Ship such as a tug with azimuting tractor drive

Abstract

 The invention concerns a ship such as a tug comprising a hull (2) with a foreship and an aft ship and a length that is less than 3.0 times and preferably less than 2.5 times a beam of the hull, located forward of the amidships two propulsion and steering units (6) with propellers (7) having approximately horizontal rotation-axes forming a main active steering equipment, and rearward of the amidships preferably near the aft ship a stationary course stabilising skeg (18).
In accordance with the invention, two rear skegs located symmetrical on both sides of the hull's centreline form the course stabilising skeg.

Description

[0001] The invention concerns a ship such as a tug in accordance with the preamble of claim 1. Such ships are known as azimuting tractor drive tugs and are in use as stable and easy manoeuvrable platforms for inland, harbour, or offshore towing and ship assistance. The known ships have a central course stabilising skeg and the configuration of the propulsion and steering units and the front of the central skeg lead to a course keeping instability. This instability means difficult and even unpredictable steering behaviour. This is a disadvantage in terms of safety in ship handling. The cause of this instability is the interaction of the two propulsion jets generated by the propulsion and steering units located in front of the central skeg. When steering, the direction of the propulsion jet makes an angle with the centre line of the hull and one propulsion jet creates an asymmetrical flow against the vertical front of the central skeg. This asymmetric flow against the central skeg can generate a steering torque on the hull that counteracts the steering torque generated by the propulsion jets of the propulsion and steering units. This leads to unexpected instability. In order to overcome the disadvantage the ship is in accordance with claim 1. It has appeared that the two rear skegs keep the ship on course and give the ship predictable steering characteristics while free sailing.

[0002] In accordance with an embodiment, the ship is according to claim 2. In this way, the skegs do not generate sideways forces during vertical movements of the skegs in the water if the skegs move in vertical direction relative to the water due to movement of the ship or due to waves.

[0003] In accordance with an embodiment, the ship is according to claim 3. In this way, the propulsion jet of one of the propulsion and steering units can flow between the rear skegs, which leads to increased predictability of the steering behaviour caused by the rear skegs.

[0004] In accordance with an embodiment, the ship is according to claim 4. In this way, while free sailing in a straight line the propulsion jets create a flow on both sides of the rear skegs, which increases the stabilising forces.

[0005] In accordance with an embodiment, the ship is according to claim 5. In this way, the rear skegs extend sufficient deep in the water so that a lateral movement of the hull creates lateral resistance on the rear skegs to improve the course keeping stability of the ship.

[0006] In accordance with an embodiment, the ship is according to claim 6. In this way, the full height of the propulsion jets influences the flow on and/or around the rear skegs, and so the course stabilising influence of the rear skegs improves.

[0007] In accordance with an embodiment, the ship is according to claim 7. In this way, the rear skegs have a minimal flow resistance.

[0008] In accordance with an embodiment, the ship is according to claim 8. In this way, the stabilizing effect of the rear skegs increases during straight course sailing and small steering angles.

[0009] In accordance with an embodiment, the ship is according to claim 9. In this way, the struts and the rear skegs can support the ship during docking.

[0010] Hereafter the invention is explained with the aid of two embodiments of a ship with the aid of a drawing. In the drawing
Figure 1 shows a side view of a tug according to the prior art,
Figure 2 shows a rear view of the tug of figure 1,
Figure 3 shows a side view of a tug according the invention, and
Figure 4 shows a rear view of the tug of figure 3.

[0011] Figures 1 and 2 show a tug 1 according to the prior art in water with a waterline 3 according to maximum draught. The tug 1 has a hull 8 with a foreship 2 and an aft ship 10 and has a length that is less than 3.0 times the beam and approximately 1.9 times the beam. Such a tug 1 is suitable for use in inland, harbour, or offshore towing and ship assistance and offers a stable platform for a crew to work on. On the aft ship 10 there is a bollard 12 and a winch 13 and around the aft ship 10 there is a fender 11; these equipments are used for assisting and towing ships and barges. The tug 1 has a wheelhouse 14 and has all necessary equipment for its intended use.

[0012] The hull 8 has two propulsion and steering units 6 that are located near the foreship 2 forward of the amidships and extending under the hull 8. There is no rudder and there is no other active steering equipment. Each propulsion and steering unit 6 has a propeller 7 that rotates around an approximately horizontal propeller axis 5. The propeller 7 also rotates around an approximately vertical steering axis 4 to change the direction of a propulsion jet generated by the propeller 7 and so steers the tug 1. In addition, for steering the tug 1 the rotational speeds of the propellers 7 can be different so that the starboard and port propulsion and steering units 6 generate different propulsion forces.

