SE541833C2 - Motorboat with flow guiding members - Google Patents

Description

Motorboat with flow guiding members FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a motorboat according to the preamble of claim 1.

When a motorboat 1 (see Fig 1) is propelled through the water it may be subjected to undesired alternating yaw movements due to periodic lateral forces on the hull 2 of the motorboat caused by a so-called Kármán vortex street 3 formed behind an aft end surface 4 at the aft end of the hull, as schematically illustrated in Fig 1. The Kármán vortex street 3 formed behind the hull 2 of the motorboat is a repeating pattern of swirling vortices 5 caused by the unsteady separation of flow of water at the aft end of the hull. This problem with undesired yaw movements induced by a Kármán vortex street mainly occurs when a motorboat of the type with an essentially planar aft end surface at the aft end of the hull is propelled through the water at a rather low speed with a lower part of the essentially planar aft end surface 4 immersed in the water. The alternating yaw movements caused by the Kármán vortex street result in an increased travelling distance for the motorboat through the water, which in its turn implies increased fuel consumption.

SUMMARY OF THE INVENTION The object of the present invention is to provide a solution to the above-mentioned problem.

According to the invention, this object is achieved by means of a motorboat having the features defined in claim 1.

The motorboat of the present invention comprises at least two rigid and rearwardly projecting flow guiding members, which are located behind the aft end surface of the hull on either side of a vertical plane coinciding with a longitudinal vertical centre plane of the hull and which are configured to prevent the formation of a Kármán vortex street behind the hull when the motorboat is propelled through the water with the flow guiding members at least partly immersed in the water. Hereby, the above-mentioned negative effect from a Kármán vortex street behind the hull of the motorboat is avoided, and the motorboat will consequently run in a straighter direction through the water as compared to a corresponding motorboat lacking such flow guiding members, which in its turn implies reduced fuel consumption for the motorboat.

The motorboat is a planing motorboat and the hull is a planing hull which is configured to assume a planing position with the aft end surface above the surface of the water when the motorboat is propelled through the water at a speed higher than a planing threshold speed and to assume a non-planing position with a lower part of the aft end surface immersed in the water when the motorboat is propelled through the water at a speed lower than said planing threshold speed, wherein the flow guiding members are fixed in such a position in relation to the hull that they are above the surface of the water when the hull is in the planing position and at least partly immersed in the water when the hull is in the non-planing position.

The flow guiding members will be in contact with the water and thereby effective to prevent the formation of a Kármán vortex street behind the hull of the motorboat when the motorboat is propelled through the water at low speed with the hull in the nonplaning position. However, the flow guiding members will not be in contact with the water when the hull of the motorboat is in the planing position and they will thereby not have any negative effect on the resistance of the hull to forward movement when the motorboat is propelled through the water at high speed.

According to an embodiment of the invention, the flow guiding members have the form of vertical plates. Hereby, the flow guiding members can be produced in a simple and cost-effective manner. Furthermore, flow guiding members in the form of vertical plates may be secured to the aft end surface in a simple manner.

Another embodiment of the invention is characterized in: - that first and second flow guiding members in the form of vertical plates are fixed to the aft end surface on a first side of a vertical centre line of the aft end surface; - that third and fourth flow guiding members in the form of vertical plates are fixed to the aft end surface on an opposite second side of the vertical centre line of the aft end surface; - that the first flow guiding member has the same size and shape as the fourth flow guiding member, wherein the first and fourth flow guiding members are located at the same distance from the vertical centre line of the aft end surface; - that the second flow guiding member has the same size and shape as the third flow guiding member, wherein the second and third flow guiding members are located at the same distance from the vertical centre line of the aft end surface; and - that the second and third flow guiding members are located between the first and fourth flow guiding members and project longer in a vertical direction rearwards from the aft end surface than the first and fourth flow guiding members.

It has turned out that four flow guiding members arranged in the above-mentioned manner will prevent the formation of a Kármán vortex street behind the hull of a motorboat in a very efficient manner.

According to another embodiment of the invention, the flow guiding members are provided with downwardly facing trim surfaces which are configured to increase the hydrodynamic lift on the aft end of the hull when the hull is in the non-planing position and thereby facilitate the movement of the hull from the non-planing position to the planing position. The abovementioned threshold speed, i.e. the speed at which the hull moves from the non-planing position to the planing position, is reduced by said trim surfaces, which results in reduced fuel consumption for the motorboat.

