[0001] The present invention relates to a marine vessel flow modifying device and a marine vessel provided with such a device. The purpose of the invention is to increase marine vessel fuel consumption efficiency.

[0002] Prospective legalisation concerning shipping dictates that C0 ₂ emissions should be reduced by 30% by 2025 and it is expected that a further efficiency increase by 2050 may be required.

[0003] Among the measures taken to date to increase fuel consumption efficiency are so called pre-swirl devices which, when connected to the exterior of the hull of a marine vessel, upstream of the propeller, alter the flow reaching the propeller to enhance its effectiveness. Fig. 1 shows a device referred to as a wake equalising duct which is positioned immediately upstream of a vessel’s propeller and tapers rearwardly in order to increase the axial velocity of flow which passes through the duct while decreasing the velocity of the flow passing immediately outside the duct. This in turn increases the propeller inflow axial velocity in the upper section of the propeller disc, making the propeller inflow velocity distribution less unequal thereby reducing propeller vibrations and the risk of cavitation. A further prior art approach is the use of reaction fins, such as those shown in Fig. 2, which provide pre-swirl to the propeller inflow which also contributes to increasing the propeller efficiency and reducing cavitation. A still further prior art approach is to provide a twisted Mewis duct, such as that shown in Fig. 3, which is essentially a combination of a wake equalising duct and reaction fins. While such devices provide some increase in propeller efficiency they do not provide efficiency improvements of the scale now sought.

[0004] When a vessel is travelling through a viscous flow, a boundary layer is formed in the immediate vicinity of the vessel’s bounding surface. For a vessel subject to an incoming flow, in the boundary layer, the velocity of fluid particles varies from zero near the vessel surface up to the vessel’s speed at the outer limit of the boundary layer. The aft portion of the bilges of a vessel narrow towards the propeller and in this region there is a tendency for the boundary layer to thicken. The consequence of this thickening of the boundary layer is that the amount of fluid dragged along by the vessel increases. Furthermore, as shown in Fig 4, separation of the boundary layer from the vessel can occur. In ship hydrodynamic analysis it is customary to consider the vessel as being static and subjected to a so-called incoming fluid flow. Fig. 4 is a schematic waterline cross-section through an aft region of a vessel subject to an incoming fluid flow of velocity U. Fig. 4 shows schematically a shoulder 6 of the hull 2 of the vessel which is the point at which narrowing of the hull commences at the waterline. At locations A to D velocity profiles are provided which show, by means of the lengths of arrows, the velocity of the flow relative to the hull 2 at various distances from a hull 2 of the vessel. The directions of the arrows show the direction of the flow relative to the hull 2. The thickness of the boundary layer 14 at the locations A, B and C are shown and are respectively t-i t ₂ and t ₃ and increase with greater distance aft of the shoulder 6. At location A a so-called “full velocity” profile can be seen which will typically be the velocity profile 8 along the majority of the portion of the vessel having parallel sides. In the full velocity profile 8 the velocity 10 close to the hull 2 is low and increases steadily to velocity 12 at the outer limit of the boundary layer 14, corresponding to the velocity of the vessel through the water 4 downstream of location A, as shown at locations B and C, the thickness of the boundary layer 14 increases to t ₂ at location B and t ₃ at location C.

[0005] At location D, an adverse pressure gradient causes the flow 14 to separate from the hull 2 and reverse, as shown by region 20 of the location D velocity profile. Low and especially reverse flow in the proximity of the propeller are particularly disadvantageous as they increase the vessel’s drag and decrease the propeller efficiency. The thickening of the boundary layer also increases the drag experienced by the vessel. A more pronounced thickening and possible separation of the boundary layer which occurs for a vessel without the device according to the invention will be described further below with reference to the drawings, some of which show the nominal wake-fields of vessels with and without the device according to the invention. The nominal wake-field is the wake-field that would exist around a vessel without propeller(s) which is situated in a fluid flow.

