1. Field of the Invention
The present invention pertains to an improved marine propulsion system. In particular, the present invention pertains to improvements to a marine propulsion system of the type comprising a surface propeller, i.e., a propeller which is designed to be only partially submerged during operation so as to reduce the drag associated with the propeller. The propulsion system is a tractor-type system whereby the propeller is positioned ahead of the drive transmission housing so as to reduce the disturbances to the water upstream of the propeller, thereby improving the efficiency of the propeller. In addition, the present invention is designed with parallel drive and propeller shafts so that spur gears can be employed in the drive transmission, thereby eliminating unnecessary thrust bearings and improving the efficiency of the transmission gear train. As a further advantage of the present invention, the propulsion system is configured to enable easy retrofit of existing boat hulls and also the propeller is protected by the system.
2. Description of the Related Art
Tractor marine propulsion systems have long been known in the art. For instance, Schmitt U.S. Pat. No. 1,605,376 discloses a prior art tractor drive propulsion system. This system comprises an engine mounted substantially forward of the boat transom and a long drive shaft extending aft from the engine through the bottom of the hull to a gearbox mounted below the transom. A propeller shaft extends forward from the gearbox at an angle relative to the drive shaft and a propeller is attached to the propeller shaft so as to be totally submerged beneath the hull. The drive shaft in this system is angled relative to the propeller shaft, thus the gearbox includes bevel gears. Due to the conical shape of the bevel gears, axial forces are produced which must be compensated for by thrust bearings or excessive wear and gear train inefficiencies occur. In addition, much of the Schmitt propulsion system is beneath the water. Thus, the brackets, shafts and gearbox increase the drag on the boat moving through the water over that of the hull alone. In addition, the shafts are exposed along much of their length, thereby suscepting them to damage from debris.
Baldwin U.S. Pat. No. 3,105,455 also discloses a tractor drive propulsion system. The drive shaft and gearbox are mounted within the boat hull, thereby reducing drag and the potential for damage. In an alternate embodiment, Baldwin discloses a system having the motor substantially aft in the hull and the drive shaft extending forward to a universal Joint connected to the propeller shaft. The drive shaft and propeller shaft are angled relative to each other and the universal joint is needed to connect the two shafts. In both embodiments the propeller is completely submerged.
Leavitt et al. U.S. Pat. No. 2,979,019 eliminated the angle between the tractor system drive shaft and propeller shaft by incorporating a belt and pulley transmission. However, the size of the housing below the waterline is enlarged to accommodate the pulley mounted to the propeller shaft, and thus the frontal area of the underwater structure is increased. This increase in frontal area increases the drag associated with the system and therefore decreases the overall efficiency. In addition, belt drive transmissions inherently cannot transmit as much power as the gearbox transmissions without slipping.
All the aforementioned prior art propulsion systems have relatively large frontal and surface areas below the waterline. Thus, the drag is increased and the propulsion system efficiency decreased. Arena U.S. Pat. No. 4,443,202 substantially reduces drag by employing a surface propeller propulsion system. The Arena system is of the pusher-type, i.e., the propeller is mounted aft of its drive shaft. The surface drive propeller is more efficient because less of the propeller drive system housing is submerged and therefore the drag associated with the system is decreased. However, a pusher-type system is often less efficient than a tractor system because of the disturbances created in the flow over the propeller by the drive transmission housing positioned forward of the propeller. In addition, the Arena system requires a significant cowling to protect personnel from the prop when entering and exiting the boat over the transom, as well as to shield over the rooster tail spray inherent in surface drive systems.