Abstract:
Stern drive assembly configured for utilization in an inboard/outboard power plant for a boat. The stern drive assembly includes a central rigid core that is configured at an upper portion to be coupled to the stern of a carrying boat. A lower portion of the core is designed to accept a boat-moving force generated by a water propulsion unit that is coupled thereto. A thin-walled housing is configured to be secured about a predominance of the centrally located rigid core. The housing has an outer surface that establishes an exterior of the stern drive assembly and an inner surface directed generally toward the central rigid core. A portion of an exterior surface of the central rigid core is configured to cooperate with a corresponding portion of the inner surface of the thin-walled housing. These two portions, when in cooperative orientation one with the other, form a functional feature for the stern drive assembly.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention is directed generally to marine engine drive systems designed to supply rotary power down from an engine carried on a boat to a submerged propeller and for imparting the propelling force generated by the rotating propeller in the water back to the boat thereby causing the boat to travel across the water. More particularly, the invention relates to a clam shell-style composite housing fixed about a core drive frame member. 
     2. Background 
     Traditionally, powerboats have been powered with either an inboard engine, an outboard engine, or an inboard/outboard engine. In the inboard configuration, the engine is typically positioned within an engine compartment or engine room that is carried on the boat. A drive shaft of the assembly extends through a bottom surface of the hull of the boat with a propeller positioned thereupon, but exteriorly to the boat. The drive shaft and propeller remain in the water during normal operation of the boat and typically cannot be removed from the water unless the boat is also taken out of the water. Often, an inboard engine is completely concealed within the engine compartment or engine room below the deck of the boat. 
     An outboard engine is a self contained unit that is most often attached to the transom of a boat. A typical outboard engine configuration includes an engine that is completely concealed within a cowling, at least one propeller attached to a lower unit, and a drive shaft contained within a drive shaft housing that extends in a generally vertical direction between the engine and the lower unit. The lower unit is typically constructed as a one piece body that is made of aluminum. Further, the lower unit contains gears for transferring torque produced by the engine, and imparted on the drive shaft, to a propeller shaft that is generally oriented perpendicularly to the drive shaft. The lower unit includes a skeg for steering purposes, an anti-cavitation plate, and a cylindrical bore that houses a forward gear, a reverse gear, and a propeller shaft. The lower unit usually also includes water intake ports for receiving raw water that is used to cool the engine. Further, the lower unit is coupled to a drive shaft housing, or upper housing, using a set of five to ten bolts. 
     Most outboard engines manufactured today further include a tilt/trim system that enables the outboard engine to be tilted through various angles to improve the performance of a boat and to rotate the lower end of the power plant out of the water. Generally, outboard engines can be trimmed between angles relative to a vertical axis of about minus 5 degrees to about plus 15 degrees and can be tilted through a range of angles between 15 degrees and about 60 degrees. The trim/tilt system generally is composed of three hydraulic cylinders, one cylinder that is the tilt mechanism and two cylinders that combine with the tilt cylinder to form a trim mechanism. The trim range includes a range of angles within which an engine can be operated to power the carrying boat while a tilt range includes a range of angles within which the engine generally will not be operated to power the boat; that is, the tilt range is for use when the engine is not running. The trim function moves the engine through the described range of angles about half as fast as the tilt function moves the engine because trim adjustment is used to adjust the drive leg through the trim range of angles when the boat is traveling, often at relatively high rates of speeds. Further, the trim feature operates at this reduced speed for safety concerns since relatively small trim adjustments can significantly affect the attitude of the boat. It also permits an operator to fine tune the travel position of the boat as it travels across the water for enhanced performance. 
     The tilt mechanism typically enables an entire outboard engine, or substantially all of the engine, to be tilted out of the water while the boat remains in the water. This feature is advantageous for many reasons. It is used during the boat launch and the retrieval process to protect the drive unit, and particularly the lower end from damaging strikes. Even if the boat is not removed from the water, the positioning of the drive leg outside the water using the tilt function prevents aquatic growth, such as algae, barnacles, and other marine plants and animals, from developing on the lower unit. Water, and especially salt water, can be highly corrosive. Salt water corrodes metals and provides a prime environment for galvanic reactions that accelerate decay of metals. Thus, removing the drive assembly from the water when not in use can increase its life dramatically. 
     The inboard/outboard engine configuration is a hybrid between the inboard and the outboard engine configuration just as the name implies. The inboard/outboard engine configuration generally includes a motor that is positioned within an engine compartment, much like the inboard engine configuration. Unlike the inboard engine which may be located mid-ship, however, the inboard/outboard engine compartment is typically located proximate the transom of the boat. The inboard/outboard engine further includes a drive assembly resembling the lower unit of an outboard engine. The drive assembly of an inboard/outboard power plant, however, is not coupled to a drive housing as described above relative to outboard engines. Instead, the drive assembly includes a shield assembly that is coupled to the transom of a boat. 
     The drive assembly of the inboard/outboard engine further includes a tilt/trim assembly that has a function similar to the tilt/trim assemblies found on outboard engines and described above. The conventional tilt/trim assemblies for inboard/outboard engines, however, are usually designed differently than those for outboard engines. Specifically, the conventional inboard/outboard tilt/trim assemblies include two hydraulic cylinders. One hydraulic cylinder is attached to one side of the drive assembly proximate the cavitation plate and the other hydraulic cylinder is attached to the other side of the assembly near the cavitation plate. Each cylinder is oriented generally parallel to the cavitation plate. With the hydraulic trim cylinders attached in this fashion, the cylinders produce unwanted water spray during operation of the engine while the boat is traveling on plane. Specifically, as the boat is planing, water travels on top of the cavitation plate and contacts the hydraulic cylinders near where they are attached to the drive assembly. The water is deflected and forms a spray that is unattractive and can cause the back portion of a boat to become wet, including any nearby passengers. 
     Though there are some drawbacks to traditional inboard/outboard designs, the design still possesses attributes that make it highly desired by many boaters. For instance, inboard/outboard engines can include a closed-loop water cooling system that uses recirculated cooling fluid. This closed-loop system eliminates corrosion problems associated with using raw salt water as encountered in outboard engine. Because a closed-loop cooling system weighs significantly more than a raw water cooling system, typical outboard motors do not have closed-loop water cooling systems. Further, the inboard/outboard engine is usually quieter than most outboard engines and thus desired by some boaters. Additionally, the low profile of inboard/outboard power plants do not provide an obstacle at the transom of the boat, such as the obstacle produced by the elevated engine portion of an outboard power plant. Instead, a relatively unobstructed transom is presented by the low-rise inboard/outboard power plant. As a result, inboard/outboard engines allow boats to include unobstructed swim platforms that extend across the entire transom of a boat which is something that is not normally possible when using an outboard engine. Thus, these and other attributes not mentioned make an inboard/outboard engine the desired engine for many boating applications. 
