Wärtsilä Midsize Waterjets

The characteristics of a waterjet make it a good propulsor for numerous applications.

• High speed vessels; waterjets have better propulsion efficiency at ship speeds over 25-30 knots.
• Shallow draft vessels; the integrated steering function provides benefits for rescue vessels, inshore passenger ferries, landing crafts & special work boats.
• High power density; the pressure built-up in the waterjet inlet allows small dimensions.
• Maneuverability; waterjets have integrated steering and reversing with quick response times.

Examples of such applications include:  

• Fast ferries
• Littoral combat ships
• Fast patrol boats
• Rescue vessels
• Fast yachts
• Fast supply vessels and workboats
• Landing craft

• A Plug and Play waterjet concept, with all auxiliary systems pre-installed on the skid. The waterjet package is welded into the hull and the yard connections are reduced to a minimum.
• The use of a water-lubricated stator bearing in the stator bowl is beneficial for the environment and enhances the reliability and maintainability of the waterjet.
• The thrust bearing block (TBB) and all hydraulics, including the cylinders, are located inside the ship. The inboard location enables excellent maintainability and allows the use of large size bearings for increased operating life. The bearing block is designed with a sump to ensure lubrication to the bearings under all circumstances.
• The cavitation margins of the axial pump are shifted backwards by 35% (see figure below).
  • As a result of this increase in the cavitation margin and the lower impeller tip speed, more power can be utilised during manoeuvring, thus giving a 15% higher manoeuvring thrust. Thanks to the additional cavitation margin, operation with a reduced number of shaft lines is possible at a higher load of the remaining engines, resulting in better operating flexibility.

During operation, water enters the waterjet through the inlet duct, which is part of the ship’s construction. After passing the pump impeller, the rotation in the flow is removed and the water is accelerated in the stator bowl. This creates the thrust necessary to propel the ship. For steerable waterjets, the jet stream is deflected by a jetavator which is mounted behind the stator bowl. The jetavator is actuated by an inboard mounted hydraulic cylinder and can be turned 30° to port and 30° to starboard.

The jetavator contains a hydraulically activated reversing plate through which part or all of the jet stream can be deflected forward. The reversing plate can be gradually moved, which makes it possible to vary the thrust from full ahead via the zero thrust position to full astern and vice versa.

The zero thrust position prevents the ship from moving when the impeller shaft is clutched in. The reverse cylinder is equipped with a counter balance valve (load holding valve). This safety device keeps the reversing plate movement controllable and prevents that, in the event of a hose failure, the reverse plate from moving to full astern without control.

In addition to the main waterjet hardware, Wärtsilä can supply other equipment that can be integrated as part of the Wärtsilä solution.

• Intermediate shafting
  Shaft line to the gearbox including the flexible elements, support bearings and bulkhead seals.
  
  
• Shaft line calculations
  Whirling and aligning calculations for the complete shaft line.
  
  
• Propulsion Control System
  
  The Wärtsilä control system can control all propulsors onboard, including the waterjets.