CONSTRUCTION OF A HULL The present invention relates to a hull construction, in particular for high speed vessels, with an approximately V-shaped or flat bottom, where the hull comprises one recessed hollow space from the bottom which, from the stern of the hull, runs in a taper up to a completion in the hull some distance from the stern, and the extension of the hollow space defines sidewise two side-hull parts between the stern section of the hull and said completion in the hull, as it is given in the introduction in the subsequent claim 1. The invention also relates to an application of the hull form.

The invention relates to a V-shaped vessel hull which is constructed so that it planes out when travelling at speed at sea. The general form of the hull can also be used for constructions of catamarans with two parallel hull shapes.

The total resistance for a vessel arises from the wave resistance and friction resistance. For a speed boat the friction resistance, as a result of the wet surface, is the largest component. With the wet surface is meant the area of the contact surface between the hull of the vessel and the sea, i.e. the stream of water which relatively arises along the hull surface.

To achieve greater speed with as little motor force as possible, speed boats are often fitted with

• extreme placing of the point of gravity in relation to the aft part (stern).

• ridges which shall ensure that there will be air between the bottom and the water.

• foils/wings that give the vessel a lift where the constructor decides.

· air intake, usually in a limited "closed" area, below larger or smaller parts of the hull.

• aerodynamic shaping of the hull which will provide extra lift. It is also known that the wet surface or contact area between the hull and water decides which friction there is between the part of the hull of the vessel and the sea. Based on this knowledge, there have been vessels with hulls that have a recess in the bottom section of the hull to reduce the resistance from the sea when at speed. The constructions are such that at speed the hull rests on the forward part only, and also on a smaller hull or bottom surface astern in each side of the stern section. In this connection reference is made to the following US patents: US 3,203,389; US 3,469,549; US 3,547,064; US 4,193,370 and US 2006/137591 , and also to a German patent DE 0667282.

Particular reference is made to the European patent 667.282 that describes a vessel where a hollow space is formed which forms a V-shape where a hollow space 4 is formed in that the rear part tilts down and back towards the stern. To the forward part of the hollow space, air is pumped in which contributes to an air overpressure in the hollow space which gives the vessel an extra lift.

Common to all these hull constructions is that they have a more or less V-shaped hull cross-section in which the hollow space is formed and there are sharp transitions.

A conventionally planning V-bottom vessel will, at higher speed, have:

• less trim as the speed increases. Here, with the notation "trim" is meant the angle of attack for the vessel hull against the sea surface

• reduction of the lift that gives optimal trim

· increased wet surface as the trim angle decreases

• the side stability is decreased when the speed increases. Therefore, catamaran constructions are often used for hulls that shall travel at very high speeds. A further development of the technology which is described above is known from the Norwegian patent NO 327066, in which it is described that along the edge of the recessed section, and running with corresponding V-shape, one or more downwardly extending flap(s) are formed. These flaps can have the same, or different, height along the whole of the length, or form a wave shape. Furthermore, each flap can be connected to the hull and be formed level with the bottom of the hull and run approximately horizontally or inclining inwards towards the centre, so that the inlet opening of the V-shaped hollow space is made smaller from the underside and that the V-shaped hollow space forms an undercut shape. Said US 2006/137591 shows a keel construction with two mutually separated ventilation tunnels 14 and 6 on opposite sides of a centre line, and where there is space for a propellor. The central hull section 25 is deflected and pushed up and down about a hinge at the front. However, there is no hollow space which is closed by said hull section as is the case for the present invention. The hull section 25 is pushed downwards so that the vessel shall be able to ride on this when travelling at sea. Furthermore, the hull according to US 2006/137591 comprises so called "fence(s)" in the form of longitudinally running, downwardly extending walls shown by 32 in figure 1A and which define the wet surface. These fences build up pressure between them when travelling at speed and will also result in an up-lift of the stern of the vessel while the forward part of the vessel sinks down, something which leads to an increased wet surface. The hull in the US patent will not lead to the vessel having a somewhat reduced wet surface as opposed to the present invention, in that only a pressure change will occur.

With the present invention one aims to improve the speed/resistance ratio and the sea characteristics across a greater speed range, i.e. to achieve the greatest speed possible with the lowest possible engine power.

Consequently, it is an aim of the invention to shape the bottom of the hull so that the hull can travel faster through the sea with the same engine power compared with known hull constructions.

