Hydrodynamics & Interaction

Videotel 2012

CONTENTS 1 Introduction 1.1 Learning outcomes 1.2 Situational awareness 2 An Introduction to Hydrodynamics & Interaction 3 What are pressure waves? 4 The formation of hydrodynamic pressure fields 4.1 The Bernoulli effect 4.2 Ship squat 5 Interaction 5.1 Bank rejection and bank suction 5.2 Ranging and surging 5.3 Passing head to head 5.4 Overtaking from astern 5.5 Boundary zones 6 Tell-Tale signs 7 Interaction at a distance 8 Summary 9 Case studies 10 Further information

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1 INTRODUCTION To provide an opportunity to reflect, two special features have been incorporated in the Reference to help you interact with the video and allow for further discussion and review.

The first of these is the Note which will provide more information about a specific subject and may provide a link to further resources and links. The second is the more

directed Observe that will ask you to apply your knowledge in practical ways,

since we believe that this is the most effective means of understanding what you have learnt. Both features are designed to help increase your confidence and skills in real life work situations. 1.1 Learning outcomes This is of particular relevance if you are instructing others. Each section of this Reference has a set of learning outcomes so that you have a clear idea which areas are covered, and why. Other suggestions about how to use this Reference are as follows:

The video follows a precise structure which should be followed carefully. That doesnt mean you should watch the entire video without pausing. Indeed we want to encourage you to use the pause function whenever you feel it necessary. However, you will not get the most from this video if you miss out sections, which are designed to follow on from each other.

As you watch the video, make notes of any areas you feel unsure about or need further clarification.

Consult this Reference. Does it give you the information you need? If not, do you know where to go to find it?

Throughout each section, questions are raised for discussion. Again, make notes if necessary.

1.2 Situational awareness We begin this Reference with a brief section on the principles of Situational Awareness. This general approach underpins the whole subject, and will help you greatly in your day to day work. If you are already familiar with Situational Awareness then you can leave out this section.

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What is Situational Awareness? It can be defined very simply recognising and responding to what is going on around you. For Situational Awareness to be successful it draws on three essential activities:

1. Gathering information 2. Interpreting information 3. Anticipating a future state

OBSERVE Look at the graphic above. What do you think is the relationship between the three different activities LOOK / THINK / ANTICIPATE? You might like to write down your response and compare it to the description below.

NOTE Another way of understanding Situational Awareness is by asking three simple questions: WHAT?

Gathering information: use perception and observation to gain as complete a picture of the situation as possible. SO WHAT?

Interpreting information: think about and understand the meaning of information gathered in step one. NOW WHAT?

Anticipating future states: think about how the situation might develop and how any action based on that development might affect your own ship and others.

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Why do we need better situational awareness? Our perception of reality can become distorted when we are stressed and overloaded with sensory information. In order to cope the brain becomes selective, filtering out information in order to concentrate on what seems most important. Thats where the danger lies.

How to improve your situational awareness WHAT? Gather information. Understand what you need to be aware of on the bridge. Understand also the means by which you gather that information. Where can those resources be found?

OBSERVE Now look at the graphic again. What do you conclude from the relationship between the three activities? It is what we call a virtuous circle, when one stage leads naturally onto the other to become a natural flow. One example from our daily life would be the skills we demonstrate in driving safely and efficiently. Can you think of another example in your own work?

NOTE In recent years, the amount of information available to the bridge watchkeeping officer has multiplied, with electronic charts and AIS. Sometimes it seems the hardest part now is not evaluating the information provided by the navigational instruments, but predicting what the other ship is going to do. Remember just looking up and out of the bridge window can give you an immediate awareness. And do not forget to look abaft the beam.

OBSERVE Make a rough list of all the instrumentation available to you on board a typical ship.

Exactly what is that instrumentation?

How might that instrumentation help you gather the information you need?

How might it prevent you?

How much reliance can you place on the data it provides?

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SO WHAT?

Process the information you have now gathered and assess it. Remember, ships operate in a highly dynamic environment and the situation changes all the time. So might your assessment

NOW WHAT? Based on your assessment, think about how the situation may develop in the future and make a decision about any necessary action. Also, consider how that action might affect your own ship and others.

Other factors in situational awareness In an ideal world the practice of Situational Awareness would naturally become part of best practice. Real life, however, has a tendency to dictate otherwise. Whatever the context, there are other factors you need to be aware of. These include Culture, Language and Leadership Styles.

NOTE We all of us have ways of looking at the world. Remember, as confidence in interpreting information increases, so does the temptation to work from certain assumptions. COLREG 7c Risk of Collision specifically addresses this issue, warning that assumptions shall not be made on the basis of scanty information, especially scanty radar information. We have already considered how we use Situational Awareness in driving. Which assumptions do we often make when driving? How do those assumptions change according to traffic or weather conditions?

OBSERVE A lecturer recently wrote One of the toughest things about learning navigation is the importance of projecting the future position of the ship. Navigation is about more than just knowing where you are; it is about knowing where the ship is going and, more critically, are we heading into danger?

Do you agree?

Can you find three reasons that support, and three that argue against this proposition?

