Working Principle :
Water Jet Propulsion system is an application similar to that of a water pump. A waterjet system generates its propulsive force or thrust from the reaction that is created when the stream of water is forced out through the rear side of the ship. It works on the principle of the Newtons third Law of motion - i.e "every action has an equal and opposite reaction".
A water jet unit is mounted in the aft section of the ships hull. Water enters the jet unit through the intake grid at the bottom of the boat, drawn through the inlet duct by the impeller which in turn increases the pressure or "Head" of the water flow. This high pressure water flow is then discharged through the nozzle with high velocity jet stream. The discharge of this high velocity water jet generates a reaction force in the opposite direction which in turn gets transferred to the ships hull thus propelling the ship in the forward direction. For making the ship to move in the astern direction, the direction of the water jet that exists from the nozzle is reversed in the forward direction with the help of the reversing buckets, thus giving a thrust in the astern direction and moving the ship in astern. The impeller shaft of the water jet unit is coupled with the main shaft by a suitable coupling which is driven through a gear box and the ships main engine.
The steering system of the water jet systems are usually of electronic type where the orders given from the bridge are processed in the form of electronic signals by microprocessors that helps in controlling the water-jet, engine and gearbox directly from the bridge. Steering is achieved by changing the direction of the stream of the water jet as it is leaving the jet units. By pointing the water jet stream in one way forces the stern of the vessel in the opposite direction thus helping in turning the vessel to the desired direction.
Water-jet systems finds a wide application in search and rescue patrol boats of the Navy and Coast Guards, Commercial car and passenger ferries, and also in a number of yachts. Speeds achieved by ships using a water jet is usually within a range of 35 to 40 knots. However, recently Australian shipyard Incat has built a High-speed dual fuel ship to be used for vehicle and passenger ferrying named Francisco, and is powered by two air craft engine-based GE gas turbines which drive a pair of water jets. The ship achieves maximum speeds upto 58.1 knots (67 miles per hour). Interestingly, apart from being the Word's fastest ferry, Francisco is also the first ship to use liquefied natural gas as its primary fuel.
Water jet type of propulsion has many advantages as compared to the other types of propulsion systems like stern drives, outboard motors and shafted propellers. The advantages include :
It provides excellent manoeuvring capabilities:
- It provided a precise steering control at all speeds.
- Sideways movement of vessel is possible by use of multiple jet installations.
- Provides a high efficiency astern thrust which makes it possible to crash stop the vessel even at high speeds.
High Efficiency system
- Water jet propulsion systems have higher efficiencies than the propeller type of systems at medium to high speeds.
- Lower Hull resistance
- Due to the absence of the underwater appendages it has lesser hull resistance.
- The intake of the water jet is flushed with the hull bottom which makes it possible to navigate in shallow water areas and beach landings with no risk of damage to the propulsion system.
- Due to the absence of the protruding propulsion gear impact dmage or snags are eliminated in this type of systems.
- Has a minimum downtime for maintenance due to simpler maintenance routines.
Smoother and Quieter Operation :
- Produces No hull vibration, no torque effect and no high speed cavitation and give maximum comfort levels to personnel on board.
- Has a Low underwater acoustic signature.
Safer System :
- Due to no exposed propeller it is a safer propulsion system when navigating around people in water and marine life.
Simpler and Easy to install :
- Water Jets are supplied as a single packed module which is fully factory tested and ready to bolt in with No difficult engine alignment problems.
Few disadvantages of this systems are :
- Can be less efficient at lower speeds.
- More expensive than the conventional propeller type propulsion systems.
-Intake grill is prone to get clogged with sea weeds, ropes, etc which can affect the performance of the system.