[0001] The present invention relates to the hydrodynamics of marine vessels, more particularly to adjuncts, appendages and auxiliary devices for affecting same.

BACKGROUND OF THE INVENTION [0002] More efficient hull designs will have a fully faired hull form which includes a tapering of the hull around the stern at and below the waterline. The hydrodynamic flow will naturally become 'turbulent' part way along the hull, and towards the stern of the boat will develop significant eddies which will add to the flow (viscous) drag of the hull. The hull will also create waves as it moves through the water and these waves become bigger as the speed increases and they generate wave induced drag on the hull.

[0003] For highly efficient hulls operating in the displacement regime (i.e. not planing) a long tapering form at the stern will reduce drag to a minimum (e.g. a rowing shell). However for many reasons such a long narrow hull form is not necessarily very helpful in terms of a working boat design, e.g. where the maximum internal volume or deck space is required within a restricted hull length. Therefore some hulls are truncated by removing the long, slender stern lines and often have a flat transom without any overhang, i.e. a transom stern which is immersed in the water, to an extent. The amount of transom immersion is measured by the 'transom area ratio' - the higher the ratio, the more of the transom is immersed - and this will vary with the displacement/loading of any vessel. Such transoms significantly increase the pressure drag of the hull because of the very turbulent water created behind the transom as the speed increases which forms a low pressure region acting on the immersed transom area. This creates a negative pressure difference on the transom which, over the immersed area of the transom, becomes an additional drag force on the hull. [0004] In the case of planing hulls, as the speed increases still further the depth of immersion reduces until at a particular hull speed the water flows cleanly away from the bottom of the transom, without impinging on the transom and just forming part of the stern wave, when this particular drag force approaches zero.

[0005] Typical cargo vessels are fitted with a propeller mounted under the hull will. In operation, the propeller will cause a cylindrical body of water to move aft along the line of the hull. The cylindrical body of water will act against the water behind the vessel to cause forward motion of the boat. As the body of water flows aft of the vessel, the upper part of the cylindrical body of water will flow along the underside of the hull at a higher velocity than the forward motion of the vessel. This, high velocity water, at high pressure, will flow toward the leading edge of the transom and will fill the low pressure region immediately behind the immersed transom, creating highly turbulent conditions which do not facilitate forward motion of the vessel. [0006] As the hull moves through the water, the water is displaced sideways and downwards to allow the hull to move through it: it is displaced far enough to enable the hull to pass, and also gains some energy from the vessel in forming the hull waves. The total quantity (weight) of water moved is equivalent to the displacement of the vessel. For a vessel with an immersed transom stern some of that displaced water is not just moved sideways and downwards, it then flows around the bottom of the transom as the vessel passes, and is accelerated upwards and forwards to fill the gap behind the transom and below the waterline. This water is then entrained behind the transom in a very turbulent area which is present as the vessel moves through the water, sapping power and providing a significant drag to forward motion.

[0007] Stern flaps have been known for a number of years; a stern flap can comprise an extension of a hull bottom surface which extends aft of a transom.

[0008] The original idea for improving the hydrodynamic efficiency of immersed transom sterns is understood to have been utilised by the German Navy in World War II. More recently, transom or stern flaps have been fitted to a range of naval style hulls by the US Navy, the US Coast Guard and the Royal Navy. There are a number of different stern flaps, which may be employed on many different hull types. A typical standard (traditional or conventional) stern flap is designed with parallel, rectilinear leading and trailing edges for orientation of these linear edges perpendicular to a ship's centreline. Stern flaps have now been proven by many bodies to reduce the requisite amount of propulsive power during navigation, with several concomitant advantages.