     As indicated above, the inboard/outboard engine has a number of disadvantages. In contrast to the outboard engine and the inboard engine, the inboard/outboard engine exposes more parts to the harsh environment of salt water. As mentioned above, the outboard engine is self-contained and is capable of being tilted completely out of the water using the trim mechanism. Further, the inboard engine has only a drive shaft and a propeller that are exposed to the water. The inboard/outboard, on the other hand, cannot usually be rotated completely out of the water when the boat is floating. Thus, anytime the boat remains in the water, the drive assembly at least partly remains in the water. Still further, the inboard/outboard engine has additional parts, such as the shield assembly, exhaust tubes, oil hoses, shift rods, covering sheaths, and a gimbal ring, that are exposed to the water, and are thus also subject to corrosion. As a result, these additional parts that are exposed to the water increase the need for service and increase the potential for breakage. 
     As described above, both outboard engines and inboard/outboard engines have lower units that transfer torque from a generally vertical drive shaft to a propeller drive shaft that is generally perpendicular to the vertical drive shaft. The lower unit typically contains a skeg, a lower chamber capable of receiving gears, bearings and the propeller drive shaft, one or more raw water intake ports that are connected to a raw water chamber and conduit system, and an anti-cavitation plate. Generally, the lower unit is a unitary member that is constructed of aluminum and formed by die-casting. The lower unit is composed of aluminum to withstand heavy blows caused by hitting submerged objects, such as logs, pilings and other debris, and from running aground. Further, the lower unit has been constructed of aluminum to handle the forces generated by the moment arm produced by the propulsion unit, most typically in the form of a rotating propeller, acting at the bottom of the lower unit. It is the lower unit coupled with either a drive housing or a shield assembly that provides the structural integrity of a typical drive system. Additionally, the lower unit is desirably designed to present an exterior surface that allows the drive system to move through water with a minimized hydrodynamic drag. 
     In addition to providing a hydrodynamically efficient exterior shape, lower units of traditional stern drives have included raw water intake ports and conduits, and exhaust conduits. The common designs for the raw water and exhaust conduits have been inefficient and susceptible to corrosion. More specifically, the conduits are inefficient flow conveyances because their configuration has previously been dictated by the die-cast manufacturing processes for these substantially solid, or thick-walled bodies. That is, the conduits or channels were conventionally formed between the outside surfaces of the exterior walls of the lower unit and the interior walls that formed the central cavity provided to receive a drive shaft therein. In other words, traditional service conduits have been formed in the wall&#39;s thickness of the lower unit. Still further, the conduits are often formed as right angle channels, or near right angle channels and resultingly include corner spaces at these right angle turns where eddies form and other flow restricting phenomena occur. Additionally, when the drive assembly is used in salt water, these areas are prone to collecting salt which accelerates corrosion. While this problem is well known, simply adding more material to eliminate these places of poor flow is not the answer because this simply adds more weight to an already over weight machine. 
     The bifurcated upper unit/lower unit design is problematic in its own right. For instance, the joint between the two units requires a seal that frequently corrodes and leaks. Further, while the lower unit has a raw water unit conduit that is formed from the inside surface of the exterior walls, the raw water conduit located within the upper unit is typically composed of a tubing or pipe. As a result, some structure for connecting the raw water conduit of the lower unit to the raw water conduit of the upper unit must be included. As with all fluid connections, each detrimentally presents a heightened potential for leakage. 
     Thus, a need exists for a drive assembly for an inboard/outboard or an outboard engine that resists corrosion while maintaining or improving the strength and hydrodynamic qualities found in conventional drive systems. Further, a need exists for an inboard/outboard engine having a tilt/trim mechanism that does not result in unwanted water spray. As always, there is a constant desire for less complex drive assemblies that are more efficient to manufacture and assemble, and are ultimately more reliable because of their simplicity. Finally, a need exists for an improved cooling and exhaust system for marine engines. 
     SUMMARY OF INVENTION 
     Set forth below are summaries of primary aspects of systems and methods configured and practiced according to the presently disclosed inventions. These features address one or more of the foregoing problems, and/or provide further benefits and advantages as will become evident from the included descriptions. 
     In at least one embodiment, one aspect of the presently disclosed inventions takes the form of a stern drive assembly that is configured for utilization in an inboard/outboard power plant for a boat. The stern drive assembly includes a central rigid core that is configured at an upper portion to be coupled to the stern of a carrying boat. A lower portion of the core is designed to accept a boat-moving force generated by a water propulsion unit that is coupled thereto. The term coupled shall be taken to mean a connection, but not necessarily a direct connection. That is, certain other components may be interstitially located, or connected between, those components that are specified as being coupled, or couplable, together. Water propulsion units take various forms; among others, single and dual rotating propellers are included, as well as marine jet propulsion systems. A thin-walled housing is configured to be secured about a predominance of the centrally located rigid core. In this context, the term predominance should be taken to mean greater than one-half or fifty percent. The housing has an outer surface that establishes an exterior of the stern drive assembly and an inner surface directed generally toward the central rigid core. A portion of an exterior surface of the central rigid core is configured to cooperate with a corresponding portion of the inner surface of the thin-walled housing. These two portions, when in cooperative orientation one with the other, form a functional feature for the stern drive assembly. 
     One example of such a functional feature is a fluid flow passage that is configured to carry fluids through the stern drive assembly during operation. A special characteristic of at least some of these flow passages is that a part of the passage curvaceously shaped for facilitating fluid flow therethrough. When the walls of such fluid flow passages are curved, as opposed to having sharp turns and corners, the resistance to movement of fluids passing therein is minimized. As specific examples, an individual fluid flow passage may exemplarily be configured as an exhaust channel or a coolant water channel, typically conveying fluids, both gases and liquids, to and/or from the powering engine of the boat. 
     The thin-walled housing is generally spaced apart from the central rigid core. In this way, a working space is formed between the two components. Preferably, the working space is provided to accommodate location of the functional feature(s), such as an exhaust channel, therein. In at least one example, the working space is maintained by one or more spacing ribs that are abuttingly positioned between the thin-walled housing and the central rigid core. Among the several spacing ribs, at least two are transversely oriented to at least one of the others. That is to say, these two exemplary ribs are not parallel to each other, but are instead positioned at some angle to one another. The angle need not necessarily measure ninety degrees, but does measure some other than zero degrees so that the thin-walled housing is fortified against flexure by the ribs&#39; inclusion. This transverse orientation of the ribs is often a natural consequence of their configuration to form a required functional feature. With respect to the thin-walled housing, the ribs are preferably oriented at approximate right angles to the housing&#39;s inner surface. 
