SH Lesson 6 Interaction Bank Effect
Summary
TLDRThis ship handling lesson delves into the effects of shallow water on ships, including scot and bank effect, and their impact on steering and maneuverability. It explains how water acceleration under the hull leads to increased sinkage or 'squat', affecting ship draft and trim. The lesson also covers the bank effect, where a ship's stern can be drawn towards the bank due to pressure differences. It discusses interactions between vessels during head-on passing and overtaking, highlighting the importance of understanding these effects for safe navigation in confined waters.
Takeaways
- 🚤 **Squat Phenomenon**: As ships approach shallow water, the water's inability to compress causes acceleration of water molecules beneath the hull, leading to a drop in pressure and an increase in draft known as 'squat'.
- 📏 **Draft Increase**: The apparent increase in draft due to shallow water is called 'squat', which can be estimated by adding 10% to the draft or 0.3 meters for every 5 knots of speed.
- 🔄 **Ship Trim**: The distribution of a ship's buoyancy affects how it trims; a forward center of buoyancy causes head trimming, while an aft center causes stern trimming.
- 🌊 **Bank Effect**: The hydrodynamic pressure distribution around a ship can cause it to be drawn towards the bank due to low pressure areas forming behind the pivot point.
- 🚦 **Interaction with Banks**: When a ship is near a bank, it may require constant corrective rudder action to counteract the strong force of the low-pressure area.
- 🛳 **Vessel Interaction**: During head-on passing, ships experience four phases of interaction, including repulsion, balance, attraction, and separation, each requiring specific helm responses.
- 🔄 **Overtaking Dangers**: Overtaking maneuvers can be dangerous due to the prolonged nature of the interaction, with particular risk during the phase where the bow of the overtaking ship overlaps the stern of the overtaken ship.
- 🚢 **Size Matters**: The interaction between vessels of different sizes is significant, with smaller vessels being more affected by the pressure changes produced by the larger vessel's hull.
- 💡 **Speed Reduction**: Reducing speed is a key strategy to mitigate the effects of squat and bank effect, as well as to improve steering control in shallow waters.
- 📉 **Steering Challenges**: Shallow water can make steering more difficult due to hydrodynamic forces acting against the rudder's turning movement, potentially requiring emergency speed reductions for effective control.
Q & A
What happens to the water underneath a ship's hull as it approaches shallow water?
-As a ship approaches shallow water, the water underneath the hull gets squeezed, causing the water molecules to accelerate due to Bernoulli's theorem, which results in a drop of internal pressure.
What is the term used to describe the apparent increase in a ship's draft due to shallow water effect?
-The term used is 'squat', which refers to the sinking of the ship as it displaces more water to maintain buoyancy in shallow water.
How does the ship's center of buoyancy influence the direction of trim when experiencing squat?
-If the center of buoyancy is forward of midships, the ship will trim by the bow. Conversely, if it is aft of midships, the ship will trim by the stern.
What types of ships are more likely to trim by the head when experiencing squat?
-Ships with fat bows and finer sterns at summer draft, such as large oil tankers and cape size bulk carriers, are more likely to trim by the head.
What is the effect of high speed in shallow water on a ship's steering ability?
-High speed in shallow water can adversely affect a ship's steering ability, as the increased water acceleration under the hull leads to increased sinkage, making it harder to control the ship's direction.
How can the effects of squat be estimated in shallow water?
-Squat can be estimated by adding 10 percent to the draft or 0.3 meters for every 5 knots of speed.
What is the bank effect and how does it influence a ship's movement?
-The bank effect is the hydrodynamic pressure distribution around a forward-moving ship that can cause the stern to be sucked towards the bank. It increases with speed and can be severe if the ship is too close to the bank.
How can a ship handler counteract the bank effect?
-A ship handler can counteract the bank effect by slowing down and steering towards the bank, which may help to balance the ship running parallel to the bank.
What are the four distinct phases of interaction between two vessels in a head-on passing situation?
-The phases are: 1) Repulsion due to positive pressure at the bow, 2) Balancing of pressure at the bow, 3) Attraction due to reduced pressure between hulls, and 4) Sterns drawing together due to negative pressure.
