EP2547581B1

EP2547581B1 - Ship with azimuting tractor drive

Info

Publication number: EP2547581B1
Application number: EP20110709410
Authority: EP
Inventors: Jochem Steven De Jong
Original assignee: Bv Scheepswerf Damen Gorinchem
Current assignee: Bv Scheepswerf Damen Gorinchem
Priority date: 2010-03-19
Filing date: 2011-03-21
Publication date: 2015-05-13
Anticipated expiration: 2031-03-21
Also published as: PL2547581T3; EP2371701A1; EP2547581A1; ES2544041T3; DK2547581T3; CY1116408T1; WO2011113964A1

Description

The invention concerns a ship 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. Document US 6698374 describes several embodiments of such a ship. One of the embodiments shows a central rear skeg for course stabilising. When the steering angle is zero the flow at both sides of the central skeg can be from one or from both propulsion and steering units. This asymmetric flow against the central skeg can generate a steering torque on the hull that counteracts the forces generated by the propulsion jets of the propulsion and steering units. This means that the forces on the hull are not well defined and this leads to instability in the course which is undesirable.
In another described embodiment, stabilising skegs are located on both sides of the centreline at the side of the hull. This means that when the ship makes an arched path the stabilising skeg at the inner circle experiences flow at a different angle compared with the stabilising skeg at the outer circle. These angles will decrease when the radius of the arch increases and the ships starts following a straight course; the changing angles lead to changing loads on the skegs unstable flows around near the skegs. In this way the changing flow conditions can lead to instability in course keeping. This instability means difficult and unpredictable steering behaviour. This is a disadvantage in terms of safety in ship handling. In order to overcome the disadvantage the ship is in accordance with claim 1. It has appeared that the indicated position of two rear skegs give the ship predictable steering characteristics while free sailing.
In accordance with an embodiment, the ship is according to claim 2. In this way, the skegs can be shorter which improves turning of the ship while maintaining the course stabilising capabilities.
In accordance with an embodiment, the ship is according to claim 3. In this way, in the situation wherein the ship is sailing a straight course and the steering angle is small each rear skeg is in the flow of the propulsion jet so that along each rear skeg a predictable flow occurs.
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 symmetrical flow on both sides of the rear skegs, which improves the resistance against disturbances.
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.
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.
In accordance with an embodiment, the ship is according to claim 7. In this way, the rear skegs have a minimal flow resistance.
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.
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.
Hereafter the invention is explained with the aid of several 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, and
Figure 5 shows top view of the underwater lay-out of the tug of figure 3.

Figures 1 and 2 show a tug 1 according to the prior art in water with a waterline 3. 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.
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.
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.
Figures 3, 4 and 5 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 a longitudinal axis 20 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 a propulsion jet 21 generated by the propeller 7 and the direction of the propulsion jet 21 makes a steering angle α with the longitudinal axis 20. Rotating the propulsion and steering unit 6 around steering axis 4 changes the steering angle α. 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 embodiment shown in figures 3 - 5 the side skegs 18 are positioned at a distance d from the longitudinal axis 20 of the tug 1 whereby the distance d is approximately 0,25 times a width or the beam w of the vessel and the distance between the side skegs 18 is at least a propeller diameter.
In other embodiments, the location of the side skegs 18 can differ. The side skegs 18 are symmetrical relative to the centreline 20 and the distance between the side skegs 18 is between 30 % and 70 % of the beam w and can be between 40 % and 60 % of the beam w. The front of each side skeg 18 is in the propulsion jet 21 of the propulsion and steering unit 6 at the same side of the tug 1 as long as the steering angle α is smaller than 5 degrees and can also be in the propulsion jet 21 as long as the steering angle α is smaller than 3 degrees.
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. 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. The front of the side skegs 18 is approximately vertical and can have a slight slope backwards, for instance 15 degrees and less than 20 degrees.
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

Ship 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 (w) of the hull, located forward of the amidships two propulsion and steering units (6) with a propulsion drive for rotating propellers (7) around a propeller axis (5) and a steering drive for positioning the propeller axis at a steering angle (α) relative to a centreline (20) of the hull, the propulsion and steering units forming a main active steering equipment, and rearward of the amidships stationary course stabilising or rear skegs (18) located symmetrically on both sides of a centreline of the hull wherein the stabilising or rear skegs (18) have a height extending the depth of the hull characterized in that the distance between the fronts of the stabilising skegs (18) is between 30 % and 70 % of the beam (w) and preferably between 40 % and 60 % of the beam.
Ship in accordance with claim 1 wherein the fronts of the rear skegs (18) are approximately vertical.
Ship in accordance with claim 1 or 2 wherein the rear skegs are located such that the front of each rear skeg (18) is in a propulsion jet (21) generated by the propulsion and steering unit (6) at the same side of the ship as the rear skeg when the steering angle (α) of that propulsion and steering unit is less than five degrees and preferably less than three degrees.
Ship in accordance with claim 1, 2 or 3 wherein the fronts of the rear skegs (18) and the vertical and steering axes (4) of the propulsion and steering units (6) have approximately the same distance from the centreline (20) of the hull.
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).
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.
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.
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).