JP5936033B2 - Stern structure and ship - Google Patents

Description

  The present invention relates to a stern structure and a ship capable of realizing energy saving by improving propulsion efficiency.

  In recent years, in the marine field, various methods for realizing energy saving have been studied due to rising fuel costs, increasing energy and environmental problems. As a method for saving energy of the ship itself excluding the ship operation method and the infrastructure such as the port, there are an increase in the efficiency of the engine and an improvement in the hull form. There are several prior arts that devised the stern structure as part of this hull form improvement.

Patent Document 1 discloses an introduction path that guides the water flow to the side of one or both sides near the stern and to a position below the propeller axis for the purpose of improving propulsion efficiency without causing an increase in hull resistance. A hull structure having a nozzle member to be formed is disclosed.
Further, Patent Document 2 discloses a rectifying device for rectifying a flow having a large wake coefficient generated in a stern flow field and then diffusing and guiding the flow to a propeller operating surface for the purpose of rectifying the stern flow in a large ship. ing.

JP-A-10-7086 Utility Model Publication Sho 56-32396

However, in the hull structure disclosed in Patent Document 1, nozzle members having the same shape are arranged on the left and right sides of the propeller as means for improving propulsion efficiency. Thus, when the same nozzle member is provided symmetrically on the left and right sides of the propeller, a beneficial action can be promoted on the side that promotes the counterflow, but on the opposite side, the action is reduced rather than reduced. May cause increased resistance. For this reason, it is not necessarily sufficient to improve the propulsion efficiency of the ship.
In addition, the rectifier described in Patent Document 2 improves performance by uniforming a violent change in the wake distribution in the enlarged ship to the propeller operating surface, and propelling the propeller using the counterflow It does not improve efficiency.
Then, an object of this invention is to provide the stern structure and ship which can implement | achieve energy saving by improving the propulsion efficiency of a ship.

The stern structure of the present invention according to claim 1 is a stern structure having a hollow duct for controlling the flow of the hull to the propeller of the stern, wherein the duct has different shapes on the left and right of the stern. The duct on the propeller rotational direction side is formed in a state where the hull is viewed from the side of the ship from the side of the ship so that the central axis of the duct extends vertically with respect to the axis of the propeller. It is formed in a shape that strengthens the counterflow with respect to the rotation direction, and is formed in a shape that strengthens the counterflow with respect to the rotation direction of the propeller by attaching the duct on the side opposite to the rotation direction at an angle at which the stern side is horizontal or lower than horizontal. To do.
By adopting a structure in which ducts are provided on the left and right sides of the stern and the shapes of the ducts are different on the left and right sides , it is possible to obtain an optimal shape that takes into account the improvement in propulsion efficiency due to the countercurrent flow in the propeller rotation direction and the increase in propulsion resistance due to the duct it can. Here, the “shape that increases the counterflow with respect to the rotation direction of the propeller” refers to a shape that can increase the counterflow received during one rotation of the propeller. For this reason, there is a shape that does not strengthen the counterflow on the counter-rotation direction side where the propeller blade rises while drawing an arc from below to the rotation direction while the propeller blade rises while drawing an arc from above to below. It is included in the “shape that enhances the counterflow with respect to the propeller rotation direction”.
Further, by forming the duct so that the center axis of the duct on the propeller rotation direction side extends vertically with respect to the propeller axis, the counter flow can be reliably guided to the propeller surface.
Further, by attaching the duct on the anti-rotation direction side at the angle at which the stern side is horizontal or lower than horizontal, a counterflow to the propeller can be formed on the anti-rotation direction side, and the propulsion efficiency can be improved.

According to a second aspect of the present invention, in the stern structure according to the first aspect, the duct on the counter-rotating direction side is in the state viewed from the side of the hull from the ship side with respect to the axis of the propeller. The center axis of the duct is formed so as to extend vertically.
By forming the duct so that the central axis of the duct extends vertically with respect to the propeller axis, the counter flow can be reliably guided to the propeller surface.

The present invention is claimed in claim 3, wherein the stern construction according to claim 1 or 2, in a state where the side view of the hull, the mounting angle of the duct in the rotating direction side of the propeller in the counter-rotational direction It is characterized in that the stern side is mounted at an angle that faces upward from the mounting angle of the duct .
With this configuration, it is possible to effectively form a counterflow with respect to a flow directed downward on the propeller rotation direction side.