It is a further essential aim of the invention to be able to reduce the resistance between the hull and water over a larger speed range than what is the case with today's known constructions.

It is a further aim of the invention to provide a solution where the hull, during normal speed of the hull/vessel by a continuous readjustment, can be adjusted from a normal V-shaped hull form (a basic hull form) to a hull form where a hollow space is established some distance above the sea.

The present invention.

The hull construction according to the invention is characterised by a separate keel section that is set up to be adjusted from one position where it covers/closes the entrance to said hollow space and runs level with the underside of the other part of the hull bottom and a raised position where the hull section is lifted up and into the hollow space. Preferably the breadth extension of the hollow space and associated keel section in the breadth of the hull between said stern section of the hull and said completion in the hull, have a triangular shape, square, rectangular or an approximate trapezoidal shape. The keel section is preferably set up to be lifted into the hollow space in the hull to provide said readjustment. The keel section can be pulled into the hollow space with the help of driving means, in particular piston/cylinder units. A gasket between said rim edge and the flange can be arranged to avoid ingress of water to the hollow space when the recess is closed by the keel section.

According to a preferred embodiment the keel section is connected to the hull via said piston/cylinder unit that is used to readjust the keel section between the position where it lies flat down on the hull flange and level with the rest of the hull surfaces and the position where it is lifted into the hollow space of the hull.

According to a preferred embodiment the forward part of the keel section adjoining said forward completion in the hull is hinged and set up to rotate about a horizontal axis to provide said adjustment where the keel section is lifted into the hollow space in the hull.

According to a preferred embodiment the cross-section profile of the keel section from said forward completion in the hull and to the stem corresponds to the profile of a corresponding hull part which could possibly be integrated as a natural permanent part of the V-shaped hull.

According to a preferred embodiment the hull construction is formed so that the aft part of the bottom which is adjusted, has a shape that gives a distance to the water so that at a design speed air circulates between said hollow space in the hull and the surrounding air outside, without the use of air fans.

Along the border line between the hull surfaces and the two transition lines, downwardly extending flaps (interceptors) are arranged that together form a V- shape. According to a preferred embodiment corresponds the hull adjoining to or a distance from the keel section comprises a crosswise flaps/interceptors that can be raised and lowered, in particular arranged close to/adjoining the point of gravity of the hull and/or that the aft part of each of the two parallel hull parts comprises corresponding crosswise downwardly extending flaps/interceptors that can be raised or lowered.

According to a preferred embodiment the hull comprises internal buoyancy means such as foam with compact cells of polyurethane, and are arranged along the centreline and out in the side parts along the whole or parts of the hull.

According to a preferred embodiment the keel section is formed in a box shape which forms an engine room, in which the propulsion engine is built in and where the drive shaft to a propeller (or the nozzles of a water-jet propulsion system) extends out, sloping down and backwards from the underside of the forward part of the box.

According to a preferred embodiment the aft part of the keel section forms a fastening point for a propulsion body in the form of an outboard engine 37 via a fitting piece 39.

According to the invention the hull construction is used for hull forms used on symmetrical and unsymmetrical catamarans.

By being able to lift up a V-formed section of the lower keel into the hull itself when the speed is increasing, one gets a marked reduction in the wet surface and thus less friction. When the speed of the vessel is low, it is the hull shape in its basic form where the hollow space is completely closed (i.e. that the keel section is lowered completely down), that gives the highest speed with the least engine power. The vessel is then formed as a so-called basis reference vessel with a conventional V- or U- shaped hull, i.e. that the hollow space is completely closed.

During starting the keel part is lowered and the hollow space is closed and will give the least resistance in the water. When the speed increases, for example, up to 20-25 knots, one conducts a controlled lifting operation of the V-formed keel sectidn into the hollow space of the hull. This leads to the remaining, wet (buoyancy) surfaces at the rear of the hull shape of the vessel changing the trim angle so that one achieves a reinforced lift of the bottom of the vessel in relation to the surface of the water.

Then, one achieves a wet surface that gives the ideal speed angle and thereby a lift between the hull and water. The contact surface between vessel and the water stream will, at higher speeds, take up a V-shape as shown by the hatched part in figure 10. The legs of the V-shape that represent a wet surface get smaller as the speed increases.