[0009] US6038995 (Karafiath et al.) teaches of a stern wedge and a stern flap attached to a combative ship and demonstrates hydrodynamic properties which, for purposes of enhancing the powering performance of a ship, are superior to those of either a solitary stern wedge or a solitary stern flap. The stern wedge portion's lower surface and the stern flap portion's lower surface are slanted at approximately equal angles with respect to the buttock line, thereby optimally consolidating the stern portion's lower surface and the flap portion's lower surface so as to effectively create an overall hydrodynamic lower surface which is slanted approximately at one and the same angle. [0010] In the 1980's some navies began to successfully apply stern wedges to larger ships up to the frigate size. The U.S. Navy designed the DDG 51 Class destroyer with a stern wedge inlaid into the hull. Model tests demonstrated that implementation of such a stern wedge resulted in a reduction of power of up to a maximum of about 6% at speeds above 24 knots. A stern wedge design was initially attempted by the U.S. Navy for the FFG 7 frigate class; however, in the course of model testing it was discovered that a stern flap was more effective than a stern wedge on this class. The model tests demonstrated approximately a 5% decrease in delivered power at speeds of 20 knots and above. In 1989, a stern flap was designed and retrofitted by the U.S. Navy on the USS Copeland frigate FFG 25 frigate class. Analysis of the ship trials data for the USS Copeland frigate having a retrofitted stern flap indicated an 8% power saving, somewhat greater than the model test results, and increased top end speed.

[0011] It is notable, from a review of the literature that whilst much research has been undertaken into "high speed" military craft, the needs of low speed 10 - 20 knot shipping vessels have not been addressed. It has been found that low speed characteristics of long, narrow military craft do not simply transpose nor would be appropriate for wide bodied merchant shipping vessels. Indeed, these references relate to the use of stern flaps on high speed vessels where the flap is generally found to be most efficient with a downward angle relative to the hull buttock run, i.e. for higher speed vessels the additional lift created by such an angle more than outweighs the additional drag caused by the flap at lower speeds. All the current body of work that has been completed has been applying the concept of a transom flap to higher speed vessels, and has not attempted to investigate the issues and mechanisms associated with a transom fitted device for low speed applications. [0012] In recent years, the price of fuel has increased exponentially; this is known only too well for automobile users, for example. In the case of commercial shipping, where amounts of fuel are measured in terms of barrels or tons rather than gallons or litres, it a simple extrapolation to realise that operators of commercial ships suffer greatly. In order to assist such plight, more hydrodynamically effective designs of hulls of ships must be created, which will enable a more economical use of fuel. Accordingly, an increasing number of companies are seeking solutions in this area. However, detailed research is sparse for commercial shipping, especially for relatively short, fat hulls which operate in the 10 to 20 knot speed range which will require different design parameters from the higher speed, long, thin naval hulls that have hitherto been addressed.

OBJECT TO THE INVENTION

[0013] In view of the foregoing, the present invention seeks to provide an improved hydrodynamic structure for a commercial vessel. The present invention also seeks to provide a hull design that can reduce the shaft power required to propel a ship, thereby reducing the engine fuel consumption.

SUMMARY OF THE INVENTION

[0014] In accordance with a first aspect of the invention, there is provided a displacement vessel having a transom and a stern flap assembly which includes generally vertically oriented baffles. Conveniently, the baffles are configured as extensions of the flap and extend along the sides of the hull, being upwardly directed with respect to the transom. Alternatively or additionally, the baffles are provided as elements separate from the transom and depend upwardly from the flaps. In the event that the flaps do not extend along the complete width of the transom, the baffles may arise from an end surface of the flap or from a top surface and extend upwardly. [0015] Conveniently, the stern flap is contiguous and faired with the surface of the hull, whereby to enable a smooth flow of water past it and on necessary occasions being designed at a fixed angle to the buttock run of the hull. The stern flap is characterised by a chord length, a thickness and a span, which characteristics are selected for each vessel application in order to optimise the effects of the device, said optimisation being functional with the speed range and operational profile of each vessel application. The baffle, to the extent it is distinct form the flap is characterised by a height, thickness and chord length, which may be variable as the baffle rises from the flap.

[0016] It has been shown that the invention materially improves (lowers) the near and far wake wave field, thus reducing the drag on the hull. It has also been shown that the invention improves the pressure gradient beneath the hull from a point ahead of the propeller disc to the aft edge of the device, thus improving the efficiency of the propeller and adding to the buoyancy of the hull at the stern. It has further been shown that the invention accelerates the wake flow velocity at the aft edge of the device, thus improving the hydrodynamic efficiency of the hull and the dynamic lift on the stern of the hull which will reduce the dynamic sinkage of the hull. [0017] It has further been shown that the invention can enable a moving vessel entrain a body of water behind the transom and within the span of the device which generates a turbulent flow and enhances the mixing of that body of water with the flow of water past the hull. This entrained body of water will effectively drain away from the transom as the speed of the vessel increases, becoming entrained with the flow of water past the vessel at the sharp edge of the device, thus reducing a 'base drag' effect on the transom. For a particular hull speed, dependent on the design and operating conditions of the vessel the transom will 'run dry', although this may not be achieved for all vessels. All the entrained body of water is effectively drained away from the transom thereby reducing the base drag effect to zero at that and any higher vessel speeds. [0018] As will be appreciated, the baffles and stern flaps may be constructed with the vessel or can be provided after construction of the vessel. The flaps can be variable, to enable trim adjustment, in the event that a ships loading characteristics differ. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein:

