Systems and methods for reducing drag on vessels

Abstract

A system for reducing drag on a vessel (2), comprising: a plurality of outlets (4) in a normally submerged region of the hull of the vessel (2) for delivering a layer of air bubbles between at least a portion of the hull and water; at least one venturi tube (6) adapted to supply air and water to one or more of the plurality of air outlets (4); at least one seawater inlet (8) disposed in a bow region of the vessel (2) adapted to supply seawater to the at least one venturi tube (6) as the vessel moves through the water; and at least one seawater inlet (8) adapted to supply seawater to the at least one venturi tube (6). at least one seawater pump (10); at least one ambient air inlet port (16) adapted to supply ambient air to the at least one venturi tube (6) such that ambient air is entrained in the seawater flowing through the at least one venturi tube (6); a compressor (18) supplying compressed air to the at least one venturi tube (6); and a controller adapted to adjust a flow rate of seawater supplied to the at least one seawater pump and to adjust a supply of compressed air from the compressor to optimize delivery of air bubbles through the multiple outlets of the hull, thereby optimizing drag reduction of the vessel.

Description

[Technical field]

[0001] The present invention relates to a system and method for reducing drag on a vessel through air lubrication, and more particularly to a system and method for providing an air lubrication layer between the vessel's hull and the water flowing beneath the vessel as the vessel moves through the water to reduce drag (friction), thereby reducing the energy required to power the vessel through the water, saving fuel and resulting in more economical propulsion. [Background technology]

[0002] International shipping is a large and growing source of greenhouse gas emissions. Maritime transport emits around 940 million tonnes of CO2 annually, accounting for around 2.5% of global greenhouse gas (GHG) emissions. Moreover, these emissions are projected to increase significantly unless urgent steps are taken to improve the efficiency of commercial shipping. As of 1 January 2018, large ships over 5,000 gross tonnage loading and unloading cargo and / or passengers at ports within the European Economic Area (EEA) are required to monitor and report their associated CO2 emissions and other relevant information.

[0003] Because water is a viscous medium, the hull of a ship propelling through water is subject to shear viscous drag forces. The magnitude of the forces increases as the ship is propelled faster through the water. It is known to reduce such drag by inserting air bubbles between the outside of the ship's hull and the body of water through which the ship moves. The drag between the resulting layer of air bubbles and the water is less than if the hull were in direct contact with the water, and a significant reduction in the viscous drag acting on the ship's hull can be achieved. Summary of the Invention [Problem to be solved by the invention]

[0004] In one known system, disclosed in EP 2817208, EP 2915735, EP 2925736, EP 3290324 and EP 3290325, an air cavity is provided in a flat area of ​​the vessel's hull, and air is supplied to the cavity so that the air is mixed into the seawater as it flows through the cavity. The air in the cavity is typically supplied by a compressor. The problem with this known system is that it works on a passive shear effect and is therefore essentially tied to and limited by the speed of the vessel through the water. The higher the speed, the greater the shear effect and the greater the amount of air injection. This results in a minimum operating speed of the system of about 9 knots, making it unsuitable for many types of vessels. Furthermore, due to the nature of the shear effect, the characteristics of the air bubbles cannot be controlled. The system cannot operate on the side walls of the vessel.

[0005] Other known systems, such as those disclosed in GB 2505281, KR 20170031425, KR 20180000968, and KR 20200055517, incorporate nozzles at the hull-water interface from which compressed air is "injected" under pressure. Ship motions cause the instantaneous hydrostatic pressures at the individual nozzle positions to continually change, in six degrees of freedom. It is believed impossible to accurately predict the ship motions (more specifically, the corresponding instantaneous depths of each nozzle) and ensure optimal compressed air supply by accurate hydrostatic pressure matching. The result is that at any given moment, over- or under-pressurized air is supplied at each injection nozzle. As a result, compressed air is either under-supplied in the case of under-pressurization or discharged at high pressure in the case of over-pressurization. This causes either an under-lubrication effect or excessive air injection, with an undetermined proportion of the supplied microbubbles being lost to the sea rather than being captured within the flat portion of the bottom boundary layer. [Means for solving the problem]

