NO340466B1 - Arrangement of stern on a vessel with two swivel nozzle propellers - Google Patents

Description

Technical field of the invention

The invention includes a method for designing a stern on a vessel, where the vessel is equipped with two rotatable nozzle propellers and a rudder or a Voith-Schneider propeller and where the vessel operates at a given speed and the vessel is given with the main dimensions Lpp (long between the perpendiculars) and B (breadth), measured from a construction waterline, and where the vessel has a tapered stern, a stern and two vessel sides.

Background for the invention

In the following, a number of terms within ship terminology will be used. These terms are defined as follows: Vessel a term for a means of transport that goes to water, a ship or a larger boat that is suitable for use in the sea. The maritime rules define vessels as "any vessel, including non-displacement vessels, seaplanes and WIG vessels, which is used or can be used as a means of transport on the water". The real meaning of the term is "tool to travel/travel with".

In its modern meaning, ship usually denotes a larger ocean-going vessel, intended for the transport of people and/or goods.

Displacement is a measure of the mass of the fluid volume that a body displaces when it floats in a fluid. The term is used as one of several measures of ship tonnage and, according to Archimedes' principle, it corresponds to the total weight of a ship with cargo. A vessel in water will therefore displace a quantity of water equal to the vessel's displacement.

The hull of ships and boats is the floating part of the ship. Different types of ships have different hull shapes according to their needs. The need is set by the desired speed, desired load capacity, desired seaworthiness. The hull of a ship includes the keel, frames (inner ribs), cladding (skin), decks and fixed fittings, but not rigging, machinery and rudder. Ships and boats can have one, two or more hulls.

The perpendiculars (main dimension) are a pair of vertical lines used when measuring ships. A ship's length is usually given as Lpp (length between the perpendiculars) or LOA (length over all). Forward perpendicular is set vertically through the point where the structural waterline meets the ship's hull. Aft perpendicular (AP) runs as a vertical line through the ship's rudder stem. The length between these perpendiculars is called Lpp.

The KVL construction water line. This is the waterline the ship must float on at the load it is designed for.

Aft denotes the rear part of a ship, aft across, i.e. everything behind amidships.

The stern is the rear part of the ship. The part that is under water is designed in such a way that the least possible turbulent water and waves are created behind the ship

Transom is the flat or slightly convex surface across the longitudinal direction at the very stern, which is found on many boat or ship hulls.

Line drawing is used in shipbuilding to describe the shape of the hull. It consists of three standard parts: A profile (also called an elevation) which shows a longitudinal section of the hull, a frame plan which shows a cross section of the hull from both the front and rear, and a waterline plan which shows horizontal sections of the hull running parallel to the waterline. A frame drawing is part of a line drawing for a ship, and shows the cross-section of the hull seen from the front and rear.

A vessel's hull can generally be classified into three categories, based on the shape of the bottom below the waterline; a displacement hull for low speeds, a semi-planing hull for medium speeds, and a planing hull for high speeds.

The displacement hull sails mainly under hydrostatic pressure with the bottom sunk deep below the waterline. In order to reduce frictional and wave resistance, the hull is often streamlined, and generally has a real cross-section that is maximum at the middle and gradually decreases towards the end of the stern. When a hull of this type is operated at a relatively high speed, it is exposed to dynamic pressure of the water which seeks to lift the front part up and pull the rear part down. At low speed, however, the situation is that the water flows along the sides, moves backwards along the tapered stern and inwards behind the transom, forming a vortex which applies a resistance to the hull which seeks to pull it backwards. For the aforementioned reasons, vessels today are often built with an optimal operating speed where the hull is designed for given operating speeds that provide the least possible resistance in the water.

A vessel moving forward in the water must overcome a resistance from the water, equal to the force exerted by the propeller, or that required to tow the vessel at the same speed. The resistance has three main components which are calculated separately: Friction occurs between the surface of the hull (the skin) and the water. Frictional resistance is proportional to the vessel's "wet surface" and approximately proportional to the square of the speed. The frictional resistance also strongly depends on the roughness and fouling of the bottom.

Wave resistance, which is due to the constant supply to the water of kinetic energy represented by the waves from the ship. The wave resistance depends on the vessel's streamlined shape and increases with a higher and variable power of the speed.

