In most surface vessels having waterjet propulsion systems, the pump is mounted within the hull adjacent the stern transom with at least a portion of the pump and the pump discharge nozzle above the surface of the water. The water jet is discharged through a discharge conduit leading from the pump that passes through the transom and impinges on a steering nozzle mounted on the outside of the stern transom. The location of the outlet from the pump discharge conduit at the water surface permits the actuators for the steering nozzle and reversing deflector of the propulsion system to be above the water, thus simplifying the installation and maintenance of the actuators and the hydraulic lines leading to the actuators. Also, it is common to provide access ports in the pump above the waterline to permit the pump to be serviced without drydocking the vessel.
Generally, the intake opening to the water supply conduit for the waterjet pump is located on the bottom of the hull a short distance forward of the pump and just far enough below the waterline to ensure that water is taken in under most operating conditions of the vessel. The location of the intake opening at a minimum height below the pump improves efficiency, as compared to a deeper location, by minimizing the vertical distance that the pump has to pump the water from the intake opening to the pump rotor.
A disadvantage of having the waterjet pump relatively close to the water surface is the reduced hydraulic head of water at the pump inlet. The reduced suction head reduces the capability of the pump to absorb high power at slow speeds due to the onset of cavitation.
The pump has to be larger than it would have to be if the suction head were greater in order to provide high power output at slow speeds without cavitation.
Another disadvantage of most previously known waterjet propulsion systems is the relative complexity of the actuators for the steering nozzle and the reversing deflector and the outboard location of the actuators. The actuators are usually hydraulic piston/cylinders and require that several hoses pass through openings in the transom, which complicates the construction of the transom and requires seals in each opening. If there is a failure of an actuator or a hose, hydraulic fluid is lost to the environment. The outboard actuator systems for the steering nozzle and the reversing deflector are also not easily repaired when the vessel is at sea.
One previously known arrangement for actuating the steering nozzle and reversing deflector of a marine waterjet propulsion system, which is described and shown in U.S. Pat. No. 3,807,346, includes concentric shafts that extend vertically downwardly from a portion of the vessel hull that is located above the steering nozzle and reversing deflector, which are pivotally mounted on a bracket for rotation about a common vertical axis that coincides with the axis of the concentric shafts. The lower end of the inner shaft is coupled to the steering nozzle, and the lower end of the outer shaft is coupled to reversing deflector. The inner shaft is driven by a piston/cylinder steering actuator that is located within the vessel hull and is coupled by a steering lever to the upper end of the inner shaft. A piston/cylinder reversing actuator is coupled between the steering lever and the upper end of the outer shaft so as to pivot the reversing deflector relative to the steering nozzle.
The steering/reversing mechanism of U.S. Pat. No. 3,807,346 has the advantages of requiring only a single penetration of the hull of the vessel and of enabling the steering and reversing actuators to be located within the vessel hull, where they are protected from the hostile water environment and can be serviced readily. The rotation of the reversing deflector about a vertical axis is, however, highly disadvantageous, inasmuch as in the retracted position for ahead propulsion, the reversing deflector resides laterally of the steering nozzle where it creates a large drag. In addition, an inactive positioning of the reversing deflector laterally of the steering nozzle requires additional athwart-ship space, which is limited in many waterjet propulsion applications.
When a waterjet propulsion system is installed at the waterline of the vessel, most parts of the installation can be located above the water surface and do not contribute drag. Locating a water jet propulsion system in a fully submerged location to attain the advantages described above presents significant problems from the points of view of minimizing drag, minimizing the number of penetrations of the hull requiring seals, constructing the system so that it can be easily maintained and repaired, and avoiding installing hydraulic or electrical apparatus outside of the hull.