Hamilton Jet HJ241, Manual

Page: 29 / 202

2.7
2 SYSTEM OPERATION
2.7 A
CCELERATION TO
H
IGH
S
PEED
HJ-241 / HSRX
2.7 ACCELERATION TO HIGH
SPEED
If leaving an area of shallows, or with debris in the
water, ensure Jets are clear of debris before
accelerating to high speed.
Refer to Section 2.9.3
Clearing Blockages . If there is any debris in the Jet
the engine will run at higher than normal RPM and
the vessel accelerate only slowly, perhaps not
reaching full (planing) speed.
Refer to Section 2.9.2
Detecting Blockages .
Daily, prior to commencing operations, the
Inspection Cover should be removed and any debris
removed from around the Impeller or Intake Screen.
Refer to Section 8.4 Servicing Intervals Item 1 .
2.8 AERATED WATER
It is possible that some Hulls may, under certain
conditions, feed aerated water into the Intake of the
Jet Units.
When operating in areas where the water may be
excessively aerated. (e.g. fast flowing rapids or surf)
The following points should be noted:
1. There may be a loss in thrust due to the Jet Unit
pumping a significant amount of air instead of
water.
2. The Impeller may unload suddenly causing the
engine RPM to fluctuate wildly.
When these symptoms occur, reduce engine RPM
until the Jet Unit maintains a steady RPM and thrust.
The operator must be prepared to lose control
temporarily in these conditions and should allow
margins of safety.
2.9 BLOCKAGES (DEBRIS IN THE
JET UNIT)
2.9.1 Avoiding Blockages
Pieces of debris, water weed or sticks, etc. will not
normally block or harm the unit. However, it is good
practice to steer around such debris where possible
as any debris caught in the Intake Screen, Impeller
or Tailpipe Stator Vanes can affect the Jet Unit's
performance.
2.9.2 Detecting Blockages
In debris laden waters it may be necessary to clear
the Intake Screens and Impellers before each run. In
many cases the debris is picked up while the Jet is
moored so it is best to clear the Screen in open or
clear waters.
Blockages of the Jet Unit are usually noticed by the
following symptoms:
1. The engine unloading (RPM increases).
2. Lack of jet thrust (vessel speed drops).
3. Excessive noise and vibration from the Jet Unit.
2.9.3 Clearing Blockages
The following methods can be used to clear a
blocked Jet Unit.
1. Slow or stop the engine driving the blocked Jet
Unit. The blockage will often clear itself. This
operation works best if the vessel is still moving
forward at speed.
2. BACKFLUSH the blocked Jet Unit (only possible if
a reversible gearbox is used) as shown below.
a) Stop or slow the Vessel to displacement
speed.
b) Move the Reverse Duct to the ZERO SPEED
position.
c) Reverse the rotation direction of the blocked
Jet Unit by engaging reverse gear and opening
the throttle slightly. This should clear the
C
AUTION
Ensure that the water level is
below the Inspection Cover level
before removing the Inspection
Cover.
C
AUTION
Running at speed with a partially
blocked inlet grill or debris on the
impeller will result in cavitation
damage to the jet unit.
W
ARNING
Extreme care is required whenever
the Inspection Cover is removed as
water may enter the vessel
through this opening.
Never leave the Inspection Cover
removed without constant
monitoring of the water levels.