Shape resistance, which is caused by local vortices behind the stern, welds, rivets, propeller boss, rudder, etc.

Normally, frictional resistance and wave resistance are considered to be the most dominant resistance components. Little or no weight is added to the shape resistance in the design and construction of ocean-going vessels.

When a streamlined hull moves in a straight horizontal line at constant speed, deeply immersed in water, currents will occur around the hull. The currents around the hull can be divided into two main groups; laminar flow and turbulent flow. Laminar flow is characterized by the water flowing along smooth lines in an ordered relationship with minimal frictional resistance. For a typical vessel, laminar flow exists for only a small distance along the hull. As the water flows along the hull, the laminar flow begins to break down, becoming chaotic and well mixed. This chaotic behavior is referred to as turbulent flow. Turbulent flow is characterized by the development of a layer of water along the hull that moves at the ship's speed. This layer of water is referred to as the boundary layer, which is a form of frictional resistance that all objects in water have. Water molecules closest to the vessel are set in motion at the ship's speed. For a vessel in motion, the boundary layer becomes visible as a white foamy band along the vessel's sides. If the speed of the vessel increases, the thickness of the boundary layer will increase and the transition between laminar and turbulent flow will gradually move closer to the bow, thereby increasing the resistance acting on the vessel.

At a point along the hull, the boundary layer will separate from the hull. This

the separation usually occurs at the aft end where the curvature of the hull is too great for the boundary layer to remain attached to the hull. A "space" is formed between the smooth flowing water and the hull which is filled with eddies. This resistance is described as form resistance. The flow separation point is a function of

the design of the hull and the ship's speed. A hull that has smooth lines aft will have a separation point that is further aft and tends to have a narrower boundary layer with less drag than a hull that has irregularities that cause the flow to separate from the hull.

As described above, the shape of a hull can greatly affect the amount of viscous pressure resistance acting on the hull. Naturally, a ship with a short length and a large beam will have greater resistance than a ship with a long length and a small beam. In addition, ships that are fuller at the bow will have greater resistance to shape than ships with narrow bows. The shape resistance that occurs at the stern of the vessel will vary depending on how the stern is designed.

However, it is known today that the hull must be streamlined so that there is the least possible resistance when the vessel moves through the water. Vessels are often designed with displacement and load capacity in mind. There are also calculation methods for calculating friction and wave resistance, which are considered to be the most dominant of the resistance components. Normally, a displacement hull has a tapered aft section, i.e. the rear part of the vessel behind amidships tapers towards the stern. This narrowing of the stern is in many cases designed with regard to experience, tradition and appearance. Little consideration has been given to the form resistance in order to reduce fuel consumption and power output.

US 3,983,829 describes the design of the aft end of a displacement hull. The hull has a tapered stern, where the vessel sides have a stern line in a waterline plane that forms an angle in relation to the vessel's longitudinal direction. The stern line is tangent to a stern line at the stern of the vessel, where the stern line is a line from a point below the construction waterline to an intersection of the construction waterline at at least one of the vessel's sides.

Amdahl, J et al. Compendium in TMR4105 - Marine engineering 1, Department of marine engineering, NTNU. Editor Fuglerud G., 2005, 3rd edition, 2nd edition, pages 215-216 describes theory related to the flow conditions around a ship describes that it is well known that curvature changes affect the water flow along a hull. At the stern of a ship, a phenomenon called separation occurs due to large changes in curvature which cause the streamlines to "leave" the hull. This separation means that a negative pressure is formed in the area, and that a reversed flow occurs in the form of eddies that follow the ship, called a wake.

Pettersen, B. Compendium in TMR4274 - Marine Engineering 3 - Hydrodynamics, Department of Marine Engineering, NTNU, 2007, pages 2.24-2,28 describes flow around bodies and explains the separation point on a body - a function of design and the relative speed between the flow and the body. This point, which lies slightly aft along the hull, will therefore give less wake and resistance as a result of separation.

Consequently, there is a need for a solution related to the design of the stern of a vessel where the above-mentioned problems with form resistance around the stern are overcome and where the vessel can operate more economically and with reduced fuel consumption.

