Abstract:
A bottom assembly for motor boats is described, of the type including at least one longitudinal tunnel, through which passes a propeller shaft exiting from the bottom, wherein it is provided at least one ground wall to close, totally or partially, the tunnel and limit the entry of water into the same during boat advancement.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention concerns a bottom assembly suitable for producing a motor boat with a so-called “surface propeller” type of propulsion system.  
         BACKGROUND OF THE INVENTION  
         [0002]    Surface propeller type propulsion systems have been known for some time, in which the propeller is partially immersed in the water and therefore works only with its lower part. The hub and the support member of the propeller, in addition to the propeller shaft providing movement to the same, stay out of the water also during hull movement, so as to reduce resistance caused by friction of the appendices.  
           [0003]    Compared to the immersed propeller propulsion systems, the surface propeller propulsion systems have a particularly reduced axis inclination. However if, on the one hand, it gives greater efficiency to the propulsion system, on the other hand it results in increased longitudinal hull dimensions.  
           [0004]    To obviate this drawback, use has been made of the so-called tunnels, i.e. the recesses made at the base of the bottom which extend longitudinally in the stern portion of the hull. Each tunnel (one for each propeller shaft exiting from the bottom) consists of one or more walls that surround sideways and above the propeller shaft exiting from the bottom. Until now, tunnels have been adopted both with conventional transmission types, in which the kinematic chain is longitudinally aligned from the motor to the propeller, as well as with V-drive transmission types, in which the motor is situated at the stern, in a particularly set back position, above the propeller shaft.  
           [0005]    However, the tunnel solution creates another problem. In fact, the presence of one or more tunnels alters the bottom by reducing its original hydrodynamic lift and giving rise to an increase in the hydrostatic pressure on the bottom itself. Consequently, under certain hull speed conditions, each tunnel tends to fill up with water during hull advancement and therefore the propeller is no longer able to work properly at the surface.  
           [0006]    Moreover, in certain rough conditions of the surrounding water, each tunnel tends to fill up with water in a variable and often irregular way, thus compromising the efficiency of the surface propeller propulsion. These drawbacks are particularly marked with “V-drive” transmission types which have a particularly set back barycenter.  
         SUMMARY OF THE INVENTION  
         [0007]    The object of the present invention is to provide a bottom assembly equipped with one or more tunnels that allows the correct operation of the propulsion systems, particularly of the surface propeller types. Another object of the present invention is to provide a bottom assembly equipped with one or more tunnels that confers the maximum efficiency to the surface propeller propulsion system independently of the boat speed conditions and/or the rough conditions of the surrounding water.  
           [0008]    These objects are achieved by the present invention, that concerns a bottom assembly for motor boats, of the type comprising at least one longitudinal tunnel arranged at least at the stern portion of the bottom, the tunnel including one or more walls that surround, above and sideways, at least one propeller shaft exiting from the bottom to rotationally operate at least one propeller, characterised in that it includes at least one ground wall radiused to the surface of the bottom to close, at least partially, the cited at least one tunnel and limit the entry of water into the same.  
           [0009]    This allows absolute minimisation of the loss of original hydrodynamic lift caused by the presence of one or more tunnels. Consequently, the propeller situated at each tunnel exit can work correctly at the surface, independently of the conditions of boat speed or wave motion around it.  
           [0010]    According to a possible embodiment of the present invention, the ground wall is made of a closing member applied to the bottom. In this case, the tunnel is used during construction to help in the assembling phase of the various seal and/or support members for the propeller shaft, and thus is covered by the closing member. A considerable practical advantage of this solution is that it is also particularly simple to apply to already existing bottoms.  
           [0011]    A bottom assembly made according to the present invention can include a hollow seat that surrounds each tunnel to allow the fixing of the closing member, hence maintaining the external surface continuity between the bottom and the closing member itself.  
           [0012]    According to another possible embodiment, the tunnel and the relative ground wall are integrally made in the bottom. This solution can be suitable, for instance, for hulls of metal construction, or anyway in the cases in which it would be possible to assemble the support and/or seal members for the propeller shaft also with the tunnel substantially closed on the bottom.  
           [0013]    According to another aspect of the present invention, the ground wall can include a suitably shaped end, or of reduced size, turned towards the propeller. The particular shape of the end portion, e.g. obtained by making a cut, may prove necessary to obtain optimum performance depending on the cavitation characteristics of the propeller exiting from the tunnel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Additional characteristics and advantages of the present invention will become apparent from the following description, with reference to the attached drawings, in which:  
         [0015]    [0015]FIG. 1 is a longitudinal section view of a motor boat equipped with tunnels according to the known technique;  
         [0016]    [0016]FIG. 2 is a perspective view of a bottom according to the known technique in a reversed position;  
         [0017]    [0017]FIG. 3 is a longitudinal section view that illustrates the behaviour of a motor boat, according to the known technique, during advancement in water;  
         [0018]    [0018]FIG. 4 is a longitudinal section view of a motor boat equipped with a bottom assembly according to the present invention;  
         [0019]    [0019]FIG. 5 is a view of the stern portion of a bottom, according to a possible embodiment of the present invention, in a reversed position;  
         [0020]    [0020]FIG. 6 is a view of the stern portion of a bottom, according to another possible embodiment of the present invention, in a reversed position; and  
         [0021]    [0021]FIG. 7 is a view of the stern portion of a bottom, according to an additional possible embodiment of the present invention, in a reversed position. 
