1. Field of Invention
The present invention relates generally to water jet propulsion systems for ships and, more particularly, to a hydrodynamically designed, integrated hull and water jet propulsion system including an inlet duct having a flush inlet and a steep inlet duct inclination angle, a water jet pump having its impeller very near the flush inlet, and a pump drive system with a short drive shaft situated so as not to interfere with flow into the impeller.
2. Brief Description of Related Art
In recent years, marine water jet propulsion has gained acceptance and has begun to challenge the long established dominance of screw propellers. Water jet propulsion offers many advantages over conventional screw propellers including: simplification of mechanical arrangement by eliminating reversing gears, controllable pitch propellers, and long propulsion shafting and associated shaft bearings; flexibility of machinery arrangements and placement of machinery in the hull; improved propulsion plant reliability since the prime mover need not be reversed during maneuvering or backing; elimination of external rudders, shafting, and propellers; improved maneuverability, including ability to turn at zero forward speed; minimized draft allowing improved shallow water operation; and reduced noise.
However, many disadvantages are well known, a significant disadvantage being low propulsive efficiency at speeds less than about 25 knots as shown in FIG. 1. As a consequence, existing water jets have been principally applied to high speed vessels such as planing and semi-planing hulls, hydrofoils, and surface effect ships where operational ranges are generally between 35 to 70 knots. Such water jet designs suffer from poor performance at off design speeds. A further disadvantage of present water jet designs is the method of designing and locating the water jet inlet with respect to the hull. Water jet inlet ducts operate under very complex three-dimensional flow conditions. Consequently, efficiency and cavitation performance of water jets is very dependent on good design of the water intake system. However, prior water jet inlet design methods have been restricted to simple two-dimensional momentum theory and two-dimensional flow regimes. As a result, the design and locating of water jet inlets have been generally confined to considering symmetric flow.
Water jet propulsion systems for marine vehicles usually comprise one or more pumps receiving water from one or more inlets in the hull bottom and discharging an accelerated flow through nozzles which are pointed generally in the direction opposite the direction of travel of the vehicle. Prior art flush and semi-flush mounted water jets have fallen into two categories: conventional shallow-ramp-angle water jets and centrifugal or Schottel type water jets.
Conventional, shallow-ramp-angle water jets, as shown in FIG. 2, are intended primarily for high speed operation. Prior art water jets of this type, and particularly the inlet duct, are designed for optimum efficiency at a particular design condition (i.e., a particular design speed and power). However, efficiency drops off at off design conditions. The inlet duct, if optimized at all, is designed to match the flow at the vehicle design condition. Usually, except in very large projects, a standard inlet duct geometry, which has been found to give acceptable performance, is used. Such water jets have flush or semi-flush inlets and ramp angles (inlet duct inclination angles) that are generally less than about 30.degree. relative to a substantially horizontal hull baseline. Shallow-ramp-angle inlet ducts incorporate a leading edge lip (upstream transition from hull surface to inlet duct) having a long radius of curvature resulting in duct length, from inlet to pump impeller, that is quite long. Consequently, viscous losses in the duct are high. Additional losses are introduced by the pump-motor arrangement. Prior art water jet systems are arranged with pumps located upstream of the inlet and driven by a substantially horizontal drive shaft that passes through the roof or upper ramp of the inlet duct into the flow chamber and, therefore, interferes with the flow into the pump impeller. Conventional water jets of this type have good efficiency at high speeds (generally &gt;30 knots) but poor efficiency at lower speeds. Furthermore, at low ship speeds, flow separation at the inlet may occur due to pump suction induced flow angles that are high relative to the ramp angle.
Schottel type water jets (manufactured by Schottel-Werft Josef Becker GmbH & Co KG, Spay, Germany) and described in U.S. Pat. Nos. 4,411,630 and 4,838,821, are intended primarily for very low speed applications and as maneuvering thrusters due to their excellent bollard pull characteristics. These water jets have a central vertical inlet and vertical drive shaft driving a centrifugal pump impeller. The unit, which is mounted in the hull bottom, includes a rotatable volute and a downwardly inclined outlet nozzle. The volute discharges through the bottom of the hull and is rotatable through 360.degree. for low speed maneuvering. Such water jets have good efficiency at low speed (generally &lt;10-15 knots). However, the efficiency diminishes rapidly above this speed.
Moreover, when water velocity through prior art water jet propulsion systems is very high, low pressure points may be created resulting in cavitation. Cavitation seriously restricts the flow rate of water through the propulsion system and, thus, lowers thrust. Kinetic energy is also wasted because of viscous losses (e.g., friction associated with high speed flow through internal ducts and passages), corner flow, and generation of vortices.
Past water jet propulsion systems have attempted to provide improved propulsive and cavitation performance over a wider speed range by using such devices as variable geometry inlets. However, these mechanically complicated schemes add weight and cost to the system.
Consequently, there is a need for a simple water jet propulsion system having high propulsive efficiency and good cavitation performance at both low speeds and high speeds. There is a further need for a system that offers flexibility of placement while minimizing the various losses associated with water jet propulsion systems.