EP2331391A1

EP2331391A1 - Hull form intended for vessels provided with an air cavity

Info

Publication number: EP2331391A1
Application number: EP08813547A
Authority: EP
Inventors: Stig Bystedt
Original assignee: Stena Rederi AB
Current assignee: Stena Rederi AB
Priority date: 2008-09-19
Filing date: 2008-09-19
Publication date: 2011-06-15
Also published as: KR101541574B1; JP5296211B2; CN102171093A; WO2010033058A9; JP2012502850A; EP2331391A4; KR20110076946A; CN102171093B; WO2010033058A1; BRPI0823077A2

Abstract

Hull form for vessels, which gives a great plane bottom area and which is suitable for and can utilize the principle of air cavity. The hull form reduces the wet surface of the hull, decreases the motions at sea and results in a vessel having reduced power need and bunker consumption and which is suitable for ocean-going traffic.

Description

Hull form intended for vessels provided with an air cavity

The present invention relates to a hull form for vessels. One of the most important parameters in all vessel construction is the relation between speed and power. For normal merchant vessels, the frictional resistance, i.e., the friction between underwater hull and water, is the totally dominating part of the total resistance of the vessel. Only when the speed increases, as e.g., for high-speed vessels, the wave resistance becomes crucial. It has been tried to affect the frictional resistance by different types of surface structures and by different types of air lubrication. However, the result for conventional merchant vessels has hitherto been negative.

Another way to decrease the frictional resistance is to decrease the area of the wet surface. The wet surface is the part of the underwater hull, which at zero speed is in contact with the surrounding water.

The wet surface can be decreased by a part of the plane bottom surface of the underwater hull being carried out as a cavity, which is filled with air - henceforth denominated as air cavity. The air has then the same pressure as the surrounding wa- ter. Because of the advance of the vessel, a part of the air will be carried away and the outflowing air then has to be compensated with new supplied air. The air may be supplied to the cavity through the ceiling thereof or the sides thereof. The cavity, which may be compared to a box turned upside down having the opening downward, has to have a well adapted shape in order not to give rise to increased resistance in the form of burble and vortex build-up, when the water passes under the cavity. With today's hull forms, the water of the forebody often flows from the side in under the vessel toward the center line of the vessel. This means that a wave crest is formed in the air cavity, which disturbs the water flow. Today's hull forms also means that the sides of the air cavity only on a relatively short distance are parallel to the center line of the vessel. Forward and astern of this portion, the sides of the air cavity have a fairly large angle in relation to the center line. The flowing water will then meet and leave, respectively, the other sides with increased resistance as a consequence. When the vessel moves in high sea, it may be rolling in the transverse direction, i.e., a reciprocating rotary motion around a longitudinal axis of the center line of the vessel or pitch motions, i.e., reciprocating rotary motion around a horizontal transverse axis in the midship point of the vessel or combinations of these, the air will flow out at the highest situated side of the air cavity. In order to decrease the outflow of air and thereby decrease the need of air supply as well as in order to avoid vortex build-ups at the sides of the air cavity, the seagoing qualities are accordingly of significant importance. The interface between air and water in the cavity aims at a horizontal level, where there is balance between air-pressure and subjacent water pressure. The interface should be lying as near the lower edge of the air cavity as possible. If the vessel is exposed to great rolling or pitch motions, the interface of certain parts will lie much above the lower edge of the air cavity. This means that powerful vortex build-up is produced along the sides of the cavity with increased resistance as a consequence. Consequently, the air cavity should be formed so that the consequences of said motions are reduced as far as possible. Therefore, in order to decrease the consequences of motions at sea, there have been proposals to by means of partition walls divide the air cavity in the transverse direction as well as the longitudinal direction. Several air cavities are then obtained, each one having a separate air supply. The risk of transverse partition walls is obvious in that they once again give rise to vortex buildup with increased resistance as a consequence.

Thus, it is easy to realize that if the principle of decreased frictional resistance - by means of cavities filled with air in the plane bottom - should work in reality, the hull form of the vessel has to be given a special shape and the sea-going qualities thereof improved. An additional complication is that the outflowing air must not come into the propeller water, since this reduces the efficiency of the propeller. In the literature, proposals have been shown where it is tried, by means of different devices, to deflect the water/air mixture, so that this does not impinge on the propeller area. Such a compul- sory guidance of the water/air flow also gives rise to extra resistance. All of said difficulties mentioned above have hitherto made that the principle of the air cavity has not led to any practical application in conventional vessels of ocean-going traffic.

