SH Lesson 6 Interaction Bank Effect

Summary

This 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

00:00

🚢 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.

05:01

⚓ 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.

10:02

🌀 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.

15:02

🛥️ 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.

Keywords

💡Ship Handling

Ship handling refers to the skills and techniques required to maneuver a ship safely and efficiently. In the context of the video, ship handling is crucial for navigating various conditions, including coming alongside a port and berthing. The video discusses how to manage the ship's behavior in shallow waters, which is a key part of ship handling.

💡Starboard Side Alongside

This term refers to the act of docking a ship so that its starboard side is next to the pier or another ship. It is a specific maneuver covered in the lesson and is part of the broader ship handling skillset.

💡Bernoulli's Theorem

Bernoulli's Theorem states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure. In the video, it is used to explain how the acceleration of water molecules under the ship's hull in shallow water leads to a decrease in pressure, which in turn affects the ship's buoyancy and can cause the ship to sink or 'squat'.

💡Squat

Squat is defined as the apparent increase in a ship's draft due to the shallow water effect. It is a phenomenon where the ship sinks lower into the water as it moves through shallow areas. The video uses the term to describe how ships react in certain water conditions, affecting their maneuverability.

💡Scott

Scott refers to the list or tilt of a ship to one side. The video explains how the shape of a ship's hull can cause it to trim by the bow or stern when experiencing the effects of shallow water, with 'Scott' causing a head trimming moment when the center of buoyancy is forward.

💡Bank Effect

Bank effect describes the influence of a riverbank or shoreline on the flow of water and the behavior of a ship. As explained in the video, when a ship is close to a bank, the pressure on the bow and the low-pressure area on the side can cause the stern to be drawn towards the bank, requiring careful navigation.

💡Hydrodynamic Forces

Hydrodynamic forces are the forces exerted by a fluid, such as water, on an object moving through it. In the video, these forces are discussed in the context of how they affect a ship's steering and maneuverability in shallow water, often acting against the rudder's turning movement.

💡Turning Circle

The turning circle is the area within which a ship can turn 360 degrees. The video mentions that in shallow waters, the hydrodynamic forces can increase a ship's turning circle, making it more challenging to maneuver.

💡VLCC

VLCC stands for Very Large Crude Carrier, a type of large oil tanker. The video uses the VLCC as an example to illustrate the water plane area of sinkage due to squat, showing how different types of ships react differently to shallow water effects.

💡Container Ship

A container ship is a type of cargo ship that carries all of its load in truck-size containers. The video contrasts the behavior of container ships with that of VLCCs, noting that container ships tend to trim by the stern when experiencing shallow water effects.

💡Interaction

Interaction in the video refers to the dynamic effects between two vessels when they are in close proximity, such as during a head-on passing or overtaking situation. The video explains how the pressure fields around the ships can cause them to be drawn together or repelled, requiring careful navigation to avoid collision.

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