WO2012128632A1

WO2012128632A1 - Tug boat with retractable skeg

Info

Publication number: WO2012128632A1
Application number: PCT/NL2012/050183
Authority: WO
Inventors: Antonie Marius Kooren
Original assignee: Sacar Holding N.V.
Priority date: 2011-03-23
Filing date: 2012-03-23
Publication date: 2012-09-27
Also published as: AU2012231900A1; AU2012231900B2; NL2006453C2

Abstract

A tug boat equipped (1) with at least one skeg (20), preferably positioned at the front side (7) of a tug boat and more specifically closest to a front extreme point (3), wherein the skeg (20) is at least partly retractable into a compartment within the hull of the boat (1), and wherein actuating means (25, 22) for the skeg are provided within the compartment and/or within the skeg.

Description

Title: Tug boat with retractable skeg

The present invention relates to a tug boat intended for escort towing and/or for use in harbour including at least a towing winch. While escort towing, the tug boat is intended to assist at high speed the steering and arresting properties of a vessel to be assisted by means of a towing cable connecting the towing winch, guided through a towing point and the vessel being assisted establishing an effective towing cable connection capable of withstanding a significant pulling force, thus improving the manoeuvrability of the escorted vessel. While working in the harbour, the tug boat can be applied to normal towing or pushing and pulling operations.

Totally different standards are set for such, so-called escort tug boats compared with conventional harbour tug boats. First the escorting speed of an escort tug boat is required to be at least as high as the lowest operating speed of the assisted vessel. The most economical escorting speed is the highest permitted speed at which trafficking is safe. In practice, this means that the escorting speed can be even 13 to 14 knots. Accordingly, the tug boat is required to carry out its escorting tasks as well as merely following the escorted vessel at this speed. Furthermore, the escort tug boat should be highly manoeuvrable in order to decrease response times between a request for steering force and the application of such steering forces on the escorted vessel. Furthermore an escort tug should be able to function in all weather conditions. Such prerequisites mandates that an escort tug boat is able to operate in all conceivable directions and, if needed, it has to be able to change the direction at maximum speed. Furthermore, an escort tug boat and/or harbour tug boat is required to apply maximum pushing and pulling power to the assisted vessel. In view of such requirements, the only useful propulsion apparatus in current tug boats is, in fact, a propulsion unit capable of delivering great propulsive thrust in all directions. Such propulsion units are well-known in the art and include, but are not limited to, azimuthal propelling units in the form of a nozzle having a screw arranged therein or for example a so-called voith- schneider propeller apparatus.

Primarily three types of tug boats appropriate for escort and/or harbour towing are considered state of the art, one of them being a so-called tractor tug boat in which the towing winch is positioned on the aft deck and in which the propulsion units have been disposed on the front side to the towing winch, closer to the bow of the tug boat. Another type is a so-called stern drive tug boat in which the towing winch is placed on the fore deck and/or the aft deck and in which the propulsion units have been arranged in the stern of the tug boat. Yet another type is a so-called rotor tug boat in which the towing winch is placed on the fore deck and/or the aft deck and in which the propulsion units have been arranged in an Isosceles triangle below the tug boat. The tractor tug boats, the stern drive tug boats and rotor tug boats thus represent the state of the art technology. Two other types of tug boats are known in the art, one of them being a combination of a prior art tug boat with a single thruster propulsion unit close to the bow of the tug boat. This type is also known as a combi-tug. Another type of tug boat is similar to the combi- tug, but substitutes the prior art, screw connected to a fixed propeller shaft, with a thruster propulsion unit in the stern of the tug boat. This type is also designated as a ship docking module.

The art distinguishes two major escort towing modes, namely the direct towing mode, directly applying the tug boats underwater power to generate a pulling force on the towing cable connection, resulting in a braking force and steering force on the escorted vessel and the indirect towing mode where the hydrodynamic forces acting on the tug boats hull generate said pulling force on the towing cable connection. The direct towing mode is most effective at low escorting speeds, while the indirect towing mode is most effect at escort speeds greater then 6 knots. The indirect towing mode requires a large lateral surface in order to most effectively generate the hydrodynamic (resistance) forces used to generate said pulling force at high speeds.