OBSERVE For more information about Situational Awareness and its application, please look at Videotels Vessel Resource Management Series on Leadership and Team Working Skills, and the Vessel Resource Management Training Course.

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2 AN INTRODUCTION TO HYDRODYNAMICS & INTERACTION

As ships have got bigger and cargoes more valuable, the financial and human costs of marine accidents have soared. Anything that helps to reduce such accidents has to be welcomed. A series of incidents in recent years has turned the spotlight on interaction as a contributing factor. This aspect of a ships behaviour is still poorly understood, even among experienced mariners. Yet the effects described in this Reference may affect and be relevant to the application of the International Regulations for Preventing Collisions at Sea and application of STCW and ISM conventions and codes. In this section you will learn the following:

Definitions of hydrodynamics and interaction

Details of a case study the Royston Grange tragedy Questions raised by the subsequent enquiry

First of all, we need some definitions. What exactly do we mean by hydrodynamics and what is interaction? HYDRODYNAMICS

The word hydrodynamic is derived from the Greek hydros, meaning water, and dunamiko, meaning work or action. This is the branch of science that deals with the dynamics of fluids in motion, especially incompressible fluids. In this video we are interested in water, which for most hydrodynamic purposes is considered virtually incompressible INTERACTION This is the reciprocal action or influence when hydrodynamic forces interact. HYDRODYNAMICS & INTERACTION

The first thing to state is that hydrodynamics and interaction are two entirely separate subjects in themselves, and can be studied individually. However, in the video you have just watched, they are linked together. Because of that link, we define hydrodynamics and interaction as the interplay of pressure fields that occurs between a vessel moving through water and its immediate surroundings.

OBSERVE In practice, mariners use a number of terms to describe hydrodynamics and the effects of interaction some of which are more accurate than others.

Do you know any of them?

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Let us now look at a case study in which this interplay had a significant effect.

The Royston Grange tragedy illustrates perfectly the effects that hydrodynamic interaction can have on a ship, leading it into disaster. On the 11th May 1972, the British cargo ship, the Royston Grange, carrying 61 crew, 12 passengers and a harbour pilot, left Buenos Aires with a cargo of frozen beef and butter. At 5:40 am and in dense fog it collided with the Liberian-registered tanker Tien Chee, which was carrying a full load of crude oil. The Tien Chee burst into flames and a series of explosions enveloped the Royston Grange. All 74 people on board the Royston Grange died, and 8 of the 40 crew on board the Tien Chee.

The enquiry concluded that the course taken by the Tien Chee had probably pushed the Royston Grange onto the bank, causing it to bounce off and hit the tanker, due to a hydrodynamic effect called bank rejection. Refer to section 5 of this work book for more details of bank rejection and the associated effect of bank suction.

A ship in motion is surrounded by pressure waves, or fields, which interact with similar fields generated by other vessels, banks and shoals. These waves are more complex when in confined waters such as canals and estuaries. This Reference will explain the main types of Interaction arising from hydrodynamic forces, and how to anticipate and deal effectively with them.

OBSERVE

What other accounts can you find on the net about the disaster?

How do their accounts differ?

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3 WHAT ARE PRESSURE WAVES?

In this Section you will learn the following:

How pressure waves in air relate to pressure waves in water

The effects generated by high and low pressure zones around a truck in motion through air

The effects generated by high and low pressure zones around a vessel in motion through water

The best way of understanding pressure waves is by looking at something we are all familiar with - the bicycle pump.

Look at the graphic. It shows the passage of air through the tube, forced by a piston. As it does so, the air becomes compressed and enters the inner tube, which traps it, causing the tyre to inflate.

1. The plunger action compresses the air in front of the piston leading to a high pressure wave.

2. Behind the piston is an area of low pressure where the air density has

been reduced. 3. Air rushing in from outside at the far end of the pump creates rising

pressure, as the external atmosphere tries to fill the partial vacuum inside the tube.

All of these pressure zones high pressure, low pressure and rising pressure are generated by any vessel moving through water. They are especially noticeable in a confined waterway such as a canal.

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Look at the vessel travelling along a canal. It performs the same function as the piston in the bicycle pump.

1. In a confined channel, the movement of the water is pushed ahead and heaps up, creating a high pressure zone ahead of the ship.

2. This draws water from the midships area resulting in a low pressure zone the water has been pushed forward to form the bow high pressure field.

3. Water rushes in, from astern, to fill the hole created, and so creates a zone of rising pressure, to form a stern pressure field.

Using the bicycle pump to describe pressure waves in air, we have a simple understanding of the first principles of hydrodynamics. But it is limited. In reality, pressure zones are stronger and much more complex, when generated by large scale objects in motion through water or air. For example, following a fast-moving truck on a motorway.

Imagine yourself driving along a motorway. Up ahead of you is a large truck. You decide you are going to overtake it. You accelerate, and pull out into the adjacent lane. You continue to accelerate as you speed past the truck, passing close to its side.

What do you feel?

What pressures is your car put under?

How do those pressures change as you pass?

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While actually concentrating on driving safely, it is often difficult to assess, but you are subject to a series of pressure waves generated by this heavy vehicle moving at speed that works in the following ways.

