A certain type of marine propulsion system mounts two counter-rotating propellers inline to rotate about a common axis of rotation. Marine propulsion systems of this kind can be used in stern drive applications, where the engine is enclosed within the hull of a boat and the propellers are mounted on a unit attached to the transom and driven by a driveshaft extending through the transom. Alternatively, an outboard motor can be provided with dual counter-rotating propellers.
The torpedo -- called the "locomotive torpedo" because it proceeded under its own power and did not have to be towed like earlier models -- made its appearance in 1866. The first models, developed by the Austrians, had a range of 370 yards at 6 knots, and packed an 18-pound explosive warhead. By 1877, the contra-rotating propeller was fitted to a torpedo, an innovation that kept the torpedo steady on course. Soon the torpedo was fitted with a horizontal rudder to keep it at constant depth as it ran to its target. By 1895, the invention of the gyroscope improved the torpedo's accuracy.
In certain applications of dual propeller systems, it is beneficial to make the counter-rotating propellers rotate at different speeds. Some of the advantages that result from having the two propellers rotate at different speeds relate to improvements in the acceleration, top speed, and performance capability of the drive system. In addition, the two propellers can be provided with different pitches for improved maneuverability during at low speed docking procedures when the two propellers are rotated at different speeds.
It has long been recognized that contra-rotating propeller systems have the advantage of eliminating the turning effect or rolling effect of torque produced by the action of a single propeller. If a single propeller is employed, for example, the craft rudder must be used to compensate for the propeller-produced torque. The rudder thus introduces a drag factor which reduces the propulsion efficiency of the engine. This problem becomes more important in high speed water craft wherein the torque produced by a propeller is greater and hence the compensatory effect of the rudder must be greater. Moreover in rough water at high speeds the submersion level of the craft is variable so that the torque balancing effect of a rudder is irregular and can cause oscillatory rolling of the hull. Contra-rotating propellers permit balancing of the torque produced by each propeller, together with the effect of flow past the propeller support struts, and permit attainment of much higher propulsion efficiency factors.
Unfortunately, prior art contra-rotating propulsion systems have suffered from practical problems which have restricted the utilization of these systems, particularly for high speed water craft. One such problem has been mechanical complexity of the propeller drive system, the complexity leading to unacceptable mechanical failure rates.
Another problem with prior art contra-rotating propellers relates to the fact that torque balance is achieved only over a small range of relative propeller speeds. Most contra-rotating propellers are driven by the same engine which limits the propeller speed ratios attainable and thereby limits the flexibility of the propulsion system. In some cases the contra-rotating propellers are driven independently by separate engines. This permits torque balancing over a wide variety of conditions; however, this arrangement introduces other limiting factors.
If the contra-rotating propellers are mounted on concentric drive shafts, this arrangement provides a larger wetted surface than a single shaft and therefore produces additional drag. Moreover, this requires that the engines be disposed along opposite sides of the concentric drive shafts, thereby requiring a wider hull than is optimum for high speed craft. Specifically, it is known that the ability of a hull to pass at high speeds over rough water with minimum impact force is closely related to the slenderness ratio of the hull. If machinery considerations dictate hull width, optimum hull design for high speed operation must suffer. Designs that drive the concentric propeller shafts via a common gear box, a single failure in either the gear box or the shaft structure can disable both propellers.
Still another problem associated with prior art contra-rotating propeller systems relates to the fixed physical orientation between the two propellers. That is, whether driven by concentric drive shafts or positioned side-by-side, no prior art contra-rotating propellers are movable relative to one another and to the craft hull. This limitation eliminates a dynamic steering capability wherein a propeller would be pivotable to produce a controllable and dynamic steering force on the craft. In addition, the fixed orientation permits no adjustment of the thrust axis direction which is a valuable capability in craft with planing-type hulls. Moreover, the fixed orientation prevents simple removal of one propeller for low speed operation so that the non-operating propeller does not remain in the water to present unwanted drag.
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