The present invention of claim 4 is the stern construction according to claim 3, the mounting angle of the duct of the rotational direction is the stern, characterized in that attach to an angle up from the horizontal.
With this configuration, by forming a counterflow against Oite propeller rotation direction, it is possible to improve the propulsion efficiency.

According to a fifth aspect of the present invention, in the stern structure according to one of the first to fourth aspects, the flow is accelerated by the duct on the rotational direction side.
With this configuration, the counter flow to the propeller on the rotational direction side can be further strengthened, and the propulsion efficiency can be improved .

Vessels of the present invention described in 請 Motomeko 6, characterized in that employing the stern structure according to the hull in one of claims claims 1-5.
With this configuration, it is possible to improve the propulsion efficiency of the ship and save energy .

The present invention described in 請 Motomeko 7, in a ship according to claim 6, wherein the hull is a stern catamaran type.
In a stern catamaran type ship, by adopting the stern structure described above, it is possible to improve the propulsion efficiency by utilizing a strong upward flow generated in the vicinity of the stern tunnel. When the hull is a stern catamaran, the left and right sides of the skeg correspond to the left and right sides of the stern. For example, when the propeller is an inward stern catamaran type, the tunnel side of the skeg corresponds to the rotation direction side, and the outer side corresponds to the counter rotation direction side.

According to the stern structure of the present invention, it is possible to comprehensively consider the improvement of propulsion efficiency and the increase in resistance due to the duct by strengthening the counterflow with respect to the rotation direction of the propeller by making the shapes of the left and right ducts themselves different. Can save energy.
Further, by forming the duct so that the central axis of the duct extends vertically with respect to the propeller axis, the counter flow can be surely guided to the propeller surface and the propulsion efficiency can be improved.
In addition, by installing the mounting angle of the duct on the anti-rotation direction side at the angle where the stern side is horizontal or lower than the horizontal, and the mounting angle of the duct on the rotation direction side at an angle where the stern side is higher than the horizontal, Propulsion efficiency can be improved by forming a counterflow to the propeller on the direction side.
Further, by the stern side from the mounting angle of the rotation direction of the counter-rotating direction side of the duct the mounting angle of the duct of the propeller is the mounting angle facing upward, the propulsion efficiency strengthen the rotational direction counter flow of the propeller Can be improved.
Further, by accelerating the flow with the duct on the rotational direction side, the counterflow to the propeller on the rotational direction side can be further strengthened, and the propulsion efficiency can be improved.
Further, by a duct internal to constitute a hollow duct, thereby improving the propulsion efficiency by a duct in consideration of the stern shape and economic aspects like.
By adopting a stern structure in which the shapes of the left and right ducts are made different in the ship, the propulsion efficiency of the ship can be improved and energy saving can be realized.
In addition, when a stern structure in which the shape of the left and right ducts is different is adopted for a stern catamaran type ship, it is possible to improve the propulsion efficiency by using a strong upward flow generated in the vicinity of the stern tunnel.

The front view which shows typically the stern structure (example 1) by the 1st Embodiment of this invention (A) The side view which looked at the stern structure of FIG. 1 from the rotation direction side of the propeller, (b) The side view which looked at the stern structure of FIG. 1 from the anti-rotation direction side of the propeller The front view which shows typically the stern structure (example 2) by the 1st Embodiment of this invention (A) The side view which looked at the stern structure of FIG. 3 from the rotation direction side of the propeller, (b) The side view which looked at the stern structure of FIG. 3 from the rotation direction side of the propeller The front view which shows typically the stern structure (example 3) by the 1st Embodiment of this invention (A) The side view which looked at the stern structure of FIG. 5 from the rotation direction side of the propeller, (b) The side view which looked at the stern structure of FIG. 5 from the anti-rotation direction side of the propeller The front view which shows typically the stern structure (example 1) by the 2nd Embodiment of this invention (A) The side view which looked at the stern structure of FIG. 7 from the rotation direction side of the propeller, (b) The side view which looked at the stern structure of FIG. 7 from the anti-rotation direction side of the propeller The front view which shows typically the stern structure (example 2) by the 2nd Embodiment of this invention

(First embodiment)
A stern structure according to a first embodiment of the present invention will be described below with reference to FIGS.
FIG. 1 is a front view schematically showing a stern structure (example 1) according to this embodiment. As shown in the figure, the stern structure 10 includes a duct (addition) 40 and a duct (addition) 50 that control the flow of the hull 20 to the propeller 30 at the stern.

