The concept of water-jet propulsion dates back to 1661 when Toogood and Hays first proposed this form of propulsion. Waterjet propulsion is often chosen instead of conventional propellers for vessels requiring high speeds, shallow draught, protected propulsion, high manoeuvrability at all speeds, low noise emissions for military applications and low vibration.
HOW A WATERJET WORKS
A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. It works in relation to Newton’s Third Law of Motion – “every action has an equal and opposite reaction”. A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful firehose.
Put simply, the discharge of a high velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft’s hull, propelling it forward (see diagram below).
Put simply, the discharge of a high velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft’s hull, propelling it forward (see diagram below).
In a boat hull the jet unit is mounted inboard in the aft section. Water enters the jet unit intake on the bottom of the boat, at boat speed, and is accelerated through the jet unit and discharged through the transom at a high velocity.
The picture below shows where water enters the jet unit via the Intake.The pumping unit, which includes the Impeller and Stator, increases the pressure, or “head”, of the flow. This high pressure flow is discharged at the nozzle as a high velocity jet stream. The driveshaft attaches at the coupling to turn the impeller.
The picture below shows where water enters the jet unit via the Intake.The pumping unit, which includes the Impeller and Stator, increases the pressure, or “head”, of the flow. This high pressure flow is discharged at the nozzle as a high velocity jet stream. The driveshaft attaches at the coupling to turn the impeller.
Steering is achieved by changing the direction of the stream of water as it leaves the jet unit. Pointing the jet stream one way forces the stern of the boat in the opposite direction which puts the vessel into a turn.
Reverse is achieved by lowering an astern deflector into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction.
Reverse is achieved by lowering an astern deflector into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction.
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 counterbalance 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.
WATERJET ADVANTAGES
Waterjet propulsion has many advantages over other forms of marine propulsion, such as stern drives, outboard motors, shafted propellers and surface drives.
These advantages include:
RELIABILITY
HamiltonJet waterjets have fewer moving parts and most major components inboard, minimizing the chance accidental damage and increasing the life of components.
EXCELLENT MANEUVERABILITY
- Precise steering control at all boat speeds
- “Zero Speed” steering effect provides 360° thrusting ability for docking and holding stationary.
- Sideways movement possible with multiple jet installations.
- High efficiency astern thrust with “power-braking” ability at speed
SPEED
HamiltonJet waterjets are designed to be optimized for vessel speeds from 25-50 knots, dependent on application’s engineering variables including hull resistance and type, engine power rating and rpm, impeller rating, etc.
HIGH EFFICIENCY
- Propulsive coefficients as good or higher than the best propeller systems achievable at medium to high planing speeds
- Flexibility when using multiple waterjets may allow operators to continue to operate efficiently on fewer drives
LOW DRAG AND SHALLOW DRAUGHT
- Absence of underwater appendages reduces hull resistance
- Shallow draught – the waterjet intake is flush with hull bottom to allow access to shallow water areas and beach landings with no risk of damage to the drive
LOW MAINTENANCE
- No protruding propulsion gear eliminates impact damage or snags
- Minimum downtime and simple maintenance routines
SMOOTH AND QUIET
- No hull vibration, no torque effect and no high speed cavitation gives maximum comfort levels on board
- Low underwater acoustic signature
TOTAL SAFETY
No exposed propeller for complete safety around people in the water and marine life
MAXIMUM ENGINE LIFE
- Jet unit impeller is finely matched to engine power
- Power absorption is the same regardless of boat speed
- No possibility of engine overload under any conditions
SIMPLICITY
- Single packaged module
- No heavy and expensive gearbox required for many installations. Simple driveline from engine to jet coupling
EASY INSTALLATION
- Complete factory tested package, ready to bolt in
- No difficult engine alignment problems
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