Dependant on a tug boats design and more specifically tug boat type both towing modes can be deployed over either the front side and stern side of the tug boat.

A drawback particularly related to the stern drive tugs is that although its lateral surface area of the hull is rather large, it is not

advantageous as far as its shape is concerned and the point of application or centre point of lateral resistance of the hydrodynamic forces is located too far back so that required lateral forces are difficult to achieve. Another drawback particularly related to tractor tug boats is the risk of capsizing at high escorting speeds. Yet another drawback particularly related to tractor tug boats is the limited manoeuvrability due to large lateral surface area. Another drawback particularly related to rotor tug boats is that the lateral surface area of the hull is relatively small compared to the other state of the art tug boat designs.

Generally a purpose of the invention can be to provide a state of the art tug boat with superior manoeuvrability and escorting characteristics at high speed as opposed to the prior art. In particular, a purpose of this invention can be to provide the possibility of an increased lateral underwater surface area for tug boats operating in direct and/or indirect towing modes.

More specifically a purpose of the invention can be to fulfil all of the previously mentioned purposes while maintaining excellent manoeuvrability capabilities and the ability to decrease response time for requests to apply steering force on the escorted vessel.

For this purpose in an aspect the invention can embody a tug boat with at least one retractable skeg. At least one arm can be provided for exerting an actuating force on the at least one skeg such that this skeg extends/retracts at least partly from/into a tug boats hull and/or at least partly from/into and/or over another skeg. Such arms in turn can retract and extend from said skeg and/or columns thereof in order to realize a significant reduction in the height used for such a retractable skeg construction.

Embodiments of a tug boat and retractable skegs shall be elucidated here after with reference to the drawings.

Figure 1 is a schematic longitudinal view of a tug boat according to the prior art, to illustrate the position of a tug boats towing point;

Figure 2 is a schematic longitudinal view of a tug boat to illustrate the position of a tug boats propulsion point;

Figure 3 is a schematic longitudinal view of a tug boat to illustrate the position of a tug boats lateral centre point;

Figure 4 is a schematic top view of a tug boat 1, to illustrate the various reference points and planes;

Figure 5 is a schematic top view of a tug boat, to illustrate a tug boat in direct towing mode and indirect towing mode with a towing point on its front side, a propulsion point and a lateral centre point of hydrodynamic forces and the resultant forces acting upon said towing point, said propulsion point and said lateral centre point.

Figure 6a and b are a schematic top view of a towing cable connection between a tug boat and escorted vessel in a direct towing mode and indirect towing mode respectively.

Figure 7 is a schematic longitudinal cross section of a tug boat illustrating its propulsion point, its lateral centre point, the location of the retractable skeg on the opposite side of said propulsion point and the influence of said retractable skeg, when deployed, on said lateral centre point.

Figure 8a - f show schematically and partially in cross section a number of embodiments of a retractable skeg, in longitudinal view with drive mechanics for the retractable skeg.

Figure 9a - f in top view schematically embodiments of a retractable skeg of the invention, wherein corresponding indices refer to the same indices of the figures in fig. 8. In this description exemplary embodiments of a tug boat and skegs of the present invention are shown, by way of example only. These should by no means be considered as limiting the scope of the invention. The drawings are schematic only. In these drawings the same or similar reference signs shall be used for the same or similar parts or features.

The invention enables the successful application of at least one retractable skeg, meaning a skeg which is constructed such that it is possible to retract the skeg at least partially within the hull and/or in or over another skeg or part thereof. The skeg can for example be substantially box shaped and/or plate shaped. This invention allows for an increased lateral surface for tug boats of any type increasing the hydrodynamic lift of the tug boat and hence the pulling force in a towing cable connection and a resulting braking force and steering forces on an escorted vessel when the skeg is deployed.