     In a like manner, such functional features can be formed, at least partially, by similarly configured ribs that extend from the central rigid core. In one particularly preferred embodiment, the functional feature, such as an exhaust channel, is formed through a cooperation of ribs that extend from the thin-walled housing and ribs that extend from the central rigid core. 
     In a preferred construction, the thin-walled housing is formed from at least two clam-shelf style cowlings that are configured for mating engagement along perimeter portions of the shells. An advantageous material of construction is impact resistant, resin and fiber based composite. To promote structural integrity of the housing, the two clam-shell style cowlings are cemented together, and as a unit, form the exterior presentation of the stern drive assembly. Further support is provided to the housing when it is cemented to the central rigid core for substantially permanent fixation together. 
     In order to affect sufficient structural fortitude while maintaining a necessary degree of flexure, an average composite thickness ranging from about 2 mm to about 10 mm is preferred. In practice, an average composite thickness of about 3 mm has been found to strike an advantageous balance between the desired performance characteristics of durability which requires a certain degree of ductility, and rigidity which is needed for efficient force transmission. 
     To enhance the performance of the stern drive assembly, the central rigid core is unitarily constructed, preferably from aluminum, thereby forming a monolithic member that extends between the upper and lower portions of the core. 
     According to another aspect, this invention is directed to a drive assembly for propelling a boat through water by coupling an engine to at least one propeller. This drive assembly replaces the conventional lower unit and its support structure that have traditionally been used in outboard and inboard/outboard power plants. 
     Specifically, the drive assembly can include, in part, a drive frame extending aft from a transom of a boat with a first cowling covering substantially all of a first side of the drive structure and a second cowling covering substantially all of another side of the drive frame opposite the first side. The cowlings provide, in part, a hydrodynamically efficient exterior shape for the drive frame. The drive frame can include an upper chamber, a middle chamber, and a lower chamber. The upper chamber is sized to receive a clutch assembly having a plurality of gears for transferring rotational motion from a generally horizontal drive shaft coupled to an engine to a generally vertical drive shaft. The middle chamber receives the generally vertical drive shaft, and the lower chamber receives a combination of gears, bearings and a propeller shaft. 
     The exterior surface of the drive frame may include one or more ribs for attaching the cowlings to the drive frame and for supporting the cowlings. The internal surface of the cowlings can contain laterally extending members configured to mate with the ribs of the drive frame to provide a surface for attaching the cowlings to the drive frame and for supporting the cowlings. Additionally, the perimeter of the cowlings contain a cowling connection system that can exemplarily take the form of a tongue and groove system. For instance, one cowling can have a tongue positioned on the inner surface of its perimeter and the other cowling can have a groove capable of receiving the tongue that is positioned on its inner surface. Alternatively, the cowling connection system can include other connection devices discussed below. 
     The cowlings establish the outside surfaces of the drive frame and provide the drive frame with a more efficient hydrodynamic exterior surface. Additionally, the cowlings have semi-conduits formed on their internal surfaces that when mated with semi-conduits formed on the exterior surfaces of the drive frame, form completed conduits. These conduits are used, for example, to transport cooling water from the water intake ports located in the cowlings to an engine cooling system. Additionally, such conduits may be used to transport exhaust gases and cooling water from the engine through a plurality of exhaust ports. 
     In one embodiment, the cowlings can be formed from a composite material. Tests have proven resin and fiber based composite material to be extremely durable and capable of performing exceptionally well in this application. A drive assembly constructed with cowlings manufactured from this material is about 40 pounds lighter than a comparable conventionally designed drive assembly, which correlates to about a 20 percent decrease in weight. Alternatively, the cowlings can be formed from other suitable materials. 
     The drive system can be assembled using an adhesive, such as an epoxy, that is placed along the perimeter of the cowlings and on the laterally extending members that mate with the ribs. Preferably, the adhesive forms a permanent bond so that once the cowlings are attached to the drive frame and to each other, they cannot be separated. Thus, the cowlings and the drive frame form a monolithic structure. Alternatively, the cowlings can be attached to the drive frame using mechanical connectors such as screws, nuts and bolts. 
     The drive frame is pivotably coupled to a gimbal ring, thereby enabling the drive frame to rotate about a generally horizontal axis so that the trim and tilt position of the drive assembly can be adjusted. Further, the gimbal ring is pivotably mounted to a shield assembly that allows the gimbal ring, together with the drive frame and attached cowlings, to rotate about a generally vertical axis for steering purposes. In addition, a single trim cylinder is coupled to the drive frame at one end and to the gimbal ring at the other end. Importantly, the single trim cylinder attaches to the drive frame on the side of the drive frame facing the transom of the boat. In this configuration, the single trim cylinder does not come in contact with the moving water as the carrying boat is on plane. Therefore, no water spray is caused by the single trim cylinder. 
     An advantage of such a drive system is that the system allows for the exterior surface of the drive system to be composed of a non-metallic material. As a result, less metal surface area is exposed to salt water and, thus, there is less corrosion. 
     Yet another advantage of the new drive system is that it contains thirty percent fewer parts than a comparable conventionally designed drive assembly. This fact is due, at least in part, to the use of an adhesive to couple the assembly together rather than bolts, as used in conventional drive systems. There is also the elimination of the interface between an upper and lower unit. The formation of fluid channels such as raw water conduits and exhaust conduits on the external surface of the drive frame and the internal surfaces of the cowlings is also new. 
     Another advantage of this system is that the drive frame provides the necessary support for the forces generated by the propeller. While the cowlings provide some support to the drive frame, the majority of the structural support is provided by the drive frame. As a result, the cowlings can be designed without taking this type of structural support into account. Instead, the cowlings are designed to a specification for withstanding leading-edge impacts such as that suffered when an object floating in the water is struck by the submerged portion of the power unit as it slices through the water. Other design features of the cowlings include a minimization of hydrodynamic drag and flow resistance to cooling water and exhaust passing through conveyances for such fluids in the drive assembly. These several features are provided by the invention, all while enclosing and protecting the working parts of the drive assembly. 
     Yet another advantage of this system is that the raw water conduit formed by the cowlings increases water circulation through an engine water cooling system by about fifty percent. 