The object of the invention is to provide a new hull form for vessels, which gives a great plane bottom area and which is suitable for and can utilize the principle of air cavity. The hull form should accordingly reduce or even eliminate the above mentioned disadvantages, decrease the motions at sea and result in a vessel having reduced power need and bunker consumption and which is suitable for ocean-going traffic.

The invention is defined in the appended claim 1. Embodiments of the invention are defined in the appended dependent claims.

Most modern merchant vessels have some form of bulb in the forebody. In order for a bulb to be in a certain position, the transverse frame in this position has to have a waist, i.e., the width of the frame should be greater above as well as below the waist. By means of a bulb in the forebody, it is possible to obtain a certain reduction of the wave resistance. The cross-section of the bulb ahead of the forward perpendicular (FP) for conventional vessels most often resembles more or less a sharpened ellipse on end. The forward perpendicular (FP) is defined as a vertical line through the point where the design waterline (dWL) intersects the stem. The design waterline (dWL) is here defined as the waterline of the greatest summer draught, which the vessel is allowed to operate in considering the strength and stability thereof.

The hull form of the forebody according to the invention is, however, characterized by a very wide, low and relatively thin bulb and the profile of the front may almost be described as a flat lying ellipse. In order to describe the forebody in more detail, it is required that certain characteristic quantities for the bulb are defined.

The bulb length counted from the most forward point of the underwater hull is the smallest of the two values below: - the horizontal distance from the most forward point of the underwater hull to the location of the transverse frame, where the waist of the transverse frame of the fore- body disappears or

- 2 times the horizontal distance counted from the most forward point of the under- 5 water hull to the most abaft point of the stem profile in the waist.

The bulb volume is the volume of the hull, starboard plus port, within the bulb length from the baseline up to the design waterline.

10 The bulb area is the area projected on a horizontal plane, starboard plus port, of the bulb below the waist within the bulb length.

An average thickness of the forebody is obtained if the bulb volume is divided by the bulb area. An average width of the bulb is obtained if the bulb area is divided by the 15 bulb length.

The bulb coefficient is now defined as the average width divided by the average thickness or by the above-mentioned quantities inserted, the following is obtained:

. .. „. . , bulb area² bulbcoeffιcιert = - ^{z υ} bulbvolyme-bulblength

Known and published hull forms most often get, depending on the dimensions and ampleness thereof a bulb coefficient of 0,5-1. By means of the hull form according to the invention, including the additionally described claims according to below, the bulb 25 coefficient should have a value of at least 1 ,5. The optimal value depends on the type of vessel and the dimension relations, but the value of the coefficient often approaches the value 3 or even exceeds the value 3.

The stem edge of the bulb becomes according to the above formula wide, flat and 30 relatively thin. The tip of the bulb should be at such a distance above the baseline that the water flow is divided into a lower flow and an upper flow. The lower flow should flow under the bulb and under the vessel while the upper flow should flow above the bulb and mainly move along the sides of the vessel. With a suitable distribution of the flow under and above the bulb, respectively, a minimum overflow is obtained from the hull side toward the bottom of the forebody. The underside of the bulb in the front part is in the transverse direction either a straight line from the center line and outward or slightly curved. In the after direction, the underside of the bulb successively transforms into becoming plane horizontally, at the same time as it increases in width as it approaches the baseline. When the underside has reached the baseline, the air cavity can start.

The following advantages are obtained by the hull form according to the invention:

• The wide and flat bulb creates possibility of the start of a very wide air cavity. The wider the bulb, the wider start is obtained of the air cavity. This in- creases the area of the air cavity, i.e., decreases the wet hull surface, which in turn gives a decrease of the frictional resistance.

• A wide start of the air cavity means that the sides of the air cavity up to the maximum width form small angles in relation to a vertical plane parallel to the center line of the underwater hull. This decreases the probability of flow of the water flow transversely over the sides of the air cavity, i.e., less risk of vortex build-up.

