Patent Publication Number: US-8109221-B2

Title: Single drive catamaran hull

Description:
TECHNICAL FIELD 
     This invention relates generally to catamaran hull construction, and more specifically concerns such a hull having two different asymmetrical hull shapes. 
     BACKGROUND 
     Catamarans typically have two identical, usually asymmetric, hulls which are mirror images of each other. Any turning force induced by one hull is thus offset by the turning force from the other hull. Catamaran hulls often are quite narrow and hence have disadvantages in terms of space and efficiency. Wide catamaran hulls have more interior space but create larger wakes which result in greater drag. In addition, catamaran hulls usually require propeller pockets configured to reduce propeller shaft angle. Conventional propeller pockets result in the water in the pockets being forced to change direction in a relatively short space, which in turn increases drag on the boat; narrow hulls in general distort the water flow less than wide hulls, thereby producing less drag. Energy loss can also be significant due to wake generation. 
     Accordingly, it is an object of the present invention to produce a catamaran hull arrangement which improves efficiency by reducing drag, and further to increase usable space within a given hull size. 
     SUMMARY 
     Accordingly, a catamaran hull is disclosed, comprising: a pair of substantially parallel separate hulls connected by an intermediate hull portion, wherein each separate hull has a shape which is asymmetric; and a propulsion assembly positioned in only one of the separate hulls, defining a propulsion hull, wherein the two separate hulls have different asymmetric configurations such that the off-center thrust created by action of the propulsion assembly in the one hull is substantially offset by the turning force created by the difference in shape of the two separate hulls. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the catamaran hull disclosed herein. 
         FIG. 2  is a view of the catamaran hull of  FIG. 1  from the bow thereof. 
         FIG. 3  is a view of the catamaran hull of  FIGS. 1 and 2  from the stern thereof. 
         FIGS. 4 and 5  are top and bottom views, respectively, of the catamaran hull of  FIGS. 1-3 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The complete catamaran hull, also referred to simply as a catamaran, disclosed herein, and is shown generally at 10, with two individual hulls  12  and  14  and an intermediate connecting portion  16 . As shown in  FIGS. 4 and 5 , the hulls  12  and  14  are substantially the same length. The catamaran hull may be of various lengths, accommodating different numbers of people, depending upon the particular application and the specific design; for instance, a 39 foot catamaran will usually accommodate four adults. 
     In the present catamaran arrangement, only one of the hulls  12 ,  14  has a drive train  18 , which comprises generally a motor, an output shaft and a propeller mounted on the output shaft. The single drive train  18  is mounted in the embodiment shown in the starboard hull, although the single drive train could alternatively be in the port hull. 
     With use of only one motor, located in one hull, that hull can be sized to accommodate the motor and the remainder of the drive train for easy access and service, while the non-propulsion hull can be narrow so as to reduce drag. The accommodations on the catamaran can be arranged to best advantage in the propulsion hull, which is wide. 
     Referring to  FIG. 4 , the outboard side hull entry angle for the starboard (propulsion) hull will vary from 8-15°, preferably approximately 10°, while the inboard side entry angle will range from 12-22°, preferably approximately 13°. This will yield a net outboard tow entry angle  21  of 2-5°. The starboard drive hull is asymmetric, off the boat centerline approximately 3′ 10″ toward the starboard side of the boat. 
     Also referring to  FIG. 4 , the outboard side hull entry angle for the port (non-propulsion) hull  14  will vary from 8-15°, preferably approximately 10°, while the inboard side entry angle will range from 7-17°, preferably approximately 9°. This will yield a net inboard tow entry angle  21  of 0-2°. The port hull is also asymmetric, off the boat centerline approximately 4′ 10″ toward the port side of the boat. 
     As indicated above, the starboard (propulsion) hull in the embodiment shown has a larger (wider) beam than the narrower port hull; typically, the starboard hull will have 20-35% more displacement than the non-propulsion hull. With respect to the drive train  18 , motor  20  is typically conventional, for instance, a 380 HP Diesel. Other motors can be used. Shaft  22  and propeller  24  are also conventional. 
     The result of the differences in hull configuration produces a net turning force in the direction of the propulsion, e.g. starboard, hull. The hulls, each being asymmetrical but different in configuration, have their individual bows shaped so that in total, an overall push to starboard results, instead of being mirror images of each other, which would have no net effect. The asymmetrical hull shapes are accomplished by varying the inboard and outboard entry angle. The arrangement of the hulls, with the resulting tendency to push the direction of the catamaran to starboard during operation, offsets the propulsion action of the single propulsion hull, which tends to push the catamaran toward the port direction. These two forces (propulsion and net hull action) counteract each other, so that the boat as a result tends to run straight. This resulting balance allows for straight tracking and a balanced helm in virtually all sea conditions, significantly increasing the efficiency of the boat. The large beam of the propulsion hull allows additional accommodation space. For instance, a 39 ft. boat will include a large cockpit and salon area, as well as a galley, dining area, two staterooms with showers and two heads. 
     A single small keel fin  28  is located near the center of the narrow port (non-propulsion) hull, extending for substantially the length of the hull, while the starboard (propulsion) hull has a small keel fin  29  which extends for only a portion of the hull, because of the propulsion equipment. If the propulsion hull is the port hull, then the keel fin arrangements will be switched. An oversized rudder  30  can be used to control balance of the helm and tracking of the boat. The rudder may also have a balancing trim-tab which is arranged for neutral helm turning. 
     Since the hull carrying the propulsion equipment is larger, it displaces more water than the non-propulsion hull. This imbalance in displacement is compensated for by careful placement of the equipment and the accommodations in the catamaran. Weight imbalance results in unequal boat motion, with pitch and roll being a function of the mass and shape of the two hull(s). In the present arrangement, since one hull has more mass than the other, the pitch and roll characteristics of the two hulls are different from each other. Generally, narrow hulls with a greater dead rise have less resistance to submersion of the hull than wider hulls with a lower dead rise. To create a balanced side-to-side rolling sensation, given the weight and shape imbalances between the two hulls, asymmetric shapes are used for both hulls. The narrower hull, i.e. the non-powered hull, has less resistance to pitch and roll. This is partially offset by that hull carrying less weight and by using larger chines, which increases hull volume near and above the water line. In calmer water, the narrow hull operates with lower resistance due to its narrow shape, while in rough waters, it submerges and gains volume more rapidly on a proportional basis than the wider hull. The larger chines and hull volume at the bow of the non-propulsion hull are balanced to match the characteristics of the propulsion hull. 
     Accordingly, a catamaran has been described which uses a drive train in only one of its two hulls. The resulting thrust in one direction is offset by differences in configuration between the two hulls, both of which are asymmetric, but instead of being mirror images of each other, as with traditional catamarans, are differently configured so as to produce a movement or thrust of the boat in the other direction, with the thrust and the hull arrangement basically compensating for each other so as to produce straight line tracking of the boat during running of the boat. 
     Although a preferred embodiment of the invention has been disclosed for purposes of illustration, it should be understood that various changes, modifications and substitutions may be incorporated in the embodiment without departing from the spirit of the invention which is defined by the claims which follow.