[0013] The aft ship 10 is V-shaped; figure 2 schematically shows the section lines 19 indicating this shape. For improving the straight-line stability of the tug 1, the aft ship 10 has a centre skeg 9 that extends in longitudinal direction under the centre of the hull 8 to the stern.

[0014] Figures 3 and 4 show an embodiment of the tug 1 wherein the aft ship 10 does not have one centre skeg 9 but has two side skegs 18 that are located symmetrically to the longitudinal axis of the tug 1. In the shown embodiment, the side skegs 18 are located at the width of the vertical steering axis 4 of the propulsion and steering unit 6 so that they extend in the propulsion jet generated by the propeller 7 and the side skegs 18 are located as far aft as possible. The side skegs 18 extend downwards at least to under the largest depth of the hull 8 and in the shown embodiment they extend as deep as the propulsion and steering units 6. The distance between the side skegs 18 is considerable so that during sailing of the tug 1 there is a stable flow of water between the side skegs 18. In the shown embodiment, the side skegs 18 are positioned at approximately 0,25 times the width of the vessel from the centre line of the tug 1; the distance between the side skegs 18 is at least a propeller diameter.

[0015] The side skegs 18 have a profiled shape with a rounded front and ending with a minimal thickness. Such a profile ensures a flow that makes an inflow angle with the longitudinal centreline of the side skeg 18 generates a lift force. A suitable profile for the described embodiment is a NACA 0018 profile, which makes an inflow angle of approximately 14° possible without loss of lift force. A similar profile with an increased thickness would allow an increased inflow angle but would lead to an increase in flow resistance as well which might be advantageous in some circumstances. A similar profile with a decreased thickness would have reduced flow resistance but would lead to loss of lift force at a smaller inflow angle and would reduce the course stabilising effect of the side skegs 18 but depending on the circumstance might be most suitable.

[0016] For ease of docking the foreship 2 can be provided with struts 15 that can be connected to a dock support 16, these are indicated in figures 3 and 4 with interrupted lines. These struts 15 are in the centreline of the tug 1 between the propulsion and steering units 6 and the dock support 16 has a depth that is more than the propulsion and steering units 6 so that during docking the propulsion and steering units remain free from a docking surface. For further ease of docking the side skegs 18 have the same depth as the dock support 16 so that the underside of the side skegs 18 and the dock support 16 form a horizontal support plane 17. In other embodiments, there are struts 15 that together with the skegs 18 support the tug in dock.

Claims

1. Ship such as a tug comprising a hull (8) with a foreship (2) and an aft ship (10) and a length that is less than 3.0 times and preferably less than 2.5 times a beam of the hull, located forward of the amidships two propulsion and steering units (6) with propellers (7) having approximately horizontal rotation-axes (5) forming a main active steering equipment, and rearward of the amidships preferably near the aft ship a stationary course stabilising skeg characterized in that two rear skegs (18) located symmetrical on both sides of the hull's centreline form the course stabilising skegs.

2. Ship in accordance with claim 1 wherein the rear skegs (18) are approximately vertical.

3. Ship in accordance with claim 1 or 2 wherein a distance between the rear skegs (18) is equal to or more than a diameter of the propellers (7).

4. Ship in accordance with claim 1, 2 or 3 wherein each of the rear skegs (18) extends in a propulsion jet of one of the propulsion and steering units (6) when the horizontal rotation-axis (5) of the propeller and steering unit is directed rearward and parallel to the hull's longitudinal axis.

5. Ship in accordance with claim 1, 2, 3 or 4
wherein the rear skegs (18) have a height extending the depth of the hull (8).

6. Ship in accordance with one of the previous
claims wherein the rear skegs (18) have a height extending to approximately the depth of the propulsion and steering units (6).

7. Ship in accordance with one of the previous
claims wherein the rear skegs (18) have a horizontal length of approximately 0.3 - 0.5 times their height.

8. Ship in accordance with one of the previous
claims wherein the two rear skegs (18) have a profiled shape with a rounded front and ending with a minimal thickness.

9. Ship in accordance with one of the previous
claims wherein between the two propulsion and steering units (6) one or more struts (15) extend downwards to below the underside of the propulsion and steering units and wherein the bottom ends of the rear skegs (18) and the bottom ends of the struts are in a horizontal plane (17).

Drawing