[0006] According to the invention there is provided a marine vessel flow modifying device positioned upstream of a propeller of the vessel including a flow modifying portion spaced outwardly of an outer surface of a hull of the vessel and having a leading edge defining a series of protuberances. Such a device has been found to effectively reduce the tendency of a boundary layer of a vessel to thicken, thereby resulting in a “fuller” velocity profile in the flow reaching the propeller. Furthermore the tendency of the boundary layer to separate from the hull, resulting in reverse flow occurring adjacent to the hull, is minimised or at least delayed. This in turn leads to a reduction in hydrodynamic drag experienced by the vessel. In general this effect is achieved by the interaction of the device with the bilge vortex. This vortex interaction scatters the bilge vortices across the boundary layer. This phenomenon also contributes to providing a more uniform flow over the area of the propeller disc leading to a more optimum loading on the propeller which in turn leads to propeller vibration reduction and an improved fuel consumption efficiency. For a given vessel speed, the propeller can be rotated at a lower rate. Also propeller cavitation and vibration are reduced leading to longer service lives for the components concerned.

[0007] Preferably the flow modifying portion has a cross-section with a rounded leading edge and a tapering trailing portion with a trailing edge. Such a form permits the advantages referred to above to be achieved without the device itself contributing to drag to an undesirable extent. The profile could for example be a NACA (National Advisory Committees of Aeronauts) hydrofoil or aerofoil profile such as a NACA 63 profile. Other hydrodynamic profiles such as IFS (Increased Foiling System) profiles are also suitable.

[0008] It has been found that the device is more effective if the series of protuberances defines a repeated pattern of protuberances more preferably the repeated pattern of protuberances defines a wave form. The repeated pattern of protuberances may advantageously define a sine-wave form.

[0009] Preferably the flow modifying portion constitutes a foil which has an angle of attack in the range of 3° to 20°, with the leading edge further from the hull than the trailing edge. Such an angle of attack results in the advantages explained above being achieved effectively without contributing an undesirable amount of hydrodynamic drag. The angle of attack could however be the other way round with the leading edge closer to the hull than the trailing edge.

[0010] Preferably the flow modifying portion has an average chord length C _av which is in the range of 0.5% to 20% of a maximum laden waterline beam of the vessel. The chord length can be measured in the direction of flow over the flow modifying portion and/or perpendicular to a longitudinal axis thereof which may extend substantially perpendicular to flow over the flow modifying portion. [0011] The optimum wavelength of the wave form will be partly determined by the size of the vessel and the device to which the device is connected and other device parameters. Preferably the wave form has a wavelength of between 1% and 30% of the average chord length C _av .

[0012] Preferably the flow modifying portion has a longitudinal axis which is disposed substantially perpendicular to streamlines flowing past the flow modifying device. This is a further factor which has been found to contribute to achieving the effects described above.

[0013] In order to achieve optimum intermingling of bilge and other vortices and optimum mixing of high and low velocity portions of the wake which leads to a fuller velocity profile with reduced velocity variation over the profile of the propeller, preferably the flow modifying portion has a vertical extent in excess of the diameter of the propeller.

[0014] Due to the fact that a typical vessel bilge shape in a region forward of the or each propeller results in flow with an upward component in a region forward of the or each propeller, preferably a horizontal centreline of the flow modifying portion is positioned below the level of a rotational axis of the propeller which is situated downstream of the device.

[0015] The longitudinal position of the device relative to the hull of the vessel has an effect on the extent of the advantages provided thereby. Where the design or ballasted waterline length of the vessel measured from an aft perpendicular of the vessel is L, and the flow modifying device is situated a distance x forward of the aft perpendicular, x/L preferably lies in the range 0.4 to 0.05 and more preferably in the range 0.3 to 0.1.

[0016] Preferably the device is positioned aft of a shoulder of the vessel, the shoulder being at the location of the hull where narrowing of the hull towards the stern starts to take place.

[0017] The device is most effective if it is positioned wholly within the boundary layer at the location of the device. [0018] For a typical vessel preferably the flow modifying portion has an average offstand of between 0.3% and 10% of a maximum waterline beam of the vessel, and more preferably between 2% and 8% thereof.

[0019] The invention will now be described by way of example only with reference to the accompanying drawings:

[0020] Fig. 1 shows a prior art wake equalising duct.