     
    
     DETAILED DESCRIPTION  
       [0022]    FIGS.  1  to  3  illustrate some views of motor boats  1  equipped with surface propeller propulsion systems with bottoms made according to the known technique. For instance, the bottoms can be made with reinforced resins or similar by means of injection molding procedures. The propulsion system of the motor boat  1  in general includes a motor unit  2 , a reversing gearbox  3 , a propeller shaft  4  and a propeller  5 , near which is envisaged a support member  6  (fixed to the bottom  10 ) for the propeller shaft  4 .  
         [0023]    To limit the inclination of the propeller axis  15  in relation to the water surface  100 , the propeller shaft  4  exiting from bottom  10  is housed in a tunnel  20 , the latter having one or more walls that surround, sideways and above, the propeller shaft  4 . The support member  6  for the propeller shaft  4  can also be housed, if necessary, also just partially in tunnel  20 .  
         [0024]    For each propeller shaft  4  exiting from the bottom  10 , a tunnel  20  is envisaged substantially set in the stern portion of the same bottom. The bottom  10  shown as an example in FIG. 2 is prepared with two tunnels  20 , each to house a respective propeller shaft  4 .  
         [0025]    [0025]FIG. 3 represents the behaviour of the motor boat  1  of FIG. 1 during its advancement in water. As can be seen, the modification of the bottom  10  set by the presence of a tunnel  20  involves a variation in the original lift of the same bottom. This results in an increase in the hydrostatic pressure on the bottom  10  that promotes the tunnel  20  to fill with water, with the result that the propeller  5  is no longer able to work properly at the surface.  
         [0026]    This phenomenon is even more marked if the motor boat  1  adopts a “V-drive type” propulsion system, in which the motor  22  (shown by the dotted line in FIG. 3) occupies a more set back position compared to a motor  2  of a conventional type of propulsion system.  
         [0027]    To obviate these drawbacks the present invention proposes a motor boat  31  like that shown in FIG. 4, which retains the same reference numbers that identify the substantially unchanged parts as regards those already identified in the previous FIGS.  1 - 3 .  
         [0028]    According to the present invention, a bottom assembly  30  is always equipped with a tunnel  20 , but a ground wall  40  is envisaged, radiused to the bottom surface  30 , allowing at least the partial closure of the tunnel  20  so as to restore, as much as possible, the original bottom lift and thus limit the entry of water into the same tunnel.  
         [0029]    Once the motor boat  31  has reached the correct planing trim, the tunnel  20  by that time has already dynamically emptied itself of water and the ground wall  40  prevents the entry of water into the tunnel during the advancement of the same hull. This has the advantage of maintaining a constant performance of the boat, independently of its speed and rough conditions of the surrounding water.  
         [0030]    A bottom assembly made according to the present invention can be identically adopted in the boats with conventional propulsion systems, in which the kinematic chain is aligned in sequence from the motor unit  2  until the propeller  5 , as in boats with “V-drive” type propulsion systems, in which the motor  22  (also here shown by a dotted line) occupies a more set back stern position.  
         [0031]    [0031]FIG. 5 illustrates a possible embodiment of the invention, according to which the ground wall  40  of the bottom assembly  30  is realized by a closing member  41  that is applied to the bottom itself.  
         [0032]    For simplicity, a closing member  41  is outlined in the form of a substantially flat plate, but it should to understood that the closing member  41  can also take on a slightly bent or shaped form according the bottom shape to be restored at the tunnel  20 . In addition, although just one closing member  41  is shown for simplicity, it is also appropriate to point out that all the tunnels  20  in the bottom  30  (both tunnels  20  in the case shown) must be closed with a respective closing member  41 .  
         [0033]    The closing member  41  is preferably fixed to the bottom  30  so as to maintain the external surface continuity between the bottom  30  and the closing member  41 . For this purpose a hollow seat  42  is envisaged that surrounds the tunnel  20  to receive the closing member  41 . In the case of hulls produced by injection molding, the seats  42  are easily obtained during injection molding by suitable inserts set around the protruding templates designed to produce the tunnels  20 .  
         [0034]    Each tunnel  20  can therefore be used to help with the assembly of the transmission parts that protrude from the bottom  30 , i.e. the propeller shaft  4 , the relative support  6  and a sealing element (not shown in FIG. 5). After assembly, the tunnel  20  can be closed, also just partially, to restore as much as possible the original configuration of the bottom  30 .  
         [0035]    This embodiment is in any case adaptable in a particularly simple way also to already existing hulls of known type. In this case, the closing member can, for instance, have protruding portions or appendices that allow it to be fixed at the tunnel walls, or anyhow in any other suitable point which ensures particularly secure fixing.  
         [0036]    In the embodiment of FIG. 6, a closing member  44  can be suitably shaped, e.g. by cutting out part of it along a line  45 , at its end portion turned towards the propeller (not shown in FIG. 6). The shape or the reduction of the end portion is made depending on the cavitation characteristics of the propeller and can also therefore take on different forms to that shown as an example.  
         [0037]    [0037]FIG. 7 shows an embodiment of a bottom assembly  50  in which the ground walls  40  of the tunnels  20  can be integrally made in the same bottom. This embodiment can be suitable, for instance, for metal hulls, or in any case, hulls for which it is possible to assemble, from the rear, the transmission members shown in FIG. 7, especially at least the propeller shaft  4  and the seal and thrust bearing  8 , in addition to the possible support member  6  in the case in which it is to be assembled (differently from that represented in FIG. 8) also only partially inside the tunnel  20 .  
         [0038]    Also in this case, the ground wall  40  can be suitably shaped near the propeller, as already explained for the closing member  44  in the embodiment of FIG. 6.