Another advantage is that by positioning the retractable skeg close, preferably closest to the tow point, the centre point of the lateral surface area is displaced towards the tow boat reducing the tug boats lateral lever and further increasing the tug boats effectiveness. Yet another advantage of the

retractable skeg can be the increased manoeuvrability of the tug boat and a decreased response time when applying steering force during indirect towing mode. Yet another advantage of the retractable skeg can be a reduced wet surface area, reducing hydrodynamic drag with the skeg in an at least partly retracted position.

In this description retracted should, with respect to a skeg, be understood as referring at least to a position of the relevant skeg with respect to the hull of a tug boat and/or with respect to another skeg or part of the skeg in which less than a maximal surface area of the skeg is exposed to water surrounding the tug boat when floating in a normal position

In this description vertical and horizontal are referred to as planes or directions in their ordinary meaning, whereas directions related to the vessel or tug boat defined by horizontally or vertically are taken when the vessel or boat is in a position afloat, in a normal, stabilized position, unless specifically otherwise defined.

Figure 1 schematically shows the contour of a tug boat 1 viewed from the side, with a number of possible towing points 8. A towing point 8 is defined hereinafter as the last physical point on tug boat 1 that fairleads a towing cable 9 from the towing winch 10 to a vessel V being assisted, establishing a towing cable connection 11 between tug boat 1 and assisted vessel V capable of sustaining significant pulling forces. With respect to this towing point 8, the towing cable 9 can at least for example turn sideways through 90° or more in a horizontal plane towards both boards. In case of the towing winch 10, the cable length can be adapted to the desired towing length and manoeuvring distance. On older models of tug boats, there is only a winch and towing point at the stern; in many modern tug boats, a towing point 8 and towing winch 10 are arranged on both front 7 and stern 6. Also such tug boats are often operating with either stern 6 or front 7 in the sailing direction.

Figure 2 schematically shows the contour of a tug boat 1 viewed from the side including the tug boats underwater part, with a number of possible propulsion points 12. A propulsion point 12 is defined hereinafter as the physical point of application of power underwater by tug boat 1. The propulsion point 12 is more specifically defined by the sum of power applied underwater by one or more propulsion units 13. A propulsion unit 13 is defined as an apparatus applying and/or directing power, especially by means of thrust, underwater. Even more specifically, the propulsion point 12 can be defined by the number of propulsion units 13 (e.g. single, twin or multiple), the type of propulsion units 13 (e.g. cyclodial, fixed and/or steerable), the location of the propulsion units 13, and the power delivered by the propulsions unit 13.

Tug boats 1 manoeuvring lever 14 is here defined as the horizontal distance between the relevant towing point 8, used by the cable 9 and the propulsion point 12 in the vertical main plane 2, which extends from stern to bow, in a longitudinal mid plane of the boat, especially of the hull, vertically. It is the leverage that at least largely enables tug boat 1 to swing its front 7 or stern 6 around the towing point 8. The tug boats primary function, pushing and/or pulling, in direct towing mode as shown in fig. 6a, on its towing point 8, is determined by the alignment of propulsion point 12, the towing point 8 and towing cable connection 11 with the escorted vessel in the horizontal plane. In case the resultant power on the propulsion point 12 is aligned with this direction, a most efficient pulling force is obtained. In all other cases a tug boat of any type is unable to apply its power in a most efficient manner. During indirect towing mode, as shown in fig. 6b, the manoeuvring lever should be increased up to a maximum in order to most efficiently achieve the optimum pulling force on the towing cable connection 11.

Figure 3 schematically shows the contour of tug boat 1 viewed from the side and including the underwater part of tug boat 1, with a possible location of the tug boats lateral centre point 15 of hydrodynamic forces. Said lateral centre point 15 is defined hereinafter as the resultant imaginary physical point of application of the lateral hydrodynamic forces acting on tug boat 1. The lateral centre point 15 is neither a fixed location nor are said lateral hydrodynamic forces a constant magnitude. It moves forward and aft as a function of hull shape and angle of water flow of tug boat 1. Speed affects the magnitude of said lateral hydrodynamic forces too.