1. As you start to pass, the high pressure wave at the rear of the truck can push your vehicle away.

2. As you continue to pass, the low pressure wave behind the truck can suck you in.

3. Drawing level with the front of the truck, an even stronger high pressure wave again pushes your vehicle aside. It can even prevent you from completing the overtaking manoeuvre, creating a wall of high pressure which cannot be penetrated.

Now imagine yourself on the bridge of a vessel travelling through either open water or a confined space. Water is eight hundred times denser than air at sea level, so the pressure waves are going to be considerably greater.

However, get it wrong and the smaller craft can be drawn into the low pressure cell with disastrous results. Now look at the video again and in particular look closely at the animation of the pressure zones around a ship in motion.

OBSERVE Can you think of any other familiar day to day situations in which you experience the same range of pressure waves? Make a note of them and discuss them with your fellow students.

NOTE Trucks often travel in convoy because then they can take advantage of the low pressure zones immediately behind each other to help them reduce fuel consumption.

Likewise, a tug following closely behind a moving vessel can be carried along on the boundary between the high pressure plume created by the turning propeller and low pressure cells generated astern. This is known as the Sweet Spot.

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One important distinction which has to be made is between vessels in open water and vessels in confined waters. In open water, the fields are relatively stable, unless distorted by cross-currents. If the vessel is navigating ahead in a straight path, the water flow around the hull is practically symmetrical. In confined waters the pressure zones are often squashed together and further distorted by being reflected by the sides and bottom. Here are some other features of pressure zones around a moving vessel which are important to remember.

1. They are rarely symmetrical. Pressure fields from other vessels, currents, tides, banks or submerged terrain will blur them.

2. The high pressure field at the stern is so large it breaks up before its fully formed.

3. All pressure zones strengthen and expand as the ship accelerates and shrink as it slows.

4. A second low pressure field is contained behind the stern wave 5. In confined and shallow waters, sub-surface fields become critical as they

interact with the sides and bottom.

Sub-surface fields

All of the above have an impact in open water, but naturally these effects are considerably greater in confined waters. All of the pressure zones become more undefinable as they interfere with each other, creating unexpected forces which can affect ship handling. As a member of the bridge team you will need to make allowances for this, and develop strategies which reduce the effects of interaction. Speed through the water is the most important factor in generating hydrodynamic forces. When reduced gradually, it can help resolve problems such as ship squat and loss of directional control.

Let us now take a step back and look at how these pressure fields are formed.

OBSERVE Why not stop the video and draw a rough copy of the animation, paying attention to the pressure zones? It does not have to be perfect but it will help fix in your minds where each pressure zone starts and finishes.

NOTE Reducing speed quickly or suddenly is known to cause ship-handling and directional control problems and should, as far as possible, be avoided.

No such issues exist with sudden speed increases.

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4 THE FORMATION OF HYDRODYNAMIC PRESSURE FIELDS 4.1 The Bernoulli effect In this Section you will learn the following:

A short history of hydrodynamic theory Bernoulli / Venturi How to detect and avoid the Bernoulli effect

To understand how the theory of hydrodynamics evolved we have to go back to 18th century Switzerland, and the Dutch born mathematician Daniel Bernoulli, who studied and taught at the University of Basel. Bernoulli came from a family of prominent mathematicians. His interest was in the application of mathematics to mechanics, and especially fluid mechanics.

In 1738 he published his book, Hydrodynamica. In it he described a method of calculating the hydrodynamic effect on flowing water of being confined.

The physical principle of Bernoullis effect is quite straightforward:

NOTE Bernoullis interest in fluid mechanics was partly due to observations he made on the nature of water when travelling by boat to St Petersburg to take up the post of Professor of

Mathematics.

NOTE Bernoullis theory was so brilliant his father with whom he had badly fallen out tried to claim it as his own in a book he predated to 1732

NOTE Bernoullis principle has some similarities with another mathematicians work Giovanni Venturi (17461822). The Venturi effect also describes the reduction in hydrodynamic

pressure that results when a fluid flows through a constricted section of pipe. The works of Bernoulli and Venturi are important milestones in the development of hydrodynamics as a science.

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1. Water forced to flow through a narrow channel between objects, for example two ships, will accelerate as it enters, then slow down as it exits the constriction.

2. At the point of greatest constriction the water flow is fastest and the hydrodynamic pressure is reduced, so a low pressure zone is generated.

3. If they are not fixed, the two objects will be drawn closer together by this low pressure zone.

In a nutshell, Bernoullis theorem says increased water speed = decreased pressure.

There is also a mathematical equation to be applied to Bernoullis effect, which increases as the speed of the water flowing through the constriction increases. It goes as follows:

1. If the speed DOUBLES, the effect increases by a factor of FOUR. 2. If the speed TRIPLES, the effect increases by a factor of NINE. 3. And so on.

This is what is known as a logarithmic progression.

OBSERVE

What are your own experiences of Bernoullis effect? Where have you seen it at its most pronounced? Think of several examples and discuss them with your fellow students.

