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 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 WaterjetPosition of the bucket for:Ahead Neutral Astern

• By changing direction of water stream as it leaves the jet unit

Steering a Water Jet

Waterflow

• By lowering an astern deflector into the jet stream – reversing the direction of the force generated

Going Astern

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– separatelyto 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

Steering

Moving sideways(multiple jet installations only)

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

CavitationCavitation causes a loss of performance and results in serious damage to the waterjet components.

CavitationCavitation 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

CavitationCavitation can be prevented by…

1 Increasing the jet intake pressure– i.e.: Increase boat speed but do not

increase power2 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

Shallow Water Operation

a) At high planing speeds, shallow water operation is not a problem until the vessel is nearly grounded.

b) At slow displacement speeds, avoid using high RPM in shallow water.

Shallow Water Operation

a) “Idle” over a shallow area into deep water before accelerating.

b) 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.