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HAMILTON WATER JETS. Nature. Newton’s Third Law of Motion “Every action has an equal and opposite reaction”. Water jet - Main parts. Cast aluminium pump u nit and intake transition duct Stainless steel impeller Astern deflector Steering nozzles. Water Jet Advantages.
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Nature • Newton’s Third Law of Motion • “Every action has an equal and opposite reaction”.
Water jet - Main parts • Cast aluminium pump unit and intake transition duct • Stainless steel impeller • Astern deflector • Steering nozzles
Water Jet Advantages • Supreme manoeuvrability • Turn around axle without touching the wheel • Speed zero possible, without switching gears • While steering capability is kept • Simplicity • Better low speed manoeuvrability and better acceleration • Precise steering control.
Water Jet Advantages • Safe for persons in water • No moving parts extending under boat
The Waterjet Position of the bucket for: Ahead Neutral Astern
Steering a Water Jet • By changing direction of water stream as it leaves the jet unit Waterflow
Going Astern • By lowering an astern deflector into the jet stream • reversing the direction of the force generated
Emergency “Crash” Stop • Warn crew to hold on • Move Astern Deflector to Reverse position while at speed (at full rev’s) • Notes: • Emergency stop should only be used in extreme situations. • Engine damage can occur because of lubricating oil moving to front of carter
Zero Speed • By lowering the astern deflector partially in the jet stream so ahead and astern thrust is equal
Waterjet Operation • A waterjet’s control functions can be used • in combination • separately • to create precise vessel control and manoeuvrability in all low and high speed situations
Throttle • Increasing rpms results in • increased acceleration • increased maneuverability
Comparison between jets and outboards • Waterjet seems to react slowly • Activate steering by giving quite a bit of rudder and turning back to central position straight afterward • Activate stopping by buckets full astern and back to neutral position straight after
Backing up • Steers other way round, compared with outboard motors / cars etc. • No banana-ing • During turn no turning of wheel when changing from ahead to astern
Debris Problems • Blockages and Debris • Any debris caught in the Intake Screen, Impeller or Tailpipe stator vanes can affect the Jet Unit’s performance
Detecting Debris • Vibrations • Noise in the jet (sometimes) • Increased rpms, but less power • Loses grip on the water • Less speed at high revolutions
Unblocking a Jet Unit • Backflush the blocked Jet • if a reversible gearbox is used • Slow or stop engine driving the blocked Jet • Bucket up • Reverse on other engine (if fitted) to create backflow
Unblock Jet Unit • Operate the manual Intake Screen Rake (if fitted) • Remove Inspection Cover on Intake • Clear manually
Vessel not stopping • Boat goes ahead, unless buckets in reverse (double jet) • One of buckets is not moving, due to broken cable • Solution • Stop motor at side of broken bucket • Unclutch jet unit
Cavitation Cavitation causes a loss of performance and results in serious damage to the waterjet components.
Cavitation Cavitation can be caused by… • Hump Conditions • Overload • Weight too far aft • Bad weather or sea conditions • Blocked intake screen • Blunt or damaged impellers • Excessive impeller tip clearance • Poor installation
Cavitation Cavitation can be prevented by… 1 Increasing the jet intake pressure – i.e.: Increase boat speed but do not increase power 2 Reduce rpm – less throttle – coarser impeller, i.e.. less rpm (steeper pitched blades) 3 Reduce flow plus fit a smaller nozzle
Shallow Water Operation Avoid pumping stones, sand and debris through the Jet Unit as this will blunt and wear the Impeller. The following diagrams illustrate good and bad practice…
Shallow Water Operation • At high planing speeds, shallow water operation is not a problem until the vessel is nearly grounded. • At slow displacement speeds, avoid using high RPM in shallow water.
Shallow Water Operation • “Idle” over a shallow area into deep water before accelerating. • If any debris has been picked up in the intake screen, momentarily stopping the engine should allow the debris to drop away from the screen. • Do not run the Jet if the vessel has run aground. Damage may occur to impellers.