• The hull form according to the invention gives a practically straight water flow and parallel to the center line of the vessel under the air cavity, which creates a calm working condition for the air of the cavity. • The wide bulb shape creates a great displacement contribution. With retained displacement of the forebody, this means especially for slender vessels that the waterline width of dWL can be decreased. This improves the motion properties of the vessel in the pitch direction - see below, which according to the above was important in order for the principle of air cavity to work.

When the forebody moves in the vertical direction, the wide bulb will pull with it a great quantity of water. This increases the so-called co-oscillating water mass and increases the polar mass-moment of inertia fore-and-aft. Simultaneously, the wide bulb may mean that the waterline width of the forebody can be decreased immediately above and below dWL. In such a way, the moment aiming to bring back the vessel to the neutral position thereof after a vertical motion is decreased. Increase of the polar moment of inertia with simultaneous decrease of the restoring moment, means that the natural frequency decreases. This is advantageous, since the risk decreases to end up in sympathetic vibration in head sea for normal wave spectra.

In addition, the wide bulb of the forebody will increase the attenuation coefficient upon vertical motion of the forebody. An increased attenuation decreases the motions in the pitch direction even if the frequency lies in the vicinity of a sympathetic vibration.

The motions in the transverse direction for a monohull vessel are to a great extent depending on the main dimensions of the vessel. The air cavity decreases the trans- verse stability and therefore it may be necessary to divide the air cavity into a plurality of longitudinally-going chambers. These air cavities are then provided with separate air supply ducts.

By the stern, the corresponding requirements of the hull form apply in order for the principle of the air cavity to work optimally. A horizontal bulb, may - like the bulb of the invention of the forebody - be arranged in the lower stern part of the afterbody.

The hull form of the afterbody according to the invention is characterized by a low and relatively thin bulb having a very large extension in the transverse direction, which increases the plane bottom surface. All transverse frames within the stern bulb length, defined according to below, should accordingly have a waist. In order to tail describe the afterbody in more de, the corresponding characteristic quantities for the stern bulb are defined.

The stern bulb length is defined as 10 % of the perpendicular length of the hull, counted from the abaft point of the bulb below the waist in the forward direction. The perpendicular length is the horizontal distance between the front and after perpendicular. The after perpendicular is a vertical line through the centre of the rudder shaft or, if the vessel lacks a conventional rudder, a vertical line through the point where the design waterline intersects the transom.

The bulb volume is the volume of the hull, starboard plus port, within the stern bulb length from the baseline up to the design waterline.

The bulb area is the area projected on a horizontal plane, starboard plus port, of the bulb below the waist within the stern bulb length.

The stern bulb coefficient for the afterbody is now defined like the forebody as:

. .. _xe ■ . bulb area² bulbcoeffιcιert = - bulb volyme- bulblength

The hull form of the afterbody according to the invention gets a stern bulb coefficient of at least 0,4 but may in certain cases become considerably higher. This depends, among other things, on whether there is a propulsion device in the afterbody and, if so, how it is arranged.

The hull form according to the invention of the afterbody gives the corresponding advantages as have been obtained above of the forebody: • The wide and flat bulb provides possibility of the ending of a very wide air cavity. The wider the bulb, the wider ending is obtained of the air cavity. This increases the area of the air cavity, i.e., decreases the wet surface, which in turn gives a decrease of the frictional resistance.

• A wide ending of the air cavity in the afterbody means that the sides of the air cavity starting from the maximum width amidships up to the ending form small angles in relation to a vertical plane parallel to the center line of the underwater hull. This decreases the probability of flow of the water flow transversely over the sides of the air cavity, i.e., less risk of vortex build-up.

• The great horizontal bulb increases the co-oscillating water mass and gives an increased polar mass-moment of inertia. Simultaneously, the bulb by the stern gives a powerful increase of the attenuation coefficient in the equation of mo- tion thereof. All quantities alter in the correct direction i.e., gives a decrease of the vertical motions in the pitch direction.

The ending of the bulb has to be carried out in view of the propulsion device and in view of possible air discharges. The following are only examples of ending:

1 -propeller vessels may suitably end the bulb on the baseline a distance in front of the propeller plane. This then gives possibility of possible air discharges passing under or outside the propeller.