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There are a number of other factors that can significantly alter the strength of Bernoullis effect. They include the following:

1. The presence of tidal flow or currents, especially in shallow or confined waters

2. Interference from hydrodynamic pressure cells generated by tugs or other nearby vessels

3. Increased water flow round the hull that results from the wash of a tug All of these must be taken in consideration.

Now we have established the principles of Bernoullis effect, we can look in more detail at some of the interaction it gives rise to, and which you are likely to encounter in your work. These tend to fall into the following two categories:

1. Vessels which are attempting to pass one another at close range. This is most noticeable in confined waters but applies also to ships passing in open seaways.

2. Vessels under way in confined waters which stray too close to a bank, shoal or quayside.

What follows is a series of practical situations that you may encounter. 4.2 Ship squat As we have already seen, pressure fields in water are less defined and more complex than is possible to show in an animation. In fact they are often difficult to see and should not be confused with the normal wash generated by a ship. However, they share some common factors. They are as follows:

1. Pressure fields are rarely symmetrical 2. Pressure fields interact with other vessels, submerged terrain including a

sloping seabed and underwater objects, and tend to blur the boundaries.

OBSERVE What other factors do you imagine would affect the water speed in constricted waters?

Make a list of them and compare it with your fellow students.

OBSERVE Remember, it is the speed of the water (STW) that is critical determining the forces, not the physical speed of the vessel over the ground (SOG).

What effect do you think a following tide or current will have on the hydrodynamic pressure fields your ship generates?

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3. All pressure fields strengthen and expand as the ship accelerates, and shrink as it slows.

4. The stern high pressure field is so large is breaks down long before it is fully formed.

5. Water moving forward, creating the following high pressure field, draws water from further astern creating a second low pressure field, and so on, forming the characteristic diminishing wave patterns that follows a moving ship.

6. A separate deep low pressure cell (hole) is created by moving ships similar to the slip stream created by a moving truck and is located immediately astern, inside the following high pressure area.

It is the interaction of the factors described above along with the speed of the vessel (or the speed of the water flowing past a stationary vessel) that will affect how the vessel sits in the water. This can result in bodily sinkage compared with the static condition and a change of the trim ratio. Speed (STW) + change of trim + vessel sitting lower = ship squat Even then the relationship is not a straightforward one. Ship squat is usually felt more when the depth to draft ratio is less than four. The strength of the effect is proportional to the square of the speed of the ship. By reducing speed by half, the squat effect is reduced by a factor of four.

NOTE Do not forget, pressure fields also project below the vessel for a considerable distance.

OBSERVE

What happens if the vessel goes faster?

Or the vessel is in shallow waters?

How does the speed of the vessel affect the squat?

What other factors do you need to take into allowance in forecasting the squat?

Can you find out some examples of accidents caused by squat or successful uses of squat to aid ship manoeuvring?

OBSERVE

Do you think a vessel moored, or anchored, in a current or tideway will generate the hydrodynamic pressure fields described?

If you think this is a possibility, what will be the effect on small craft or vessels operating nearby?

OBSERVE

Do you think a moored vessel might experience squat?

If so, in what circumstances might this occur?

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Now we can apply the theory you have learnt to some practical examples of hydrodynamics and interaction. All of the following have been simplified in order to show the effects more clearly.

5 INTERACTION 5.1 Bank rejection and bank suction

Factors 1. The ship is travelling along a canal. 2. The rudder is set amidships.

Q: What happens when the vessel is taken too close to the bank? A: The high pressure zone at the bow takes effect first. Q: What impact does that high pressure zone at the bow of the ship have? A: It causes the ship to be turned back towards mid-channel. This is bank rejection.

Q: With the vessel now at an angle to the bank, should there be a similar stern

rejection effect? A: That may be the case, but if the stern gets very close, a localised Bernoullis

effect is created which is stronger than the bank rejection effect and takes over. Q: What does Bernoullis effect do? A: It generates a low pressure zone in the constriction between the ships side and

the bank that draws the vessel toward the bank. It is most noticeable on the quarter.

This is bank suction. Consequences Although the rudder is set to starboard in order to regain a straight course, bank suction effects may exert a force that can be as much as five times greater than a standard rudder can manage.

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Bank Suction + Bank Rejection + Bernoulli effect + Standard manoeuvring capability = Contact with bank ACCIDENT!

When closing on a bank, speed should be reduced and, if possible, the distance between the ship and the bank should be increased. A reserve rudder force must be available at all times to counter any sudden increase in force at the bow (Bow cushion, or bank suction aft). 5.2 Ranging and surging

Factors

1. A moving ship is approaching a moored vessel at close quarters. 2. The moored ship will possibly move briefly towards the advancing vessel

before surging forward on its moorings. 3. The high pressure zone generated by the advancing bow wave will push

forward on the stationary ships stern causing the stern moorings to tighten. 4. Forward moorings slacken off. 5. This forward motion on the moorings will be reversed as the ship completes

its pass. This fore and aft motion is what we call ranging or surging.

Q: What happens as the moving ships bow reaches the moored ships stern? A: The bow waves lateral pressure field pushes the moored ships stern towards

the quayside, causing the stern moorings to slacken. The moored ships bow is forced out towards mid-stream by this movement, tightening the forward moorings.

Q: What happens as the passing vessels bow moves abeam? A: The moving ships bow pressure field pushes the moored ships bow back

towards the quay. Q: What happens to the stern? A: It is drawn out by the low pressure field amidships the moving vessel.