Figure 1 shows a simple view of a displacement hull;

Figure 2, there is shown a diagrammatic representation of a transom of a vessel;

Figure 3 shows the transom of a first embodiment of the invention; Figure 4a shows the transom of a second embodiment of the invention ;

Figure 4b details a craft made with an arrangement in accordance with the second embodiment of the invention; Figure 5 shows the transom of a third embodiment of the invention; and,

Figure 6 details a craft made with an arrangement in accordance with the third embodiment of the invention;

DETAILED DESCRIPTION OF THE INVENTION

[0020] There will now be described, by way of example only, the best modes contemplated by the inventor for carrying out the present invention. In the following description, numerous specific details are set out in order to provide a complete understanding to the present invention. It will be apparent to those skilled in the art, that the present invention may be put into practice with variations of the specific. [0021] Figure 1 shows a simple view of a displacement hull 10 of a general short and wide configuration. The aft of the vessel comprises a transom 12, being a generally flat truncation to the otherwise curvilinear hull design. The hull sits in the water, with the waterline 14 being the uppermost level the water sits at with respect to the hull. Rather than being designed for speed, armament etc, it is designed to carry goods - containers, fluids, powders and grains, fish and fishing equipment etc., volume of a cargo hold being more important than top speed. Referring now to Figure 2, there is shown a diagrammatic representation of a stern flap 20 at a transom 22 of a vessel, having a centre line CL, the stern flap having a span across one half of the transom and a length referred to as a chord. The stern flap is an appendage built in form of a plate that extends aft of the transom in an angle a relative to the buttock plane of the ship. Its interaction with the hull modifies the ship running trim, reduces propulsion resistance and increases maximum attainable speed. The critical parameters for a stern flap geometry design are: chord length (Lf), flap angle (a) referenced to an extension of the hull bottom and flap span across the transom. Secondary design aspects have previously been determined to be: determination of platform shape, transverse thickness variations, and the detailed fairing into the hull, with special attention to the outboard edges. Generally, a simple radius corner has been used to simplify design and construction. [0022] The principal benefits that stern flaps provide are: i) a reduction in powering resistance - Experience has shown that this reduction is between 5 to 12%; ii) An increase of maximum attainable speed - or an increase in range for a given speed - together with a reduction in waste gases; iii) A beneficial propulsion interaction can be achieved; & iv) A modification of transom wave characteristics due, inter alia from propeller loading, cavitation etc..

[0023] In practice, stern flaps can comprise a relatively small appendage comprising, typically, internal metal bracing beams and external metal plate material, which is fitted to the ship's transom. The main purpose of a stern flap device is to reduce the shaft power required to propel a ship through the water, thereby reducing the engine's fuel consumption and increasing the ship's top speed and range. In "Hydrodynamic analysis of the performance of stern flaps in a semi-displacement hull" by M Sala et al, (ISSN 0327-0793 Lat. Am. appl. res. v.34 n.4 Bahia Blanca oct./dic. 2004), it is claimed that the hydrodynamically significant stern flap surface is its lower surface. In principle, a stern flap is coupled with a hull stern so that the hull bottom surface and the stern flap lower surface essentially represent a kind of surface continuum, thereby effectively altering the hydrodynamic character of the hull.