How does the interaction between a large and a small ship during overtaking affect the smaller vessel?
-The smaller vessel is affected more by the interaction due to the larger vessel's pressure field, which can cause the tug to be drawn towards the larger ship, requiring constant corrective action to avoid collision.
What is the Venturi effect as it relates to the interaction between a tugboat and a larger ship?
-The Venturi effect develops between the tug's bow and the larger ship's side, causing the tug to turn in towards the larger ship due to the reduced pressure in this area.
Outlines
🚢 Ship Handling in Shallow Waters
This paragraph discusses ship handling in shallow waters, focusing on the phenomena of scot and bank effect. As a ship approaches shallow water, the water underneath the hull accelerates due to Bernoulli's theorem, leading to a drop in pressure and causing the ship to sink, known as squat. This effect varies depending on the ship's design, with fuller bows leading to head trimming and stern trimming in finer hulls. The paragraph also covers the impact of high speeds on steering ability and the estimation of squat. To mitigate squat, it is advised to reduce ship speed.
⚓ Impact of Shallow Waters on Steering
The second paragraph delves into how shallow waters affect a ship's steering. In such conditions, hydrodynamic forces oppose the rudder's turning movement, increasing the ship's directional stability and potentially making it difficult to steer. The turning circle increases, and stopping distances may lengthen. It is crucial to consider the ship's maneuvering characteristics, especially in emergencies where reducing speed may not yield the expected results. The bank effect is also introduced, explaining how a ship's movement near a bank can lead to being 'sucked' towards it due to pressure differences.
🌀 Interaction Between Vessels
This section examines the interaction between two vessels during head-on passing and overtaking scenarios. In a head-on situation, positive pressure causes ships to repel each other, requiring corrective rudder action. Overtaking involves four phases, with the most dangerous being the second phase where Bernoulli's effect can draw ships closer together. The paragraph advises on the necessary helm actions to maintain safe navigation during these interactions.
🛥️ Effects on Tugboats and Large Ships
The final paragraph explores the interaction between a large ship and a small tugboat during overtaking. The smaller vessel is more affected by the pressure changes around the larger hull. The paragraph outlines the different phases of interaction, emphasizing the need for constant corrective action to prevent collisions. It concludes with a reminder of the importance of adjusting helm and speed to ensure safety during such maneuvers.
Mindmap
Keywords
💡Ship Handling
💡Starboard Side Alongside
💡Bernoulli's Theorem
💡Squat
💡Scott
💡Bank Effect
💡Hydrodynamic Forces
💡Turning Circle
💡VLCC
💡Container Ship
💡Interaction
Highlights
Ship handling situations in shallow water and alongside berthing are discussed.
Approaching shallow water causes water to accelerate under the ship's hull, leading to a drop in pressure.
The ship must displace more water to maintain buoyancy, resulting in sinkage known as squat.
Ships with different designs will trim either by bow or by stern due to the center of buoyancy.
The sinkage due to squat can be visually observed in water plane areas.
Large oil tankers and bulk carriers tend to trim by the head, while container ships trim by the stern.
Bottom effects can be experienced up to 15 times the draft of the ship.
High speed in shallow water increases the squat effect and can affect steering.
Squat can be estimated by adding 10% to the draft or 0.3 meters for every 5 knots of speed.
Reducing ship speed is the most effective way to reduce squat.
Squat can cause an increase in the list of a ship moving in shallow water with an angle of heel.
Shallow water affects steering by acting against the rudder's turning movement.
The bank effect is caused by hydrodynamic pressure distribution around a moving ship.
Bank effect increases with speed and can cause a ship to be drawn towards the bank.
Interaction between two vessels during head-on passing involves four distinct phases.
Overtaking interaction between two similar-sized ships can be dangerous and involves four phases.
When overtaking, the smaller vessel is affected more by interaction than the larger one.
Tugboats working with larger ships experience significant interaction effects.
Constant corrective rudder action is required when interacting with larger vessels.
