Abstract:
A boat propeller drive with an underwater housing which is connected in a fixed manner to a boat hull and has tractor propellers which are arranged on that side of the housing facing ahead and the blade areas of which are adapted to one another in such a manner that, at least under certain operating conditions, the aft propeller works in a cavity-generating manner whereas the fore propeller works in a cavitation-free manner.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a drive assembly in a boat, comprising a propeller drive which is arranged in a fixed manner on the outside of a boat hull and has an at least essentially vertical drive shaft which, via an angle gear enclosed in an underwater housing, drives in a counter-rotating manner a pair of at least essentially horizontal propeller shafts each with their own propeller, and a drive unit which is arranged on the inside of the hull and to which the vertical drive shaft is drivably connected. 
     It is a known fact that, in fast motor boats, it is possible to achieve considerably higher overall efficiency with an outboard drive with twin counter-rotating propellers coupled to an inboard engine than with an inboard engine coupled to a straight shaft with a single propeller. Until now, outboard drives in fast boats have with few exceptions been of the type which is suspended steerably as well as trimmably and tiltably in the transom stern of the boat. Such an exception is disclosed and described in SE 8305066-6, where a special embodiment of a drive with a pusher propeller and a tractor propeller is installed in a fixed manner and projects down from the bottom of the hull. The advantage of being able to trim the drive at different angles in relation to the transom stern of the boat is that the drive angle can be adapted to the position of the boat in the water, which depends on loading, speed and weather conditions, so that optimum propulsion can be achieved under different operating conditions. The advantages of being able to trim the drive are most apparent in smaller and medium-sized fast-moving boats up to about 40 feet. The larger and heavier the boat is, the less its position in the water is affected by said factors and the smaller the need to be able to trim the drive. At the same time, the cost of the drive increases considerably, the greater the power that it is to transmit. For these reasons inter alia, outboard drives are seldom used in boats in the size class over 40 feet, but in this case the engines drive straight propeller shafts with a single propeller via inboard-mounted reversing gears. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is generally to provide a drive assembly of the type referred to in the introduction, which is primarily but not exclusively intended to replace a conventional inboard installation with reversing gear and a straight shaft in larger boats, and in this connection, compared with the inboard installation, to bring about not only higher overall efficiency and better performance but also simplified installation and lower installation weight. 
     According to the invention, this is achieved primarily by virtue of the fact that the propellers are tractor propellers and that the blade areas of the propellers are adapted to one another in such a manner that, at least under certain operating conditions, the aft propeller works in a cavity-generating manner whereas the fore propeller works in a cavitation-free manner. 
     It is previously known to use a propeller combination of a fore and an aft propeller together with steerable outboard drives, in which combination, at least at higher speeds, the aft propeller works in a cavity-generating manner whereas the fore propeller works in a cavitation-free manner. In this way, it is possible to reduce the grip of the propellers in the water slightly during turning, so that a certain sideways sliding occurs and uneven movement is avoided, which is essential as far as boats with a Froude number close on  5  are concerned. However, it has been found that such a propeller combination can lead to various advantages even in boats with a lower Froude number and with fixed drives with tractor propellers. When the aft propeller generates cavities, the resistance in the water from the underwater housing lying behind is reduced. Moreover, cavitation damage to the underwater housing is prevented when the aft propeller generates cavities. A further advantage is that the cavitation threshold is raised in the speed range, which means that the upper speed limit of the propeller can be raised or alternatively that the blade area can be reduced. 
     According to a development of the drive according to the invention, another possibility afforded by a drive with tractor propellers is the positioning of an exhaust discharge outlet in the aft side of the underwater housing, which means that it is possible inter alia to utilize the ejector effect which the water flowing past exerts on the exhaust gases streaming out in the same manner as when the exhaust gases are conveyed out through the propeller hubs. When the exhaust gases are conveyed out in the rear edge of the underwater housing instead of through the hubs, the hub diameter and thus the overall propeller diameter can be reduced, which is advantageous in a number of respects. On the one hand, the mass and the mass forces are reduced and, on the other hand, the space requirement under the bottom of the hull is reduced, which means that the drive shaft housing can be designed so as to be shorter and consequently lighter than if propellers with an exhaust discharge outlet in the hubs were to be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is described in greater detail with reference to exemplary embodiments shown in the appended drawings, in which 
     FIG. 1 shows a diagrammatic partly cut-away side view of an embodiment of a drive assembly according to the invention, 
     FIG. 2 shows a plain side view of the drive assembly in FIG. 1, 
     FIG. 3 shows a perspective view of a drive installation comprising two drive assemblies according to FIGS. 1 and 2, 
     FIG. 4 shows a side view of a second embodiment of a drive assembly according to the invention, 
     FIG. 5 shows a perspective view of a drive installation comprising two drive assemblies according to FIG. 4, 
     FIG. 6 shows a diagram of the overall efficiency of a drive assembly according to the invention compared with a conventional inboard installation, and 
     FIG. 7 shows a diagram illustrating the increase in speed of a boat with a drive assembly according to the invention in relation to a boat with a conventional inboard installation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, reference number  1  designates generally a drive unit consisting of an engine  1   a  and a reversing gear mechanism  1   b  which are fixed to an inner surface  2  on the bottom  4  of a boat hull. An underwater housing  5  has a fastening plate  7  which is fastened to an outer surface  8  on the bottom  4 . The engine  1   a  drives, via an angle gear in the reversing gear  1   b,  an output shaft  9  which in turn drives, via an angle gear comprising conical gearwheels  10 ,  11  and  12 , a pair of propeller shafts  13  and  14 , of which the shaft  14  is a hollow shaft, through which the shaft  13  extends. The shaft  13  bears a propeller  15  with a hub  15   a  and blades  15   b,  and the shaft  14  bears a propeller  16  with a hub  16   a  and blades  16   b.    