2-propeller vessels of so-called twin-skeg type may have a corresponding ending as for 1 -propeller vessels for the respective propeller. The width and the ending of the bulb on the inside of the respective propeller then have to be adapted so that the propeller gets enough water flow.

An interesting application of the invention arises for two hull vessels, so-called catamaran vessels, where propulsion devices are placed in the center line of the vessel. The hull arrangement is then based on the fact that a local body is built in the center line of the vessel, i.e., between the catamaran hulls, under the strength deck which unites the two hulls. The local body is present both in the stem and in the stern and extends down toward the design waterline. In slack water, the lower edge of the body is at or immediately above the design waterline. The propulsive device is applied to the lower edge of the respective body in the stem and in the stern, respectively. The afterbody of the two catamaran hulls can now be formed with great wide bulbs without consideration needing to be given to the propulsion devices. The plane bottom and the air cavity can be ended in a completely optimal way.

In order not to create unnecessary contribution in the wet surface, the aft side of the bulb according to the above catamaran arrangement may and should end in a more or less rounded tip in the vicinity of the after perpendicular a bit above the baseline. The underside of the bulb in the after part is either straight horizontally in the transverse direction or slightly curved, which successively forward transforms to a plane horizontal underside, which increases in width the closer it approaches the baseline. When the underside has reached the baseline, the width is considerable and can therefore meet a good ending of the air cavity.

For all applications, a certain quantity of air may be carried away by the water and pass out in the afterbody under the hull. In order to decrease the risk of extra resistance when the water passes the aft side of the air cavity, this is provided with a leaning plane, which extends from the ceiling of the air cavity down toward the interface between air/water. A smaller angle of the leaning plane then gives less risk of vortex build-up when the water and possibly a smaller quantity of air leave the air cavity. The leaning plane should of course have the same width as the air cavity.

In order to be able to keep the interface between air/water as near the lower edge of the hull along the entire air cavity, it is essential to hold the lower edge of the hull horizontal, i.e., the vessel should be on a so-called even keel, i.e., without trim. Because of hydrodynamic effects, the vessel will, however, change its trim when the speed is changed. Therefore, a manual or automatic system should be installed, which by means of water ballast pumping in forebody and afterbody aims to hold the vessel without trim.

The water pressure in the lower edge of the hull will vary in the longitudinal direction depending on wave troughs and wave crests. Therefore, there is no set value of the air pressure given in advance. Instead, an automatic control system has to be installed, where level meters controls the fans, which feed the air cavity and thereby the pressure. The level meter or level meters that shows/show the lowest levels of the interface between water/air should then be compared with the desired level of the interface and constitute control signal to the fans.

The height of the cavity, i.e., the distance from the lower edge of the hull to the ceiling of the air cavity has to be adjusted to the internal arrangement of the vessel in question and to the maximum wave height, at which the air cavity in an effective way should be able to reduce the resistance of the vessel. The longitudinal stability becomes much less than for conventional vessels due to the existence of a great free liquid surface in the air cavity. This means that the vessel becomes extra trim-susceptible when the vessel is in port during loading or unloading. A movement of a weight in the longitudinal direction gives rise to a much greater change of the trim than for the corresponding vessel without air cavity. In order to reduce this extra trim increase, the cavity may - when the vessel is in port - need to be divided fore-and-aft into a plurality of sections. This may take place by the fact that one or more transverse walls are lowered down or are turned down in the air cavity. Each section of the air cavity should then be provided with its own air supply. When the vessel is under speed the transverse walls should be pulled up or raised.

Depending on the dimensions of the vessel and the relation speed/length, different types of waves are produced along the hull. These waves may occasionally also ex- tend into the air cavity. At a certain speed, such a wave crest may occur in a position under the leaning plane of the stern. This wave is stationary in relation to the vessel and can be utilized. If it is desired to utilize such a wave crest, the abaft limiting surface of the air cavity as well as adjacent underside of the bulb astern of the air cavity should abut against and meet the crest of said wave. The lower part of the aft side of the air cavity is accordingly closed by means of the wave crest and there is no hull portion astern of the same. The aft side of the leaning plane should then meet the crest of this wave. This means that the leaning plane has to be adapted to or be possible to be turned or be lowered/be raised, so that the lower portion thereof can be connected to the height in question of the wave. In such a way, the air is contained to the greatest feasible extent. There should be no hull portion astern of and under the lower portion of the leaning plane, since this creates extra resistance. The higher height of the wave, the higher up is the ending of the leaning plane. In this way, the resistance decreases and the vessel gets increased power/speed forward. If the draught in port is critical, the leaning plane should in port be possible to be lowered/be turned down, so that the lower edge thereof comes in flush with the lower edge of the hull. Then, the air cavity is maximally utilized and the draught of the vessel is decreased. An embodiment example of the hull form and hull arrangement according to the invention is described in the following, reference being made to the appended drawings, where