A tug boats turning lever 16 is here defined as the horizontal distance between the towing point 8 and lateral centre point 15. It is the leverage that is generated by the lateral hydrodynamic resistance acting on tug boats 1 hull and is compensated by the power applied on the propulsion point 12. It is obvious for people skilled in the art that in case to create a most efficient escort tug boat this turning lever 16 should be reduced to a minimum in direct towing mode, while this should be maximized during indirect towing mode.

Figure 4 schematically shows the contour of a tug boat 1 viewed from the top. Viewed in the transverse direction, the tug boat 1 is substantially symmetric relative to a vertical main plane of symmetry 2, extending in the longitudinal direction of the tug boat 1. The tug boat 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 tug boat 1. Herein below, horizontal length positions will be indicated as measured relative to the rear extreme point 4. By reference numeral 5, a transverse vertical plane is indicated in the Figures which is perpendicular to the main plane of symmetry 2, halfway the front and rear extreme points 3 and 4. That intersection of these planes 2, 5 defines a vertical line M which will be referred to as the centre M of tug boat 1, and the vertical plane 5 will be referred to as the transverse centre plane of tug boat 1. Herein below, horizontal width positions will be indicated as measured perpendicularly to and relative to the main plane of symmetry 2. The body portion of tug boat 1 located behind the transverse centre plane 5 will be referred to as the stern 6, and the body portion of tug boat 1 located before the transverse centre plane 5 will be referred to as the front 7. Hereinafter, it is assumed that tug boat 1 is afloat in unloaded condition, and the expression 'horizontal' and 'vertical' are meant as defined before. For purposes of these claims both stern 6 or front 7 revert to the sailing direction Vs of a tug boat and are thus considered inter- exchangeable.

In the prior art one or more propulsion units are usually located on the stern beneath the tug boat by means of a screw connected to a fixed propeller shaft. In this case the power, usually by means of thrust, is applied predominantly in the longitudinal direction of the tug boat. This power is also directed partially sideways by means of rudders. The state of the art tug boats are often equipped with so-called omni-directional propulsion units. In this case the propulsion unit can deliver power in all directions in the horizontal plane in, including, but not limited to, the form of azimuthal propelling units with a nozzle having a screw arranged therein or so-called Voith-Schneider and/or cyclodial propulsion units.

Primarily three types of tug boats appropriate for escort and/or harbour towing are considered state of the art, one of them being a so-called tractor tug boat in which the towing winch is usually positioned on the stern and in which the propulsion point is positioned on a line between two propulsion units closer to the front of said tug boat. Another type is a so-called stern drive tug boat in which a towing winch is placed on the stern and/or the front and in which the propulsion point is positioned on a line with length position closer to the stern of said tug boat. Yet another type is a so-called rotor tug boat in which deck lay-out is similar to said stern drive tug boat and in which the propulsion point is positioned in an Isosceles triangle below the tug boat. The tractor tug boats, the stern drive tug boats and rotor tug boats thus represent the state of the art technology. Two other types of tug boats are known in the art, one of them being a combination of a prior art tug boat with a single thruster propulsion unit close to the front of the tug boat. This type is also known as a combi-tug. Another type of tug boat is similar to the combi- tug, but substitutes the prior art screw connected to a fixed propeller shaft, with yet another thruster propulsion unit on the stern of the tug boat. This type is also designated as a ship docking module.

Figure 5 and 6 illustrate the two major escort towing modes and forces applied by a tug boat 1 on an escorted vessel. During direct towing mode (fig. 6a), tug boat 1 directly applies its underwater power to generate a pulling force 17 on the towing cable connection 11 and a resultant braking force 18 and steering force 19 on the escorted vessel. During indirect towing mode (fig. 6b) the hydrodynamic forces acting on tug boats 1 hull generate said pulling force on the towing cable connection 11. The direct towing mode is most effective at low escorting speeds, while the indirect towing mode is most effective at escort speeds greater than about 6 knots. The indirect towing or escort mode requires a large lateral surface in order to most effectively generate the hydrodynamic (resistance) forces used to generate steering force 19 at relatively high speeds. Dependent on a tug boats design and more specifically tug boat type both towing modes can be deployed over either or both of the front and stern of a tug boat. It is obvious to people skilled in the art that in case of indirect towing or escort mode the tug boats propulsion point is preferably situated in the tug boats stern in order to obtain the oblique angle required to generate the hydrodynamic lateral forces or hydrodynamic lift on the lateral centre point.

