The present invention concerns a bottom assembly suitable for producing a motor boat with a so-called xe2x80x9csurface propellerxe2x80x9d type of propulsion system.
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.
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.
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.
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.
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 xe2x80x9cV-drivexe2x80x9d transmission types which have a particularly set back barycenter.
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 funnels 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.
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.
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.
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.
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.
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.
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.