WO2003091090A1

WO2003091090A1 - A vessel

Info

Publication number: WO2003091090A1
Application number: PCT/NO2003/000132
Authority: WO
Inventors: Gunnar Saehle
Original assignee: Fjellstrand As
Priority date: 2002-04-26
Filing date: 2003-04-23
Publication date: 2003-11-06
Also published as: AU2003241219A1; NO20022005L; NO20022005D0; NO317619B1

Abstract

A hull for a multi-hulled vessel is described where the hull is laterally symmetrical and where the body plan is stepped below the designed water line (8), whereupon the hull continues substantially vertically in a downward direction from the step before it is terminated in a substantially horizontal direction.

Description

A VESSEL

The present application relates to the design of a laterally symmetric hull for a multihulled vessel.

Since 1972 different forms of high-speed catamarans have been used and developed to an ever increasing extent. These were for the most part relatively small vessels, less than 40 metres in total length, and were constructed to transport passengers at speeds varying from a modest 25 knots in the beginning to more than 40 knots in recent years. The last decade has seen the development of substantially larger, but still high-speed, catamarans designed to transport a combination of passengers and cars. A typical speed for these catamarans is between 30 and 40 knots. A feature that is common to the said catamarans is that they are generally used for long-haul routes, and that they have a defined bow section and a defined stern section. This means that the vessels must turn in port before setting out on their next journey. To avoid large movements, it is desirable to minimise the water line area, whilst moving the buoyancy-providing volume as far down in the water as possible. This results in a vessel that will barely be affected by waves, and which at the same time will create insignificant waves when propelled through the water. This type of catamaran exists in a limited number and is termed SWATH (Small Waterplane Area Twin Hull).

Catamarans that are longitudinally symmetric, and which therefore move at the same speed in both directions, have also been developed. Thus, these catamarans have both a bow and stern at both ends of the vessel. For relatively short-haul routes, this represents a considerable advantage because the vessel does not have to turn in port. This is an advantage both because there may be little space for turning in a port, and because time is saved by not having to turn. The solution also means that vehicles drive into the "stern" in the port of embarkation and out of the "bow" in the port of disembarkation.

However, in terms of shipbuilding, this poses problems because the bow section is usually built with narrow water lines to avoid large bow waves and associated resistance. When the water lines are as narrow as required to avoid bow waves, this will normally result in substantial changes in trim during the embarkation and disembarkation of heavy vehicles when this is done via the "bow section".

If the vessel trims significantly during the embarkation and disembarkation of heavy vehicles, this can be compensated for by having adjustable ramps. However, these are costly and unwieldy. A very important property of vessels of this type is therefore the so-called "unit trim moment", that is to say the moment required to give the vessel a trim of 1 cm. Thus, for a vessel to be successful in its transport activity, the unit trim moment must be as large as possible. The unit trim moment is a function of the area moment of inertia of the water line area about the longitudinal centre of flotation divided by the immersed volume. The area moment of inertia is in turn a function of how great the area of the water line is at the ends of the vessel (bow and stern), that is to say, the aim must be to have water lines at the ends of the vessel that are as broad as possible. Normally, this will mean a huge increase in the vessel's wave resistance. However, in the case of the present invention, the hull design is especially adapted to the desire for broad water lines whilst avoiding the creation of large waves. This is achieved by a special design of the frame profile, especially the width of the frame in the water compared with the width of the frame at the level of the water line.

Thus, according to the invention there is provided a hull for a multiple hulled vessel where the hull is stepped at a vertical distance below the designed water line, whereupon the hull continues substantially vertically in the downward direction from the step before it is terminated in a substantially horizontal direction.

For conventional, existing hull shapes, it is difficult to avoid the formation of large waves even at low speed/length ratios. The term "speed/length ratio" as used herein means the ratio of the quotient of the speed in knots to the root of the water line length in feet. This wave formation will contribute substantially to a vessel's resistance, and thus the power requirement at a given speed, but will in addition give an unwanted aft trim under normal sailing conditions, which will also increase the magnitude of the stern-generated wave resistance. For vessels of this type, where both ends of the vessel are both bow and stern, depending on which way the vessel is sailing, this problem will be exacerbated because both ends must be shaped so that they perform in a satisfactory manner both when they function as bow and when they function as stern. The requirements for a bow design and for a stern design will to a large extent be conflicting requirements. The present invention relates to a technical solution to satisfy these requirements.

Furthermore, the wave that is formed, either from the bow or from the stern, will to a great extent depend upon how the volume of the vessel is distributed, both in the longitudinal direction and in the water from the designed water line in any section. To avoid the formation of large waves, it is essential that as much of the immersed volume is as far away from the water line as possible, whilst it is also favourable to have a maximum water line breadth in order to reduce the trim change as a result of different load as much as possible. This results in conflicting requirements, which the stepped hull design according to the invention meets by maintaining the large breadth at the water line whilst the volume is placed as far down as possible at each individual transverse section.