The hollow space and the length of the keel section can extend over a larger part of the length of the vessel dependent on the weight of the vessel and design speed, i.e. the speed which the vessel is designed for.

By lifting the keel section the vessel gets a greater side stability, as follows:

1 ) If the keel section is a water-tight (engine) box the side stability increases as the keel section is lifted.

2) If the keel section is a bottom section and a stem section (only panels) the side stability will increase as soon as it is opened for ingress of water if it has been pumped out after it was used last.

Said US patent US 2006/137591 refers to none of the features that the present invention is based on, which relate to adjusting a sector of the hull between a position where it is pulled up and into the hull construction and a position where it is lowered to lie level with the rest of the hull.

The rear part of the bottom that is readjusted preferably has a shape that gives a distance to the water so that at design speed air circulates between a hollow space in the hull and the surrounding air outside without one having to use air fans.

With the use of the present invention the angle of attack (trim angle) of the vessel is increased at design speed so that the wet surface is reduced so that the load between the bottom and the water increases. According to the invention as much of the bottom construction is adjusted as possible, depending on the defined speed range of the vessel, that one achieves maximum lift with the least possible wet surface. If the angle of attack (the trim angle) at design speed (the speed which the hull/bottom is constructed for) is increased sufficiently, one will be able to reduce the wet surface so that the load between the bottom and the water can be more than one tonne per m ² . The trim of the vessel, at design speed ought to lie at 1 -8° in relation to the baseline. The optimal trim is 2-5°. The term trim relates to the baseline of the hull in relation to the water plane, which can be set equal to 0°. The angle of this baseline in relation to the water plane defines the trim of the hull when at speed. In addition, the invention will contribute to lifting up the whole of the hull from the sea when travelling at speed.

The aft, lowered part of the bottom that is adjusted has a form that gives a distance to the water so that at design speed air circulates freely between the recessed volume/space in the bottom and the air (the atmosphere) outside, via the backward-facing opening, without one having to use air fans.

In the application of the hull form according to the present invention one achieves that:

1 ) the vessel retains it optimal trim over a longer range and to a higher speed range,

2) retains the lift which is a result of optimal trim over a greater speed range

3) it provides a smaller wet surface, and

4) one retains and maintains the good side-stability at increased speed. By utilising the present invention one can increase the top speed or keep the top speed as a conventional vessel, but with a reduced engine power. Consequently, one also achieves a reduction in release of exhaust gases, such as CO ₂ , NO _x and similar harmful gases into the atmosphere. As mentioned the aim of the V-shaped recess is to give the vessel a much reduced wet surface during speed, i.e. the hull that has the lowest possible contact surface with the water under the hull when travelling on water, something which is desirable according to the present invention.

The invention shall now be explained in more detail with reference to the enclosed figures, in which: Figure 1 shows a longitudinal section partly in outline of the hull and shows the movable keel part in the hull according to the invention in its lowered position, cf. figure 4. Figure 2 shows a longitudinal section partly in outline corresponding to figure 1 , but where the movable keel part is pulled into the hull.

Figure 3 shows a perspective section seen from below of the movable (can be lifted) keel part adjusted to a frame part which is built into the hull.

Figure 4A shows a plane outline seen from below of the movable keel part with a triangular plane section, and also a forward crosswise interceptor.

Figure 4B shows a plane outline seen from below of the movable keel part with an approximate trapezoidal cross-section, and also a forward crosswise interceptor.

Figure 5 shows a side outline of the movable keel part, and its mounting to the hull inside the hollow space via hydraulic cylinders is indicated. Figures 6 and 7 show two variants of how the movable keel section rests against the main hull in the closed position, cf. also figure 3.

Figure 8 shows a side outline of the keel section according to the invention seen from below. It shows the aft part with a formed recessed hollow space formed by projecting of the hull where the movable keel part shown in figure 3 not yet fitted into the hull.

Figure 9 shows a perspective of the keel section according to the invention seen from below (from the under side) where the keel part, in its lowered position, rests against the frame part.

Figure 10 shows a similar side outline as in figure 7 where the movable keel part is pulled up and into the hull, i.e. as it is set when the vessel is at speed, something which reduces the wet surface of the hull.