[0006] According to a first aspect of the present invention there is provided a system for reducing drag on a marine vessel comprising: a plurality of outlets provided in a normally submerged region of a hull of the marine vessel for delivering a layer of air bubbles between at least a portion of the hull and the water; at least one Venturi tube having a converging inlet region, a diverging outlet region, and a throat between the converging inlet region and the diverging outlet region, at least one air inlet disposed within or downstream of the throat, the outlet region communicating with one or more of the plurality of air outlets; at least one water inlet disposed in a bow region of the vessel adapted to supply water to the inlet region of the at least one Venturi tube as the vessel moves through water; at least one water pump adapted to supply water to the inlet region of the at least one Venturi; at least one ambient air inlet port adapted to supply ambient air to one or more of the at least one air inlets of the at least one Venturi tube, wherein air from the at least one ambient air inlet port is entrained in water flowing through the at least one Venturi tube to deliver air bubbles to one or more of the plurality of outlets; a controller adapted to adjust a flow rate of water provided from the at least one water pump to the at least one venturi to optimize delivery of air bubbles through the plurality of outlets of the vessel, thereby optimizing drag reduction of the vessel. Includes.

[0007] The system may further include selectively supplying additional air to at least one of the at least one air inlet of the at least one venturi tube or to the water flow upstream or downstream of the air inlet of the at least one venturi tube.

[0008] At least a portion of the additional air may include compressed air provided by an air compression device, and the controller is adapted to regulate a supply of compressed air from the air compression device to at least one venturi to optimize delivery of the air bubbles through a plurality of outlets of the hull. The air compression device may include at least one air compressor, such as a variable frequency drive screw compressor.

[0009] At least a portion of said additional air may be in the form of air bubbles, more preferably nanoscale air bubbles, that are mixed into the water flow upstream or downstream of the at least one venturi air inlet.

[0010] In one embodiment, the system may include a plurality of said venturis, each feeding a respective one or respective group of said outlets.

[0011] The at least one water pump may be associated with each sea chest inside the hull in communication with a water inlet in communication with the water surrounding the vessel, or the at least one water pump may include an existing pump provided within the vessel, such as a ballast pump, a cooling pump, a fire pump or a jockey pump.

[0012] Preferably, the at least one ambient air inlet port is located above the surface of the water on which the marine vessel floats.

[0013] Preferably, the controller is programmed to adjust the flow rate of water supplied from the at least one water pump to the at least one venturi tube as a function of the speed of the vessel through the water. The controller may be further programmed to adjust the flow rate of water supplied from the at least one water pump to the at least one venturi tube as a function of the draft of the vessel and / or as a function of hydrostatic back pressure on the at least one water pump. One or more sensors may be provided to determine: one or more of the flow rates of water supplied from the at least one water inlet to the at least one venturi tube; the flow rate of water supplied from the at least one water pump to the at least one venturi tube; the flow rate of air supplied from the at least one ambient air inlet port to the at least one venturi tube; the flow rates of air and water supplied from the at least one venturi tube to a plurality of outlets in the vessel's hull; and the hydrostatic back pressure of the at least one water pump. The one or more sensors provide inputs to the controller.

[0014] According to a further aspect of the present invention there is provided a method of reducing drag on a marine vessel comprising providing a plurality of outlets in a normally submerged region of a hull of said marine vessel; providing at least one Venturi tube having a converging inlet region, a diverging outlet region, and a throat between said converging inlet region and said diverging outlet region, at least one air inlet being disposed within or downstream of said throat, said outlet region communicating with one or more of said plurality of air outlets; supplying water from at least one water inlet located in a bow region of the vessel to an inlet region of the at least one Venturi tube as the vessel moves through the water; pumping with at least one water pump into the inlet region of the at least one venturi; wherein the flow of water through said at least one Venturi tube entrains ambient air from said at least one ambient air inlet port through at least one of said at least one air inlet of said at least one Venturi tube; adjusting a flow rate of water supplied by the at least one water pump to the at least one venturi to optimize delivery of air bubbles through the plurality of outlets of the vessel, thereby optimizing drag reduction of the vessel. Includes.