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Q: What impact does Bernoullis effect have? A: Where the low pressure field exerts it will try and draw both ships together. The

result of all the motions acting together often results in a circular motion in the moored ship, as it tugs at its moorings.

Consequences Hydrodynamic interaction between the two vessels can result in considerable strain on the moorings. Fore surging + aft surging + successive bow and stern rejection + Bernoulli effect = pulled off berth ACCIDENT! 5.3 Head to head Originally developed on Britains Manchester Ship Canal in the late 1890s, but also much used on the Houston Ship Channel (hence its nickname, Texas Chicken), this manoeuvre is specifically intended to allow the passing of two vessels in a very restricted waterway. Consequently, not only ship to ship interaction, but also bank effect and squat are factors to be considered. This manoeuvre is normally only undertaken by very experienced ship handlers and properly trained pilots. The two ships are lined up, head to head, with their turning points located over the mathematical middle of the channel and approach each other on what is, essentially, a collision course, but at a carefully regulated speed. Shortly before they collide, at a distance that varies according to the size, blockage factor and the space available but usually not more than one to two ship lengths

OBSERVE The following are some other factors that can play a part in ranging or surging speed of passing vessel, depth of water, distance apart of the two ships and tidal flow or currents.

Find some information on them.

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apart, each vessel will make a slight alteration of course to starboard so as to disrupt the pressure field symmetry surrounding them (otherwise they would simply collide). Once offset, the bow of each vessel is rejected from the other, forcing them apart. When the bow pressure field of each ship interacts with the midships negative pressure field of the other, the rejection effect is neutralised and the vessels steady briefly, before a bank rejection effect causes both bows to move to port in unison, forcing the vessels to straighten up in the channel. Once the midships negative pressure fields coincide, they combine very intensely and the vessels draw bodily together. At this point vessels may also notice a change in squat or a slight heeling towards the other as the Bernoulli effect acting between the two passing hulls can be extreme. As long as both ships proceed at the agreed speed, they are prevented from colliding by the interacting bow and stern positive pressure fields causing rejection and keeping the ships apart. Very shortly afterwards, the stern positive pressure systems interact causing the sterns to reject and, as a consequence, the bow of each ship will sheer across the stern of the other. At the same time, the bow of each ship will also be drawn into the negative hole astern and the sheer will become quite dramatic, requiring considerable helm and engine movements to counteract. The effects can vary considerably if the participating vessels are very different in size. However, properly executed, this manoeuvre allows ships to pass safely in very confined situations with a separation of just a few metres. Factors

1. Even in open waters, the interaction between the hydrodynamic pressure waves generated by two moving ships that pass close to each other can influence the handling of both.

2. This interaction can result in unusual behaviour. 3. The power and effect of this interaction can vary according to the vessels

relative sizes, their speed, distance apart and the depth of water.

Q: What do vessels travelling in opposite directions experience as they pass? A: Mutual bow rejection. Q: Why is that? A: Because their advancing positive pressure fields are interacting.

OBSERVE Can you think of any instances when you have experienced such unusual handling behaviour caused by interaction when part of a Bridge team? Share those experiences with your fellow students.

What can you learn from the experiences of others?

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Q: What happens when the forward high pressure fields and amidships low

pressure fields interact? A: They stabilise, but only briefly. Q: What effect does this have on the vessels? A: The vessels draw together, despite the combined effect of the high pressure

fields.

Finally, just as the manoeuvre seems complete, their sterns are pushed apart as the high pressure zones aft combine to cause rejection. Each vessel is then in danger of steering into the low pressure field astern of the other.

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Consequences Hydrodynamic interaction between the stern high pressure zones continues to affect the manoeuvre as it is completed, so that the sterns are pushed apart. Sterns mutually reject + loss of steering control = vessel sheers into low pressure field astern of other

ACCIDENT! 5.4 Overtaking from astern

Factors 1. Overtaking another vessel from astern can be a hazardous manoeuvre. It

should be planned to maximize passing distance as far as possible. 2. The relative speed of both vessels can result in a longer encounter. 3. So far as circumstances and safe operational parameters allow, the

overtaking vessel should proceed as quickly as possible (without embarrassing the overtaken vessel) and must always bear in mind the Colreg 13 obligation to keep out of the way of the overtaken vessel.

4. In confined channels, or under pilotage, those conducting the navigation of passing vessels may arrange speed differentials that are appropriate to the prevailing situation but this will largely be a matter of experience.

5. Once initiated, bridge teams should carefully monitor the overtaking manoeuvre closely and immediately seek clarification where uncertainty arises.

6. The relative sizes of the vessels can also increase the dangers. The larger vessel has greater influence over the smaller vessel.

Q: What does the overtaken vessel feel at some distance, especially in confined or shallow water?

A: The advance high pressure field of the approaching vessel.

NOTE Any interaction between vessels may be complicated by secondary forces such as bank rejection from the edge of a channel, the bottom, or from the pressure fields of other vessels

operating in the vicinity.

OBSERVE Can you think of any instances when you have experienced such unusual handling behaviour caused by interaction when part of a Bridge team? Share those experiences with your fellow students.

What can you learn from the experiences of others?

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Q: What does the vessel being overtaken experience? A: A slight increase in speed as it surfs on the encroaching pressure field.