Figure 1 1 shows a plane outline of a hull according to the invention seen from below and shows the wet surface as a hatched V-shape. Figure 12 shows a diagram of the relationship between resistance to propulsion and speed for a hull with the movable hull construction according to the invention taken from measurements carried out during a test of the hull. Figures 13-16 show examples of the extent of wet surfaces for a vessel according to the invention which, with the keel section completely lowered (closed) (figure 13), is driven up to about 20 knots (one knot corresponds to a speed of around 1.85 kilometres per hour). Figure 14 shows the situation at about 25 knots where the lifting up of the keel section 40 has started. Figure 15 shows where the lifting up has continued and the speed is about 30 knots, while figure 16 shows the situation where the speed is above 30 knots and the keel shows a wet surface as shown in figure 11. Basic form of the hull.

Figure 1 shows a side outline of the hull 10 with a V-formed keel part with plane, straight bottom surfaces without any abrasions or ridges. The keel part of the hull can, all the way to the aft part, have a marked V-shape, particularly at the forward part of the hull, but one also has hull forms where the V-shape in the backwards direction goes gradually over into a U-shape or a flat-formed horizontal aft part.

The bow of the hull 10 is shown by 12 and the stern is shown by 14. The hull of the vessel is arranged to plane when travelling at speed, i.e. an upwardly directed dynamic force is set up so that the hull is partly lifted up from the sea.

The hull is shown with a propulsion body in the form of an outboard engine 37 that is connected to the rear part of the keel section 40 via a fitting piece 39.

With reference to figure 8 also, the bottom of the hull 20, from the stern section 14 and forward to about midships, is formed with a hollow space 22. The hollow space is widest at the stern 14 of the hull, and tapers forward to a completion about midships, so that the lower and aft parts of the hull, from midships and back, form two separate hull parts with an approximate catamaran shape. Generally, the hollow space 22 can have many forms in the horizontal plane, a V- shape or a more square shape, with rounded corners, as a rectangle or a trapeze. According to a preferred embodiment the hollow space has a horizontal plane, a V-shape or a trapeze shape, where the pointed end of the V 26 that faces forwards lies in the centreline 36. The hollow space 22 is defined by the vertical, opposite facing sidewalls, one on either side of the recess and the hull roof formed by plate parts, where the bottom surface parts 30,32 can be broadest at the V tip 26 and narrowest at the stern, to form said two hull parts. These surfaces, shown by 30 and 32 in figure 8 represent the bottom surfaces that make contact with the sea/waterline. In the following parts of this description the hollow space and the associated keel section are described as the V-shape, although in practice many forms can be used, for example, a trapeze form as shown in figure 4B.

The V-shaped bottom of the hull has straight surface sections as shown in figure 9, without there being any ridges, edges or the like. This contributes to reduce the friction resistance when travelling at speed. But as an alternative solution the surfaces can be formed with ridges.

From the pointed end of the V-shape one can consider the bottom section so that it is formed by two mutually separate hull parts which between them form a recess/hollow space, said hull parts are gradually narrower in the backwards direction towards the stern. Consequently, the recess is also gradually and evenly wider in the direction back towards the stern. At the very back where the opening can be the widest, a free air passage or an air exchange between the air outside and the hollow space 22 inside the hull/vessel is formed when travelling at speed.

The hollow space preferably has a length I (see figure 1 ) in the range 10-80% of the length L of the hull. The figures show that the recess can have a length of about 60% of the hull length. The transition between the horizontal surfaces 30 and 32, respectively, and the vertical surfaces in the hull in towards the hollow space, define sharp edges.

The entrance to this hollow or recess 22 is set up to be covered by a separate keel section 40 which has a contour outline that corresponds to the approximately horizontal plane of the recess 22. The section 40 is set up to be adjusted between a position where it covers the inlet opening of the recess and a position where it is pulled into (up into) the recess. In figure 1 the vessel is shown with the fitted keel section in lowered position so that the hull is a vessel with a standard V-shaped bottom contour, cf. the perspective in figure 9. In figure 2 the vessel is shown with the keel section pulled up into the hull. This corresponds to the perspective in figure 10. The separate keel section 40 is shown in a plane outline and seen from below in figure 3, in a plane outline seen from below in the figures 4A and 4B, and a side outline in figure 5. The centreline is shown by 36 in figures 4Α,Β· The keel section 40 in figures 3 and 4A has a triangular plane outline corresponding to the entrance of the recess in the hull, the keel line is shown by 41 , and the underside of the section has the contour of the V-shape of the hull which, from a forward marked and clear V-shaped cross-section form, goes more and more over to a U- shaped cross-section with an approximately flat aft bottom part.