Key criteria for escort tug boats include, but are not limited to, the steering force and response time. Said response time is defined as the time between a request for steering force and application of said steering force. State of the art escort tug boats are often, but not always, equipped with a fixed skeg, either plate or box-like, in order to increase the tug boats lateral underwater surface, and thus achieving substantial steering forces during indirect towing mode. A major drawback of said fixed skegs is that it limits the manoeuvrability and thus simultaneously increases the tug boats response time. The present invention combines both the capability of applying an increased pulling force 17 on the towing cable connection 11 during indirect towing mode and decreasing tug boats 1 response time.

The invention relates to a tug boat 1 with one or more propulsion units 13, either fixed, steerable or retractable, located at the front 7 and/or stern 6 of tug boat 1. Tug boat 1 is equipped with an apparatus to reduce tug boats 1 turning lever 16 and increase its underwater lateral surface while simultaneously decreasing tug boats 1 response time compared to state of the art tug boats. Such an apparatus is also more specifically described as a retractable skeg 20. Said retractable skeg 20 is preferably located closest to the towing point 8. More specifically said retractable skeg 20 can be located in a position relatively close to tug boats 1 front extreme point 3.

Figure 8a - f illustrates a side view for a number of different configurations in relation to a retractable skegs 20 interior. A retractable skeg 20 is defined as a device, capable of moving up and down in a vertical longitudinal plane either by moving up and down linearly and/or pivoting in the vertical direction or askew with a vertical direction by means of the invention and capable of sustaining significant hydrodynamic forces associated with high speed indirect escort towing. More specifically such hydrodynamic forces may range between 50 and 100 ton or more in a transverse direction to a longitudinal center plane 2 of tug boat 1. In the present invention the skeg is to be retractable under load of inter alia an escorted vessel and during movement of the tug boat.

A retractable skeg's 20 main advantage as opposed to the other state of the art designs is that it decreases tug boats 1 response time. Furthermore a retractable skeg 20 increases the pulling force in the towing cable connection 11 in a most efficient manner by reducing a turning lever 16 by moving the lateral centre point 15 closer to a towing point 8. More specifically the invention temporarily increases the lateral water surface area, especially or preferably near the front extreme point 3. The additional underwater lateral surface has the added benefit of increasing the magnitude of hydrodynamic (lateral) resistance acting on tug boat 1 and thus positively influencing a pulling force 17 and steering force 19 in the towing cable connection 11. A further advantage is that application of this invention enables the design of a tug boat most suitable for both escort towing and/or harbour towing. During (direct) harbour towing a large fixed skeg limits the manoeuvrability of the tug boat, increasing the tug boats response time. State of the art escort tug boats equipped with large fixed skegs are restricted in harbour towing operations. Yet another advantage of the retractable skeg 20 is a reduced wet surface area in transit, reducing hydrodynamic resistance with the skeg 20 retracted in the tug boats 1 hull. This means that even though the forces acting on the skeg 20 are high, retractability is preferable.

The invention can be embodied at least partly by one or more arms 22 exerting a significant actuating force on a retractable skeg 20, thus enabling the vertical movement of said skeg 20. An arm 22 can be constructed such that in case a skeg 20 is retracted within tug boats 1 hull, said arm 22 retracts within the construction of skeg 20. Figure 8 illustrates different configurations for such actuating arms 22. More specifically and in case of a vertically moving skeg, such arms can comprise or consist of one, two or a multitude of arms transmitting an actuating force on the retractable skeg 20. Such arms can, dependent on design specification, either be telescopic, fixed or moving in nature to further reduce the amount of space occupied by the retractable skeg construction. More specifically said arm 22 could also be constructed in an actuating manner moving a skeg 20 up and down in a vertical direction. Such an actuating arm 22 could for example, comprise or be embodied by a hydraulic arm constituting a cylinder moved by hydraulic means well known in the art.