Q: How does the approaching vessels bow wave interact with the slower vessels

quarter? A: It causes the stern to be rejected. Q: What can be the result of this? A: A slight alteration towards the path of the overtaking ship.

Consequences This hydrodynamic interaction is initially very difficult to detect and will often be countered, unnoticed, by the auto-helm until the effect has increased to such a degree that the off course alarm sounds. By this time, the slower vessel may already be sheering into the path of the overtaking ship.

NOTE There is an extra risk if the faster vessel is also larger. The smaller ship may become trapped in the bigger vessels intense amidships pressure field and be sucked in.

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Counter helm applied + overtaken ship displaced from course = both vessels dangerously close + Bernoulli effect ACCIDENT! 5.5 Boundary zones There is one area that requires special attention, and that is the boundary zone between high and low pressure fields. It is here that forces can change very rapidly and with significant impact. The most common example of this is when a tug is manoeuvring at close quarters to a moving vessel. This can sometimes work to the tugs advantage, as it gets a free ride in the low pressure zone astern the Sweet Spot mentioned earlier - but boundary zones can change in an instant, and become deadly traps.

Here is a case study which illustrates what can happen, and the dangers involved. CASE STUDY

1. A 1,600 GT cargo vessel in ballast. A harbour tug was assigned to assist her to berth.

2. The tug was instructed to make fast on starboard bow 3. The speed of both vessels was about 4 knots. 4. As the tow line was being passed the tug took a sheer to port and the two

vessels touched the vessels stern striking the tugs port quarter. 5. The tug took an immediate starboard list and within seconds it capsized. 6. One man was drowned.

OBSERVE

What is your personal experience of the hydrodynamic interaction of boundary zones?

Have you been part of a bridge team that has had to deal with their effect?

Share your experiences with others.

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6 TELL-TALE SIGNS We have looked at a whole series of situations that can arise as part of your everyday work. Weve looked at what causes them and what effect they can have. But in order to be able to respond to them effectively, you first need to know what to look for. You need some tell-tale signs. Here is a list of those tell-tale signs, how they are caused, and how you can recognise them. Also included is a recommended action.

Tell-tale signs

Type: Cause: How to recognise: Action:

Engine noise

Sound waves produced by the engine bounce back from the channel bed and banks in confined water. Sound changes as banks get closer or water more shallow

Learn to identify the changes in sound which warn of hydrodynamic effects

Observe .. Note: Changes in sound indicates a potential risk .. Check the chart

and echo sounder .. Reduce speed

slowly if necessary

Frothy wash and wave changes

1. Shallow water more liable to turbulence as following wave pattern wavelengths and frequency shorten .. 2. Changes may cause water to be more foamy or frothy .. 3. Frothy water less dense so propellers meet less resistance 4. Engine can race

Changes in the ships wash and excessive aeration in the form of bubbles are both good indicators

Reduce speed slowly, reduce engine revolutions or change propeller pitch

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Change in following wave height (amplitude) and length (period)

. Wave making may change at the bow

Waves bunch together as they impact on the sea-bed. As they get higher, they also get slower and so catch up with each other . Ship bow waves are likely to change and increase in dimension

Careful observation of how stern waves forms in deep and shallow water Particularly noticeable on a shoalling beach as the vessel passes

Observe notify engine room/ Master/pilot if necessary . Check the chart and echo sounder . Put engines on standby . Reduce speed

slowly if necessary

Steering

Affected steering leads to: 1. Slow rudder response . 2. Stalling . 3. Reversed rudder

application. . As a result, vessel strays from track

Identify changes in steering pattern. Vessel turning circle may increase considerably as the water shallows

Reduce speed slowly, reduce engine revolutions or change propeller pitch

Speed

Increasing engine power results in greater resistance and slower speed. Especially relevant in confined waters. . It is also worth noting that a positive pressure field ahead of a vessel approaching from astern can result in similar problems for the ship ahead.

Constant monitoring of engine revolutions and speed through the water is the ship slowing down for no apparent reason? Stopping distance and times will increase dramatically.

Reduce speed in small increments. This avoids: 1. The vessel

surfing forward.

. 2. Loss of

propulsion as entering a dock or lock.

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Ship movement and attitude may change

Cushioning effect of reducing water layer under the hull has a dampening effect on vessel motion . Squat is likely to increase as water shallows

Rolling, pitching and heaving motions may reduce as the vessel moves from deeper to shallower water Vessels trim may change

Observe roll and pitch timing periods . Observe draught gauges. They may show a change in the vessels fore and aft readings . Note changes.

Notify Master / pilot / engine room

Mud plumes, rising around the vessel caused by flocculation of bottom mud

. Unusual eddies around the vessel . Engine cooling water intakes becoming clogged and thus less effective

Vessel hull is coming close to the bottom and the underwater pressure is interacting with sea bed

Watch for mud plumes in the ships wash and wake Unusual readings on echo sounder Unusual water speed (STW) readings Engine temperature readings change rapidly Unexpected increase in vibration

Observe and be

aware notify engine room/Master/pilot if necessary . Check the chart

and echo sounder . Engines on

standby . Reduce speed if necessary . Change engine

room sea suctions from low to high if necessary

Now we have looked at the impact of hydrodynamic interaction on your ship. But what about the impact you are having on other vessels, even those far away? This brings us on to the final interaction we need to consider.