According to another embodiment of the invention, the motorboat comprises a shelf which is configured to rest on at least some of the flow guiding members in order to form a supporting platform for a person entering the motorboat from the water or descending into the water from the motorboat. In this embodiment, the flow guiding members are used as supports for said shelf, which for instance may be used when bathing or when rescuing a person that has fallen into the water.

Further advantageous features of the motorboat according to the present invention will appear from the description following below and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, a specific description of embodiments of the invention cited as examples follows below. In the drawings: Fig 1 is a schematic planar view from above of a motorboat according to prior art, Fig 2 is a schematic planar view from above of a motorboat according to a first embodiment of the present invention, Fig 3 is a schematic rear view of the motorboat of Fig 2, Fig 4 is a schematic lateral view of the motorboat of Fig 2, Fig 5 is another schematic planar view from above of the motorboat of Fig 2, Fig 6 is a schematic planar view from above of a motorboat according to a second embodiment of the invention, Fig 7 is a schematic rear view of the motorboat of Fig 6, Fig 8 is a schematic lateral view of the motorboat of Fig 6, Fig 9 is a schematic planar view from above of a motorboat according to a third embodiment of the invention, Fig 10 is a schematic rear view of the motorboat of Fig 9, Fig 11 is a schematic lateral view of the motorboat of Fig 9, Fig 12 is a schematic rear view of a motorboat according to a fourth embodiment of the invention, and Fig 13 is a schematic planar view from above of the motorboat of Fig 12.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Different embodiments of a motorboat 10 according to the present invention are illustrated in Figs 2-13. The motorboat 10 comprises a hull 11 and at least one motor driven propeller 12 for propelling the motorboat through the water. The propeller 12 may be driven by an inboard motor or an outboard motor.

A rearwardly facing and vertically extending aft end surface 13 is provided at the aft end of the hull 1 1. A lower part of the aft end surface 13 is configured to be immersed in the water, at least when the motorboat 10 is at rest or propelled through the water at lower speed. In the illustrated embodiments, the aft end surface 13 is planar.

The motorboat 10 comprises at least two rigid and rearwardly projecting flow guiding members 14a, 14b, 14c, 14d, 14a’, 14b’, 14a”, 14b” which are located behind the aft end surface 13 of the hull on either side of a vertical plane 15 (see Figs 2 and 6) coinciding with a longitudinal vertical centre plane of the hull 11 and which are configured to prevent the formation of a Kármán vortex street behind the hull 11 when the motorboat 10 is propelled through the water with the flow guiding members 14a, 14b, 14c, 14d, 14a’, 14b’, 14a”, 14b” at least partly immersed in the water.

In the illustrated embodiments, the flow guiding members 14a, 14b, 14c, 14d, 14a’, 14b’, 14a”, 14b” are fixed to the aft end surface 13 of the hull and extend rearwards therefrom. As an alternative (not illustrated), the flow guiding members may be fixed to a separate holder, which is arranged to keep the flow guiding members positioned behind the aft end surface 13 of the hull and which is fixed to the hull 11 or fixed to a component mounted to the hull 11.

In the embodiment illustrated in Figs 2-5 and in the embodiment illustrated in Figs 12 and 13, the motor boat 10 comprises four flow guiding members 14a, 14b, 14c, 14d in the form of vertical plates, wherein: - a first flow guiding member 14a and a second flow guiding member 14b are fixed to the aft end surface 13 of the hull on a first side of a vertical centre line 16 (see Figs 3 and 12) of the aft end surface 13; and - a third flow guiding member 14c and a fourth flow guiding member 14d are fixed to the aft end surface 13 of the hull on an opposite second side of the vertical centre line 16 of the aft end surface.

The first flow guiding member 14a has the same size and shape as the fourth flow guiding member 14d, wherein the first and fourth flow guiding members 14a, 14d are located at the same distance from the vertical centre line 16 of the aft end surface. Furthermore, the second flow guiding member 14b has the same size and shape as the third flow guiding member 14c, wherein the second and third flow guiding members 14b, 14c are located at the same distance from the vertical centre line 16 of the aft end surface. The second and third flow guiding members 14b, 14c are located between the first and fourth flow guiding members 14a, 14d and project longer in a vertical direction rearwards from the aft end surface 13 than the first and fourth flow guiding members 14a, 14d. In the illustrated example, each flow guiding member 14a, 14b, 14c, 14d has a rectangular shape. However, the flow guiding members 14a, 14b, 14c, 14d may as an alternative have any other suitable shape.

