WO2000007872A1

WO2000007872A1 - Hull shape i

Info

Publication number: WO2000007872A1
Application number: PCT/NO1999/000238
Authority: WO
Inventors: Knut BØRSETH
Original assignee: Petroleum Geo-Services As
Priority date: 1998-07-21
Filing date: 1999-07-16
Publication date: 2000-02-17
Also published as: AU5658399A; NO983369L; AU5658299A; NO983369D0; WO2000007873A1

Abstract

Hull shape having a sharp bow with a bulb, and a broad, essentially squarily cut off stern in and below the design waterline, the waterlines in the stern part of the hull being essentially perpendicular to the centre line of the hull, the waterlines being gradually shifted astern with increasing draught from the base plane approximately midships, so that an approximate sloped plane is laid through the respective waterlines transverse cutting-off providing a broad cutting-off astern on the hull. The waterlines between the bow and the stern in the longitudinal direction of the hull shape extends through a curved, unabridged, convex path in which the stern of the hull is positioned at the widest part of the hull.

Description

HULL SHAPE I

This invention relates to a hull shape having a sharp bow and bulb, and a broad, essentially squarily cut off stern, in and below the design waterline, in which the waterline in the stern end of the hull is essentially perpendicular to the centre line of the hull shape and the waterline is shifted gradually toward the stern with increasing draught from the ground plane approximately midships, so that an approximate sloped plane positioned through the respective waterlines thwart ships make a broad end in the stern.

A vessel of this type is described in European patent 0,134,767 (Ramde) . Ramde's solution, however, comprises sinusoidal waterlines in the area between the bow and stern of the vessel. The sinusoidal lines stretching over half a period from the least separation defined by the minimum value of the sinusoidal shape to the largest separation defined by the maximum value by the vessels stern. The hull shape defined by Ramde's patent gives the vessel a number of advantages compared to conventional hull shapes, as it has been possible to improve the vessels dead weight tonnage, lateral stability, manoeuvrability and sailing characteristics and to reduce the strains on the hull beams both when sailing in quiet waters and against the waves. As discussed in the abovementioned Ramde patent will with certain main dimensions such as length, breadth and draught of the design waterline, conventional hull shapes be able to obtain larger dead weight tonnage by increasing the body of the under water part of the hull, so that the total displacement increases. To improve the lateral stability of a conventionally shaped hull, expressed a the initial metacentre, the width of the hull may be increased so that a larger moment of inertia occurs at the waterline, and possibly also the volumetric centre of gravity of the underwater hull is increased.

Changes of this type (the increase of the displacement and width) as required for increasing the lateral stability and speed, will, however, at the end result in an unacceptable increase in the conventional vessels propulsion resistance in quiet water as well as with high waves. In order to improve the sea characteristics for a conventional hull shape, expressed as the angular movements of the ship relative to a lateral axis (pitching) , vertical movements (heave) , accelerations and increase in the resistance to propulsion as compared to the resistance in quiet waters, one will try to change the natural frequencies at pitching and heaving so that this frequency as far as possible does not correspond to the frequency of the wavelengths the vessel will meet.

Regarding the conventional hull constructions the structural changes only results in minor improvements in the vessels sea properties, and it will give extreme pitch and heave movements as well as a strong increase in the resistance to propulsion when the ship sails against waves with a dominant wavelength corresponding to the length of the vessel at the waterline. Depending on the type of vessel and its velocity such movements will always make it necessary to reduce the speed or change the course relative to the waves, so as to change the periods for meeting the waves, so that the wave period does not correspond to the natural frequency of the vessel for pitch and heave.

In the previously mentioned Ramde patent the use of some relations is described which has been found to be non- optimal. Also it has been found other differences providing significant improvements over the performance of ships constructed according to the previous Ramde patent, at the same time as problems are solve being related to the use of such ships.

The performance of the hull shape according to the present embodiments of the inventions at sea is improved so that the pitch and heave properties of the hull is reduced when compared to the movements of conventional hulls, as well as the earlier Ramform hull. Also, these movements are damped so that the improved hull does not show such large movements, while the resistance to propulsion of the improved hull is reduced at the same time. Also, according to the invention, a very even, essentially homogeneous two dimensional water flow is obtained under the hull and passed the stern, resulting in very low turbulence and very smooth sailing. In addition the invention implies an improved position of the propulsion to take advantage of the smooth water flow. In Norwegian patent application 95.1511 Ramde has tried to improve these properties of the vessel even further. Trials have shown that especially the bow part of Ramde's hull shape is not optimal, and that the changes according to this invention gives significant and surprising improvements of the motion properties, especially when the vessel is laying still and has a possibility to chose the best direction toward the waves. This is obtained using a vessel as described above and being characterized as described in claim 1.