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7 INTERACTION AT A DISTANCE As a member of the bridge team it is not enough to maintain a constant awareness of pressure fields around your own ship. What kind of effect do you think you are having on other craft in the vicinity? And what influences that effect? When making a judgment the following factors need to be taken into consideration:

1. Size. A big difference in relative sizes increases the impact on the smaller vessel.

2. Speed. This influences the size and strength of all the pressure fields generated by the vessel.

3. Open or confined waters. What difference does it make to the distance hydrodynamic effects can be felt?

There is evidence gathered from open water collisions to suggest that the pressure field at the bow of a moving vessel can extend up to 3 times the ships length. For a Very Large Crude Carrier this can mean affecting the handling of another ship over half a mile away.

Imagine having an impact on the manoeuvring abilities of another ship so far away it cannot even be seen. That is the range and extent of hydrodynamic interaction, in confined waters.

OBSERVE Have you ever had the experience of being in a small craft caught up in the pressure field of a much larger vessel, either in open or confined waters?

Can you remember what it was like? Share your experiences with others.

NOTE Examples of this have been recorded in places such as port entrances near pilot stations, relatively shallow straits, e.g. Dover and Malacca, and in tidal estuaries.

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8 SUMMARY

It is the intention of this Reference to help you understand the basic principles of Hydrodynamics and Interaction as they impact upon you in your daily work. As you will no doubt agree, it is a complex subject but we believe it can be summarised as follows.

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Interaction is an aspect of ship handling that is often insufficiently understood, in both its causes and effects. We hope by now you have gained a better understanding and knowledge of hydrodynamics and interaction, and feel more confident in dealing with situations as they arise in your daily work. Please refer back to the video for more information, and to review any section.

9 CASE STUDIES Case Study 1 Running aground At approximately 16.32 on 3 June 2004, a double hulled chemical tanker ran aground on Lymington Banks in the west Solent. The vessel suffered bottom plate indentation forward but no hull penetration. Nobody on board was injured and there was no pollution. Synopsis

Having completed loading a cargo at Fawley Marine Terminal, Southampton, UK, a pilot was ordered, and the vessel sailed at 15.15. The Master had decided to proceed to the English Channel via the west Solent and Needles Channel, as he had done on a previous occasion 6 weeks before. This decision was contrary to his companys

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standing instructions that required its vessels to use the east Solent route when arriving or sailing from Southampton. The pilot disembarked, automatic steering was engaged and the vessel continued her passage through the west Solent. It was a clear day, with little recreational craft traffic and no other commercial traffic in the west Solent. Neither the second officer nor the cadet were sure of who was responsible for plotting positions on the chart, although both did some rudimentary checking off of buoys passed. The Master was not paying attention to the navigation of the vessel, and was distracted, using the ships mobile telephone. The vessel ran aground on Lymington Banks at about 16.32, at a speed of about 11 knots. At this point she was approximately 0.5 mile north of her intended track.

How could knowledge of hydrodynamics and interaction have helped here?

What tell-tale signs could have warned of imminent danger?

What action could have been taken? Learning points

1. Lack of training meant an inability to recognise early indications associated with shallow water effects, for example increased engine vibration, bank rejection, the autopilot applying counterhelm.

2. Poor bridge team management on the vessel resulted in a lack of accurate vessel positional awareness and an inappropriate division of tasks.

3. The use of the mobile telephone distracted the Master from his primary responsibilities.

4. The routine transit of large vessels, some carrying hazardous cargoes and some carrying large numbers of passengers through the west Solent and Needles Channel, is a cause for concern. The route passes through an environmentally sensitive area but the navigable channel is narrow. Given the shifting shingle of the seabed, there is no pilotage available and the area is not monitored by any local Vessel Traffic System.

Case Study 2 Collision. On Saturday, January 23, 2010, about 09.35 an oil tankship collided with a 597-foot-long general cargo vessel at the Port of Port Arthur, Texas, USA. A 297-foot-long barge, which was being pushed by a towboat, subsequently collided with the oil tankship. Synopsis The tankship was inbound in the Sabine-Neches Canal with a load of crude oil en route to an oil facility in Beaumont, Texas. Two pilots were on board, as called for by local waterway protocol. When the tankship approached the Port of Port Arthur, it experienced several unintended heading diversions culminating in the tankship striking the cargo vessel, which was berthed at the port unloading cargo. A short distance upriver from the collision site, the towboat was outbound with two barges. The towboat Master saw the tankship move toward his side of the canal, and he put his engines full astern but could not avoid the subsequent collision. The forward barge collided with the tankship and breached its starboard ballast tank and the No. 1 centre cargo tank a few feet above the waterline. As a result of the breach, 862,344

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gallons of oil were released from the cargo tank, and an estimated 462,000 gallons of that spilled into the water. The three vessels remained together in the centre of the canal while pollution response procedures were initiated. No crewmember on board any of the three vessels was injured.




1. The vessel was out of position with respect to the channels mathematical middle and the hydrodynamic effects of bank rejection and bank suction combined to rob the vessel of effective steering.