Fig 5 schematically illustrates how the flow guiding members 14a, 14b, 14c, 14d affect the vortices that are formed behind the aft end surface 13 of the hull when the motorboat 10 is propelled through the water at a rather low speed with the flow guiding members 14a, 14b, 14c, 14d in contact with the water. As illustrated in Fig 5, the flow guiding members 14a, 14b, 14c, 14d affect the water flow behind the hull 11 in such a manner that the vortices formed behind the aft end surface 13 of the hull are broken up and prevented from forming a Kármán vortex street behind the hull.

In the embodiment illustrated in Figs 6-8, the motor boat 10 comprises two flow guiding members 14a’, 14b’ in the form of vertical plates, wherein a first flow guiding member 14a’ is fixed to the aft end surface 13 of the hull at a first lateral edge 13a thereof and a second flow guiding member 14b’ is fixed to the aft end surface 13 of the hull at an opposite second lateral edge 13b thereof. In the illustrated example, each flow guiding member 14a’, 14b’ has a rectangular shape. However, the flow guiding members 14a’, 14b’ may as an alternative have any other suitable shape.

In the embodiment illustrated in Figs 9-11, the motor boat 10 comprises two flow guiding members 14a”, 14b”, wherein a first flow guiding member 14a” is fixed to the aft end surface 13 of the hull on a first side of the vertical centre line 16 of the aft end surface 13 and a second flow guiding member 14b” is fixed to the aft end surface 13 of the hull on an opposite second side of the vertical centre line 16 of the aft end surface.

The motorboat 10 according to the present invention is a planing motorboat, wherein the hull 11 is a planing hull which is configured to assume a planing position with the aft end surface 13 of the hull above the surface of the water when the motorboat 10 is propelled through the water at a speed higher than a planing threshold speed and to assume a non-planing position with a lower part of the aft end surface 13 of the hull immersed in the water when the motorboat 10 is propelled through the water at a speed lower than said planing threshold speed. The flow guiding members 14a, 14b, 14c, 14d, 14a’, 14b’, 14a”, 14b” are fixed in such a position in relation to the hull 11 that they are above the surface of the water when the hull 11 is in the planing position and at least partly immersed in the water when the hull 11 is in the non-planing position. The flow guiding members 14a, 14b, 14c, 14d, 14a’, 14b’, 14a”, 14b” are preferably positioned so as to be completely immersed in the water when the hull 11 is in the non-planing position. When the planing motorboat 10 is at rest, the weight of the motorboat 10 is borne entirely by the buoyant force on the hull 11. At low speeds, the hull 11 acts as a displacement hull, which implies that the motorboat 10 is mainly supported in the water by the buoyant force. As the speed of the motorboat 10 increases, the hydrodynamic lift on the hull 11 increases and the hull is lifted upwards in the water, which in its turn entails that the buoyant force on the hull 11 decreases. At a specific speed, here denominated planing threshold speed, the hydrodynamic lift becomes the predominant upwards force on the hull 11 and the motorboat 10 is planing with the hull 11 gliding on top of the water. The value of the planing threshold speed depends on the specific shape and design of the hull 1 1.

In the embodiment illustrated in Figs 9-11, the flow guiding members 14a”, 14b” are provided with downwardly facing trim surfaces 17 which are configured to increase the hydrodynamic lift on the aft end of the hull 11 when the hull is in the nonplaning position, i.e. when the motorboat 10 is propelled through the water at a speed lower than the planing threshold speed. The increased hydrodynamic lift provided by the trim surfaces 17 facilitates the transition from the non-planing position to the planing position. The trim surfaces 17 are fixed in relation to the hull 11 and they are consequently non-adjustable. The trim surfaces 17 are above the surface of the water when the hull 11 is the planing position and in contact with the water when the hull 11 is in the non-planing position.