     Still another advantage of this system is that the exhaust conduit formed by the drive frame and the cowlings reduces the back pressure that is typically found in engine systems using conventional drive systems. 
     Another advantage of this system is that the raw water conduit and the exhaust conduit do not include spaces where eddies can form and salt can accumulate. 
     Still another advantage of this system is that the exterior shape of the cowlings results in ten percent less hydrodynamic drag than conventional drive assemblies. Further, the cowling generates vertical lifting forces that are translated to the drive assembly and to the boat, thereby enhancing the performance of the boat. 
     Another advantage of this invention is that the drive assembly includes a single hydraulic trim cylinder for trimming and tilting the engine. This reduces a manufacturer&#39;s warranty costs significantly and reduces the number of parts susceptible to failure. 
     Yet another advantage of this system is that the single hydraulic trim cylinder is coupled to the drive assembly so that it is not in contact with the moving water while the carrying boat is on plane. Therefore, the single hydraulic trim cylinder does not produce undesirable water spray that is typical of hydraulic trim cylinders used with convention inboard/outboard drive cylinders. 
     These and additional advantages will become evident to those skilled in the art from the drawings and detailed description that is provided herewith. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings, which are incorporated in, and form a part of the specification, illustrate preferred embodiments of the presently disclosed invention(s) and, together with the description, disclose the principles of the invention(s). The several illustrative figures include the following: 
     FIG. 1A is a side view of an exemplary marine drive system configured according to the present invention(s) installed upon a boat. 
     FIG. 1B is a perspective view of the marine drive system. 
     FIG. 2 is an exploded perspective view of the marine drive system. 
     FIG. 3 is a side view of a drive frame of the marine drive system. 
     FIG. 4 is a rear view of the drive frame. 
     FIG. 5 is a perspective view of an internal surface of a cowling. 
     FIG. 6 is a perspective view of an external surface of a cowling. 
     FIG. 7 is an elevational view of the internal surface of a cowling. 
     FIG. 8 is a cross-sectional view taken along section line A—A as shown in FIG.  1 B. 
     FIG. 9A is a perspective view of the marine drive system in a tilt-up position showing a single trim cylinder. 
     FIG. 9B is a perspective view of the marine drive system in a tilt-down position. 
     FIG. 10 is a perspective view of a shield assembly. 
     FIG. 11 is a perspective view of a gimbal ring assembly. 
     FIG. 12 is a perspective view of a single trim cylinder assembly. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 1A, a drive assembly  20  according to a preferred embodiment of the invention is configured to be installed upon a boat  23  proximate to the boat&#39;s  23  transom  27  and is coupled between an engine  21  that is mounted to a hull  29  of boat  23  and at least one propeller  25 . For the purposes of this invention, engine  21  can be a gasoline engine, a diesel engine, or other mechanical power generating engine. Further, engine  21  can be cooled using an air cooling system, a closed-loop water system, or an open-loop system using water taken from the body of water in which boat  23  floats. In addition, boat  23  is not limited to a particular size, model or application; the drive assembly  20  can be employed across a wide range of boats. Details of the exterior of the drive assembly  20  may be best appreciated in FIG.  1 B. 
     FIG. 2, an exploded view, shows the drive assembly  20  including, in part, a drive frame or central rigid core  22  and a thin-walled drive housing  31 . The drive housing  31  is further composed of a first cowling  24  that covers substantially all of a first side of the drive frame  22  and a second cowling  26  covering substantially all of a second side of the drive frame  22 . The first and second sides are located on opposite sides of drive frame  22 . Substantially all of one side shall be interpreted as meaning a predominance of one side of drive frame  22 , taking into account certain exceptions in the form of windows, apertures, and ports such as an aperture proximate to lower chamber  32  for accommodating propeller  25 , another aperture for connecting drive frame  22  to gimbal ring  64 , and an aperture for receiving cap  33 . 
     Generally, drive frame  22  provides the drive assembly  20  with a rigid frame or super structure capable of containing and positioning a plurality of gears, bearings and shafts for transferring power from an engine to a propeller, and at times a plurality of propellers, such as two counter-rotating propellers. Further, first and second cowlings  22 ,  24  provide drive assembly  20  with a hydrodynamic skin that can strengthen drive frame  22 . Additionally, the first and second cowlings  22 ,  24 , together with the drive frame  22 , provide the drive assembly  20  with a plurality of conduits or channels for transferring raw water, exhaust, and waste water between the engine  21  and the outside environment. 
     The drive frame  22  supports drive shafts that are used to transfer power from engine  21  to at least one propeller  25 . Additionally, drive frame  22  is mechanically coupled between propeller  25  and boat  23  for transferring forces generated by the propeller  25  to boat  23 . More specifically, drive frame  22  acts as a moment arm when receiving forces generated by propeller  25 . Further, drive frame  22  is sized and constructed of materials that provide rigidity and allow the frame  22  to translate the forces generated by propeller  25  to the boat  23  without any substantial deflection. For instance, drive frame  22  is preferably constructed of aluminum. Drive frame  22 , however, can be composed of other materials including, but not limited to, stainless steel, titanium, brass, composites and similarly performing materials. Aluminum is preferred for among other qualities, it rigidity, light weight character, and anticorrosive behavior. 
     When drive frame  22  is coupled within the drive housing  31  as described in more detail below, drive frame  22  accepts substantially all of the forces generated by the propeller or caused by the propeller and conveys them to the carrying boat  23 . The drive housing  31  provides some structural integrity to the drive assembly  20 , however, drive housing  31  does not translate any significant forces generated by the propeller to the boat. Instead, this is done substantially alone by the drive frame  22 . 
     Drive frame  22  can have various designs. For instance, one embodiment of drive frame  22  includes an upper chamber  28 , a middle chamber  30 , and a lower chamber  32 . Upper chamber  26  is sized to receive and house a plurality of gears and a clutch assembly for transferring torque from an engine drive shaft  34  coupled to engine  21  to an upright drive shaft  38  that is capable of being positioned within the middle chamber  30 . A clutch assembly suitable for use in the present invention is disclosed and described in U.S. Pat. No. 4,397,198, that is hereby expressly incorporated by reference in its entirety. Further, other suitable clutch assemblies can be used without deviating from the scope of the present application. 
     Middle chamber  30  includes a plurality of bearings that position the upright drive shaft  38  within an interior space of the middle chamber  30 , and correctly position the upright drive shaft  38  in relation to engine drive shaft  34  and the propeller drive shaft  40 . The lower chamber  32  houses a plurality of bearings that position propeller drive shaft  40 . Additionally, lower chamber  32  houses gears capable of rotating propeller drive shaft  40  clockwise or counterclockwise for forward or reverse operation. Lower chamber  32  can be sized to accommodate a single propeller, or a dual propeller configuration that includes counter rotating propellers as is known in the art. 