[0021] Fig. 2 shows prior art reaction fins.

[0022] Fig. 3 shows a prior art twisted Mewis duct.

[0023] Fig. 4 shows a schematic horizontal waterline flow velocity distribution on a vessel’s starboard side aft of a shoulder of the vessel.

[0024] Fig. 5 shows a perspective view of a device according to the invention attached to a hull of a vessel.

[0025] Fig. 6 shows a cross-section on the line A-A of the device shown in Fig.

5.

[0026] Fig. 7 shows a perspective cross-sectional view through the flow modifying portion of the device shown in Fig. 5.

[0027] Fig. 8 is a schematic perspective view of an underside port quarter of a vessel with a device according to the invention connected thereto.

[0028] Fig. 9 is a schematic starboard side elevation of a vessel with a device according to the invention connected thereto.

[0029] Fig. 10 is a schematic cross-section transverse or frame view of a vessel.

[0030] Fig. 11 is a schematic diagram of the underside of the port side of a vessel (without a device according to the invention) in a region forward of the rudder (propeller omitted) showing the development of vortices.

[0031] Fig. 12 is a view similar to Fig. 11 of a vessel with a device according to the invention showing vortex interaction. [0032] Fig. 13 is a schematic diagram showing axial flow velocity excluding the effect of the propeller at the aft region of a vessel (a) without a device according to the invention and (b) with such a device.

[0033] Fig. 14 is a schematic diagram of the underside of the port quarter of a vessel without a device according to the invention showing the thickening of the boundary layer.

[0034] Fig. 15 is a view similar to Fig. 14 of a vessel with a device according to the invention (device itself not shown in the Fig.).

[0035] Fig. 16 shows the nominal wake-field at the propeller region of a vessel (a) without a device according to the invention and (b) with such a device.

[0036] In the description reference is made to the positioning and effect of a device according to the invention which is attached to the outside of the hull and upstream of a propeller of the vessel. It will be understood however that such a device will be connected to each side of the vessel. Furthermore two such devices (one on each side of the vessel) could be attached not only to a vessel with a single propeller but also to vessels having two or more propellers. It is also possible that more than one such device could be connected to each side of the vessel.

[0037] A description of the thickening and separation of the boundary layer for a vessel without a device according to the invention has been described above with particular reference to Fig. 4. Fig. 16a shows a representation of the nominal wake-field of a vessel without any devices according to the invention attached to its hull. The ship wake is the region of uneven flow velocity distribution that occurs at the aft end of a moving vessel. Nominal wake-fields explained above are used to assess the flow arriving to the propeller and facilitates the assessment of any potential of cavitation and vibration. Fig. 13 is a schematic diagram showing hull cross-section boundary layer axial velocity fields just ahead of the propeller and corresponds roughly to the location labelled T in Fig. 12. The velocity fields shown in Fig. 13 are so-called “non-dimensional” velocity fields. In Fig 13 the flow velocity relative to the hull is shown non-dimensional and graphically. U _x is the velocity of flow relative to the vessel and U is the velocity of the vessel. Larger negative numbers represent higher axial flow velocities (U _X /I ) towards the stern and smaller negative numbers represent regions of slower flow. Positive values of U _x /U represent reverse flow conditions. This representational method is employed in Figs. 13 and 16. Fig. 14 shows the boundary layer inception point and the hull shoulder 6. Aft of the shoulder 6, the boundary layer steadily tends to thicken towards the section identified in Fig. 14 as FR2. A cross-section through the boundary layer slightly aft of the section FR2, just ahead of the propeller (omitted from Fig. 14), is shown in Fig. 13a. It is clear particularly from Fig. 13a that, without devices according to the invention, a thick boundary layer with slow flow and some reversed flow travels towards the propeller. This flow affects the propeller performance and may present a risk for cavitation and propeller vibrations issues. The tendency for this to occur is however significantly reduced when devices according to the invention are employed as shown in Figs. 13b and 16b.

[0038] Fig. 11 shows the development of bilge vortices in the boundary layer of a vessel without devices according to the invention. All of the bilge vortices at the locations marked T to 5' rotate in the same direction shown in Fig. 11 and referred to as positive or “+ve” vortices, and are represented by areas with horizontal hatching.