     The propeller shafts  13  and  14  are mounted in a torpedo-like part  20  of the underwater housing  5 . The housing part  21  between the torpedo  20  and the fastening plate  7  has a wing-like profile with slightly domed side surfaces on both sides of a vertical plane of symmetry. On the aft side of the housing part  21 , a rudder flap  22  is mounted for pivoting about a vertical pivoting axis. The front end portion  23  of the rudder flap  22  has a semi-circular cross section and projects into a semi-circular channel  24 , as shown most clearly in FIG. 3, where the starboard drive assembly is shown with the rudder blade removed. The side surfaces of the rudder flap lie, at the front edge, in the same plane as the rear edge of the side surfaces of the housing part  21 , so that a smooth transition is obtained between the housing part  21  and the rudder flap  22 . Together, these two extend over the entire length of the torpedo  20 . 
     At its aft end, the torpedo  20  has a discharge opening  25 , in which an exhaust pipe  26  opens, which runs from the engine  1   a  and through the underwater housing  5 . As a result, the propellers will work in completely undisturbed water, on the one hand on account of their being positioned in front of the underwater housing and on the other hand on account of the positioning of the exhaust discharge outlet, which moreover, on account of the ejector effect which arises during motion, contributes to minimum exhaust back-pressure. As can be seen from the figures, the torpedo is at its rear edge designed with a screen  27  towards the rudder flap  22  in order to screen the rudder blade from the exhaust gas flow. By virtue of the fact that the exhaust gases are conveyed out through the underwater housing and not through the propeller hubs  15   a  and  16   a,  the diameter of the hubs and thus the diameter of the propeller as a whole can be reduced. In steerable outboard drives with pusher propellers, the maximum diameter of the hubs is normally the same as the maximum diameter of the adjacent part of the underwater housing, whereas the maximum hub diameter of the propellers  15  and  16  shown in FIGS. 2-5 is roughly 60-65% of the maximum diameter of the torpedo  20  in the portion adjacent to the propellers. As the propellers require a certain minimum distance from the surface of the bottom of the boat above, the length of the underwater housing in the vertical direction is also affected by the propeller diameter, which means that the smaller the propeller diameter is, the shorter the underwater housing needs to be in the vertical direction. 
     FIG. 2 shows a propeller drive of the type described in connection with FIG. 1, that is to say a drive with an underwater housing  5  which is fixed directly to the bottom surface of the boat hull by its fastening plate  7 . The drive has two propellers  15  and  16 , of which the fore propeller has three blades whereas the aft propeller has four blades, which is known per se in steerable outboard drives. In a preferred embodiment, moreover, the blade areas of the propellers are adapted to one another in such a manner that, within a predetermined upper speed range, the aft propeller works in a cavity-generating manner whereas the fore propeller works in a non-cavity-generating manner. 
     The propeller drive in FIG. 2 is mounted on one side of and at a distance from the centre line  30  of the bottom. A corresponding propeller drive is mounted on the other side of the centre line, as shown in greater detail in FIG.  3 . As mentioned above, the rudder flap of the right-hand drive has been removed in order to illustrate the design of the wing-like part  21  of the underwater housing  5 . With twin-mounted drives, means (not shown) can advantageously be arranged, which make it possible to disconnect the normal synchronous operation of the rudder blades and instead steer the rudder blades in a mirror-inverted manner, that is to say in such a manner that a given deflection of one rudder to, for example, port leads to a corresponding deflection of the other to starboard. In this way, the steering deflections cancel each other out and the rudders instead function as brake flaps without any steering effect. 
     FIG. 4 shows an embodiment of a propeller drive according to the invention, which differs from that described above in that the underwater housing  5  is connected to a housing  32  which is mounted against the transom stern  31  of the hull and contains an angle gear and a reversing gear mechanism with an output shaft connected to the shaft  9  (FIG.  1 ). In the transition between the housing  32  and the underwater housing  5 , the latter is designed with a cavitation plate  33  which extends up to the transom stern  31 . The front edge of the cavitation plate  33  is sealed against the surface of the transom stern, so that the cavitation plate  33  forms an extension of the bottom of the boat. Like the drive in FIGS. 1-3, the drive in FIG. 4 has a three-bladed fore propeller and a four-bladed aft propeller which is preferably, within a given upper speed range, a cavity-generating propeller. FIG. 5 shows a boat hull with two drives of the type shown in FIG. 4 mounted on the transom stern at an equal distance from the centre line  30 . 
     The diagram in FIG. 6 illustrates the overall efficiency as a function of the speed of the boat for one and the same boat type with on the one hand a conventional inboard installation, that is to say straight shafts and a single propeller (broken line), and on the other hand the drive assemblies according to the invention described above (solid line). As can be seen from the diagram, the difference at, for example, 38 knots is as much as 20 percentage units, in other words an increase in overall efficiency of no less than roughly 40% is obtained with the installation according to the invention compared with a conventional inboard installation. The diagram in FIG. 7 illustrates in a corresponding manner the increase in speed of a boat with a drive assembly according to the invention in relation to the same boat with a conventional inboard installation. It can be seen from the diagram, for example, that if the top speed of a boat with a drive assembly according to the invention is 40 knots when equipped with a given engine, the top speed of the same boat and engine with a conventional inboard installation is roughly 35 knots.