[0024] Referring now to FIG. 3, there is shown a first embodiment of the invention, where an extension edge flap (hereinafter referred to as a "baffle") is shown extending from the complete edge of the outside hull surfaces with respect to the transom 22. In tests, the baffle has comprised an extension to the hull lines beyond an immersed transom stern around the periphery of the transom, from the waterline on one side of the hull to the waterline on the other side of the hull. For boats ranging in a waterline length of 6 - 200m, it has been tested and / or calculated that the chord length is conveniently up to five per cent of the waterline length with a flap angle in the range 0° - 20° trailing edge down with respect to the hull at the transom. [0025] The beneficial effect of the flap and baffle arrangement is believed to occur in part due to the fractional lengthening of the waterline of the hull which reduces the Froude Number at any specific speed. It is also believed that the benefit i s due to the effect on the entrained water behind the transom which has a less turbulent region due to the capturing effect of the baffle on the water within it. The present invention also modifies the stern wave immediately behind the transom due to the longer run of the bottom of the hull - which moves the stern wave crest further away from the transom - and the sharp edge to the baffle which creates a cleaner exit for the water away from the transom. It has been demonstrated in tests that this embodiment of the present invention modifies (reduces) the wave making resistance of the hull around the stern quarter, reducing the crest height of the stern quarter waves.

[0026] The length of the extension will vary depending on the selected speed/Froude number at which the hull is designed to operate at, the waterline length (LWL) of the hull and the depth of immersion of the transom. This extension to the hull will marginally increase the skin friction drag of the hull, but the hydrodynamic benefits of the device far outweigh this feature.

[0027] In a first alternative, the span of the flap and baffle arrangement may be constrained for operational reasons not to extend to the topsides of the hull, but may run upwards (either vertically or at a divergent angle away from the centre line of the vessel) from the lower buttock line at the bottom of the transom from the edge of the flap commencing at some point between the centre line and the turn of the bilge of the hull, to a point above the deepest operational waterline of the vessel (the baffles). The device will be approximately perpendicular to the surface of the transom.

[0028] One of the beneficial effects of the present invention is to entrain a body of water adjacent to the transom against which the turbulent flow arising from water flow around the aft edge of the baffle impinges, as the hull moves through the water. This is believed, in part, to reduce the pressure difference which the transom normally experiences and hence reduces the drag on the hull. As the speed of the vessel increases, the level of water entrained behind the transom will also reduce, creating a difference in water level between the outside surface of the stern/baffle and its uppermost/inside surface. With an increase in speed, this difference in water level can also increase until the transom is evacuated of water - when the drag effects from this particular drag effect will fall to zero. This feature is common with planing boats, but it will be appreciated that economical running of a bulk carrier would not arise at planing speeds - were this to be possible. Nonetheless, with an arrangement in accordance with the present invention, the speed at which the transom runs dry will be lower than without the device, thus continuing to improve the overall hull drag above such a speed. The sharper exit for the water from the hull created by an aft peripheral edge of the flap and baffle also helps to encourage an earlier separation of the flow from the hull so that the 'running dry' speed will be lower than without the device fitted.

[0029] The exact flow patterns of the water entrained within an arrangement in accordance with the present invention changes as the speed increases. The planform of this device approximately follows the shape of the transom such that the additional water flow length is consistent for all water flowing past the device, thus ensuring a consistent pressure gradient and flow velocity past the device and at the end edge of the device.

[0030] In accordance with a second embodiment, and with reference to Figure 4a, the vertical baffles 24 are arranged more closely spaced with respect to a centre line of the vessel. The flap 23 and baffle has a similar hull resistance improvement capability to the first embodiment but entrains the water around the transom to a lesser degree and therefore does not deliver quite the same performance advantage. The volume of entrained water within this flap is smaller than for the version f irst discussed and the water circulation between the baffles is therefore lower. Figure 4b shows a model of the second embodiment.

[0031] In accordance with a third aspect of the invention, there is provided, for each side of the transom, first and second baffles. Conveniently, the outer baffle is contiguous with the sides of the hull, at least below the waterline. It has been found that the advantages of both the first and second embodiments can work well together. It combines the advantages of each, and delivers the best performance improvement of both the versions. The addition of the baffles has a significant effect on the circulation of the entrained water behind the transom, breaking up the reverse circulation flow of the wake back towards and across the face of the transom. This then forms three lower velocity portions of turbulent flow across and to and from the transom face and the flow regime changes materially as the speed of the vessel increases. The height of the water entrained behind the transom in the three areas varies with the speed of the vessel and has an effect on the resistance of the hull.