[0039] The device 24 according to the invention is shown in Fig. 5. The device 24 includes a flow modifying portion 26 or main hydrofoil and two support legs 28. Each support leg connects an end of the flow modifying portion 26 to the hull 2 of a vessel 22. The direction of flow 30 resulting from forward motion of the vessel 22 is shown by an arrow in Fig. 5. Each support leg 28 preferably has a hydrofoil shape with a rounded leg leading edge 32 and a tapered leg trailing portion 34. The flow modifying portion 26 has a longitudinal axis 36 which is preferably disposed substantially parallel to an outer surface 38 of the hull 2. The longitudinal axis 36 is also preferably disposed substantially perpendicular to the flow direction 30.

[0040] The flow modifying portion 26 will now be described with particular reference to Figs. 5 and 6. Fig. 6 shows a cross-section on the line A-A of the device 24 shown in Fig. 5.

[0041] The flow modifying portion 26 has a transverse cross-section, perpendicular to its longitudinal axis 36 with a rounded leading edge 40. It also has a tapering trailing portion 42 and a trailing edge 44. The flow modifying portion 26 is positioned such that it has an angle of attack a ⁰ with respect to the outer surface 38 of the hull 2. The angle of attack is preferably in the range 3° to 20°, and more preferably in the range 5° to 12° and most preferably approximately 10°, with the leading edge 40 positioned an offset distance O _max from the hull outer surface 38 and the trailing edge 44 positioned a smaller offset distance O _min therefrom. The average offset distance O _av (to a central plane 46 of the flow modifying device) is given by:

O av ₌ o max + Omin

2

[0042] The leading edge 40 defines a series of protuberances 48. The protuberances 48 are preferably evenly spaced although unevenly and/or irregular spacings are also possible. The cross-section shown in Fig. 6 is through the apex of one such protuberance. A maximum chord length C _max of the flow modifying portion 26 is shown in Figs. 5 and 6 and is the distance between the leading 40 and trailing 44 edges at the location of the protuberances 48. Between each pair of protuberance 48 is a recess 50. A minimum chord length C _min of the flow modifying portion is shown in Fig. 5. The average chord length C _av is given by:

[0043] The cross-sectional shape of the flow modifying portion 26 between a base of each recess 50 and the trailing edge 44 is preferably substantially identical to that at cross-section A-A in that it has a rounded leading edge and a tapering trailing portion with a trailing edge although other configurations are possible. The protuberances 48 and recesses preferably define a wave form with a wavelength l and amplitude a. The wave-form is more preferably a sine wave-form.

[0044] The device 24 may be fabricated from any suitable material. It could for example be cast and/or machined and made of solid metal or metal alloy such as bronze, phosphor-bronze, steel or stainless steel. Parts thereof could however contain apertures or voids. It could be fabricated as a single monolithic component or fabricated from plural components connected together by welding fasteners or other means. It is also envisaged that it could be fabricated as a composite structure including different materials which could be fastened together or possibly co-formed by casting or moulding. It could also be formed from a fibre reinforced material including fibre reinforcement such as carbon-fibre reinforcement. [0045] The device can be connected to the hull 2 of the vessel 22 by any suitable means such as by welding or by the use of fasteners. A fitting could be supplied on the outer surface 38 of the vessel to which the device 24 is connected. The connection means has been omitted from the drawings for the purpose of clarity.

[0046] The positioning of the flow modifying portion of the device with respect to the hull 2 will be critical since the position must be chosen to maximise the effect on the flow impinging on the propeller. The optimum dimensions and position for the device 24 for any given hull can be determined by a mathematical modelling and/or tank testing. For a typical cargo vessel however the dimensions and positions described below have been found to be effective. A perspective view of a device according to the invention connected to the underside of a port quarter of a vessel 22 is shown in Fig. 8.

[0047] The wave form has a wavelength (l) which is preferably between 1% and 30% of the average chord length C _av of the flow modifying portion.