Fig. 1 shows a profile of a stem having inserted definition of bulb length. Fig. 2 shows an example of a body plan of a forebody having air cavity. Fig. 3 shows an example of a body plan of an afterbody having air cavity. Fig. 4 shows an example of a body plan of an afterbody for a catamaran vessel. Fig. 5 shows a profile of an afterbody having a wave in the ending of the air cavity.

Fig.1 shows a profile of a stem having inserted definitions. The baseline (BL) is a line parallel to dWL (Design water line) through the lowest point of the vessel. The waist (W) of the stem is the most abaft point of the stem in a position over the stem edge of the bulb. In the figure, also the bulb length is defined. This definition applies on the assumption that all transverse frames have a waist within this length. If this is not the case, the length is reduced to become from the stem edge of the bulb up to the transverse frame, which is the last frame of the forebody which has a waist.

Fig. 2 shows an example of a body plan of the forward part of a forebody up to dWL. The shown frames correspond to the locations 85, 90, 95, 100 and 101 ,25 % of the perpendicular length. 50 % corresponds to amidships and 100 % corresponds to the frame of the forward perpendicular (FP). 101 ,25 % of the perpendicular length corresponds in this example to a frame halfway between FP and stem edge of the bulb. In the figure, it is seen how the underside (1) of the bulb successively transforms to a plane underside (2) when it approaches the baseline (BL) (3). In the example shown, the underside of the bulb reaches the plane bottom immediately forward of the frame location 90 % and there the plane bottom has been replaced by the starting of an air cavity, shown by dashed lines (5). On frame 85 %, the width of the air cavity has in- creased additionally, shown by dashed lines (6). On frame 101 ,25 %, an ellipse (4) has been drawn in as comparison. Here it is clearly seen the great horizontal extension of the bulb and the resemblance thereof with a horizontally lying ellipse, even if the upper edge of the ellipse has been provided with a crest to meet the connection to the stem.

Fig. 3 shows an example of a body plan according to the invention of a 1 -propeller afterbody. The shown frames correspond to the locations 0, 5, 10, 15 and 20 % of the perpendicular length. 0 % of the vessel length corresponds to the after perpendicular (AP). In this case it is clearly seen how the underside of the bulb coincides with the plane through the baseline (BL) and entails a considerable increase of the width of the air cavity. The air cavity has full height (7) on frame 20 %. The height of the air cavity decreases successively by the stern by a leaning plane and reaches BL exactly astern of frame 15 %. This means that the ending of the air cavity in this example has a width (8) of approx. 55 % of the maximum width of the air cavity amidships. The lower part of frame 2,5 % is drawn in to indicate the ending of the bulb, which ends immediately before the propeller plane (P). This means that a great part of the air that has been carried away by the flowing water under the air cavity now will pass under and outside the propeller area.

Fig. 4 shows an example of a body plan according to the invention of an afterbody for a catamaran vessel, where the afterbody of the respective frame lacks propulsion de- vices. Accordingly, here the afterbody can only be formed in view of resistance and maximum utilization of the principle of air cavity. The shown frames correspond to the locations 0, 5, 10, 15 and 20 % of the perpendicular length. In this example, the bulb having a rounded tip (8) ends immediately astern of AP and at a suitable height from resistance point of view. Here, the underside (9) of the bulb is drawn with straight lines in the transverse direction. Forward the width increases fast and approaches the baseline approximately at the 10 % frame. Here, the ending (10) of the air chamber may be arranged. In this example, the width of the ending of the air cavity becomes almost 80 % of the maximum width of the air cavity. The leaning plane extends in this example from frame 10 % up to frame 15 %, where full height of the air chamber pre- vails (11). Fig. 5 shows a profile of the afterbody having body plan according to fig. 4. Here, it has been chosen to utilize the pressure from a stationary wave generated by the vessel in position around frame 10 %. The pressure is greater than the pressure in the air chamber and the air chamber can therefore end at a higher level. This is indicated by the dashed line (12). The leaning plane is shown by the line (13) and there can be no hull under and straight horizontally astern of the ending of the leaning plane. Schematically, a wave (14) has been drawn in, which fills up the area from the ending of the leaning plane down to the baseline. The pressure in the air chamber then gives an extra propulsion force on the vessel ahead.