[0015] The method may include selectively supplying additional air to one or more of the at least one air inlet of the at least one venturi or to the water flow upstream or downstream of the at least one venturi.

[0016] At least a portion of said additional air may be in the form of air bubbles, more preferably nanoscale air bubbles, that are mixed into the water flow upstream or downstream of the at least one venturi air inlet.

[0017] At least a portion of the additional air may include compressed air supplied by an air compression device, the method including supplying the compressed air to one or more of the at least one air inlet of at least one Venturi tube and regulating the supply of compressed air from the air compression device to the at least one Venturi tube to optimize delivery of air bubbles through a plurality of outlets of the hull.

[0018] The flow rate of water supplied from the at least one water pump to the at least one venturi tube is preferably adjusted as a function of the speed of the vessel through the water. The flow rate of air supplied from the air compressor to the at least one venturi tube may be adjusted as a function of the speed of the vessel through the water. The flow rate of water supplied from the at least one water pump to the at least one venturi tube may further be adjusted as a function of the draft of the vessel and / or the hydrostatic back pressure on the at least one water pump.

[0019] The flow rate of water supplied from the at least one water pump to the at least one Venturi tube may be regulated as a function of one or more of the following, measured by suitable sensors: the flow rate of water supplied from the at least one water inlet to the at least one Venturi tube; the flow rate of water supplied from the at least one water pump to the at least one Venturi tube; the flow rate of air supplied from the at least one ambient air inlet port to the at least one Venturi tube; the flow rates of air and water supplied from the at least one Venturi tube to multiple outlets in the hull of the vessel; and the hydrostatic back pressure of the at least one water pump.

[0020] A system for reducing drag on a vessel by air lubrication in accordance with an embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0021] [Figure 1] 1 is a schematic side view of a marine vessel incorporating a system for reducing marine vessel drag through air lubrication, in accordance with one embodiment of the present invention; FIG. [Diagram 2] FIG. 2 is a schematic plan view of the system of FIG. 1. [Diagram 3] FIG. 2 is a detailed view of the Venturi tube of the system of FIG. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] As shown in the drawings, a system for reducing drag on a vessel 2 through air lubrication controls the injection of air bubbles into the boundary layer of at least a portion of the vessel's submerged hull. The vessel's hull is provided with a number of outlets 4 for injecting air bubbles into the boundary layer, such that the air bubbles propagate from the rear towards the stern of the hull, covering a majority of the submerged surface of the hull, thereby significantly reducing viscous drag acting on the vessel's hull.

[0023] The present invention provides a system that allows for control of the size of air bubbles and the amount of air injected through said multiple outlets 4 into the boundary layer of the vessel for air lubrication purposes. This is achieved by using multiple venturis 6 to inject air into the water in the vessel's internal piping system and controlling the flow rates of both air and water into the venturis 6 to entrain the air into the water flow. This allows for greater control of the injection of air bubbles into the boundary layer and allows the flow around the vessel to be utilized to reduce power requirements. Injecting a precisely controlled mixture of air and water into the boundary layer avoids the creation of turbulence and also reduces the spread of air bubbles at the bottom (and potentially the side walls) of the vessel.

[0024] In the embodiment shown in the drawings, a series of Venturi tubes 6 (e.g. six) are provided, each feeding a separate outlet or group of outlets 4 within the hull of the vessel 2. A single Venturi tube 6 may be used, or any number of multiple Venturi tubes may be used to suit the size of the vessel and / or the number of outlets 4 provided in the hull of the vessel.