According to the different embodiments of this invention a hull according to the invention is provided with a sloped plane having an angle being under approximately 14 degrees between the basis line and the centre plane, and the a line stretching from the (squarily cut off) stern end to another point on the surface at 0.2-0.3L from the stern, L being the length of the ship at the waterline (dwl) . Further according to the invention a bulb at the bow continuous to meet an upward tilting element essentially under the design waterline and extending close to the surface of the design waterline.

According to yet another embodiment of the invention a displacement type ship is provided with a hull according to the invention, having squarily cut off in the longitudinal direction and convex waterlines, and having at the bottom of the stern part a sloped plane between the basis plane and the stern, said sloped surface making an angle with the basic plane and extending tangentially into the base plane at approximately L/2, and the angle of the sloped surface is relative to the base plane and a line connecting a first point in a longitudinal section in parallel to or equal to the centre section at the lower edge of the straight ended stern, and another point in the same section as the first in the sloped plane at 0,2L. According to a preferred embodiment of the invention the angle of the sloped surface is 14°. According to another embodiment a ship of the displacement type is provided with a hull according to the invention, having an essentially squarily cut off longitudinal and convex waterlines comprising a sloped plane at the bottom of the ship, formed between the base plane and the stern, the sloped plane extending tangentially into the base plane at approximately L/2.

According to yet another embodiment of the invention a ship of the displacement type is provided with a hull according to the invention, comprising squarily cut off longitudinal and convex waterlines comprising a sloped plane at the bottom of the ship, formed between the base plane and the stern, the sloped plane extending tangentially into the base plane at approximately L/2, with a bulb which at the cross section in the middle between the forward perpendicular and the a cross section through an upper part of the bulb has a larger breadth than hight and with a rounded upper surface and a V-shape down toward the base plane. According to a special embodiment the length of the bulb measured from the forward perpendicular up to the cross section corresponding to the upmost part of the bulb, is in the range of 0.1 to 0.12 Bmax, and the cross section of the bulb in the middle of this length has a width/depth relationship being approximately 1.8.

According to another embodiment a ship of the displacement type is provided with a hull according to the invention, having an essentially squarily cut off longitudinal and convex waterlines comprising a sloped plane at the bottom between the base plane and the stern of the ship, the sloped plane extending tangentially into the base plane at approximately L/2.

The abovementioned embodiments are only given as examples. They are not meant to give any limitation to the invention to any special feature or combination of the abovementioned examples, as will be obvious for a person known in the art .

The invention will be described below with reference to the drawings, of which: Figure 1 is a horizontal projection of a hull formed according to an embodiment of the invention. Figure 2 is an elevation of the hull according to figure 1. Figure 3 is an elevation of the bottom of the hull according to figure 1. Figure 4 is an elevation of a hull with two keels.

Figure 5 illustrates the same hull shape as figure ty , as seen from the side. Figure 6 is an elevation of a hull according to a preferred embodiment of the invention. Figure 7 illustrates the same hull shape as figure 6 in a horizontal projection. Figure 8 illustrates the hull shape in figures 6 and 7 as seen from one side.

Reference is now made to figure 1. According to an embodiment of the present invention a hull 10 is provided having more rounded lines than conventional hull shapes, which is evident from the expression for the length/displacement ratio L/V¹³, where L is the length of the hull at the design waterline (dwl) corresponding to the depth at summer free board (see figure 2) and V is the displacement volume of the hull at the design waterline. According to this embodiment of the invention L/V¹³ is 3,7 or lower. At the same time this embodiment results in a hull width being such that the L/B ratio is between 2 and 2.5. The preferred ratio has been found to be approximately 2.1. According to this embodiment B is the maximum width of the hull at the stern by the design waterline (dwl) . The height of the metacentre of the hull 10 is in this embodiment multiplied relative to the conventional hull shapes having the same length.

According to another embodiment of the invention the displacement distribution in the longitudinal direction becomes an approximate Rayleigh distribution. This type of distribution is obtained in the present embodiment with the essentially squarily cut off convex waterlines (figure 3: dwl, 1,2, 3) with extreme or stationary points 12 and 14 at the ends of the hull both afore and astern, while the basis lines for the waterlines (0_dwl, 0_X, 0₂, 0₃₎ from (G) is shifted astern with increasing draught so long that an essentially sloped plane (S) , which may be straight, is defined. According to this embodiment the plane (S) further comprises the stern half of the hull 10 and allows for the use of different propulsion means or positioning devices. Reference is now made to figures 2 and 3. An additional embodiment of this invention a ratio Bl/tl is defined at a cross section through the hull 10 under the design waterline (dwl) at a distance 0.15L from the stern, where Bl is the width an the design waterline (dwl) and tl is 25%T of the draught (T) measured from the same water line. According to this embodiment the ratio Bl/tl approximately equals 18.