Shown in figures 4A and figure 5 the forward part 42 of the movable keel section 40 forms a storing unit 141 for connection to the hull so that the section 40 can be flipped up and down about a horizontal axis shown by 43 in figure 4.

While figure 4A shows the hollow space 40 as an approximately triangular shape, the version in figure 4B has a trapezoidal shape, where the forward keel section forms the shorter side of the two approximately parallel sides in the trapeze.

The keel section 40 can be rotated upwards and downwards as shown in the figures 1 and 2, i.e. the section 40 can be swung into the hollow space 22 as shown in figure 10 and in that the aft part is connected to one or more drive bodies, preferably in the form of piston/cylinder units 47 (figure 5) the one upper end of which is connected to the framework 45 of the hull, while the other end is connected to the upper side of the keel section 40. In its lower position as shown in figure 9, the keel section 40 covers the entrance to the hollow space 22 completely, for example, with sealing contact with the hull flange 23 (figure 7).

In the lower position the movable (can be lifted) rim flange of the keel part around the circumference is set up to be put down against a correspondingly formed flange edge in a frame part which is a part of the hull. The figures 6 and 7 show, in vertical sections, two different embodiments of the principle for this construction part. The flange edge 35 of the recess 22 and the lower edge 37 of the keel section have a mutually corresponding shape so that they form a sealing contact surface, something which will reduce the friction resistance when the keel section closes the entrance to the hollow space when travelling at speed. According to figure 6 the two adjoining surfaces 35,137 define hook-formed or L-formed flanges, while in the version in figure 7 they define mutually corresponding surfaces cut at an angle.

With the help of the piston/cylinder unit 47 the keel section 40 is adjusted by rotating about the axis 43 from the closed position where it closes the entrance from below to the hollow space and to the use position during travelling where the section 40 is rotated all the way into the hollow space 22.

The keel section 40 can also be formed with a box shape 140 in which the propulsion engine itself is built in and where the driveshaft to a propeller extends out at an angle downwards and backwards from the lower forward part of the box. At the hinge part the necessary equipment can be placed for sending control signals to the operation of the engine and for the supply of fuel and cooling water. The hull is constructed to have one or more propulsion systems, such as propellors and a water jet aggregate.

The hatched sections in the figures show the contact surface between the hull and the sea stream. The situation is shown in figure 13 where the whole of the hull ploughs through the sea as a usual V-shaped hull and partially starts to plane. The figures 14 and 15 show that the keel section 40, as the speed increases, to a smaller and smaller extent makes contact with the flow of water shown by 201 until it is, according to figure 16 (figure 1 1 ), completely pulled into the hull and the contact surface with the sea stream has the form of a V-shape with small/thin legs.

According to a preferred embodiment one can lock the position of the section 40 in a position that gives a water flow surface as shown at about 25 knots (figure 14) or about 30 knots (figure 15). This means that the hull can be produced with a permanently pulled up section 40 without the need for hydraulic or other lifting mechanisms. This is applicable if the form of propulsion that is achieved as according to the figures 14 and 15 implies that the shape is sufficient for rational operation of the vessel.

Alternative variant.

According to another preferred variant, there can be arranged, along the border between the hull surfaces 30,32 and the two transition lines 42 and 44, respectively, a downwardly extending longitudinal flap/flange or an edge section (a so-called interceptor) 46,46 (in a corresponding V-shape, see figure 8) which can also be called a downwardly extending beard. When travelling at speed, these two flaps/flanges have some of the same effects as the bow section of the vessel, namely pushing the sea outwards and sideways backwards. At the same time the wet surface (the downwardly facing bottom surface internally in the hollow space) is reduced as mentioned previously when the vessel reaches service speed. This effect arises by the pressure from the water against the beard giving the vessel an additional upward force, it is given an extra lift. Thereby the wet surface that is the subject of friction resistance when travelling is reduced, and also the water set up in parts of the bottom increases, something which results in lifting the vessel up in the sea when travelling at speed.

The height of the flaps can vary depending on the total length of the hull. For a speedboat with a length of 40 metres they can have a "height" in the area 5-20 mm.