[0025] Water is first supplied to each Venturi 6 from a passive water inlet 8 located at the bow of the vessel so that water enters the inlet as the vessel moves through the water. Secondly, water is selectively pumped through each Venturi 6 via one or more water pumps 10 such that the flow rate of water to each Venturi 6 may be controlled by controlling the operation of each water pump 10.

[0026] In the embodiment shown, two water pumps 10 are provided, each supplying water to a group of venturi tubes 6 from a respective sea chest 12 located in the hull of the vessel 2 with an inlet for receiving water from outside the vessel's hull. Alternatively, it is contemplated to use existing ballast / cooling / fire extinguisher / jockey jumps to supply water to the venturi tubes 6, and such pumps would typically be located in the engine room of the vessel 2.

[0027] Air mixed into the water flowing through each Venturi tube 6 is supplied to each Venturi tube 6 from an atmospheric port 16 (e.g., installed on the outer deck of the ship) that communicates with the atmosphere, and to each air inlet 14 located at the throat of each Venturi tube, preferably each Venturi tube.

[0028] Additionally, air may be provided into each Venturi tube 6 or into the water stream upstream or downstream of each Venturi tube.

[0029] In one embodiment, at least a portion of said additional air may comprise air bubbles, more preferably nano-scale air bubbles, generated by a suitable air bubble generator and introduced into the water flow upstream or downstream of said ambient air inlet 14 of each venturi tube. Nano-scale air bubbles may reduce slime growth in the system and on wetted surface areas on the exterior of the hull.

[0030] In one embodiment, at least a portion of the additional air may include compressed air from a compressed air source, such as a compressor 18, preferably via a second set of air inlets 20 of each Venturi tube 6. The second set of air inlets 20 may include annular air injection nozzles disposed at the throat of each Venturi tube 6 (or at the diverging section of the venture tube downstream of the throat, or elsewhere upstream or downstream of the throat) to supply air to the respective Venturi tube 6 from the compressed air source.

[0031] A control system is provided to control the supply of water, and optionally air, to each Venturi tube 6 to generate an optimum size and flow rate of air bubbles into the boundary layer of the vessel 2 via a number of outlets 4 located in the submerged portion of the vessel's hull.

[0032] The combined effect of the active and passive elements of the system allows precise control of the output (air bubbles) and reduces energy requirements compared to current best-in-class systems. The passive and active water systems and optional air / air bubble supply system to the Venturi tube allow for optimization of the air bubble flow rate into the boundary layer of vessel 2 regardless of the operating conditions of vessel 2, especially speed.

[0033] Air can be injected into the or each Venturi tube both passively (by pressure drop occurring within the Venturi tube) and actively (by compressed air supply from an air compressor and / or bubbler) via a control system, and the air supply (velocity and pressure) from the injection nozzles can be adjusted to suit the prevailing operating conditions.

[0034] The size of the passive bubble jet (due to the pressure drop created within the Venturi) is determined by the flow rate through each Venturi 6. Active control of this flow rate through the use of water pumps 10 to control the flow rate of seawater through each Venturi 6 allows control of the passive air jet conditions. Control of the water flow rate through the Venturi 6 allows the air / water mixture to be injected into the boundary layer at the correct velocity, thus reducing boundary layer turbulence and reducing bubble dispersion.

[0035] A passive water inlet 8 is preferably located near the stagnation point at the bow, through which water ("system water") enters as the vessel moves through the water.

[0036] If required and to provide optimum performance, the or each seawater pump 10 adds water to each Venturi tube 6 to create an eductor effect. The eductor effect creates a suction which increases or maintains water flow in the system and overcomes friction losses and back pressure created by the Venturi tubes 6. The additional water supplied by each water pump 10 supplements the passive water taken in at the passive water inlets and careful control of pump speed ensures the correct rate and amount of water and air is expelled from the vessel via outlets 4 in the bottom and side walls of the hull.