  The duct 40 and the duct 50 are both ducts having a semicircular shape inside. In FIG. 1, thick lines indicate the outer shells at the stern side ends of the duct 40 and the duct 50, and the inner side of the thick lines indicates the opening. As described above, by making the shapes of the duct 40 and the duct 50 different from each other, the function of strengthening the counterflow with respect to the rotation direction of the propeller 30 and the function of rectifying the flow to the propeller 30 in accordance with the actual state of the wake distribution. Can be shared by the left and right sides of the stern. Therefore, the propulsion efficiency of the ship can be improved.

Below, the duct 40 which mainly acts to strengthen the counterflow with respect to the propeller 30 and the duct 50 which functions to rectify the flow with respect to the propeller 30 will be described with reference to FIGS. 2 (a) and 2 (b).
Fig.2 (a) is the side view which looked at the stern structure of FIG. 1 from the rotation direction side of the propeller. As shown in the figure, the duct 40 provided on the rotation direction side of the propeller 30 (the side where the propeller blades of the propeller 30 rotate from the top to the bottom, the side indicated by A in FIG. 1) has a low bow side. It is attached at an angle that faces upward with an elevation angle so that the stern side becomes higher. For this reason, it is possible to improve the propulsion efficiency by strengthening the counterflow opposite to the rotation direction of the propeller.
In addition, the duct 40 has a center axis C extending vertically with respect to the propeller shaft 31 when the hull 20 is viewed from the ship side at the height of the propeller shaft (rotating shaft of the propeller 30) 31 and the propeller shaft 31. It is formed at the overlapping position. Therefore, the duct 40 can reliably guide the counterflow to the propeller surface and strengthen the counterflow to the propeller 30.

The duct 40 is formed so that the water flow inlet is relatively wide and the outlet is narrow, that is, the inlet cross-sectional area is larger than the outlet cross-sectional area. For this reason, when passing through the duct 40, the water flow is accelerated, and the accelerated water flow becomes a counter flow with respect to the propeller 30, and the counter flow can be further strengthened. Therefore, the propulsion efficiency of the propeller 30 is improved. That is, by increasing the flow in the direction opposite to the rotation of the propeller 30, the circulation of the propeller surface of the propeller 30 is increased, and the propulsion efficiency is improved by the double reversal effect.
Note that circulation is not only the circulation in fluid mechanics, which is obtained by integrating the product of the tangential vector and line segment of each point along the closed curve in the flow, but also the circle around which the propeller rotates. A concept that includes what is determined cyclically using a flow vector along the circumference.

FIG. 2B is a side view of the stern structure of FIG. 1 viewed from the side opposite to the rotation direction of the propeller. As shown in the figure, a duct 50 provided on the side of the propeller 30 in the counter-rotating direction (the side where the propeller blades of the propeller 30 rotate from the bottom to the top, the side indicated by B in FIG. 1) is provided on the propeller shaft. It is attached substantially horizontally so as to be parallel to 31. Further, the duct 50 is formed at a position overlapping the propeller shaft 31 so that the center axis C is substantially parallel to the propeller shaft 31 when the hull 20 is viewed from the ship side at the height of the propeller shaft 31. . The water flow inlet and outlet of the duct 50 have the same size (cross section). For this reason, the duct 50 provided on the side opposite to the rotation direction of the propeller 30 functions to rectify the water flow by passing through the inside thereof.
It can be said that the flow is rectified on the counter-rotation direction side, so that the counter flow is formed by so-called negative action.

  However, the duct 50 may be attached so that the bow side is high and the stern side is low. In this case, the duct 50 is formed at a position overlapping the propeller shaft 31 so that the center axis C extends vertically with respect to the propeller shaft 31 when the hull 2 is viewed from the ship side at the height of the propeller shaft 31. According to this configuration, in addition to the rectifying action, an action of forming a counterflow with respect to the propeller 30 on the counter-rotating direction side rotating from the bottom to the top can also be achieved. When the action of forming the counterflow is imparted to the duct 50, the profit obtained by the increase of the propulsion efficiency of the ship due to the increase of the counterflow is greater than the disadvantage of the decrease of the propulsion efficiency of the ship due to the increase of the resistance. To.