[0048] The angle of attack a ⁰ , as explained above, is preferably in the range of 3° to 18°.

[0049] For a vessel with a maximum laden or design waterline beam B (shown in Fig. 10) the average chord length C _av of the flow modifying portion 24, as defined above, is preferably in the range of 2% to 8% of the maximum laden or design waterline beam B.

[0050] The longitudinal axis 36 is preferably disposed substantially perpendicular to streamlines 52 flowing past the flow modifying device, as shown for example in Figs. 5 and 12. Different orientations are however possible.

[0051] The vertical extent h of the flow modifying portion 26 preferably has a vertical extent in excess of the diameter E of the propeller 62 of the vessel 22, as shown in Fig. 9. This height is more preferably between 110% and 130% of the diameter E of the propeller 62.

[0052] As a consequence of the typical hull geometry between the shoulder 6 and the propeller 62, in this region flow tends to travel upwards. For this reason the horizontal centre-line 64 of the device 24 is preferably positioned below the level of a rotational axis 66 of a propeller 62 situated downstream of the device 24 (as indicated by dimension d in Fig. 9).

[0053] The aft perpendicular 54 of a vessel is a notional vertical line through the point at which the stern meets the water (when the vessel is laden). The vessel also has a waterline length L (when laden). The device 24 is preferably situated a distance x forward of the aft perpendicular 54, wherein x/L lies in the range 0.4 to 0.05 and more preferably in the range 0.1 to 0.3 and most preferably is around 0.18.

[0054] A typical cargo vessel has a central portion 56, referred to as the parallel body, with a non-varying exterior profile between a forward line 58 and an aft line 60 as shown in Fig. 9. Aft of the aft line 60 the hull tapers towards the propeller. A so-called shoulder 6 of the vessel is situated at the aft line 60. The device 24 is preferably situated aft of the shoulder 6.

[0055] The device is also preferably positioned wholly within the boundary layer created by forward movement of the vessel, and in particular within the boundary layer of the vessel when travelling at its normal cruising speed.

[0056] The average offstand O _av (as defined above) of the flow modifying portion 26 from an outer surface 38 of the hull 2 is preferably between 0.3% and 10% of the maximum laden waterline beam B of the vessel (shown in Fig. 10).

Figs. 13(b) and 15 show schematically the boundary layer for a vessel with devices according to the invention attached to its hull. Fig. 13(b) corresponds to Fig. 13(a) with respect to its location and how the information that it conveys is represented. By comparing Fig. 15 with Fig. 14 it can be seen that the steady thickening of the boundary layer seen in Fig. 14 is significantly reduced in Fig. 15. Low momentum flow close to the hull is mixed with higher momentum flow from the upper and outer regions of the boundary layer in such a way as to yield a fuller velocity profile. This results in a redistribution of momentum in the boundary layer and suppresses or delays the flow separation point. This phenomenon reduces hydrodynamic drag.

In addition, bilge vortices, labelled “+ve” in Figs. 11 and 12 (represented by areas with horizontal hatching) interact with so-called streamwise vortices from upper portions of the wake, labelled “-ve” in Fig. 12 (represented by areas with vertical hatching) thus scattering the bilge vortex intensity across the boundary layer instead of being concentrated in a region of flow impinging on the propeller (propeller not shown in Figs. 11, 12 and 14). As shown in Fig. 16(b) this results in a more uniform flow impinging on the propeller thereby increasing vessel propulsion efficiency by achieving a more optimum loading on the propeller. Therefore, for a given vessel speed, the rate of rotation of the propeller can be reduced which will allow the engine to operate in a more efficient region of it fuel map. Furthermore reduced propeller vibration and cavitation can be achieved. Furthermore the reduction of propeller vibration will have the consequence of reducing noise generated by rotation of the propeller. Mathematical modelling comparing typical cargo vessels with and without devices according to the invention have shown that by using the devices a thrust gain and a reduced drag resistance can be achieved. As a result, a gain in total efficiency of 10% has been achieved for a typical cargo vessel. While the embodiment described refers to a single device being used on each side of a vessel to modify the flow impinging on a propeller, an array comprising a plurality of devices could be used instead.