[0037] The combined system water (passive inlet + actively pumped) passes through each Venturi 6. When the two water streams meet upstream of the Venturi 6, the total water flow rate increases. This increase in flow rate creates a localized pressure drop (Bernoulli's Law). The converging inlet section of each Venturi is designed (proportioned) to keep the pressure drop below atmospheric pressure, resulting in a net suction effect. Ambient air is preferably drawn from the water surface and / or deck level into the ambient air port 16 and is entrained in the water flowing through each Venturi 6 as air bubbles.

[0038] The size of the bubbles is determined by the flow rate of water through each Venturi 6. Thus, the internal shape of each Venturi itself, and particularly the cross-sectional area of ​​its throat region, provides a "rough" control over the characteristics of the bubbles produced. It is envisioned that variable size venturis with adjustable throat areas could be provided to facilitate control of the bubbles.

[0039] An optional annular air injection nozzle assembly 20 may be provided to supply air from a compressor having multiple air injection nozzles therein. The air injection nozzles of the air injection nozzle assembly 20 may be located in the throat section of each venturi tube, immediately downstream of the fresh air inlet 14. The compressor 18 may include a variable frequency drive (VFD) screw compressor that may be used to generate compressed air at the precise pressure and volumetric flow rate required to ensure optimal air lubrication for a particular vessel operating condition (i.e., minimize net energy usage).

[0040] The shape and design of the injection nozzles of the air injection nozzle assembly 20 may be adapted to allow precise control (homogenization) of the air bubbles injected into the water stream passing through each venturi tube 6 .

[0041] A sensor-based automated control system may be provided to manage the interaction of physical components based on the vessel's prevailing operating conditions, particularly the vessel's speed, to ensure optimal power output and reduce energy usage. The control system may be developed with machine learning capabilities to enable continuous optimization.

[0042] The control system can be programmed to automatically customize the passive ambient air entering each venturi tube, optionally supplementing the amount of passive air with additional active air from the compressor, and even controlling the water flow rate through each venturi tube by controlling the operation of the water pump to maintain precise control of the air and water mixture produced by the venturi tubes. The controller can receive inputs from various sensors, such as a vessel speed sensor, an air flow meter measuring the air flow rate from the ambient air inlet port 16, a pressure sensor determining the ambient air pressure, the air pressure and / or the flow rate from the compressor 18, and / or a water flow meter measuring the water flow from the passive water inlet 8 and the water pump 10 (and / or a pump speed sensor), and / or a pressure sensor to determine the hydrostatic back pressure of the water pump 10. The controller can also receive information from the sensors to determine the draft of the vessel, for example so that the system can compensate for the load being carried by the vessel. The controller may be programmed to control the flow rate of water and preferably the flow rate of air supplied to each venturi tube in response to information received from these various sensors so as to provide optimal operation of the system over a wide range of typical operating conditions of the vessel, and in particular to control the flow rate of water and preferably the flow rate of air supplied to each venturi tube in response to the speed of the vessel.

[0043] Depending on i) the local water flow velocity at the outlet 4, and ii) the hydrostatic back pressure of the water pump 10, the water pump 10 may have to work harder to overcome the pressure at the outlet 4, and therefore the energy "demand" of the water pump 10 increases with the vessel's draft. The vessel's speed through the water is preferably the primary determinant of the operating speed of the water pump 10. However, the vessel's draft through the water may have a secondary effect that adapts the required output from the water pump 10. The pump 10 may be designed to deliver a higher or lower flow rate / velocity to overcome the higher / lower pressure at the outlet 4. In short, pump speed, mass flow rate, pressure and injection rate are all intrinsically related.

[0044] To avoid turbulence at the injection point, it is desirable to target an exit injection velocity equal to the velocity of the flow in the boundary layer at the injection point (location of the hull outlet 4). The water pump 10 is adapted and controlled to ensure that the relative flow difference between the injected system water and the seawater passing through the outlet 4 is zero. Factors that affect the velocity of the boundary layer flow at the injection point, and therefore the demand for the water pump, are: 1. The vessel's forward speed (speed through the water) - can be obtained from measurements on the vessel's bridge; 2. Location of hull outlet 4 - determined through design / installation and CFD analysis; 3. Draft can also affect local current speeds - this can be detected by echo sounders and draft gauges; 4. Fouling of internal pipelines causes increased back pressure in the system / water pump – this can be detected by a pressure gauge.