     Drive frame  22  is preferably a unitary piece and as previously indicated, constructed from aluminum. Drive frame  22 , however, can be composed of any number of pieces. Further, it should be understood by those skilled in the art that drive frame  22  can be further compartmentalized, and drive frame  22  can comprise fewer than the three chambers described above. 
     As can best be seen in FIGS. 3 and 4, drive frame  22  preferably includes a plurality of ribs  41  protruding generally laterally from the exterior surface of the body of the drive frame  22 . Ribs  41  can be formed on drive frame  22 , for example, during the frame&#39;s  22  manufacture. Among other purposes, ribs  41  are configured to fortify the drive frame  22  against loads experienced as the boat  23  moves through a body of water and as the drive frame  22  is pivotally adjusted in the water while the boat is under power. In addition to providing structural stiffness, ribs  41  may act as a tongue to matingly engage corresponding laterally extending members  43  formed on cowlings  24 ,  26  as described in greater detail below. Alternatively, ribs  41  can include a groove for engaging corresponding laterally extending members  43  from the cowlings  24 ,  26 . 
     Drive assembly  20  also preferably includes at least one exhaust inlet  42 . In the illustrated embodiment, an exhaust inlet  42  is formed on each side of the drive frame  22 . Exhaust inlets  42  are formed as apertures or channels that allow exhaust gasses and waste cooling water to flow from engine  21  and out of the drive assembly  20  through exhaust port  44  and the propeller  25  as is illustrated by the dashed-arrowed line in FIG.  7 . Each exhaust inlet  42  is arranged in fluid communication with an exhaust semi-conduit  46 . An exhaust semi-conduit  46  is located on an exterior surface at each side of the drive frame  22  and is configured to cooperate with correspondingly formed semi-conduits on first or second cowling  24 ,  26  to form an exhaust passage. Ultimately, each passage terminates in an exhaust port  44 . Exhaust port  44  provides an exit for the exhaust gases produced by the engine  36 . 
     Drive frame  22  can also include a shifting rod chamber that is capable of receiving a shifting rod for actuating shiftable gears located in the lower chamber  32 . Further, drive frame  22  can include at least one conduit system for transferring oil from lower chamber  32  to a filter located on the boat  23 , and back to lower chamber  32 . This system enables the oil to be monitored for water contamination or for the presence of metal shavings. Both of these conditions are indicators of potentially serious and life limiting problems for a marine engine. 
     Drive frame  22  further includes mounting arms  47  configured to be coupled to a mounting assembly on a transom  27  of boat  23 . Mounting arms  47  can take the form of posts adapted to be received within receiving bores on the transom  27 . Alternatively, mounting arms  47  can include bores from receiving insert posts that form an axle for pivotation as shown in FIG.  3 . FIG. 1B shows this pivot connection  87  as it appears from the exterior of the drive assembly  20 . 
     Referring again to the drive assembly  20 , which in one embodiment includes two cowlings. As described before, first cowling  24  and second cowling  26  provide drive assembly  20  with a hydrodynamic skin surface. Specifically, once constructed, the drive housing  31  has an exterior surface that contacts ambient water in which the boat  23  floats. Generally, first cowling  24  covers the first side of the drive frame  22  and second cowling  26  covers the second side of the drive frame  22 . As illustrated in FIGS. 5-7, first cowling  24  is a near mirror image of second cowling  26 . However, this invention in not restricted to the cowlings  24 ,  26  being even near mirror images of each other. For instance, first cowling  24  could cover substantially all of a first side of drive frame  22  and could form a substantial entirety of skeg  48 . In such an embodiment, second cowling  26  could cover a substantial portion of a second side of drive frame  22 , but not form skeg  48 . Rather, second cowling  26  could form an intersection with first cowling  24  at a location proximate to skeg  48 . In yet another embodiment, cowlings  24 ,  26  could join each other on the side of drive frame  22  rather than at front edge  58  and rear edge  59 . FIG. 2 shows an asymmetric aspect in the form of a window  35  that is provided in only one of the cowlings  26  that permits access to a shift mechanism for maintenance and repair purposes. It should be appreciated, however, that other designs incorporating the spirit of the invention will be obvious to one of ordinary skill in the art and are not detailed herein, such as drive housing  31  having more than two sections. 
     For the sake of brevity, first cowling  24  as shown in FIGS. 5 and 6 will be discussed in detail with the understanding that second cowling  26 , as illustrated at least in part in FIG. 7, is a substantial mirror image of first cowling  24 , but will typically have select different and custom features. As described briefly above, first cowling  24  substantially covers an entire side of drive frame  22  and forms an exterior shell resembling conventional upper and lower units of a drive assembly. First cowling  24  includes skeg portion  48 , raw water pickup ports  52 , raw water semi-conduit  54 , and exhaust semi-conduit  56 . Raw water pickup ports  52  are positioned near front edge  58  of the first cowling  24  for receiving water for the engine cooling system. Alternatively, the ports  52  may be advantageously located on the leading edge of the assembly  20 . Ports  52  are configured to receive water at both low and high speeds. 
     Semi-conduits are essentially half of a conduit that are formed at the internal surface of first cowling  24 . Semi-conduits can also be referred to as recessed surfaces. Further, a semi-conduit can form more than or less than half of a conduit. These semi-conduit portions  54 ,  56  are positioned and designed to at least cooperate, and preferably mate with semi-conduit portions formed on the external surface of drive frame  22 . Specifically, once first cowling  24  is properly positioned upon drive frame  22 , respective semi-conduits form a complete conduit, thereby eliminating the need for other components, such as hoses, to form such conduits. As a result, the number of parts needed to construct drive assembly  20  is reduced, thereby simplifying the design and increasing the durability and reliability of the drive assembly. Further, the reduction in the number of parts also reduces the cost of production. 
     FIG. 6 shows first cowling  24  further includes a portion for forming half of an anti-cavitation plate  60 . The entire anti-cavitation plate  60  is formed with the combination of first cowling  24  and second cowling  26 . Anti-cavitation plate  60  begins at front edge  58  of first cowling  24  and extends aft, terminating at and forming exhaust port  44 . Exhaust port  44  is placed in fluid communication with an exhaust inlet  42  through an exhaust passage formed by the semi-conduits of one of the cowling  24 ,  26  and the drive frame  22 . 