  On the counter-rotation direction side of the propeller 30, unlike the rotation direction side, a flow in the same direction as the rotation direction of the propeller 30 is formed near the stern. For this reason, it is difficult to form the counterflow of the propeller 30 by the duct 50 as in the case of the duct 40. If the duct 50 has the same configuration as the duct 40, it does not contribute to the improvement of the propulsion efficiency of the propeller 30, but rather may disturb the water flow in the vicinity of the propeller 30 and increase the resistance of the ship.

  Therefore, in the stern structure 11 of the present embodiment, the duct 40 and the duct 50 are provided on the left and right sides of the stern, and both have different shapes. With this configuration, as a comprehensive action of the duct 40 and the duct 50, the increase in propulsion efficiency by increasing the counterflow with respect to the rotation direction of the propeller 30 and the decrease in propulsion efficiency due to the increase in resistance of the hull are adjusted. In order to improve the propulsion efficiency, it is possible to obtain an optimum shape. Improving ship propulsion efficiency leads to energy savings.

  The rotation direction side duct 40 of the propeller 30 is attached at an angle in which the stern side faces upward relative to the anti-rotation direction side duct 50 in a state in which the hull 20 is viewed from the side. That is, the duct 40 is attached such that the attachment angle α (see FIG. 2A) of the central axis C with respect to the horizontal L is larger than that of the duct 50. With this configuration, the counterflow on the rotation direction side can be strengthened and the propulsion efficiency of the propeller 30 can be improved.

  Further, the duct 40 on the rotation direction side of the propeller 30 is attached so that the stern side is higher than the horizontal L, that is, the attachment angle α is formed above the horizontal L, and the duct on the counter rotation direction side of the propeller 30 is also installed. 50 may be attached such that the stern side is lower than the horizontal L, that is, the attachment angle α is formed below the horizontal L. According to this configuration, it is possible to improve the propulsion efficiency by forming a counterflow with respect to the propeller 3 on both the rotation direction side and the counter rotation direction side. As described above, the attachment angle α may be determined in consideration of the benefits of increasing the counterflow with respect to the propeller 30 and the disadvantages of increasing resistance. Since the duct 40 and the duct 50 are hollow additions, the duct 40 and the duct 50 have an advantage that a flow can be accurately guided and a counter flow can be formed.

  FIG. 3 is a front view schematically showing the stern structure (example 2) according to the present embodiment. As shown in the figure, the stern structure 11 has a plurality of fin-like appendages as fins (additions) 41 and fins (additions) 51 that control the flow of the hull 20 to the stern propeller 30. doing. The “fin shape” includes a plate shape and a wing shape.

  Fig.4 (a) is the side view which looked at the stern structure of FIG. 3 from the rotation direction side of the propeller. As shown in the figure, a fin 41 including a fin 41A and a fin 41B is attached to the rotation direction side A of the propeller 30. The fins 41 are attached at an angle so that the bow side is low and the stern side is high. The fin 41 is formed at a position where the central axis C extends vertically with respect to the propeller shaft 31 and overlaps with the propeller shaft 31 when the hull 20 is viewed from the side of the ship at the height of the propeller shaft 31. For this reason, like the duct 40, a counterflow on the rotation direction side of the propeller 30 can be formed.

  The fins 41 </ b> A and 41 </ b> B are formed such that the distance between the inlet side (the bow side) of the water flow is relatively wide and the distance between the outlet side (the stern side) is relatively narrow. For this reason, the water flow accelerated when passing through the fin 41 becomes a counter flow with respect to the propeller 30. Therefore, the propulsion efficiency of the propeller 30 can be improved. That is, by increasing the counter flow that is the flow in the direction opposite to the rotation of the propeller 30, the circulation of the propeller surface is increased and the propulsion efficiency is improved by the counter-rotating effect.