The purpose of the invention is to remedy or reduce at least one of the disadvantages of the current design of stern ends on vessels, or at least to provide a useful alternative to known technology.

The purpose is achieved by features that are stated in the description below and in subsequent patent claims.

Summary of the invention

An overall problem the invention seeks to solve is to design an improved stern for a vessel that has an angle at the stern that eliminates the form resistance acting on the hull.

Another problem the invention seeks to solve is to reduce fuel consumption for a vessel in transit.

Another problem the invention seeks to solve is to provide a design for the stern of a vessel that provides increased payload and better utilization of available space, as well as providing a stable and safe work platform for all forms of offshore operations.

Another problem the invention seeks to solve is to provide a directionally stable vessel with better predictability during manoeuvring.

In a first aspect, the invention relates to a method for designing a stern on a vessel where the vessel operates at a speed (V) and the vessel is given with the main dimensions length Lpp and width B, measured from a construction waterline, and where the vessel has a tapered stern, a stern and two vessel sides.

In what follows, a stern line for at least one of the vessel's sides shall be understood as a line in a waterline plane for the stern of the vessel with an angle C in relation to the ship's longitudinal direction and which is tangent to a stern line in the stern of the vessel.

In what follows, a stern line shall be understood as a line from a point below the construction waterline in the stern, to an intersection of the construction waterline at at least one of the vessel's sides.

The angle C on the waterline plane is determined from the speed (V) of the vessel and a constant water filling speed (F) 90 degrees to the direction of movement, with the formula tanC = V/F or C=arctan(V/F).

When a vessel moves through the water, a layer of water called a boundary layer will form along the hull as described in the background technology. At a point along the hull, the boundary layer will separate from the hull. This separation usually occurs aft or near the stern, where the curvature of the hull is too great for the boundary layer to remain attached to the hull. A space is therefore formed between the smooth flowing water and the hull which is filled with eddies. These eddies apply a resistance to the hull which seeks to pull it backwards, it therefore becomes harder for the vessel to maintain the same speed. In other words, the effect is the same as if the vessel were towing on water. We call this resistance entrainment. Entrainment is another term for form resistance. Vessels moving forward must therefore overcome this extra load, in the form of drag, and this is done by increasing the force exerted by the propeller.

A vessel moving through the water will displace a quantity of liquid equal to the vessel's displacement. For each ship's length the vessel moves, it must therefore be "replenished" with water in the space between the smooth flowing water and the hull, which is equal to the displacement of the vessel. In what follows, this phenomenon will be described as water filling the stern.

An overarching problem that the invention seeks to solve is to provide a method for designing an acting on a vessel that eliminates entrainment, i.e. that the vessel "drags" on water. This is achieved with a design of the stern of the vessel according to the invention which provides optimal water filling around the stern.

Water is a liquid that has definite volume but indefinite shape. We basically think that the water is at rest and that the vessel moves through the water. When a level difference occurs in the water, the surrounding water will try to equalize the difference with the help of gravity, so that the water will always move towards the lowest level. A vessel moving through the water will therefore, for every length of ship the vessel moves through the water, leave behind a level difference equal to the ship's displacement. Displacement is a measure of the mass of the volume of water that the vessel displaces when it floats in water. For every length of ship the vessel moves, it must therefore be topped up with water equal to the displacement of the vessel. The surrounding water will immediately try to equalize this level difference, and a water flow will occur inwards towards the hull, across the vessel's direction of movement. For a vessel moving at a given speed, a water filling will therefore occur which occurs at 90 degrees to the direction of movement and inwards towards the hull. Optimal water filling will therefore exist in those cases where the water flow inwards towards the vessel hits the vessel's hull side and vortex formation at the stern of the vessel is avoided. This can be achieved by the vessel's stern is designed for optimal water filling.

If the design of a vessel's stern does not allow such water filling, but that the flow of water towards the hull does not hit the hull, eddies and a drag will occur which make the vessel heavier to drive because it "drags" on the water.

Optimal water filling can be easily demonstrated using a model test of a towboat. The model is applied with wet paint along the hull below the waterline. When the model moves through the water, which is initially at rest, paint will be dragged out into the pile and onto vertical surfaces. If the design of the hull at the aft end does not allow the boundary layer to remain attached to the hull and a "room" with eddies is formed, the paint in this section will remain untouched. The water filling is not optimal and the test shows that the model has a drag equal to the model's speed through the water.