     First cowling  24  and second cowling  26  are preferably formed from a composite material, which is a combination of two or more materials that has properties that the constituent materials typically do not have by themselves. It is preferred that the composite material exhibit characteristics of toughness, stiffness, and be suitable for use in applications subject to impacts and rough handling. In a preferred embodiment, the composite is a vinyl ester-based sheet-molding compound. The composite preferably comprises a glass fiber content in the range of less than about seventy percent. In a particularly preferred embodiment, the glass fiber content is about sixty-three percent. The glass fibers are preferably about twenty to thirty millimeters long. In a particularly preferred embodiment, the glass fibers are about 25 millimeters long. 
     The composite material employed in the preferred embodiment of the present invention offers several advantages over conventionally employed metals. For example, the composite material has a higher tensile strength than aluminum that is normally used to form conventional lower units (344 Mpa vs. 250 Mpa) and a lower density (1.9 specific gravity vs. 2.7 specific gravity for aluminum). Such a reduction in density offers a substantial reduction in the overall weight of the drive assembly  20 . In an exemplary application, a weight reduction of approximately forty pounds has been achieved over a comparable conventionally construction drive assembly. This weight reduction is about twenty percent of the weight of drive assembly  20 . Moreover, the preferred composite material has a lower modulus of elasticity, which means that for a given stress, the composite material will flex more than aluminum, steel, or other metals. Consequently, the lower modulus of elasticity provides increased resistance to impacts. 
     This increased resistance to impacts is appreciated especially when the cowlings  24 ,  26  withstand repeated blows to front edge  58 . Specifically, drive assembly  20  has been subjected to a log test that entails driving a boat using a drive assembly  20  for propulsion across a submerged wooden telephone pole having a diameter of about twelve to sixteen inches. In the test, the telephone pole is floating in the water and generally perpendicular to the boat&#39;s  23  heading. The boat  23  was driven at speeds of twenty miles per hour, thirty-five miles per hour, and forty-five miles per hour. As the boat crossed the log, the front edge  58  of cowlings  24 ,  26  collided with the telephone pole. The drive assembly  20  was subjected to multiple collisions with such a telephone pole at each of the speeds identified above. After each collision, drive assembly  20  remained in working condition. Further, drive assembly  20  was closely inspected after each collision and showed no visual signs of performance-compromising damage, such as cracks or stress marks. Instead, only cosmetic damage occurred to the outside surface of cowlings  24 ,  26  at the local region surrounding the impact point(s) on front edge  58 . 
     The composite material is preferably formed from composite sheets that are, for example, about three millimeters thick and tacky to the touch. Suitable sheets having thicknesses ranging from about two to ten millimeters may be utilized without drastically departing from the spirit of the invention. A molding assembly is used to cut the composite sheets into desired lengths. These lengths of composite material are then placed into a mold which is referred to as the charge pattern. The mold is then subjected to pressure and heat until the cowling is properly molded. The composite is then allowed to cure and set. Using this method, first and second cowlings can be pre-formed with the semi-conduits described above. 
     As may be appreciated in FIGS. 2,  5  and  7 , this composite material allows a plurality of laterally extending members  43  to be formed on cowlings  24  and  26  in a cost-effective manner compared to conventional metal housings or cover portions. Further, the composite material and the design of cowlings  24  and  26  allow raw water intake semi-conduit  54  and exhaust semi-conduit  56  to be formed in the most efficient configuration possible. Specifically, sharp corners that are found in other stern drive assemblies, which provide a haven for salt to accumulate and significantly reduce the life of an stern drive, are eliminated. Furthermore, the configuration of the semi-conduits  54  and  56  reduces the friction flow loss associated with less efficient systems having abrupt paths for containing raw cooling water and exhaust gases and water. In an alternative embodiment, cowlings  24 ,  26  can be composed of composite materials other than the composite material described above. Further, cowlings  24 ,  26  can be made of materials such as, but not limited to, aluminum, titanium, brass, or galvanized steel. 
     Cowlings  24 ,  26  can include a cowling connection system for connecting cowlings  24 ,  26  to each other. Preferably, the cowling connection system includes a tongue and groove system for locking the cowlings  24 ,  26  together. Specifically, first cowling  24  can include a groove  50  for receiving a tongue  51 , and second cowling  26  can include tongue  51  capable of fitting within the groove  50 , or vice versa. This tongue and groove system can be located at the perimeter  53  of the cowlings  24 ,  26  and provides cowlings  24 ,  26  with a reinforced connection means. The tongue and groove system can be composed of a single groove located at the perimeter  53 , or it can include multiple grooves  50  for receiving tongues  51 . Further, grooves  50  can have rounded cross-section, as shown in FIG. 8, or the cross-section can be in the shape of a rectangle, a triangle, a trapezoid, or any other configuration allowing first cowling  24  to mate with second cowling  26 . Only one tongue and groove is illustrated; it should be appreciated, however, that multiple tongues and mating grooves may be employed without departing from the spirit of this aspect of the invention. 
     In another embodiment, the cowling connection system can be composed of a plurality of snap fittings. More particularly, the perimeter  53  of the inside surfaces of cowlings  24 ,  26  can include snap fittings forming an L orj shape that lock together after being pressed against each other. 
     The cowling connection system, as described above, can also be used for mating drive frame  22  to cowlings  24 ,  26 . Specifically, laterally extending members  43  on cowlings  24 ,  26  can each have a groove  50  for receiving tongues  51  formed by ribs  41  on drive frame  22 , or vice versa. It is preferable that the tongues  51  and grooves  50  be sized to accommodate a layer of adhesive between each. Alternatively, laterally extending members  43  and ribs  41  can include a snap fitting connection system as described above or any other suitable connection system. 
     In a preferred embodiment, drive frame  22  and cowlings  24 ,  26  are bonded together using an adhesive, such as epoxy or other suitable material. This epoxy can be used to bond together tongues  51  and grooves  50  formed in drive frame  22  and grooves  50  formed in cowlings  24 ,  26 . Preferably, the adhesive produces a permanent bond between first cowling  24  and second cowling  26  and between drive frame  22  and cowlings  24 ,  26 . In this embodiment, cowlings  24 , and  26  are not readily removed from drive assembly  20  after being assembled. Rather, a monolithic structure is formed once cowlings  24 ,  26  have been attached to drive frame  22  and to each other. 