In embodiments of the present invention preferably the space occupied by a retracted skeg 20 is minimal, enclosed between a deck of the boat and the underlying hull portion.

The invention enables an effective design for a retractable skeg 20 occupying a limited amount of space within tug boats 1 hull. More specifically the height between the retracted skegs 20 construction and tug boats 1 main deck 21 may be limited due to general design specifications of a tug boat. Such design specifications include, but are not limited to, the reinforced structure required to support significant forces acting on tug boats 1 towing point 8 or towing points 8. Furthermore tug boats 1 primary purpose requires a substantial working area on tug boats 1 main deck 22 situated on either front 6 or stern 7 for purposes of establishing effective towing cable connections 11. Constructions extending above tug boats 1 main deck 21 severely reduce the effective working area on tug boats 1 stern 7 or front 6. The invention thus increases the practical applicability of the retractable skeg 20 design concept, as published at the Southampton, UK Tugnology 2007 conference, directly enabling the design of a multi-purpose tug boat 1 suited for both escort and harbour operations.

Another design criterion for the successful implementation of the retractable skeg 20 concept is a short deployment time wherein the skeg 20 should be able to retract or extend within a 5 to 10 second time period and during indirect towing operations, in loaded condition. During indirect towing operations significant lateral forces act on a retractable skeg 20 further increasing friction forces between slides and thus forces required to extend and retract a skeg 20. Alternate inventions, like for example US 4,316,425 are known within the art using a screw spindle to extend and retract a box-like structure within a hull and capable of exerting significant actuating fore on a skeg 20. Yet 4,316,425 is not suitable for the application of this invention due to the response time annotation within the art.

In order to further minimize the actuating apparatus 25 -meaning an apparatus applying power to an arm 22 such that a skeg 20 moves up and down- dimensions, actuating forces should be minimized in line. Unless properly sealed, retracting and extending a skeg 20 in or from tug boats 1 hull results in unacceptable water ingress in tug boat 1. Proper seals are costly, difficult to install properly and prone to leakage. Alternatively a shaft or compartment 23 is constructed as a 'wetted' shaft in combination with a

'closed' skeg 20. A 'closed' skeg 20 meaning that a skeg 20 is constructed in a closed box-like manner such that water can not enter said skeg 20, providing a limited amount of buoyancy by means of water displacement. In case a skeg 20 is subject to water ingress, the actuated weight would increase significantly. Drawback to a closed skeg is that such a construction leaves no room for an arm 22 retracting the skeg 20. In an aspect embodiments of the invention can circumvent this problem by creating a column 24 inside a skeg 20 allowing an arm to extend and retract inside said skeg 20. The permeability of a column 24 increases/decreases in a linear relation to the extended or retracted position of skeg 20 due to the arm retracting in said column 24. This feature significantly reduces water ingress in column 24 and thus reduces excess water weight in skeg 20 to acceptable proportions.

Fig. 8a shows schematically an embodiment of a retractable skeg 20, movable by a telescopic arm 22 as described here above, which can for example be driven by a hydraulic mechanism 25. Alternatively this could be driven by different means, for example pneumatically, electrically, by combustion engine or combinations thereof. In this embodiment the hull is provided, below the deck 21, with compartment, opening downward into the body of water below the hull. In fig. 9a a top view is shown, in which the skeg 20 is shown as a box like embodiment, having centrally a column 24 in which the arm 22 extends. By withdrawing the arm 22 into the compartment 23 the skeg 22 is lifted into the compartment 23, enclosing the arm 22 and mechanism 25 within the column 23. By extending the arm 22 by the mechanism 25 the skeg 20 is extended from the hull into the body of water below the hull. The skeg 20 is led and supported by appropriate guide means, provided by and/or integrated in or provided on at least one of the internal walls the compartment 24, such as but not limited to sliding gears, flat guide surfaces or the like.