For the type of vessel that is described here, and which is to be built with both bow and stern at both ends of the vessel, it will be undesirable to use waterjets, and therefore space must be allocated for a propulsor, preferably at each end of the vessel. This results in a problem for conventional vessels, as the position of a propulsor will normally interfere with a bow design. In the present design, the combined bow and stern is designed in such manner that it will have space for a propulsor, preferably, but not necessarily, of the azimuth propeller type. When an azimuth propeller is used, the need for and desirability of a rudder is also eliminated, which will greatly improve the steering properties, especially in a following sea. In a following sea, all vessels run the risk of timing substantially off course at the same time as the bow is pressed down into the water (broaching). The invention also takes into account that it may be expedient or desirable to mount a propeller shaft and accompanying propeller at each end of the vessel, which then must also be equipped with a rudder.

One of the objects of the present invention is to avoid, or significantly reduce the risk of broaching in that the end of the vessel functioning as bow at any given time has a reduced centre of lateral resistance whilst the end functioning as stem at any given time has a direction-stabilising body.

The invention will now be explained in more detail with reference to the drawings in which, for the sake of simplicity, a catamaran is shown. The same hull design according to the invention also applies to trimarans and other multiple-hulled vessels.

Figure 1 is a longitudinal, schematic side view of a hull according to the invention;

Figure 2 is a cross-section of the hull in Figure 1 taken along the line A- A;

Figure 3 is a cross-section of the hull in Figure 1 taken along the line B-B;

Figure 4 is an enlarged partial view of the hull in Figure 2; and Figure 5 is an enlarged partial view of an end of the hull in Figure 1 and a cross-section of the same taken along the line C-C.

Figure 1 shows a typical profile of a catamaran according to the invention. The figure shows the keel line 1 which across the mid-section of the hull is approximately horizontal, but which at each end 2 turns sharply upwards and forms an almost vertical keel line 3 before it bends sharply again and stays almost horizontal 4. The water line length 5 is also shown, and will be used in the descriptions of the following figures.

Figure 2 shows a cross-section of the hull in Figure 1 taken along the line A- A. This cross-section is typical for that part of the longitudinal direction that is limited to the central part of the vessel, i.e., not the bow and stem part. In an advantageous embodiment of the vessel, this central part constitutes about 75% of the total water line length. The draught 6 is shown as the distance from the base line 7 to the designed water line 8. A typical frame profile 9 is shown. In an advantageous embodiment of the invention, the frame profile, from less than about 15% of the draught reckoned from the baseline and up to more than 70% of the draught, is an almost straight vertical line 10. In another advantageous embodiment of the hull according to the invention the frame profile, from less than 10% of the draught reckoned from the baseline and up to more than 90% of the draught, is an almost straight vertical line 10. This vertical line 10 can also continue right down to the baseline 7.

Figure 3 shows a cross-section of the hull in Figure 1 taken along the line B-B. This figure shows the designed water line 11 that extends along the whole of the water line length. A broken line shows a typical extent of the lower deck 12 of a catamaran of this type. On the typical designed water line for a preferred vessel according to the invention, the maximum width 13 is located at half the water line length (0-cross). The extent of the middle 90% of the designed water line is marked by the lines 14, 15. Within this limit, the designed water line for a preferred embodiment of the hull according to the invention is greater than 75% of the maximum width 13, and then goes more or less evenly in towards the centre line 16 of each of the two hulls that form the immersed hull of the catamaran.

Figure 4 is an enlarged partial view of Figure 2. The figure shows a typical cross- section of the hull within about the middle 85% of the designed water line. For any section, the width 17 of the vertical part of the frame 10 is preferably less than 85% of the associated width of the designed water line 18, more preferably it is less than 75% and most preferably it is less than 65% of the width of the associated designed water line 18. Such a design of the hull gives good trim properties during the embarkation and disembarkation of heavy vehicles, whilst the hull volume is as deep in the water as possible to avoid the formation of large waves.

Figure 5 shows an enlarged partial section of a stern/bow end of the hull in Figure 1. We see a profile of the keel line 1, where the almost vertical part of the keel line 3 has been given a projecting form, a bulb 19. The same drawing shows a typical section 20 through the projecting form which, because of its shape, can be used as a propeller shaft 21. The projecting bulb 19 can also be used to give expedient water flow to or from a propeller.

Claims

P a t e n t c l a i m s

1.

A hull for a multihulled vessel, wherein said hull is laterally symmetrical, characterised in that the body plan of the hull, along the entire length of the hull, has a marked step under the designed water line (8) which limits the width (17) of the frames to less than 80% of the associated width of the designed water line (18), whereupon the hull continues substantially vertically in a downward direction from the step before it is terminated in a substantially horizontal direction.

2.

A hull according to claim 1, characterised in that the step is approximately horizontal.

3. A hull according to claim 1, characterised in that the step is at a distance of about 30% or less below the designed water line (8) in relation to the distance from the designed water line to the baseline.

4. A hull according to claim 1 , characterised in that the step is at a distance of about 10% or less below the designed water line (8) in relation to the distance from the designed water line to the baseline.

5. A hull according to claim 1, characterised in that the vertical keel line (3) of the hull comprises a bulb (19).