[0032] It has been found that the vertical elements of the invention act to provide a flow constraint with respect to entrained water behind the transom, guiding the water flow in a recirculating phase and enabling the water level behind the transom to reduce as the hull speed increases without additional inflow of water from the side of the hull. In this way the water level behind the transom can reduce more quickly than would otherwise occur, without the invention, thus eventually enabling the transom to 'run clear' at a lower speed with the flap and baffle arrangement fitted.

[0033] The entrained water behind the transom and within the confines of the flap and baffle arrangement moves with the hull and has a recirculating flow that gains some water from around the edge of the flap and baffle arrangement while losing some water to the flow past the hull. As the speed increases the outflow of water from this region is greater than the inflow such that the water level behind the transom gradually reduces as the speed increases until the transom 'runs dry'. This flow regime is consistently different to that which occurs behind a flat transom without a flap and baffle arrangement fitted where the water is continuously changing and is extremely turbulent and at a lower pressure, thus exerting a greater degree of 'base drag' to the transom than when fitted with the flap and baffle arrangement. [0034] Stern flaps have been fitted to USN and RN ships for many years and have been optimised for reducing the hull resistance over medium to high speed ranges (i.e. 15 - 30 knots) which are more commonly associated with warships. Significant fuel savings have been repeatedly demonstrated from operational ships over a range of hull sizes, ranging from 10% to 15% depending on the speeds and operational profiles of the ships. However one key feature has been prominent in all the trials and modelling activities undertaken for these ships - the model test data has without exception, always underestimated the actual fuel savings achieved by up to 50%.

[0035] Indeed, the present invention has proven to overcome certain disadvantages and preconceptions of transom-flaps as fitted to fast and planing craft with respect to displacement craft. As a rule, the stern flap itself was considered to produce drag at all speeds, under all conditions. It is believed that these preconceptions have arisen from a poor understanding of certain flow characteristics of displacement craft. The improvements witnessed in tests of the present invention show that it is possible to use modified stern flaps to modify the afterbody flow field to provide significant performance enhancements in a displacement hull. The stern flap causes the flow to slow down under the hull at a location extending from its position to a point generally forward of the propellers. This decreased flow velocity will cause an increase in pressure under the hull, which in turn, causes reduced resistance due to the reduced suction force generated on the afterbody. The flow exit velocity from the trailing edge of the flap is increased in comparison to the transom trailing edge (no flap), leading to a lower speed for clean transom flow separation. Localized flow around the transom, which represents lost energy through eddy-making, wave breaking and turbulence, is modified by the stern flap. Wave heights in the near field stern wave system and far field wave energy, are both reduced. [0036] Secondary stern flap hydrodynamic effects provide an effective lengthening of the ship whereby to further reduce wave resistance. The conditions of decreased flow velocity/increased pressure, and reduced propeller loading, are both favourable to the powering (and cavitation) performance of the propeller. The upward force on the afterbody generated by the increased pressure, coupled with the lift force developed on the flap, results in increased bow down trim, and also a slight reduction in the ship's dynamic sinkage. As a rule, the stern flap itself produces drag at all speeds, under all conditions. However, the beneficial interaction with the hull and propulsor consistently results in a decrease in ship power requirements for a given speed. [0037] Incidentally, much research has been performed upon scale models with flaps operating in towing tanks. It has usually been found that scale model test results specifically of this type of device have significantly under predicted the improvement in hull performance that has been measured at full scale. Model testing is a well understood tool that has been used by naval architects for over 200 years, and the need for correlation coefficients to account for scale effects between model and full sized vessels is well understood. However in this case these coefficients do not account for this very significant discrepancy between the results achieved for the transom flaps - it is therefore generally accepted that there are further flow mechanisms at work around such flaps which causes these exceptional results to be achieved, and which is not susceptible to being modelled at scale size. In comparing their overall results the Applicant design team has concluded that greater model data adjustments are necessary at lower speeds and smaller model scales.

[0038] In the special case where a vessel is towing something (e.g. a tug, or a fishing vessel when trawling) then the speed difference between the forward motion of the vessel and this accelerated body of water is considerably higher, i.e. the exit wash from the propeller is much faster than the forward speed of the vessel in order to generate the thrust to achieve the tow, and the turbulence is even more marked aft of the transom. This accelerated flow past the transom accentuates the 'squat' of the hull, i.e. increases the stern trim, which also increases the drag of the hull and reduces the propulsive efficiency of the propeller.