The motorboat 10 may with advantage comprise a shelf 18 (see Figs 12 and 13) which is configured to rest on two or more flow guiding members 14a, 14b, 14c, 14d in order to form a supporting platform for a person entering the motorboat 10 from the water or descending into the water from the motorboat 10. Thus, the shelf 18 is dimensioned so as to be able to carry the weight of at least one person. The shelf 18 may be permanently or detachably mounted to the flow guiding members 14a, 14b, 14c, 14d and may for instance have the form of a plate.

The invention is of course not in any way restricted to the embodiments described above. On the contrary, many possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention such as defined in the appended claims. The flow guiding members may for instance have any other suitable design in addition to the designs illustrated in Figs 2-13. Furthermore, a large motorboat with an aft end surface of larger size may be provided with more than four flow guiding members.

Claims (7)

1. A planing motorboat with a hull (11) , wherein a rearwardly facing aft end surface (13) is provided at the aft end of the hull (11) and wherein the hull (11) is a planing hull which is configured to assume a planing position with the aft end surface (13) above the surface of the water when the motorboat (10) is propelled through the water at a speed higher than a planing threshold speed and to assume a nonplaning position with a lower part of the aft end surface (13) immersed in the water when the motorboat (10) is propelled through the water at a speed lower than said planing threshold speed, characterized in: - that the motorboat (10) comprises at least two rigid and rearwardly projecting flow guiding members (14a, 14b, 14c, 14d; 14a’, 14b’; 14a”, 14b”), which are located behind the aft end surface (13) on either side of a vertical plane (15) coinciding with a longitudinal vertical centre plane of the hull (11) and which are configured to prevent the formation of a Kármán vortex street behind the hull (11) when the motorboat (10) is propelled through the water with the flow guiding members (14a, 14b, 14c, 14d; 14a’, 14b’; 14a”, 14b”) at least partly immersed in the water; and - that the flow guiding members (14a, 14b, 14c, 14d; 14a’, 14b’; 14a”, 14b”) are fixed in such a position in relation to the hull (1 1) that they are above the surface of the water when the hull (11) is in the planing position and at least partly immersed in the water when the hull (11) is in the non-planing position.

2. A motorboat according to claim 1, characterized in that the flow guiding members (14a, 14b, 14c, 14d; 14a’, 14b’; 14a”, 14b”) are fixed to the aft end surface (13) and extend rearwards from the aft end surface (13).

3. A motorboat according to claim 1 or 2, characterized in that the flow guiding members (14a, 14b, 14c, 14d; 14a’, 14b’) have the form of vertical plates.

4. A motorboat according to claim 3, characterized in: - that first and second flow guiding members (14a, 14b) are fixed to the aft end surface (13) on a first side of a vertical centre line (16) of the aft end surface (13); - that third and fourth flow guiding members (14c, 14d) are fixed to the aft end surface (13) on an opposite second side of the vertical centre line (16) of the aft end surface (13); - that the first flow guiding member (14a) has the same size and shape as the fourth flow guiding member (14d), wherein the first and fourth flow guiding members (14a, 14d) are located at the same distance from the vertical centre line (16) of the aft end surface (13); - that the second flow guiding member (14b) has the same size and shape as the third flow guiding member (14c), wherein the second and third flow guiding members (14b, 14c) are located at the same distance from the vertical centre line (16) of the aft end surface (13); and - that the second and third flow guiding members (14b, 14c) are located between the first and fourth flow guiding members (14a, 14d) and project longer in a vertical direction rearwards from the aft end surface (13) than the first and fourth flow guiding members (14a, 14d).

5. A motorboat according to claim 3, characterized in: - that a first flow guiding member (14a’) is fixed to the aft end surface (13) at a first lateral edge (13a) thereof; and - that a second flow guiding member (14b’) is fixed to the aft end surface (13) at an opposite second lateral edge (13b) thereof.

6. A motorboat according to any of claims 1-5, characterized in that the flow guiding members (14a”, 14b”) are provided with downwardly facing trim surfaces (17) which are configured to increase the hydrodynamic lift on the aft end of the hull (11) when the hull is in the non-planing position and thereby facilitate the movement of the hull (11) from the non-planing position to the planing position.

7. A motorboat according to any of claims 1-6, characterized in that the motorboat (10) comprises a shelf (18) which is configured to rest on at least two of the flow guiding members (14a, 14b, 14c, 14d) in order to form a supporting platform for a person entering the motorboat (10) from the water or descending into the water from the motorboat (10).