FIG. 4B is a side view of the stern structure of FIG. 3 viewed from the counter-rotation direction side of the propeller. As shown in the figure, on the counter-rotation direction side B of the propeller 30, a fin 51 composed of a fin 51A and a fin 51B is attached. The fins 51 are attached substantially parallel (substantially horizontally) to the propeller shaft 31. The fin 51 is formed at a position overlapping the propeller shaft 31 so that the center axis C thereof is substantially parallel to the propeller shaft 31 when the hull 20 is viewed from the side of the ship at the height of the propeller shaft 31. . For this reason, like the duct 50, the fin 51 applies the water flow rectified by passing through the fin 51 to the propeller 30.
Further, the fin 51A and the fin 51B of the fin 51 are formed so that the distance between the inlet side (the bow side) and the outlet side (the stern side) of the water flow is the same. In this way, the propulsion efficiency of the ship is improved by making the fins 51 and the fins 41 have different shapes and different actions on the left and right sides of the stern.

  The fins 51 may be attached with an elevation so that the bow side is high and the stern side is low. In this case, the fin 51 is formed at a position overlapping the propeller shaft 31 so that the central axis C extends vertically with respect to the propeller shaft 31 when the hull 20 is viewed from the side of the ship at the height of the propeller shaft 31. According to this configuration, it is possible to form a counterflow with respect to the propeller 30 on the counter-rotating direction side in which the propeller 30 rotates from the bottom to the top. Note that, as with the duct 50, the attachment angle α may be determined in consideration of the benefits of increasing the counterflow with respect to the propeller 30 and the disadvantages of increasing resistance. Since the fin 41 and the fin 51 are open on the sides, the fin 41 and the fin 51 have an advantage that the flow can escape somewhat, but the configuration is simple and can be provided at low cost.

  FIG. 5 is a front view schematically showing the stern structure (example 3) according to the present embodiment. As shown in the figure, the stern structure 12 has a hollow appendage having no opening on the surface as an appendage 42 and an appendage 52 that control the flow of the hull 20 to the propeller 30 of the stern.

  Fig.6 (a) is the side view which looked at the stern structure of FIG. 5 from the rotation direction side of the propeller. As shown in the figure, an appendage 42 is attached to the propeller 30 in the rotational direction side with an elevation angle so that the bow side is low and the stern side is high. Further, when the hull 20 is viewed from the side of the hull 20 from the ship side at the height of the propeller shaft 31, the appendage 42 is formed at a position where the center axis C extends vertically with respect to the propeller shaft 31 and overlaps with the propeller shaft 31. . Therefore, like the duct 40 and the fin 41, the counterflow to the propeller 30 can be strengthened.

  FIG. 6B is a side view of the stern structure of FIG. 5 viewed from the side opposite to the rotation direction of the propeller. As shown in the figure, on the side opposite to the rotation direction of the propeller 30, an appendage 52 is attached substantially horizontally so as to be parallel to the rotation axis of the propeller 30. Further, the appendage 52 is formed at a position overlapping the propeller shaft 31 so that the center axis C is substantially parallel to the propeller shaft 31 when the hull 20 is viewed from the ship side at the height of the propeller shaft 31. Yes. For this reason, the water flow rectified by the adduct 52 can be applied to the propeller 30.

  The adduct 42 and the adduct 52 described with reference to FIG. 5 and FIG. 6 may be configured in a hollow shape having a space inside or a solid shape having no space in the inside. Since the adduct 42 and the adduct 52 are hollow or solid adducts that do not have openings on the surface, the effect of guiding the flow is slightly inferior to that of the duct-like adduct, but can be strengthened in terms of strength. Has advantages.

  When implemented as a ship adopting the stern structure 10, 11, 12 described with reference to FIG. 1 to FIG. 6, as a configuration in which a plurality of types of ducts 40, 50, fins 41, 51, and appendages 42, 52 are provided in combination. Also good. The ducts 40 and 50, the fins 41 and 51, and the appendages 42 and 52 may be formed on the hull 20 as a new ship from the beginning, or may be formed later on an existing ship. It can also be replaced if it is damaged, etc. or removed and maintained, and then reattached.

(Second Embodiment)
A stern catamaran vessel according to a second embodiment of the present invention will be described below with reference to FIGS. The stern catamaran ship of this embodiment is different from the ship of the first embodiment in that it is a stern catamaran type. In addition, about the member demonstrated in 1st Embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted in this embodiment.
FIG. 7 is a front view schematically showing the stern structure (example 1) according to the present embodiment. As shown in the figure, the stern structure 13 includes a duct 40 and a duct 50 that control the flow of the hull 23 to the stern propeller 30.

