Effect on tides on Ships using a propeller

1. EFFECT OF TIDE

2.  As compared to the effect of wind, it should be
borne in mind that effect of tide/a mass of
water on the move is several hundred times
denser than air and thus by comparison is
capable of generating forces of enormous
magnitude.
 How tides can affect the handling characteristics
of a vessel can be easily explained. However,
because of tidal flow in and around jetties and
waterways the situation can be extremely
complex.
 Intimate local knowledge will be required and
only an experienced senior pilot can offer advice
concerning the handling of a ship in such specific
locations.
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3. Effect of tide upon a Ship’s
Handling Characteristics
 It is a general misconception that tides have
an adverse effect upon the ship handling
characteristics.
 Provided a ship is clear of any external
features which might obstruct tide, such as
shallow water, nearby shoals or manmade
structures, and no attempt is made to
restrict the tidal drift of the ship with tugs,
anchors, or moorings, it can be handled in
much the same way as normal with no
particular adverse effects(see dig).
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4.  The only difference is that during the period of
maneuver the area of water that encompasses
the vessel is moving en masse along with the
vessel.
 Thus the position of the vessel at the end of a
said maneuver could be quite a large distance
away from the original starting position. E.g., if
it takes 15mins to turn a ship short round in a
2kt tide, the ship will have travelled ½ a mile
over the ground and downstream during the
course of that movement.
 It is therefore very important that the ship
handler assesses the tidal strength and direction
with great care prior to commencement of any
maneuver, in order to ascertain if there is
sufficient time and space to complete it.
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6. Working in a tide
 When a tide flows across a berth, it can be
effectively used to improve slow speed
control and create lateral motion.
With the tide from ahead
 By using low RPM or short kicks ahead in
order to maintain a small headway through
the water and into the tide, it is possible to
maintain good steering lever and heading
control even at a considerably lower speed
over ground. This is called “stemming the
tide”(see dig).
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7. With the tide from astern
 This is a most unsatisfactory situation where
it is extremely difficult to maintain positive
control of the ship.
 Consider a ship running with a tide of 1.5kts
from astern(see dig).
 To maintain steering/rudder effectiveness, it
will be necessary to have some
headway/speed ahead though water.
 This results in quite a substantial speed over
ground which is considerably faster than the
speed of tide which will be too fast!
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8.  To reduce these persistent high speeds over
the ground, we will have to put the engines
astern perhaps frequently and for prolonged
periods of time.
 This will result in the ship backing up hard
against the tide and resulting in the pivot
point shifting to the aft of the ship.
 The ship is now purely at the whim of the
transverse thrust.
 It can be very difficult to keep control of the
ship with a following tide.
 If practicable it is always preferable to stem
the tide.
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9. Working across the tide
 Once a reasonable balance is struck between the
tidal stream and the ship’s speed through the water
so that the speed over ground is minimal, it becomes
possible to ‘work the tide’ and create a sideways or
‘lateral motion’.
 To achieve this, just ease the tide around to fine on
the bow by slight change of heading.
 The resultant of the two vectors, i.e. the ship’s
headway and tidal stream will now be acting
sideways on the ship giving it a lateral movement
across the tide and onto the berth(see dig).
 To move the ship away from the berth, bring the tide
around to the other bow and the ship will ‘walk’ in
the opposite direction away from the berth(see dig).
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11.  To stop or correct this sideways drift, it will be
necessary to bring the ship’s head directly into
the tide so that it is once again dead ahead.
 When using the tide in this way it is important to
be patient and not swing the bow around too
much hoping to get more/faster lateral motion.
 It may so happen that a strong tide may catch
the vessel with too large a bow angle and we will
require a good long distance ahead to be able to
give sufficient power and rudder to bring the
ship’s head around back into the tide and control
the situation.
 It is therefore better to wait with the tide fine
on the bow and see what is happening rather
than rush the maneuver.
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12. Tidal Forces – General
 Water being several hundred times denser than air, and
during any attempt to restrict its flow e.g. with moorings,
anchors, tugs, it can generate enormous force!
 The magnitude of the force is influenced by:-
Draft and depth of water
 When calculating the force of tide accurately, the draft
and depth are important particularly in the case of large
vessels like VLCCs.
 The velocity of tide can vary substantially with depth, e.g.
a tidal difference of 2.5kts over a depth of 4.5m has been
known to be recorded.
 It is important to be aware that published ‘tidal stream’
information may be based only upon data recorded at
limited depths.
 In the absence of reliable tidal information, any
calculations to ascertain strength of tide, may be very
inaccurate.
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13. The ship’s bow configuration
 The shape of the ship’s bow has a significant
effect upon tidal force. e.g. when calculating
current loads of a ship in order to determine
mooring parameters at terminals, in the interest
of accuracy, the difference between a ship with
a modern conventional bulbous bow and the
more traditional rounded bow, will have to be
taken into account.
The velocity of the tide
 The force of the tide acting upon a ship,
measured in tones, is directly proportional to the
square of the velocity of the tide.
 This means that for even a small increase in the
velocity of the tide, there is an enormous
increase in the force exerted upon a ship.
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14. Under keel clearance
 The single greatest influence upon the
magnitude of the tidal force is under keel
clearance.
 As the UKC reduces, the vessel has a blocking
effect since the tide can no longer flow
under the vessel but is forced to flow around
the ship.
 The ratio of the vessel’s draft to the depth of
water is therefore very important. e.g. with
a depth to draft ratio of 1.05, the tidal force
is 3 times stronger than with a depth to draft
ratio of 3.0.
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