For a vessel moving at a given speed, a water filling will occur which occurs at 90 degrees to the direction of movement and inwards towards the hull. Full-scale practical trials show that the water's filling speed is approximately 5m/s (10 knots) regardless of the speed of the vessel.

In an embodiment according to the invention, the filling speed of the water can be in the range of 4-6 m/s, 90 degrees to the direction of movement of the vessel.

In a preferred embodiment according to the invention, the filling speed of the water is

(F) constant equal to 5m/s, 90 degrees to the direction of movement of the vessel.

This can be illustrated using vectors that form a right-angled triangle, where

the speed (V) of the vessel is called b, and the filling rate of the water (F) is called c. Both the speed of the vessel and the filling rate of the water are known factors. This provides a tool where the angle of the waterline plane in the stern is determined at a specific operating speed.

The angle C on the waterline plane is determined from the speed of the vessel and a constant water filling rate 90 degrees to the direction of movement, with the formula tanC = V/F or C=arctan(V/F). The angle C is the angle between a line on the waterline plane between the longitudinal direction and the hull shape in the form of the stern line at the stern of the vessel.

In an embodiment according to the invention, the stern line is a line from a point in the cross-sectional plane of the stern below the structural waterline, to an intersection point on the structural waterline at at least one of the vessel's sides.

In a preferred embodiment according to the invention, the stern line is a line from the lowest point in AP (stern perpendicular) on the hull to an intersection point in the construction waterline on at least one of the vessel's sides, and where the stern line lies in the cross-sectional plane of the stern. This stern line also forms one of the vessel's stern sections or one of the sides in the transom. According to the invention, in a given waterline plane, the stern of the hull (the stern line) will have an angle C in relation to the longitudinal direction of the vessel and be tangent to the stern line in the stern. By moving in different planes parallel to the waterline, it is possible to outline the line drawings showing horizontal sections of the hull.

Although lines, vectors and right-angled triangles are described here, it will be obvious to a person skilled in the art that the vessel's sides and end edges are rounded so that the hull is as streamlined as possible.

In another embodiment according to the present invention, the stern line is a curved line.

In another embodiment according to the present invention, the stern line is a line in different vertical planes, i.e. that the stern line has an angle in relation to the cross-sectional plane. This means that the stern and the transom do not necessarily have to be vertical, but also inclined.

In yet another embodiment according to the present invention, the stern line is a curved line.

A transom designed in accordance with the invention provides increased payload and better utilization of available space in the transom by the stern end and the deck being relatively large. At the same time, the stern part provides a stable and safe working platform during all forms of offshore operations.

In a second aspect, the invention relates to a device for a stern on a vessel where the stern end is designed according to the above method, and where the device further comprises; two rotatable nozzle propellers and an intermediate Voith Schneider propeller for propulsion and dynamic positioning. The combination of rotatable nozzle propellers and Voith Schneider provides a stable and safe work platform for all forms of offshore operations.

A Voith Schneider propeller is characterized by being both a drive and steering mechanism, so that a rudder is redundant. The Voith Schneider propeller is used on ferries and tugs where high maneuverability is of particular importance. The blades are mounted vertically on a rotating disc that lies flush with the bottom of the ship. The blades constantly twist in relation to the disc so that they get a suitable angle of incidence and thereby push the water in the desired direction. In similar applications, screw propellers are also used that can be rotated about a vertical axis, often called rudder propellers.

The rudder can be installed in the usual way, but can also be looped if the Voith Schneider propeller is used to steer with. Voith Schneider propellers can be used to control the stern in DP because the propeller is quick to correct movements in the x- and y-axis. Submersible thrusters are often too slow to react to changes in the stern's movements.

AP (stern perpendicular) is, as described earlier, a vertical line through the vessel's rudder stem. According to an embodiment of the present invention where a Voith Schneider propeller is used, AP is defined as a vertical line through the central axis of the Voith Scheider propeller.