Fig.8 (a) is the side view which looked at the stern structure of FIG. 7 from the rotation direction side (inner side of skeg) of a propeller. As shown in the figure, on the rotational direction side A of the propeller 30, when the duct 40 is viewed from the side of the ship body 20 at the height of the propeller shaft 31, the center axis C extends vertically with respect to the propeller shaft 31. And it is formed in the position which overlaps with propeller axis 31.
FIG.8 (b) is the side view which looked at the stern structure of FIG. 7 from the anti-rotation direction side (outside of skeg) of the propeller. As shown in the drawing, the central axis C of the duct 50 is attached to the counter-rotation direction side B of the propeller 30 so as to be substantially parallel to the propeller shaft 31.

  The stern catamaran vessel 23 includes two skegs 60 at the stern, and a very strong upward flow is generated in a tunnel portion between the seg 60. In order to utilize this upward flow, the propellers 30 provided on the stern sides of the two skegs 60 are used in a so-called inward rotation that rotates downward inside the tunnel. Therefore, by providing the duct 40 inside the skeg 60, the upward flow can be used more effectively as the counter flow of the propeller 30, and the propulsion efficiency can be further improved. Further, by providing the duct 50 outside the skeg 60, it becomes possible to improve the propulsion efficiency by arranging the water flow.

  As described above, since the propeller 30 of the stern catamaran vessel 23 uses the upward flow in the tunnel, it is normally used in an inward rotation. However, for example, it may be used in outward rotation for the purpose of preventing the net from being rolled up. In the following, based on FIG. 9, a mode suitable for use in outward rotation will be described.

  FIG. 9 is a front view schematically showing the stern structure (No. 2) according to the present embodiment. The stern structure 14 shown in the figure is different from the stern structure 13 shown in FIG. 7 in that the propeller 30 is outward. For this reason, the outer side of the skeg 60 is the rotation direction side, and the inner side of the skeg 60 is the anti-rotation direction, which is different from the inner one. However, the side view of the stern structure viewed from the propeller rotation direction side and the counter-rotation direction side is the same as FIGS. 8 (a) and 8 (b).

As described above, in the stern catamaran vessel, even when the upward flow of the tunnel portion between the skegs cannot be used, the propeller propulsion efficiency is improved by providing asymmetrical additions on the left and right sides of the skegs. be able to.
In the above description based on FIG. 7 to FIG. 9, the configuration in which the duct-shaped appendage is provided has been described. However, in the stern catamaran vessel, another type of appendage described in the first embodiment is used alone or You may use together.

  The present invention can be widely applied to various ships. Further, the stern structure of the present invention can be provided at the time of construction of a ship, or can be provided later on an existing ship.

10, 11, 12, 13, 14 Stern structure 20, 23 Hull 30 Propeller 31 Propeller shaft 40, 50 Duct (Addition)
41, 51 Fins (additions)
42, 52 Additives

Claims (7)

A stern structure having a hollow duct for controlling the flow of the hull to the stern propeller, the duct being formed in different shapes on the left and right of the stern, and provided on the rotation direction side of the propeller The duct is formed in a shape that reinforces the counterflow with respect to the rotation direction of the propeller so that the central axis of the duct extends vertically with respect to the propeller axis in a state where the hull is viewed from the side of the ship. A stern structure characterized in that the duct on the rotation direction side is attached at an angle at which the stern side is horizontal or lower than the horizontal to enhance the counterflow with respect to the rotation direction of the propeller . The duct on the counter-rotating direction side is formed so that a central axis of the duct extends vertically with respect to the axis of the propeller in a state in which the hull is viewed from the side of the ship. Stern structure as described. The mounting angle of the duct on the rotation direction side of the propeller is mounted at an angle such that the stern side is directed upward from the mounting angle of the duct on the anti-rotation direction side in a state where the hull is viewed from the side. The stern structure according to 1 or 2. Stern structure according to claim 3, characterized in that the mounting angle of the duct of the rotational direction is the stern side attach to an angle up from the horizontal. 5. The stern structure according to claim 1, wherein a flow is accelerated by the duct on the rotation direction side. Ship, characterized in that employing the stern structure according to the hull in one of claims claims 1-5. The ship according to claim 6 , wherein the hull is a stern catamaran.