In fig. 8b and 9b an embodiment of a skeg 20 and part of a hull is shown, similar to the embodiment of fig. 8a and 9a, but having a skeg guiding portion 26 fixed to a lower side of the hull, comprising part of the compartment 23. The retractable skeg 20 can therefore in the extended position as shown in fig. 8b be supported by the skeg guiding portion 26, thus providing extra support. Alternatively the skeg 20 can be extended further down into the body of water and/or the skeg 20 can have a smaller height 27, thus providing for the possibility in more compact building of the hull and/or skeg 20. Obviously the drag of the portion 26 will add to the overall side drag of the boat. As can be seen in fig. 9b in this embodiment the skeg 20 has an overall wing or torpedo shaped cross section perpendicular to the direction P of retraction and extension of the skeg 20. This will streamline the skeg at least in a normal sailing direction V_s of the tug boat 1. In 8c and 9c an embodiment is shown, similar to that of fig. 8a and 8b, wherein however two arms 22 and drive mechanisms 25 are shown, in two parallel columns 24, such that the skeg 20 can be moved up and down in the direction P, from and into the body of water, into the retracted and extended positions relative to the compartment 23. In this embodiment the skeg 20 has a substantially oblong, rounded shape in the cross section perpendicular to the direction P.

In fig. 8d and 9d an embodiment of a skeg 20 with actuating means and part of a hull of a tug boat 1 are shown, wherein the skeg 20 is connected to the hull by at least one and, in the embodiment shown, two pinion and rack drive mechanisms 25, the racks 27 extending substantially parallel to the direction P within the compartment 23. The racks 27 can extend into columns 24 within the keg 20, especially lower ends thereof when the skeg is extended fully, as shown in fig. 8d. The pinions 28 are connected to the skeg 20, for example on a top side thereof, such that upon rotation of the pinions 28 by a drive mechanism 25, the skeg 20 is moved up or down. As can be seen from fig. 9d in this embodiment the skeg 20 is substantially plate shaped, leading to a low resistance when moving into or out of the body of water.

In fig. 8e and 9e embodiments are shown of a skeg 20, movable in the direction P into and from a compartment 23 by actuating means 22, 25 similar to the means of fig. 8a - c. In fig. 8e only one arm 22 and corresponding mechanism 25 with column 24 is shown, wherein in fig. 9e two parallel systems are shown. In these embodiments at the opposite fore and aft sides of the skeg 20 struts 29 are provided, extending from the hull downward, forming guides for the skeg 20. Moreover these struts can in embodiments form part of a support for the boat when for example resting on a floor. Furthermore the struts 29 can be protective for the skeg and/or hull. As can be seen from fig. 9e the skeg 20 can have a substantially oblong cross section, perpendicularly to the direction P, provided with guide channels 30 at the fore and aft ends 31. The struts 29 have corresponding glide wall elements 32 at facing sides fitting in the channels 30.

In fig. 8f and 9f a further retractable skeg assembly is shown, having two substantially parallel drive assemblies 25, 22, 24 as shown in and discussed in reference to fig. 8a - c and e, wherein the skeg 20 is divided into two skeg elements 20A, 20B, provided the one in front of the other, seen in the fore-aft direction of the hull. As can be seen from fig. 9f, the one skeg element 20A has a channel 30 extending along a side thereof, into which a glide wall element 32 of the other skeg element 20B fits. Each of the elements 20A, B is connected to a drive assembly 22, 24, 25, such that they can be retracted and extended individually into and from the compartment 23. This can be advantageous, for example in reducing drag, in reducing resistance against insertion into and/or retraction from the water body and in further limiting response time.

In embodiments a flap or auxiliary appendage can extend from the at least one retractable skeg, especially from a lower end and/or from sides thereof, opposite to the hull side, which flap or flaps can be rigid or may at least partly be flexible and/or can be connected to the skeg in a movable manner, for example by hinges or slides. Flaps can further increase the lateral surface of the boat or can provide lift or other forces due to movement of the hull relative to the body of water in which the boat floats. Such flap can for example comprise elastic materials, such as elastomers and/or rubber, and can be provided with a core, which may be more rigid than its outer surface.