Transcripts
so in ship handling lesson five we
covered
ship handling situations to come
starboard side alongside
and also on birthing in various
conditions
in this lesson i will cover scots at c
bank effect and interaction so let's get
started
as we approach the shallows water
underneath the ship's
hull tends to get squeezed
since water cannot be compressed its
molecules accelerate
according to bernoulli's theorem
acceleration in any medium results in a
drop of internal pressure in that medium
in our case acceleration of water
molecules
results in a drop of pressure underneath
the hull of the ship
since the weight of the ship is
unchanged ship
has to displace more water to support
our buoyancy
resulting in sinkage of the ship
the sinkage is also known as squat
which could be defined as apparent
increase in her draft
due to the shallow water effect
since most ships are not box shaped
therefore they not only sink but
also trim either by bow or by stern
if a ship's center of buoyancy is
forward of a midships
the hull is fuller in the bow than the
stun
there is more immersed area forward than
aft
therefore scott will produce a head
trimming moment
when the center of the buoyancy is aft
of the midships
as is generally the case in finer line
hulls
scott causes a stern trimming moment
on our left is the water plane area of
the sinkage due to squats suffered by a
vlcc
and on the right is the water plane area
of the sinkage of a container ship
while comparing the two you can see that
ships
with fats boughs and finer stones at
summer draft
will tend to trim by head while
experiencing scots
and these ship types are the large oil
tankers
cape size bulk carriers etc
and at the same time the fine flared
ships
such as container ships passenger
vessels car carriers and reefer ships
will tend to trim by stern
bottom effects can be experienced in
water depths
up to 15 times the draft of the ship
but the effects will not be significant
until we are in a depth less than
two and a half times our draft
if a ship is experiencing squats the
wake will widen considerably and you may
also notice
hull vibration and forward facing
bow waves that are also higher than
normal
in shallow water squat can be estimated
by adding 10 percent
to the draft or 0.3 meters
for every 5 knots of speed
high speed in shallow water can also
adversely affect a ship's ability
to steer
effect will vary from ship to ship
as the scot is a direct result of a drop
in pressure due to the water
acceleration under the hull
increased speeds result in increased
water acceleration and increased sinkage
the most effective way to reduce squats
is by reducing the acceleration of under
kill water
and that can be achieved by reducing the
speed of the ship
squats also has an impact on the angle
of your heel
a ship moving in shallow water with an
angle of heel
will experience an increase in the list
due to the squats concentrating in the
region of
least under kill clearance
similarly an upright vessel will suffer
scot
induced list if it moves over a shelving
seabed
that becomes progressively shallower
under one side of the ship
apparent increase in ships list due to
squats
can be reduced also by reducing the ship
speed
another impact of shallow water is its
effect on ship
steering in shallower waters
the hydrodynamic forces on the hull
actually
act against the turning movement of the
rudder
the directional stability of the ship
however increases
and she may become almost impossible to
steer
by the rudder and may behave as running
along
tram tracks dictated by the seabed
characteristics
the turning circle of most ships
increases in shallow waters
some ships may also have longer stopping
distances in shallow waters
it is important that you check and
consider your
own ship's maneuvering characteristics
data from the ship
trials you should also
note that high or low frequency runner
cycling
to reduce the ship speed in an emergency
may not yield the desired results as one
may
expect in deeper waters
let's now have a look at the bank effect
in its
most simplistic form the hydrodynamic
pressure distribution system
around the forward-moving ship can be
seen
as a boundary layer of water that
surrounds a ship
when it is making headway forward at the
pivot point
a positive pressure area builds up
however
after the pivot point the flow of water
down the ship side
creates a low pressure area
this area extends out from the ship
and does not cause any concern in deep
waters
or when the ship is clear of any
obstruction
or traffic along her route
however when the ship closes in on a
bank
the pressure on the bow works on a short
turning lever forward of the pivot point
but the lower pressure or suction area
on the other hand
works well after the pivot point
and is consequently a very strong force
suction area further drops in pressure
as the water gets squeezed and
accelerates
as a result of the two forces the stern
of the ship is
likely to be sucked into the bank
it can be very difficult to break out of
this hold
the ship requiring constant corrective
rudder
action and power sometimes
hard over even in order to control the
hedging
bank effect increases with increase in
speed
if speed is too high bank effect can be