As shown in figures 4A, 4B and 8, a downwardly extending flap/flange 52, the height of which can be regulated, can be formed with a horizontal extension across the longitudinal direction 36 of the vessel arranged adjoining the rotation mounting at 43 of the keel section in the hull.

As can be seen in figure 4B the flap 52 is placed some distance forward of the forward mounting at 42 of the keel section, parallel with the shorter side of the trapeze form. Either fitted completely in the hull at the front of the hollow space of the hull as it is set up to be lowered down and form said lift-providing flap/flange. It is preferred that the flap is placed near the point of gravity of the vessel.

Correspondingly, with reference to figure 9, at the aft end 14 of the hull, corresponding flaps or height regulated flaps (interceptors) 54,56 can be formed with one in each parallel hull part. Correspondingly, these are set up to extend down into the sea as clearly shown in the figure and will give the aft part of the hull an extra lifting force. The flaps are arranged to extend straight down into the sea or are set somewhat at an angle backwards.

With the use of the hull construction according to the invention it is shown that one gets a large reduction in the fuel consumption and reduced release of harmful gases at higher speeds. Said in a different way one can achieve higher speed for the same power.

Catamarans can also be designed with this described principle and one then achieves:

1 ) that keeps its optimal trim over a longer speed range and to a higher speed

2) the vessel keeps the lift which the optimal trim provides over a larger speed range

3) one will thereby have a smaller wet surface at increased speed and catamarans give the vessel, as we know, an extra good side stability.

With regard to stability, the hull will, according to the invention, demonstrate better side stability at higher speeds (i.e. when the keel section is in its upper position) than for a conventional V-shaped hull bottom.

Today, there is a requirement that all vessels have a given side stability when lying stationary. Passenger vessels with all passengers standing on the one side shall, for example, not tilt more than about 10° (degrees).

One aspect of the advantages of the invention is that as water can circulate freely into and out of the hull parts that demarcate the recess, less buoyancy is also achieved along the centreline of the vessel when it is lies stationary, something which contributes to the improved stability of the vessel. A corresponding vessel with a hull form with a keel section that can be pulled up, can permit taking onboard more passengers than a vessel with a V-shaped hull.

According to the present invention, in each of the side hulls numbered 30 and 32, and as shown in figure 10, buoyancy means can be inserted, for example, in the form of one or more foam cushions 60 in case the hull surfaces are damaged so much that water penetrates into the hull. This provides sufficient side stability and can safeguard the vessel against shipwrecking or capsizing.

There can also be buoyancy means placed in the forward part of the hull, such as foam cushions with closed cells as shown by 62 in figure 10. Figure 1 1 shows a lower plane outline of a vessel hull according to the invention and shows the V-shaped wet surface hatched and given the reference number 200. Figure 12 shows a diagram over the relationship between propulsion resistance and speed for a hull with the movable hull construction according to the invention taken from measurements made during test driving of the hull. The curves in the figure are drawn where the Y-axis denotes propulsion resistance (friction) while the X-axis denotes the speed of the hull.

The upper curve, marked k indicates how the friction resistance (in KN=Kilo Newton) is changed as a function of the speed (metres/second) for a standard V- shaped hull, while the curve K indicates the same relationship for the hull form where the keel section 40 is withdrawn into the hull.

One can see that for driving with the V-shaped hull in the sea from a stationary position in the sea at the starting point S1 , the curve k has a gently arch-shaped and is approximately linear. Now one drives the hull in the sea and increases the speed correspondingly up to the speed S2 is reached. Then one starts the withdrawing of the keel section 40 into the hull, something which leads to that the speed increases initially up to a speed S3 without the resistance changing. From the point S3 one gets a smaller gradient for the curve compared to the corresponding curve for a conventional V- shaped hull, i.e. that they move away from each other. One can see from the curve for the further increased speed level that the resistance is lower than for a corresponding traditional V-shaped hull.

The figure clearly indicates that by using a hull form as defined by the invention, the resistance to propulsion decreases, something which will lead to reduced consumption of fuel.

In practice, it will be rational to drive the hull according to the invention with the hollow space closed, i.e. as a traditional V-shaped hull until the speed is up to about 15-20 knots which can correspond to the point S2 in the figure. Then the keel section is gradually withdrawn into the hull and one achieves the immediate speed increase up to S3 as the figure shows, without having to increase the engine power.