     These bonds and the addition of the adhesive increase the overall structural integrity of the drive frame  22 . In addition, by bonding cowlings  24 ,  26  to drive frame  22 , cowlings  24 ,  26  are able to transmit hydrodynamic steering loads to drive frame  22  and gimbal ring  64 , which increases the overall efficiency of the system. It is also preferred that first and second cowlings  24 ,  26  are also bonded together using an adhesive applied to the perimeter  53  of each cover portion. If needed, bolts or other suitable mechanisms could be used in addition to the adhesive but are not necessary. Alternatively, the adhesive could be substituted with mechanical fasteners, such as, but not limited to, screws, nuts and bolts. 
     Bonding cowlings  24 ,  26  together and to drive frame  22  provides significant manufacturing advantageous. For instance, it is no longer necessary to use bolts to couple a lower unit to an upper unit. Instead, cowlings  24 ,  26  can be quickly coupled to each other and to drive frame  22  using an adhesive, as described above. Further, the bonding process can be completed with robotic machines, rather than human labor. As a result, significant reductions in labor costs are realized by using the bonding process. In addition, the need to pressure test an upper unit, a lower unit, and the seal between each has been eliminated because the upper and lower units have been eliminated. Instead, only a single test is need to test the drive assembly after cowlings  24 ,  26  have been coupled to drive frame  22 . 
     FIG. 1B shows that drive frame  22  is pivotably coupled to a gimbal ring  64  at axis  87 . Pivotation of drive frame  22  is controlled by a single hydraulic trim cylinder  78 , as described in detail below and pivotably connected upon axle  85  through first aperture  74  and at second aperture  76  on ears connected to the gimbal ring  64 . Gimbal ring  64  is pivotably mounted to a shield assembly  62  that allows the gimbal ring  64 , together with drive frame  22  and attached cowlings  24 ,  26 , to rotate about a generally vertical axis for steering purposes. Shield assembly  62  can be coupled to transom  27  of a boat  23  and typically provides at least water tight seals for the engine drive shaft  34  and an exhaust conduit  65  across the transom  27 . Further, shield assembly  62  can be made of the composite material described above. However, shield assembly  62  can be composed of materials including, but not limited to aluminum, stainless steel, titanium and other suitable composite materials. 
     Referring to FIG. 10, shield assembly  62  also includes an exhaust water outlet  67  that is preferably in fluid flow communication with exhaust pipe  66 . As is well known in the art, the exhaust pipe  66  extends from engine  21  in fluid flow communication with the engine cooling system so that both exhaust gases and exhaust cooling water flow together through exhaust pipe  66 . As the cooling water flows through exhaust pipe  66  and enters exhaust passage  42 , a majority of the cooling water is diverted through exhaust water outlet  67 . The exhaust gases and any remaining exhaust cooling water are passed through exhaust inlet  42  and thereafter through the exhaust passages formed between cowlings  24 ,  26  and drive frame  22  that ultimately discharge through an exhaust port  44  and/or a propeller assembly  25 . Exhaust water outlet  67  opens either to the atmosphere or under the water in which the boat  23  is floating. In this fashion, the spent cooling water can be returned to the body of water in a simple and inexpensive manner. 
     Gimbal ring  64  is coupled to shield assembly  62  for pivotal movement about axis A as represented in FIG. 10 so that the drive assembly  20  can be rotated to turn the boat  23 . Gimbal ring  64  can be coupled to an upper portion  68  of shield assembly  62  via a steering shaft  70  and key  71  as illustrated in FIG.  11 . In addition, gimbal ring  64  can be coupled to a lower portion  72  of shield assembly  62  using a pin or other suitable mechanisms. Gimbal ring  64  can be formed of the composite material described above. Alternatively, gimbal ring  64  can be composed of materials including, but not limited to aluminum, stainless steel, titanium and other composite materials. 
     Shield assembly  62  and gimbal ring  64  each contain a centrally located opening. These openings are sized to receive and provide passage for at least a drive shaft, an exhaust system, a raw cooling water system, and a shifting rod. Further, both shield assembly  62  and gimbal ring  64  can provided passage for other parts of drive assembly  22 . 
     As shown in FIGS. 9A and 9B, an hydraulic trim assembly  78  exemplarily illustrated in FIG. 12 controls the pivot position of drive frame  22  and cowlings  24 ,  26 . Specifically, hydraulic trim cylinder  78  enables drive frame  22  and cowlings  24 ,  26  to be pivoted for trim and tilt purposes through a range of about minus five degrees, as represented in FIG. 9B, to about plus sixty degrees, as represented in FIG. 9A, relative to a vertical axis. In addition, the lower limit of minus five degrees can vary, either up or down, without departing from the scope of this invention. Likewise, the upper limit of sixty degrees can vary, either up or down, without departing from the scope of this invention. 
     Now referring to FIG. 12, hydraulic trim assembly  78  includes a cylinder  80  having a bore  82 , a piston (not shown) slideably housed in cylinder  80  and a piston rod  83  having a first end coupled to the piston and a second opposite or drive end  84  extending from the cylinder  80  and pivotably coupled to drive frame  22 . For instance, the drive end  84  of piston rod  82  can be pivotably coupled to drive frame  22  via a connecting member, such as a pin, dowel, or other suitable mechanism extending through both the opposite end of piston rod  82  and drive frame  22 . Drive frame  22  preferably has defined therein a bore for also accepting such a connecting member. In this configuration, the tilt and trim of drive assembly  20  can be controlled from the helm of boat  23 . For instance, hydraulic trim assembly  78  can be actuated using a remote control unit typically positioned near the steering wheel of boat  23  and composed of a toggle switch. Actuating hydraulic trim assembly  78  either causes piston rod  82  to run in or out, depending on which direction the toggle switch is depressed. While the piston rod runs in or out, drive frame  22  pivots about a generally horizontal axis formed by the axle  87 . This rotation is generally referred to as adjusting the tilt or the trim of the drive frame  22 . As discussed above, trim refers to the rotational range of between about minus five degrees and about plus fifteen degrees relative to a vertical axis that a drive assembly can move through, and tilt refers to the rotational range of between about plus fifteen degrees and about plus sixty degrees relative to a vertical axis. Adjusting the trim angle changes the performance of the boat by changing, among other things, the height of the bow while the boat is on plane and the amount of cavitation present at propeller  25 . Running piston rod  82  all the way out is referred to as a tilt-up position as shown in FIG.  9 A and running the piston rod  82  all the way into the cylinder  80  is referred to as a tilt-down position as represented in FIG.  9 B. Generally, the tilt-down position is used while running the boat  23 . Further, the trim-up position is generally used when the boat is in shallow water or the bow of the boat is relatively heavy. The tilt-up position is most typically used for trailering the boat. The single hydraulic trim assembly  78  can move drive frame  22  through both the tilt and trim ranges discussed above, and the single trim assembly  78  preferably moves drive frame through this range at the same rate of speed. In a preferred embodiment of the present invention, a single hydraulic trim assembly  78  is employed. The single hydraulic trim assembly  78  dictates the use of dual exhaust passages  42 , which in a preferred embodiment requires a substantially Y-shaped manifold ahead of the cylinder  80 . The exhaust system is composed of exhaust pipe  66  that is coupled to engine  21 . Exhaust pipe  66  receives both exhaust water and gases. Exhaust pipe  66  is coupled to shield assembly  62 . At shield assembly  62 , the exhaust system is split into two parallel exhaust conduits  39  using a Y-manifold or adapter. Flexible hoses make up the exhaust conduits  39  between shield assembly  62  and drive frame  22 . Each exhaust conduit  39  is fluidly connected to an exhaust passage extending through drive frame  22  and ultimately form the exhaust passage that is in fluid communication with exhaust port  44 . The Y-manifold or connecter diverts the exhaust around single hydraulic trim assembly  78 . Additionally, this exhaust configuration presents less resistance to fluid flow than conventionally plumbed exhaust systems. Thus, less back pressure develops and engine  21  is able to run more efficiently. 