According to yet another embodiment of the invention yet another hull ratio e=C_p/C_dwl is defined, in which C_p is the longitudinal prism coefficient expressed by the following equation

C_p= V/ (A_L/2 x L), where C_dwl= A^/LB, as L is the design waterline length, A is the area of the cross section up to the waterline at L/2, V is the displacement volume for the design waterline, A_dWl is the waterline area and B is the maximum width of at the waterline. According to this embodiment the hull parameter e equals approximately 1.1 or less. Reference is again made to figure 1. In yet another embodiment of the invention the centre of gravity of the design waterline (LCF) is positioned approximately 0.14 L astern of the midship, and the volumetric centre of gravity (buoyancy) (LCB) of the improved hull at the depth of the design waterline (dwl) approximately 0.083L in front of the area centre, which may be expressed as LCF - LCB = 0.083 L. Again reference is made to figure 1 in which the hull 10 is shown with convex waterlines by the design waterline (dwl) with extremes or end points by the bow and stern ends, in which the area centre point (LCF) is approximately 0.14 L astern of L/2 and where the length/breadth ratio L/B at the design waterline equals approximately 2.1.

Figure 2 shows an embodiment of the hull according to the invention under the design waterline (dwl) in a vertical section in which it is shown that the base lines is essentially squarily cut off. According to this embodiment convex waterlines (O_dwl, C 0₂, 0₃) along a sloped, essentially plane surface (S) , being combined with the base plane (G) at approximately L/2, and being astern with increasing draught. Further, the distance between the area centre of gravity (LCF) and the buoyancy centre (LCB) for the hull 10 at the depth of the design waterline (dwl) is approximately equal to 0.083L. The essentially plane surface (S) is in some embodiments a curved surface with a very large radius .

In figure 3 the hull shape of figure 2 is shown in a horizontal projection with the waterlines dwl, 1,2,3 and G in the example with a triangular shape 9 afore in the ship. Figures 4 and 5 illustrates an embodiment of the invention comprising two skegs or keels 300. These keels contributes in stabilizing the roll and pitch movements and increasing the directional stability and displacement of the ship, and also provides a possibility for laying the whole ship in the keels in dock, e.g. for maintenance. The arrangement of the keels will especially, when the ship is at rest, contain a large volume of water in addition to the displacement of the ship. This increased mass will substantially reduce the roll and pitch movements of the ship. As is evident from figure 5 the keels 300 will preferably stretch in the vertical direction down to the base plane G. In figure 5 the vessel is also provided with a tongue shaped bulb 100 of a per se known type.

Preferably the keels 300 stretches astern to a position in front of the ship propellers. The keels preferably ends at a distance from the stern corresponding to approximately 4-7% of the ship length. Preferably 5%, but this will to some degree depend on the steering and propulsion systems of the vessel. Figures 6, 7 and 8 show an especially preferred embodiment of the hull 10 according to the invention, and which, with proper scaling of the drawings, is the basis of the prototype of the hull being tested. In the figures the hull also has a bulb 100. Figure 6 shows the bow part of the hull in perspective using frame profiles and waterlines, figure 7 illustrates, using waterlines, the hull as a whole, while figure 8 shows a side elevation of the hull with longitudinal profiles. In figure 8 the hull 10 is shown with keels 300, in a manner similar to figure 5.

Claims

C l a i m s

1. Hull shape having a sharp bow with a bulb, and a broad, essentially squarily cut off stern in and below the design waterline, the waterlines in the stern part of the hull being essentially perpendicular to the centre line of the hull, the waterlines being gradually shifted astern with increasing draught from the base plane approximately midships, so that an approximate sloped plane is laid through the respective waterlines transverse cut off providing a broad cut off astern on the hull, c h a r a c t e r i z e d in that the waterlines between the bow and the stern in the longitudinal direction of the hull shape extends through a curved, unabridged, convex path in which the stern of the hull is positioned at the widest part of the hull.

2. Hull shape according to claim 1, c h a r a c t e r i z e d in that it has a length/width ratio being between 2.0 and 2.5, preferably 2.1.

3. Hull shape according to claims 1 or 2 , c h a r a c t e r i z e d in that the vessel has a length/displacement ratio being 3.7 or less.

