first and second
azimuthal propelling units being juxtaposed about said
longitudinal main plane of symmetry and at a first distance
from said rear extreme point, said third azimuthal
propelling unit being positioned substantially in said
longitudinally extending main plane of symmetry at a second
distance, different from said first distance, from said rear
extreme point.
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Thus, it is possible to produce a greater thrust which is
distributed over the tugboat in a bette: manner. in comparison
with known "tractor" tugs, that greater thrust can be realized
through the use of smaller propelling units, as a result of
1 5 which the tugboat according to the present invention can have
a reduced draft. In practice, the purchase costs of three
azimuthal propelling units according to the present invention
are about as high as those of two azimuthal propelling units
having the same total power.
if it is desired that the boat be provided with a fire
extinguishing installation having two fire extinguishing
pumps, driven by two of the three driving engines, according
to the present invention it is sufficient to include one slip
coupling between one of those fire extinguishing pumps and the
associated engine. During use of the fire extinguishing
installation, the propelling unit of the second engine can be
switched off completely, and the ship is entirely controllable
by using the propelling unit of the first engine (with slip
coupling) and the propelling unit of the third engine, which
third engine is not connected to a fire extinguishing pump and
can hence be used freely (without slip coupling).
In a simpler variant, wherein the boat comprises a fire
extinguishing installation having one fire extinguishing pump,
driven by one of the three driving engines, preferably the
third, a slip coupling is not even necessary at all: during
use of the fire extinguishing installation, the propelling
unit of that one engine can be swithed off completely, and the
ship is entirely controllable by utilizing the two other
propelling units.
These and other aspects, characteristics and advantages
of the present invention will be specified by the following
description of a preferred embndiment of a tugboat according
to the invention, with reference to the accompanying drawings,
wherein:
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Fig. 1 is a schematic top plan view of a tugboat according to
the present invention, to illustrate the positions of the
three propelling units;
Fig. 2 is a schematic longitudinal section taken on the line
II-II in Fig. 1;
Fig. 3 is a schematic cross section taken on the line III-III
in Fig. 1.
Fig. 1 schematically shows the contour of a tugboat 1,
viewed from the top. Hereinafter, it is assumed that the
tugboat 1 is afloat in unloaded condition, and the expression
'horizontal' and 'vertical' are meant relative to the water
surface.
Viewed in the transverse direction, the tugboat 1 is
substantially symmetric relative to a vertical main plane of
symmetry 2, extending in the longitudinal direction of the
tugboat 1. The tugboat 1 has a front extreme point 3 located
in the main plane of symmetry 2, and a rear extreme point 4
also located in the main plane of symmetry 2. The horizontal
distance between the front and rear extreme points 3 and 4 is
indicated as the length L of the tugboat 1. Hereinbelow,
horizontal length positions will ;;e indicated as measured
relative to the rear extreme point 4.
By reference numeral 5, a vertical plane is indicated in
the Figures which is perpendicular to the main plane of
symmetry 2, and which intersects that main plane of synttnetry 2
according to a vertical line M precisely halfway the front and
rear extreme points 3 and 4. That vertical line M will be
referred to as the center M of the tugboat 1, and the vertical
plane 5 will be referred to as the transverse center plane of
the tugboat 1. Hereinbelow, horizontal width positions will be
indicated as measured relative to the main plane of
symmetry 2. The body portion of the t-igboat 1 located behind the
transverse center plane 5 will be referred to as the stern 6,
and the body portion of the tugboat 1 located before the
transverse center plane 5 will be referred to as the stem 7.
By a circle 8, the horizontal position is indicated of a
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towing point provided on the stern 6, i.e. a point intended
for securing a towing cable or the like thereto, or for
guiding, via that point, a towing cable or the like to a
towing winch. The tugboat 1 can have several towing points;
5 for i~~,,,tance, a towing point can be provided on the stem 7. If
the tiigboat 1 has several towing points on the stern 6, the
towing X,oint 8 is meant to be the rear towing point, i.e. the
towing point whose length position LB is minimal.
The tugboat 1 comprises three azimuthal propelling units
10, 20 and 30, whose propelling direction in horizontal
direction can be varied through 360 relative to the
respective vertical axes 11, 21 and 31 associated with the
propelling units 10, 20 and 30. Each propelling unit is driven
by a separate drivina engine, not shown for simplicity's sake.
Such azimuthal propelling units are known per se, for instance
in the form of a screw, a nozzle having a screw arranged
therein, or a so-called Voith Schneider unit. As the nature
and construction of such azimuthal propelling units do not
constitute a subject of the present invention, and a skilled
person need not have knowledge thereof for a proper
understanding of the present invention, they will not be
further described.