severe and sudden
and it can catch the ship handler
unaware
you should slow down and steer towards
the bank
by doing so it may be possible to strike
a balance
with your ship running parallel to the
bank
bank effect is also felt on bends
in a waterway when proximity to the
outer bank
may actually help the bow around a tight
bend
let's now have a look at the interaction
between two vessels in a head-on passing
situation
this situation can be split into four
distinct phases
as a ship moves forward with the pivot
point
a positive pressure area builds up
however
after the pivot point a low pressure
area builds up
so in phase one as both ships approach
on a head-on situation
positive pressure at the bar will cause
them to repel each
other a helm order in the direction of
passing
and in our case a helm order to port
side
is required to balance the interaction
effect
a burst in the propeller wash may be
briefly required to enhance the rudder
thrust
in phase two for a short interval
pressure at the boughs is balanced
you may apply midship's helm or even
some
starboard helm to neutralize suction if
required
in phase 3 both vessels are drawn
together by reduced pressure between
their hulls
this may cause swing to port which
should be controlled with starboard helm
in phase four as vessels clear each
other
the sterns will be drawn together due to
the negative pressure
use the helm to control swing but
keep vessels turning to starboard until
they return to the course
let's now look at interaction between
two similar sized
ships during overtaking
unlike head-on overtaking interaction
can be quite dangerous as the maneuver
is quite prolonged
we'll divide it into four phases in
phase one
the high pressure regions at the bow of
overtaking ship
b causes the ships to repel each other
initially so pressure buildup at the bow
of the overtaking vessel
can cause the other vessel to turn
across the bow
if uncorrected
vessels being overtaken that is ship a
must take corrective action
in this case ports helm to neutralize
the swing
in phase two if both vessels use helm to
maintain coarse
then as the bow of ship b overlaps the
stern of shepay
the bernoulli's effect between the two
ships
turns the high pressure into low
pressure
drawing the boughs of the ship b and
stern of ship a
closer this is generally the most
dangerous point in overtaking
as there is both a turning movement and
bodily suction
drawing the ships closer to each other
in phase three mutual attraction of the
ship's turns
due to decreased pressure causes the
overtaken vessel
ship a to swing to port
starboard helm is required to control
the swing
in phase 4 ship a will observe
suction or bow to starboard and this can
be corrected by
slight counter helm in shipping
finally let's look at interaction
between a large and a small
ship during overtaking when two very
dissimilar size ships are passing in
close proximity
the smaller vessel is affected by
interaction
considerably more than the larger one as
the pressure changes produced by
flow around the larger hull are much
greater than those of the smaller vessel
tugboats are frequently in this
situation when they work with larger
ships
the tug is moving in water flow that is
dominated by the pressure field
surrounding the tanker for example in
our case
particularly if the larger vessel is
moving in shallow water
relative to its depth this means that
the water flow
around the thug is not necessarily
coming from right ahead
even if the tug moves ahead with the
helm midships
if we divide the situation into
different phases
in phase one the tongue experiences flow
coming on the
outside bow which is pushing it in
towards the tanker's
aft quarter in phase two
the venturi effect develops between the
tug's bow
and the tanker side so the tug turns in
towards the tanker
and can be drawn bodily to its side
in phase three the tug is now
in a region of the tanker's uniform
pressure field
so the forces are due to the tug's own
much smaller pressure distribution
system
a small suction force accompanied by a
weak
turning out movement is created in the
same way
that the tug would react to the close
proximity of a channel
bank in phase 4
the tug encounters increasing pressure
at the bow
as it moves past the tanker's forward
shoulder
whilst adventure effect remains at the
tug's turn
so these forces combine to suck the
tug stern in towards the tanker while
swinging the bow outward
in phase 5 as the tug starts to clear
the tankers bow water flow is still
directed onto the
inward side of its rudder and so
causes an inward turning moment
if left uncorrected tug may turn
across the bow of the tanker and a
relative speed
with the tanker may significantly drop
in a very brief interval
this may cause a collision particularly
when the positive pressure at the bow of
the tanker is insignificant
you have noticed in each of the five
situations
a constant counter helm and increase and
decrease of the tug
speed is going to be required to ensure
that at no point the tug comes too close
to the tanker or gets sucked in
alright guys this brings us to the close
of this lesson
i hope you enjoyed the session and i
will see you soon
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