4. Hull shape according to claims 1, 2 or 3, c h a r a c t e r i z e d in that the hull has a ratio e=C_p/C_dwl being less or equal to 1.1, C_p being the longitudinal prism coefficient expressed as C_p= V/ (A_L/2 x L) , and where C_dwl= A^/LB, as L is the length at the design waterline, A is the area of a cross section up to the waterline at L/2, V is the displacement volume for the design waterline, A_<j_wl is the waterline area and B is the maximum width at the waterline.

5. Hull shape according to any one of the preceding claims, c h a r a c t e r i z e d in that the design waterline centre of gravity (LCF) is positioned approximately 0.14 L astern of midship, and that the volumetric centre of gravity (buoyancy) (LCB) at the design waterline depth (dwl) is approximately 0.083 L in front of the area centre of gravity, which may be expressed as LCF - LCB = 0.083L.

6. Hull shape according to any one of the preceding claims, c h a r a c t e r i z e d in that said sloped plane comprises two keels positioned a chosen distance apart stretching in the longitudinal direction of the hull along a substantial part of the planes length.

7. Hull shape according to claim 6, c h a r a c t e r i z e d in that the keels along their whole length stretches in the vertical direction to the same depth as the base plane (G) of the hull.

8. Hull shape according to claim 6 or 7, c h a r a c t e r i z e d in that the keels are positioned a distance apart corresponding to between 20 and 40% of the largest width of the hull.

9. Hull shape according to claims 6, 7 or 8, c h a r a c t e r i z e d in that the keels end astern at a distance from the stern corresponding to 4-7% of the hull length. AMENDED CLAIMS

[received by the International Bureau on 14 December 1999 (14.12.99); original claim 3 cancelled ; original claims 1 and 2 amended ; original claims 4-9 renumbered as claims 3-8 ; other claims unchanged (2 pages)]

1. Hull shape having a sharp bow with a bulb, and a broad, essentially squarily cut off stern in and below the design waterline, the waterlines in the stern part of the hull being essentially perpendicular to the centre line of the hull, the waterlines being gradually shifted astern with increasing draught from the base plane approximately midships, so that an approximate sloped plane is laid through the respective waterlines transverse cut off providing a broad cut off astern on the hull, c h a r a c t e r i z e d in that the waterlines between the bow and the stern in the longitudinal direction of the hull shape extends through a curved, unabridged, convex path in which the stern of the hull is positioned at the widest part of the hull, the hull having a length/width ratio being between 2.0 and 2.5, and a length/displacement ratio being 3.7 or less.

2. Hull shape according to claim 1, c h a r a c t e r i z e d in that it has a length/width ratio being 2.1.

3. Hull shape according to claims 1 or 2 , c h a r a c t e r i z e d in that the hull has a ratio e=C_p/C_dwl being less or equal to 1.1, C_p being the longitudinal prism coefficient expressed as C_p= V/ (A_L/2 x L) , and where C_dwl= A^/LB, as L is the length at the design waterline, A is the area of a cross section up to the waterline at L/2, V is the displacement volume for the design waterline, _dWl is the waterline area and B is the maximum width at the waterline.

4. Hull shape according to any one of the preceding claims, c h a r a c t e r i z e d in that the design waterline centre of gravity (LCF) is positioned approximately 0.14 L astern of midship, and that the volumetric centre of gravity (buoyancy) (LCB) at the design waterline depth (dwl) is approximately 0.083 L in front of the area centre of gravity, which may be expressed as LCF - LCB = 0.083L.

5. Hull shape according to any one of the preceding claims, c h a r a c t e r i z e d in that said sloped plane comprises two keels positioned a chosen distance apart stretching in the longitudinal direction of the hull along a substantial part of the planes length.

6. Hull shape according to claim 5, c h a r a c t e r i z e d in that the keels along their whole length stretches in the vertical direction to the same depth as the base plane (G) of the hull .

7. Hull shape according to claim 5 or 6, c h a r a c t e r i z e d in that the keels are positioned a distance apart corresponding to between 20 and 40% of the largest width of the hull .

8. Hull shape according to claims 5, 6 or 7, c h a r a c t e r i z e d in that the keels end astern at a distance from the stern corresponding to 4-7% of the hull length. STATEMENT UNDER ARTICLE 19

In the enclosed amended claims 1-8 claim 3 and most of claim 2 is incorporated in claim 1.

Although none of the cited documents may be said to indicate that the waterlines between the bow and the stern in the longitudinal direction of the hull shape extends through a curved, unabridged, convex path in which the stern of the hull is positioned at the widest part of the hull, the claims have been amended to state even more clearly the difference between WO 9724256 and the present invention.

The new claim 1 is now limited to a hull as described above having a length/width ratio being between 2.0 and 2.5, and a length/displacement ratio being 3.7 or less, which is clearly different from the characteristics of the vessel in the abovementioned publication.