Viewed in horizontal direction, the three azimuthal
propelling units 10, 20 and 30 are arranged according to an
isosceles triangle, symmetrically relative to the main plane
of symmetry 2. It is preferred that two azimuthal propelling
units 10 and 20 be located on one side of the transverse
center plane 5, and that the third azimuthal propelling
unit 30 be located on the other side of the transverse center
plane 5.
In the preferred embodiment illustrated, a first
azimuthal propelling unit 10 and a second azimuthal propelling
unit 20 are located below the stem 7, symmetrically on both
sides of the tranaverse center plane 5. By this it is meant
that the length position Llo of the vertical axis of rotatiozi4i
symmetry 11 of the first azimuthal propelling unit 10 is equal
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to the length position L20 of the vertical axis of rotational
symmetry 21 of the second azinnithal propelling unit 20, and
that these positions are greater than 0.5 L, while the width
position Blo of the vertical axis of rotational symmetry 11 of
the first azimuthal propelling unit 10 is equal (but oppcsite)
to the width position B20 of the vertical axis of rotational
symmetry 21 of the second azimuthal propelling unit 20. As
regards the length positions Llo and L20, they are preferably
smaller than 0.8 L, more preferably smaller than 0.65 L. If a
towing point is provided on the stem 7, the length position
thereof is preferably greater than or equal to Llo and L20.
The vertical axis of rotational symmetry 31 of the third
azimuthal propelling unit 30 lies in the main plane of
symmetry 2, and has a length position L30 smaller than 0.5 L,
and preferably greater than or equal to 0.15 L. Preferably, L30
is smaller than or equal to 0.4 L, more preferably smaller
than or equal to 0.25 L. It is preferred that L30 be greater
than or equal to L8.
The three azimuthal propelling units 10, 20 and 30 can be
mounted entirely below the bottom 9 of the tugboat 1. However,
as is indicated in dotted lines in Figs. 2 and 3, it is also
possible that the azimuthal propelling units 10, 20 and 30 are
-partly recessed in the bottom 9 of the tugboat 1, so that the
tugboat 1 will have a less great draft. This applies in
particular to the third azimuthal propelling unit 30, located
at the center of the tugboat 1, because, viewed in cross
section, the bottom 9 of the tugboat 1 is generally more or
less v-shaped, so that in fact, the lowermost point of the
third azimuthal propelling unit 30 determines the draft of the
tugboat 1.
Conventional tugboats having azimuthal propelling units
have only two of such azimuthal propelling units, which are
comparable with the first and second azimathal propelling
units 10 and 20 according to the present invention. Through
the addition of a third azimuthal propelling unit 30 in the
main plane of symmetry 2, but at a different length position,
the following advantages are realized.
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if one of the propelling units is damaged, this means, in
conventional tugboats, a loss of 50% of the thrust, while in
the tugboat according to the present invention, only about 33%
of the thrust will in that case be lost.
in conventional "tractor" tugboats, the propelling units
are disposed at equal length positions. A consequence thereof
is thaz when the boat is moved truly transversely to the
longitudinal direction, and, moreover, a pulling or pushing
force is to be exerted in that direction, a fairly large part
of the installed power is lost: this loss can be about 25%,
depending on the type of the propelling units installed. in
conventional tugboats with the screws mounted at the rear of
the boat, that loss may even be 70%. Owing to the presence of
a third propelling unit 30 at a length position different from
that of the other two propelling units, the maneuverability in
lateral direction is improved, and the maximally producible
pulling or pushing force transverse to the longitudinal
direction is increased considerably.
Since the third azimuthal propelling unit 30 is located
in the main plane of symmetry 2, it is possible in an easy
manner to travel straight on utilizing only one propelling
unit, viz. the third propelling unit 30. This possibility,
which can for instance be used when the tugboat 1 travels in
unloaded condition, provides a saving of fuel and a reduced
wear.
The three propelling units according to the present
invention can jointly develop a thrust greater than the thrust
that can be produced by two propelling units at an equal
draft. in accordance with the present invention, it is even
possible to realize a greater total thrust while the three
propelling units are individually chosen to be smaller than
the individual propelling units of the conventional "tractor"
tugboats, whereby the draft of the tugboat is reduced as well.
It will be understood by anyone skilled in the art that
changes and modifications of the embodiment described are
possible, which fall within the framework of the present
invention and within the protective scope of the claims. For
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instance, during the building of a conventional "tractor tug",
it is possible to reserve space for later incorporation of a
third propelling unit, with the construction of the boat being
designed for such incorporation.
It is also possible that one or several, for instance the V
third one, of the propelling units are retractably mounted,
enabling a propelling unit that is not being used to be
retracted to a position within the profile of the bottom of
the boat. As a result, the resistance during travelling will
be reduced, which means a saving of fuel.
In tugboats that mainly operate according to the so-
called "push/pull" system, the position of the propelling
units can be reversed, i.e. one unit at the front and two
units at the rear.