It shall be understood that all skeg forms as shown can be used in all of the embodiments shown and discussed, as well as that they can be combined. Similarly the different drive mechanisms and assemblies can be used by the different skeg configurations.

The present invention is by no means limited to the embodiments shown and described herein. Many variations are possible within the scope of the invention as defined by the claims. For example all combinations of elements of the embodiments are considered to have been disclosed herein. Moreover, in stead of one or more columns in the skeg, the compartment can form and/or can be provided with one or more columns 24 in which at least part of the drive mechanism can be provided, in stead of or additional to the drive assemblies shown. For example one or more racks can be provided on an outer side of the skeg, the pinion or pinions being provided in the compartment 23. Multiple skegs can be provided all or some of which may be retractable. Skegs may be provided for and aft or side by side under the hull. These and many other variations are considered to have been disclosed herein too.

Claims

1. A tug boat (1) equipped with at least one skeg (20), preferably positioned at the front side (7) of a tug boat (1) and more specifically closest to a front extreme point (3), wherein the skeg (20) is at least partly retractable into a compartment (23) within the hull of the boat (1), and wherein actuating means (25, 22) for the skeg (20) are provided within the compartment (23) and/or within the skeg (20).

2. A tug boat (1) according to claim 1, wherein said at least one skeg (20) is provided with one or multiple columns (24), wherein the actuating means comprise one or more arms (22) for transmitting an actuating force on the skeg (20) capable of moving the skeg (20) up and down in at least a vertical direction and said arms (22) are retractable into and/or extendable from said at least one column (24).

3. A tug boat (1) according to claims 1 or 2, with one or more of the skegs (20) retractably disposed out of and/or on a longitudinal vertical centre plane (2) of the tug boat (l).

4. A tug boat (1) according to any one of claims 1 - 3 , wherein at least one retractable skeg (20) is constructed such that it is possible to partially or fully retract and/or extend said skeg (20) within and/or over another skeg (20) attached to the tug boat (1) or to another skeg (20).

5. A tug boat (1) according to any one of the previous claims, with an auxiliary appendage or movable flap attached to said at least one retractable skeg (20).

6. A tug boat (1) according to any one of the previous claims, wherein the at least one retractable skeg (20) is supported by appendages fixed to a hull of the tug boat (1).

7. A tug boat (1) according to claim 6, wherein said appendages are movable in relation to the hull of the tug boat (1).

8. A tug boat (1) according to any one of the previous claims, wherein the at least one column (24) has at least partly an oblong, semi-circle or oval-like cross-section.

9. A tug boat (1) according to any claim 2, wherein the at least one column (24) has at least partly a rectangular-type cross section.

10. A tug boat according to any one of claims, wherein the actuating means are at least partly hydraulic means.

11. A tug boat according to any one of claims 1 - 10, wherein the at least one skeg (20) is a substantially closed skeg (20).

12. A tug boat (1) according to any one of claims 1 - 11, wherein the actuating means comprise at least two arms or racks, extending substantially parallel to each other and preferably substantially parallel to a direction of movement of the skeg.

13. A tug boat (1) according to any one of claims 1 - 11, wherein the at least one skeg (2) is at least partly pivotable between a retracted position within the compartment and an extended position substantially outside said

compartment.

14. Assembly of a skeg (20), actuating means and compartment (23) for a tug boat according to any one of claims 1 - 13.

15. A method for towing and assisting vessels on a water surface, wherein a tug boat is connected to the vessel by at least one line, wherein during towing and/or assisting the vessel the tub boat is brought from a direct towing mode in an indirect towing mode and/or vice versa, and wherein:

in the indirect towing mode at least one retractable skeg is brought into or held in an extending position, such that it extends at least partly below the hull of the tow boat; and

in the direct towing mode said at least one retractable skeg is brought into or held in a retracted position within a compartment in the hull of the boat, such that in the indirect towing mode the lateral surface of the boat is larger than in the indirect towing mode.