[0045] An outlet 4 in the hull delivers the air and water mixture into the hydrodynamic boundary layer at the bottom and sidewall of the vessel. The outlet can be adapted to minimise flow turbulence and ensure flow into the boundary layer.

[0046] A surfactant may be added to the water either upstream or downstream of the Venturi to promote the formation of bubbles at the outlet 4 of the vessel hull.

[0047] The system provides continuous air lubrication to the bottom and sides of the vessel, reducing drag through the water. The control system can be programmed to automatically optimize the interaction between the passive and active water and air supplied to each Venturi tube to ensure the vessel's total energy usage is minimized and adjust the "recipe" of the air / water mixture supplied from the outlets. This can allow 3D control of the system output regardless of operating conditions / speed / vessel type. Thus, air output / volume is decoupled from the vessel's speed.

[0048] By controlling the flow rate of the water through each Venturi 6, the shear stress at the injection point can be controlled. This ensures that the size of the air bubbles generated can be controlled. Simply put, the smaller the air bubble, the greater the surface area to volume ratio of that bubble and therefore the more lubrication per unit of air produced. This improves the final performance of the system compared to competing systems.

[0049] By introducing air into the water inside the ship, air bubbles can be pumped out through outlets on the sides of the hull in addition to the bottom of the ship, allowing the sides of the hull to be lubricated, effectively doubling the surface area available for lubrication.

[0050] The passive nature of the Venturi tube allows at least a portion of the bubbles generated by a zero energy input system to be generated using only a passive supply of water and air from the passive water inlet and ambient air inlet ports, respectively, providing a net performance improvement to the system compared to prior art fully active systems (e.g., systems where all air is supplied from a compressor). Meanwhile, the active supply of water and optionally air from a water pump and air compressor, respectively, significantly improves the performance and flexibility of the system compared to prior art fully passive systems.

[0051] Systems and methods according to the present invention may be applied to any waterborne vessel, such as commercial ships, ferries, pleasure craft, inland waterway vessels, and any other vessel configured to move through water.

[0052] The present invention is not limited to the embodiments described herein, which can be modified or varied without departing from the scope of the invention as defined by the appended claims.

Claims

1. 1. A system for reducing drag on a marine vessel, comprising: a plurality of outlets disposed in a normally submerged area of ​​the hull of the marine vessel for delivering a layer of air bubbles between at least a portion of the hull and the water; at least one Venturi tube having a converging inlet region, a diverging outlet region, and a throat between the converging inlet region and the diverging outlet region, at least one air inlet disposed within or downstream of the throat, the outlet region communicating with one or more of the plurality of air outlets; at least one water inlet disposed in a bow region of the vessel adapted to supply water to the inlet region of the at least one Venturi tube as the vessel moves through water; at least one water pump adapted to supply water to the inlet region of the at least one Venturi; at least one ambient air inlet port adapted to supply ambient air to one or more of the at least one air inlets of the at least one Venturi tube, wherein air from the at least one ambient air inlet port is entrained in the water flowing through the at least one Venturi tube to deliver air bubbles to one or more of the plurality of outlets; a controller adapted to adjust a flow rate of water provided from the at least one water pump to the at least one venturi to optimize delivery of air bubbles through the plurality of outlets of the vessel, thereby optimizing drag reduction of the vessel. Including, the system.

2. further comprising an air compressor configured to compress air to generate compressed air and selectively supply compressed air to one or more of the at least one air inlet of the at least one Venturi; 2. The system of claim 1, wherein the controller is adapted to regulate a supply of compressed air from the air compressor to the at least one venturi to optimize delivery of air bubbles through the multiple outlets of the hull.