In a preferred embodiment according to the invention, the nozzle propellers can be turned from a vertical to a horizontal position, and provide thrust both in the horizontal plane and in the vertical plane. In horizontal position, the nozzle propellers can be used as active dampeners of sway. This provides a safer work platform and a significant reduction in time lost in bad weather.

In an embodiment according to the invention, the attachment of the nozzle propeller to the hull is angled in relation to the horizontal plane in order to operate freely.

In a preferred embodiment according to the invention, the attachment of the nozzle propeller to the hull is angled 20 degrees to the horizontal plane.

In yet another preferred embodiment according to the invention, the Voith Schneider propeller is arranged in a recess at the stern of the vessel. In icy waters, the nozzle propellers can be set horizontally to blow away the ice, while the Voith Schneider propeller provides propulsion.

In yet another preferred embodiment according to the invention, both the Voith Schneider propeller and the nozzle propellers have electric propulsion motors. Electric propulsion motor can be placed directly on the shaft of the nozzle propellers. One main engine can drive both nozzle propellers during transit. If mechanical power transmission is chosen, this advantage is lost.

In another embodiment, the nozzle propellers can have a mechanical forward operating system.

A professional will agree that the final design of the stern depends on the space requirements for the jet propeller's engines and the Voith Schneider propeller.

Brief description of the drawings

In what follows, an example of a preferred embodiment is described which is illustrated in the accompanying drawings, where;

Fig. 1a - shows a section in a waterline plane for a stern according to the present invention where a right-angled triangle illustrates the waterline plane's angle C. Fig. 1b - shows a line drawing of a waterline plane for the hull showing horizontal sections of the hull running parallel to the water line. Fig. 2a shows a cross-section of a vessel at AP and the tang lines that form the vessel's stern section. Fig. 2b shows a frame view of the hull seen from the rear with the nozzle propellers in a vertical position. Fig. 2c shows a frame view of the hull seen from the rear with the nozzle propellers in an approximately horizontal position. Fig. 3 shows a vessel with a stern designed according to the present invention.

Detailed description of the solution shown in the figures

Figure 1a shows a section in a waterline plane 14 for a stern 11 for a vessel 10 according to the present invention. The vessel is given with the main dimensions length Lpp and width B measured from a construction waterline, KVL 19. The figure shows the angle C on the waterline plane 14 based on the speed V of the vessel and a constant water filling rate F. This is illustrated using vectors that form a right-angled triangle where the speed of the vessel V is measured b, and the filling speed F of the water is measured c. The water filling speed F is constant approximately equal to 5m/s, 90 degrees to the direction of movement and towards the vessel's 10 stern 11. The angle C is calculated with the formula tanC=V/F or C=arethane(V/F).

A stern line 15 describes the curvature of the hull at the stern end 11 in a given waterline plane 14. A stern line 15 for at least one of the vessel sides is a line 15 in a waterline plane 14, with angle C in relation to the longitudinal direction of the vessel, and which is tangent to a stern line 16 in the stern 12 of the vessel 10. The stern line 15 according to the present invention can be a straight line but also a curved line as shown in figure 1 b.

Figure 1b shows a line drawing of a waterline plan 14 for the aft end 11 of the hull. The waterline plane 14 shows horizontal sections of the hull that run parallel to the waterline. The figure shows a preferred embodiment according to the invention where the stern line 15 is a curved line. The figure shows a recess in the hull for the placement of a Voith Schneider propeller 17, and two rotatable nozzle propellers 18 are attached to the stern end 11. The figure also shows the nozzle propellers 18 in a horizontal position.

Figure 2a shows a stern section 12 in the AP (aft perpendicular) cross-sectional plane, where AP runs as a vertical line through the vessel's rudder stem 10 or in this case through the center axis of the Voith Schneider propeller. A stern line 16 is a line 16 from the lowest point AP on the hull to an intersection point on the construction waterline 19 at at least one of the vessel's sides. The design waterline 19 is the waterline the vessel 10 must float on at the load for which it is designed. The stern line 16 thus forms the stern 12 of the vessel 10, where the stern lines 16 on each of the vessel sides form the side edges of the transom 13. The stern line 16 can be a straight line but also a curved line as shown in Figure 2b.