2. The US National Transportation Safety Board (NTSB) determined that the probable cause of the collision of the tankship with the cargo vessel, and the subsequent collision with the barge was the failure of the first pilot, who had navigational control of the tankship to correct the sheering motions that began as a result of the late initiation of a turn at a mild bend in the waterway.

3. Contributing to the accident was the first pilots fatigue, caused by his untreated obstructive sleep apnoea and his work schedule.

4. Guidelines were not followed which meant a radio call was not answered by the second pilot. Also contributing was the lack of oversight by the Jefferson and Orange County Board of Pilot Commissioners.

Case Study 3 Dont pass on bends. Two vessels collided while passing one another on a bend of a busy river. Both vessels had pilots on board and were regular visitors to the river. Synopsis The inbound vessel was running with the tide at about 7 knots over the ground, while the outbound vessel was making 5 knots over the ground against the 4 knot tide. Both vessels had been responding normally to helm, and the pilots had agreed by VHF that they would pass on a particular bend in the river. This meant that neither vessel would have to slow down, and that the bend was relatively wide and open. Both pilots had passed vessels on this bend previously. Although the outbound vessel was known to handle poorly at low speeds, at a speed through the water of about 9 knots this was not seen as a problem. However, this speed was reduced as the vessel approached the bend by bringing back the pitch on the controllable pitch propeller. As the vessel entered the bend, the inbound vessel was about 180m ahead, on a reciprocal course, and looked safe to pass about 30m apart. The pilot on the outbound vessel applied 10 starboard helm to bring his vessel more toward mid-river, but noticed the bow swinging slowly to port. He therefore increased helm to hard a-starboard, but, since there was no effect, he instructed the Master to kick the engine ahead. When the vessels were about 50m apart, with the outbound vessels bow still falling off to port, the pilot ordered full astern to reduce the inevitable impact. The vessels collided seconds later.


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1. In trying to pass on a bend, multiple hydrodynamic effects come into play, including bank rejection and bank suction, loss of steering capability and loss of power. Good Situational Awareness would have prepared for this.

2. This accident would not have occurred had the pilots agreed to pass on a straight section of the river. A slight delay of one vessel was all that was needed to ensure that the vessels met in a straight, either side of the bend.

3. The manoeuvrability of the outbound vessel was known to be poor at slow speeds, and reducing speed on a controllable pitch propeller would have had the effect of creating a baffle to the flow of water over the rudder, exacerbating this problem. This effect could have been minimised by ensuring that the speed was reduced gradually. An earlier reduction of speed would also have allowed power to be increased again just before the helm was applied, thus maximising the flow over the rudder.

10 FURTHER INFORMATION Regulations

The International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW), as amended

The International Regulations for Preventing Collision at Sea 1972 (COLREGS) as amended

The International Safety Management (ISM) Code

Standard Marine Communication Phrases (SMCP) Publications

The Behaviour and Handling of Ships by Henry H. Hooyer FNI, published by Cornell Maritime Press

Guide to Port Entry

Manoeuvring Information for the Pilot/Navigator. Its Source, Value and Limitations by Thomas G. Knierim, Sandy Hook Pilot 1991. Published by the Society of Naval Architects and Marine Engineers USA

Manoeuvring Single Screw Vessels Fitted with Controllable Pitch Propellers in Confined Waters by Captain H. Hensen FNI, published by The Nautical Institute

MCA Marine Guidance Notice MGN 199 (M), Dangers of Interaction

New Thinking on Ship Generated Hydrodynamic Fields (RINA STS-11 Conference. Proceedings for 2nd International Conference on Ship Manoeuvring in Shallow and Confined Water, Trondheim 2011)

NP100 The Mariners Handbook

Ship Design and Performance for Masters and Mates by Dr C.B. Barrass

Ship Dynamics for Mariners by Ian C. Clark MSc FNI, published by The Nautical Institute

Ship Generated Hydrodynamic Fields Nautical Institute Seaways Magazine, August 2010

The Ship Handlers Guide by Captain R. W. Rowe FNI, 2nd Edition, published by The Nautical Institute

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Tug Use in Port by Captain H. Hensen FNI, 2nd Edition, published by The Nautical Institute

Websites

http://www.shallowwater.ugent.be/EN/kc_EN.htm

Ship Squat: Are We Out of Our Depth http://www.pilotmag.co.uk/2008/02/01/squat-part-1/

www.nwint.net Videotel training programmes

Leadership and Team Working Skills - Vessel Resource Management Series: Part 1 An Introduction to Resource Management (Code 1142) Part 4 - Bridge Watchkeeping (Code 1145) Part 6 Working With Pilots (Code 1147) Part 7 Resource Management & Accident Prevention (Code 1148) Part 8 Five Case Studies (Code 1149)

Pilot On Board! Working Together (Code 945) Ship Handling in Head Seas (Code 661)

Ship Handling in Restricted Waters Ship Squat and Shallow Water Effect (Code 697)

Ship Handling in Restricted Waters Bank Effect & Interaction between Two Ships (Code 748)

Vessel Resource Management Training Course (Code 884)

Working With Tugs (Code 972)

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Hydrodynamics & Interaction

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