     In addition, it is preferable that hydraulic trim assembly  78  be coupled to drive frame  22  at a point substantially midway along the length of drive frame  22  for reduced stress on the parts, to provide a greater moment arm for lifting the drive frame  22 , and to alleviate the need for using two hydraulic trim cylinders. It should be apparent to one having ordinary skill in the art, however, that hydraulic trim assembly  78  may be mounted at any point along the length of drive frame  22 . Further, hydraulic trim assembly  78 is mounted in its substantial entirety within gimbal ring  64 , thereby eliminating the water spray conventionally caused by mounting hydraulic trim assemblies outside of gimbal ring  64 . 
     As may be appreciated from FIG. 9A, the hydraulic trim assembly can be coupled between the drive frame  22  and the gimbal ring  64 . Further, in this configuration, hydraulic trim assembly  78  is at least partially covered by gimbal ring  64  and shield assembly  62 , as may be appreciated from FIG.  9 B. In addition, hydraulic trim assembly  78  is at least partially covered by drive frame  22  and drive housing  31 ″s two cowlings  24 ,  26 . As a result, hydraulic trim assembly  78  is shielded, at least partially from spray water during use. Specifically, while the hydraulic trim cylinder  78  can be exposed to ambient water while boat  23  is at rest or is off plane, hydraulic trim cylinder  78  is not in contact with ambient water as the boat  23  is running on plane. This, increases the life of hydraulic trim cylinder  78  and reduces the likelihood of failure. Further, maintenance of the hydraulic trim cylinder  78  is not required as often, thus, producing a cost savings for a boat owner. 
     In addition, as represented in FIG. 9A, hydraulic trim assembly  78  is positioned at a centerline of drive frame  22  and within a plane also containing the vertical axis about which gimbal ring  64  rotates. This axis is identified as axis A as shown in FIG.  10 . Further, hydraulic trim assembly  78  passes through axis A. Positioning hydraulic trim assembly  78  in this fashion reduces stress on assembly  78 , drive frame  22  and gimbal ring  64 . 
     Drive system  20  transfers torque or momentum between engine  21  and propeller  25 . Further, drive system  20  can change the direction of propeller  25  for steering boat  23  and can change the angular position of the propeller  25  relative to a vertical axis. As described in detail above and depicted in FIG. 1, drive system  20  can advantageously be coupled with an inboard/outboard engine. However, the invention is not limited to use only with an inboard/outboard engine. Rather, drive system  20  can be used in cooperation with an outboard engine as well. It is recognized that some modification to the drive system  20  would be required to couple it with an outboard engine; however, the spirit of the drive frame  22  and cowlings  24 ,  26  combination invention would remain substantially unchanged. 
     For example, an outboard engine could receive a drive frame  22  that is adapted to couple directly to the power head of an outboard engine or couple to a support member that is proximate to the power head. In this configuration, drive frame  22  would resemble the drive frame portrayed in FIGS. 3 and 4; however, upper chamber  28  would be removed because drive assembly would accept a generally vertical engine drive shaft, rather than a generally horizontal drive shaft from an inboard/outboard engine. Further, drive frame  22  would be capable of receiving a generally vertical drive shaft and include a lower chamber  32  for containing the gears, bearings and propeller drive shaft necessary to convert the rotational motion of the generally vertical drive shaft to a propeller drive shaft positioned generally perpendicular to the vertical drive shaft. Also, cowlings  24 ,  26  could maintain the same general exterior shape as the cowlings depicted in FIGS. 5 and 6; however, top portions of cowlings  24 ,  26  would have to be configured to be received by an engine cowling covering the outboard engine. Also, raw water semi-conduit  54  and exhaust semi-conduit  56  would function in a similar manner as described above, except that their route would differ in order to connect to an outboard engine that is positioned above drive assembly  20  rather than to its side, as is the case with an inboard/outboard engine. 
     The system  20  described herein includes numerous advantages when compared with conventional systems. For instance, the design of the cowlings  24  and  26  and the drive frame  22  allow for cowlings  24 ,  26  to be formed of a composite material that is not susceptible to corrosion. Further, the system  20  includes fewer parts than conventional systems, thereby yielding a more reliable and efficient system. In addition, the system  20  is lighter than conventional system which results in better fuel economy. The cowlings  24  and  26  also have less hydrodynamic drag than conventional systems. Moreover, the system  20  is more maneuverable than previous systems. The curvaceous design of the passages in the assembly  20  promote fluid flow by their avoidance of sharp turns and dead-space corners. This design is not only enabled by the clam-shell design of the cowlings  24 , 26  about the central frame member  22 , but also by the elimination of the traditional die-cast housings that have composed the lower and upper end units of stern drive systems. Through the molding process enabled by the use of composites, curvaceous channels are easily formed merely by the provision of such features as channel-defining ribs or vanes that establish walls, or parts of walls that delimit such channels. Preferably, the cowling features cooperate with corresponding features on the core member to form such functional features as exhaust and cooling water paths or passages. As but one example, because of the freedom these configurations allow, the exhaust system of the present invention is more efficient than conventional designs because the amount of back pressure generated within the system while it is operating is minimized. Additionally, the drive assembly  20  produces less unwanted water spray than was caused in conventional systems utilizing two hydraulic trim cylinders, each located along side the stern drive assembly. 
     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention or the following claims.