Ship Squat

SHIP SQUAT
 Squat is the decrease in under-keel water, that is,

the difference between her under-keel clearence
when making way and when stopped over the
water.
 It is not the increase in draft as visually read or as
shown on draft indicators.
 Bernoulli’s theorem states that in any moving
fluid, the sum of the potential energy, the kinetic
energy and the pressure energy is a constant.
 The fact that the ship is floating in that water
does not alter the level or height of water there.
 Therefore the potential energy of that water is
unchanged.
 As the vessel makes way through the water, she
leaves behind a hollow in the water flows aft, its
kinetic energy increases.
 According to Bernoulli’s theorem, when the
kinetic energy of the water increases, it’s pressure
energy must reduces.
 Since the ship is supported by the pressure
energy of the water, as the pressure energy has
reduced, the ship sinks to a longer draft.
 In addition to the bodily sinkage that occurs, the
ship also trims by the head or by the stern.
 With a static even keel trim, full form vessels such
as tankers and bulk carries with C b more than 0.7
trim by the head.
 Fine form vessels such as passenger ships and
containers vessels with Cb less than 0.7 trim by
the stern.
1
 The overall decrease in under –keel clearence due
to sinkage and trim is the squat forward or aft.
The factors that affect the amount of

squat are
1) The ships speed over the water
 The squat varies approximately directly as
the speed over the water in knots squared.
 Squat occurs even when the ship is moored,
if a tide is running.
 As stated under the chapter on draft surveys,
this should be taken into account when
conducting draft surveys.
 Also, when loading to a particular draft,
squat could result in under loading if the
drafts are read when a tide is running.
2) The block coefficient, Cb
 The squat varies directly as the Cb. The Cb
values generally vary from about 0.85 for very
large tankers to about 0.75 for bulkers, about
0.7 for general cargo vessels to about 0.6 or
less for passenger vessels and container
ships.
3) The blockage factor, S
 The blockage factor, S, is the ratio between
the immersed cross sectional are of the vessel
and the cross sectional area of the water in
the canal.
S  b x Static draft
B X depth of water
where
‘b’ is the breadth of the ship and
‘B’ is the width of the canal.
 Even in open waters, this factor is to be
considered using the width of influence ‘B’ in
place of the width of the canal B.
2
 The width of influence ‘B’ in open waters is
obtained as
‘B’ = [ 7.7 + 20 (1-Cb)2] b
where ‘b’ is the breadth of the ship.
 The ‘B’ value in open waters varies from
about 8 b for large tankers to about 9.5 b for
general cargo vessels to about 12 b for
container and passenger ships.
 In open waters where the depth of water to
draft of ship ratio is about 1.2, the value of
the blockage factor S will be around 0.1.
4) The static under keel clearance
 The lesser the under-keel clearance, the more
is the squat because the stream lines of
return flow aft of the water, past the vessel
increases due to the reduced clearance under
the vessel.
 This increases the kinetic energy and
therefore further reduces the pressure energy
of the water.
 Thus as the ratio of depth of water to draft to
ship reduces, the squat increases.
5) The at rest trim of the vessel
 The squat at the bow increases to a greater
extant if her at rest trim was by the head.
 The squat at the stern will increase to a
greater extent if her at rest trim was by the
stern. The calculated maximum squat should
therefore be applied to the greater of the two
end drafts to obtain the minimum under keel
clearance.
6) Passing another ship in a river or canal
 When the ship is passing or overtaking
another vessel in a river or canal, the squat
can increase upto twice the normal value as
3
the combined blockage factor, S, becomes the
sum of the blockage factor of each ship.
7) The squat increases if the ship is close to the
bank of a river or canal.
 Various empirical formulae have been
suggested for estimating the maximum squat.
 As there are so many variables and so many
factors, the exact values of which may not be
readily available, none of the formulae are
likely to provide absolutely accurate squat
values.
 However, from the analysis of many
measured squat values on ships and results
of ship model tests some empirical formulae
have been developed for satisfactorily
estimating the maximum squat is confined
and one waters.
 Obviously the squat is greater in confined
waters and lesser in open waters.
 For a vessel at an even keel static trim when
the ratio of the depth of water to the draft of
ship is in the range of 1.1 to 1.4, the
maximum squat in open or confined waters
may be predicted fairly accurately by either of
the expressions:-
(i) Maximum squat = Cb x S0.81 x V2.08
20
or
(ii) Maximum squat = Cb x S2 2/3 x V2.08
30
S As
where ‘S’2 = 1 S
= Ac  As
in the above expressions:

4
‘S’ is the blockage factor.
‘V’ is the ship’s speed over the water in knots.
‘S2’ is the velocity return factor.
‘As’ is the immersed cross sectional area of the
ship.
‘Ac’ is the cross sectional area of the water in the
canal.
Other approximate formulae are:-

1) Maximum squat in open waters = Cb x V2
100
Maximum squat in confined waters
Where S is between 0.1 and 0.265) = Cb x V2
50
Both the above approximate formulae slightly over
estimates the maximum squat thereby erring on
the safer side.
Indication that the ship is in shallow waters
include
(i) Wave making by the ship, especially forward,
increases.
(ii) Manoeuvring becomes sluggish.
(iii) The propeller RPM reduces.
(iv) The ship’s speed over the water reduces.
(v) Stopping distances and time increases.
(vi) The diameter of the turning circle increases
to a great extent.
(vii) Rolling and pitching reduces.
(viii) The ship may start to vibrate.
 At this point, a consideration may arise as

to the depth of water, which can be
considered shallow.
5
 This depends on the depth of influence of
the ship, which is approximately 5/Cb x
draft.
 In depths above the depth of influence the
ship may be considered in shallow waters.
 Since the depth of influence is more than
5 times the draft, though the ship’s squat
may commence to increase slightly at such
depths it is not of much consequence.
 The increase in squat is significant when
the depth to draft ratio is less than 2.
 It is much more pronounced and of
consequence when this ratio is less than
1.5.
 The best course of action to reduce squat is
to reduce the ship’s speed, because the
squat varies directly as the ship’s speed
squared.
 Halving the speed will reduce the squat to
a quarter.
 However, the fact that manoeuvring which
is already sluggish in shallow waters may
deteriorate further should also be
considered when reducing the speed.

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SHIP SQUAT

 Squat is the decrease in under-keel water, that is,

The factors that affect the amount of

(i) Maximum squat = Cb x S0.81 x V2.08

in the above expressions:

Other approximate formulae are:-

Maximum squat in confined waters

Where S is between 0.1 and 0.265) = Cb x V2

 At this point, a consideration may arise as

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