Figure 2b shows a frame view of the hull seen from the rear. As shown in the figure, the stern line 16 is a curved line which forms the end edges/corners of the stern 12. Corners and end edges are slightly rounded to obtain a streamlined shape. The nozzle propeller's 18 attachment to the hull is angled 20 degrees on the horizontal plane to operate freely.

Figure 2c shows an actuation of a vessel according to the present invention. The stern line 16 is shown in Figure 2 as a slightly curved curve. Edges along the hull are rounded and streamlined. The figure also shows various waterline planes 14 for the vessel in the form of horizontal sections of the hull that run parallel to the waterline. The figure also shows that the attachment of the nozzle propellers 18 to the hull is angled in relation to the horizontal plane and that the nozzle propellers can be turned to an approximately horizontal position.

The vessel in figures 1b, 2b, 2c, and 3 is equipped with two rotatable nozzle propellers 18 and an intermediate Voith Schneider propeller 17. The Voith Schneider propeller 17 is arranged in a recess in the stern end 11 of the vessel 10. The nozzle propellers 18 can be turned from vertical to horizontal position, and give thrust both in the horizontal plane and in the vertical plane. The attachment of the nozzle propeller 18 to the hull is angled 20 degrees on the horizontal plane in order to operate freely, other angles may also be appropriate for attachment of the nozzle propeller, depending on the type of forward operating system and space requirements. In Figure 2c, the nozzle propellers 18 are shown in a substantially horizontal position with thrust in the vertical plane. This is particularly appropriate in dynamic positioning operations, where the nozzle propellers 18 are used as active dampeners of sway.

Figure 3 shows a three-dimensional sketch of the stern end 11 of a vessel according to the invention. Both the stern line 15 and stern line 16 are shown in Figure 3 as a slightly curved curve. As shown in the figure, the nozzle propellers are angled 20 degrees to the horizontal plane to operate freely. The nozzle propellers 18 can also be turned from a vertical to a horizontal position. The Voith Schneider propeller 18 is arranged in a recess in the stern end 11 so that parts of the transom 13 fall away. The figure also shows different waterline planes 14 for the stern section 12 of the vessel. The stern line 16 forms the side edges of the transom 13 and appears in the figure as a curved line. The vessel is shown with a flat or U-shaped keel, alternatively the keel can also be pointed or V-shaped (not shown).

Claims (7)

1. An arrangement of a stern (11) on an offshore vessel which is equipped with two rotatable nozzle propellers (18) and a rudder or a Voith-Schneider propeller (17), characterized in that the stern (11) of the vessel (10) is designed as follows that the nozzle propellers (18) can be turned from a vertical to a horizontal position and provide thrust both in the horizontal plane and in the vertical plane. 2. Arrangement of the stern end (11) of an offshore vessel according to claim 1, where the attachment of the nozzle propeller (18) to the hull forms an angle of 20 degrees on the horizontal plane in order to operate freely. 3. Arrangement of the stern end (11) of an offshore vessel according to claim 1 or 2, where the nozzle propellers (18) have a mechanical forward operating system. 4. Arrangement of the stern end (11) of an offshore vessel according to one of claims 1 to 3, where said Voith-Schneider propeller (17) is arranged on a surface in a recess in the stern end (11) of the vessel (10). 5. Device at the stern end (11) of an offshore vessel according to claim 4, where the surface is located at such a level that the lower end of the Voith Schneider propeller (17) is level with the vessel's bottom plane. 6. Arrangement of the stern end (11) of an offshore vessel according to one of claims 1 to 5, where the Voith Schneider propeller (17) and the nozzle propellers (18) have an electric propulsion motor. 7. Arrangement of the stern end (11) of an offshore vessel according to one of claims 1 to 6, where a stern line (15) for at least one of the vessel sides, where a line (15) in a waterline plan (14) for the stern end (11) of the vessel (10) with an angle C in relation to the longitudinal direction of the vessel (10) and which is tangent to a stern line (16) in the stern (12) of the vessel (10), where the angle C on the waterline plane (14) is determined from the speed (V) of the vessel (10) and a constant water filling speed (F) 90 degrees to the direction of movement, with the formula tanC = V/F or C=arctan(V/F), and where the stern line (16) is a line from a point below the construction waterline (19), to an intersection on the construction waterline (19) at at least one of the vessel's sides.