Claims

1. Hull form and arrangement for vessels, characterized in

• that a forebody has a wide and horizontal bulb, which divides the water flow into a lower flow, which passes under the vessel, and into an upper water flow, which mainly moves along the sides of the vessel and

• that a bulb coefficient defined according to the below has a lowest value of 1 ,5

bulb area² bulbcoefficiert = - bulb volyime- bulblength

where the bulb length counted from the most forward point of the underwater hull is the smallest of two of the values below:

- the horizontal distance from the most forward point of the underwater hull to the location of the transverse frame, where the waist of the transverse frame of the fore- body disappears, or

- 2 times the horizontal distance counted from the most forward point of the underwater hull to the most abaft point of the stem profile in the waist, and where the bulb volume is the volume of the hull, starboard plus port, within the bulb length from the baseline up to the design waterline, and where the bulb area is the area projected on a horizontal plane, starboard plus port, of the bulb below the waist within the bulb length and

• that the underside of the bulb in the front part of the bulb length is a straight line from the center line and outward or slightly curved in the transverse direction, which then in the abaft direction successively transforms in the transverse direction into becoming entirely horizontal and when the underside reaches the baseline or immediately astern of the same position the greater part of the plane horizontal bottom area is replaced by a cavity, which is open downward and which by means of inflow pipes placed in the upper surface of the cavity or in one or some of the sides thereof is filled with air having a pressure corresponding to the pressure of the surrounding water.

2. Hull form according to claim 1 characterized in that the bulb coefficient has a lowest value of 2,0.

3. Hull form according to claim 1 characterized in that the bulb coefficient has a lowest value of 2,5.

4. Hull form according to any one of the above claims characterized in

• that the transverse frame in the lower after part of the afterbody within at least one stern bulb length defined according to below, has a waist, and • that the same part has a wide and horizontal bulb, which increases the plane horizontal part of the bottom surface, and

• that a stern bulb coefficient defined according to the below has a lowest value of 0,4,

. .. „. . . bulb area² bulbcoeffιcιert = bulbvolyme bulblength

where a stern bulb length is defined as 10 % of the perpendicular length of the hull, counted from the abaft point of the bulb below the waist in the forward direction, and where bulb volume is the volume of the hull, starboard plus port, within the stern bulb length from the baseline up to the design waterline, and where the bulb area is the area projected on a horizontal plane, starboard plus port, of the stern bulb below the waist within the stern bulb length, and

• that the stern bulb at the after edge thereof ends on or above the baseline with a rounded tip as seen from the side and which then forward suc- cessively increases in width and where the underside of the stern bulb in the after part of the bulb length in the transverse direction is either a straight line from the center line and outward or slightly curved and which then in the forward direction successively transforms in the transverse direction into becoming entirely horizontal and when the underside reaches the baseline or immediately forward of this position, the plane horizontal bottom area is replaced by the air cavity.

5. Arrangement according to any one of the above claims, characterized in that the lower edge of the after ending of the air cavity lies above the baseline and that from this level down to the baseline, the air cavity becomes closed by a stationary wave generated by the vessel.

6. Arrangement according to claim 5 above, characterized in that the lower edge of the after ending of the air cavity can be raised and be lowered, respectively, in order to at different speeds meet different heights of the stationary wave.

7. Arrangement according to claim 5 or 6 above, characterized in that when the vessel is in port or at a low speed, the lower edge of the after ending of the air cavity is lowered down to the lower edge of the air cavity.

8. Arrangement according to any one of the above claims, characterized in that when the vessel is in port, one or more transverse partition walls temporary are lowered and divide the air cavity fore-and-aft into a plurality of air chambers and that these partition walls are raised, when the vessel makes speed and util- izes the power reducing principle of the air cavity.