3. The system of claim 1 including a plurality of said venturis, each venturi feeding a respective one or respective group of said outlets.

4. The system of claim 1 , wherein the at least one water pump is associated with a respective sea chest within the hull that communicates with an inlet that receives water from outside the hull.

5. 4. The system of claim 2 or 3, wherein the at least one water pump comprises an existing pump provided on board the vessel, such as a ballast pump, a cooling pump, a fire pump or a jockey pump.

6. The system of claim 1 , wherein the at least one ambient air inlet port is located above the water level and / or above deck.

7. The system of claim 2 , wherein the air compression device comprises at least one air compressor.

8. The system of claim 7 , wherein the at least one compressor comprises a variable frequency drive screw compressor.

9. 3. The system of claim 2, wherein the controller is programmed to adjust a flow rate of water supplied from the at least one water pump to the at least one Venturi tube and / or to adjust a supply of compressed air from the air compressor to the at least one Venturi tube as a function of a speed of the vessel through water.

10. 3. The system of claim 2, wherein one or more sensors are provided to measure one or more of the following flow rates, the one or more sensors providing input to the controller: a flow rate of water supplied from the at least one water inlet to the at least one venturi; a flow rate of water supplied from the at least one water pump to the at least one venturi and / or a speed of the pump; a flow rate of air supplied from the at least one ambient air inlet port to the at least one venturi; a flow rate of air supplied from the air compressor to the at least one venturi; a flow rate of air and water supplied from the at least one venturi to the multiple outlets of the hull of the vessel; and a hydrostatic back pressure on the at least one water pump.

11. A method of reducing drag on a marine vessel comprising providing a plurality of outlets in a normally submerged region of a hull of said vessel; providing at least one Venturi tube having a converging inlet region, a diverging outlet region, and a throat between said converging inlet region and said diverging outlet region, at least one air inlet being disposed within or downstream of said throat, said outlet region communicating with one or more of said plurality of air outlets; supplying water from at least one water inlet located in a bow region of the vessel to an inlet region of the at least one Venturi tube as the vessel moves through the water; pumping with at least one water pump into the inlet region of the at least one Venturi; wherein the flow of water through the at least one Venturi tube entrains ambient air from the at least one ambient air inlet port through at least one of the at least one air inlets of the at least one Venturi tube; adjusting a flow rate of water supplied by the at least one water pump to the at least one venturi to optimize delivery of air bubbles through the plurality of outlets of the vessel, thereby optimizing drag reduction of the vessel. A method comprising:

12. 12. The method of claim 11, further comprising selectively supplying compressed air from an air compression device to one or more of the at least one air inlet of the at least one Venturi tube to regulate the supply of compressed air from the air compression device to the at least one Venturi tube to optimize delivery of air bubbles through the multiple outlets of the hull.

13. 13. The method of claim 11 or 12, wherein the flow rate of water supplied from the at least one water pump to the at least one venturi is adjusted as a function of the speed of the vessel through the water.

14. 14. A method as claimed in claim 13 when dependent on claim 12, wherein a flow rate of air supplied from the air compressor to the at least one venturi tube is regulated as a function of the speed of the vessel through water.

15. 13. The method of claim 12, wherein the flow rate of water supplied from the at least one water pump to the at least one Venturi tube and / or the supply of compressed air from the air compressor to the at least one Venturi tube are adjusted as a function of one or more of the following: the flow rate of water supplied from the at least one water inlet to the at least one Venturi tube; the flow rate of water supplied from the at least one water pump to the at least one Venturi tube; the flow rate of air supplied from the at least one ambient air inlet port to the at least one Venturi tube; the flow rate of air supplied from the air compressor to the at least one Venturi tube; the flow rates of air and water supplied from the at least one Venturi tube to the multiple outlets of the hull of the marine vessel; and the hydrostatic back pressure on the at least one water pump.

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