217-Navigation and Watch Keeping / 748
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g
ation and
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atc
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Kee
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in
g
BOS-PRD-07.900502
Version 04. Original date: 01/06/2012. Version date: 31/12/2014. Valid version only on QMS site. Uncontrolled document when printed.
Procedure prepared by: BOS Head of Operations. Approved by: BOS Head of Operations. Validated by: BOS Quality Representative.
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landmarks, or other distinctive sighting marks, soundings and the use of radar parallel indexing.
GPS, as an electronic aid, may be used to verify fixes;
x
Ensure that the echo sounder is operational, accurate and fully utilised. The date and time of
switching on
should
be marked on the recorder chart. In addition, the date and time of passing
significant land or seamarks
should
be marked on the recorder.
During all shallow water passages and/or operations, the engine room is to be manned by a certified Engine
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atc
h
O
ff
ice
r
who, in addition to the duties given in chapter 4.3 is responsible for ensuring the following:
x
The use of high sea-suctions when required and the continuous monitoring of the sea water flow;
x
Checking that the main engine alarms and shutdowns are operational and function tested;
x
Seawater isolation valves are free and operational;
x
Seawater strainers are clean;
x
A spare seawater strainer is available for replacement at short notice;
x
Main and standby seawater pumps are operational and any automatic start-ups have been function
tested;
x
That the seawater low-pressure alarms are operational and correctly set;
x
That all underwater section watertight doors are closed and that watertight integrity is maintained.
4
.
3
.
5
Th
e
s
q
u
at e
ff
ect
Squat effect is approximately proportional to the square of the speed of the ship. Thus, by reducing speed by
half, the squat effect is reduced by a factor of four. The squat effect is usually felt more when the depth/draft
ratio is less than four.
The simplified squat formula for vessels in unrestricted waters is as follows:
Squat = C
B
x V
2
/100 (metres)
Where C
B
= Ships block coefficient* and V = Speed through water in knots
Example: C
B
= 0.72, V = 12 kts, Squat = 1.04 metres
The simplified squat formula for vessels in restricted waters is as follows:
Squat = 2C
B
x V
2
/100 (metres)
Example: C
B
= 0.72, V = 6 kts, Squat = 0.52 metres
*
NOTE: Block Coefficient is the ratio of the volume of water displaced by a ship to the product of the ship's length, breadth, and draft.
Naval Architects determine it for a number of configurations during commissioning. Block Coefficient may be found in the
Stability
Booklet
.
Ho
w
to
r
eco
g
ni
s
e
s
q
u
at
If a vessel reaches shallow wat
ers and “feels the ground” following changes
shall
be observed:
x
Change of wave pattern at the stern and the bow of the vessel;
x
The vessel becomes sluggish when carrying out manoeuvres;
x
The revolution of main engine decreases;
x
The ship’s speed decreases
;
x
Appreciable vibr
ations of the ship’s hull occur.
Ho
w
to
pr
e
v
ent
s
q
u
at
The main factors of squat are as follows:
x
Existing water depth;
x
Ve
ssel’s speed through the water;
x
The only effective measure to minimise squat is the immediate reduction of speed.
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