Tender and Stiff Ships

If the name of this article suggests that it has something to do with the strength of the ship structure, then leave your presumptions at the door. But rinsing up the basic concepts of stability would reap a better understanding of what tender and stiff ships are all about. Form the designer's point of view, is that, deciding on the suitable metacentric height (GM) of the ship is a very important factor in setting the stability and comfort standards of the ship. Why? Well, this has everything to do with the behaviour and response of the ship in rolling motion. 

A ship having very high center of gravity (CG) will end up having a low metacentric height (GM), resulting in a reduced righting lever (GZ). Which means:

• More force is required for the heeling ship to return to the upright position.
• If you look into the formula for the roll period of a ship (below), the roll period will be high for a ship with low GM. 

Roll Period

The rolling pattern of this ship will be sluggish. It will take longer time to roll back to an upright position, rendering the ship to be comfortable. Such kind of a vessel will be a tender ship. Cruise ships and ocean liners (ships that are designed for more comfort) are designed to be slightly on the tender side, with roll periods around 10 to 12 seconds. At the same time, the designer maintains a minimum required GM to provide the ship with stability. A ship too tender (very low or zero metacentric height), would be unstable. A tender ship will also have more probability of capsizing in case of large weight shifts or high speed turns and strong beam winds. So optimum level of safety and comfort is attained midway between a very high and very low metacentric height. This comes primarily from experience and feedback from existing ships. The Korean ferry MV Sewol (article) is now said to have capsized due to an overloading. Its CG was actually higher than what it was designed for, which resulted in a very low GM, resulting in the rapid capsize. This is would not have happened, had the Captain made a check on the tenderness of the ship that day.

On the other hand, a ship having very high metacentric height (GM) would show the following behaviour:

• A stiff ship will tend to respond to the wave profile more rapidly, tending to assume the slope of the passing wave. 
• So, even though a stiff ship will develop rolling moment easily in a passing wave, it will also require less force to return to an upright position, rendering the ship more stable.
• Also, the time period of the rolls would be shorter. 

But if you're assuming that a stiff ship would be the better option, you probably need to see the bigger picture. Look at it from the angle of the roll period. The roll period of a very stiff ship would be quite low, making it uncomfortable for a person on-board (often called motion sickness). So a stiff ship, though highly stable, is never preferred from the comfort point of view. Again, it is in the hands of the designer to make a balance between the sufficient stability and motion response of the ship. 

In many cases, ships were found to be too tender after trials. Redesigning and rebuilding is an uneconomical and meaningless option to think about. So in such cases, permanent ballasting often proved to be useful, wherein one or more void tank spaces in the lower compartments of the hull were filled with calculated amounts iron ore or similar high specific weight material that resulted in lowering the center of gravity by required levels to attain the required metacentric height. For ships that were on the stiffer side, motion dampeners like stabilizer fins and anti-roll tanks were incorporated. 

A lot of experience goes into designing a ship for its required metacentric height according to stability standards and at the same time, keep the roll periods in favourable limits. Still doubting the importance of these terms? Imagine you own two cruise ships. One that can capsize faster than your childhood paper boats and the other that tosses your passengers around with every passing wave! LSD

Article By: Soumya Chakraborty

Why Midships Fail?

If you look into any of the recent or past accident cases where a ship's hull developed a crack or entirely split into two, the striking factor will be the region of occurrence of this phenomenon. In all the cases, the cracks or split-offs have originated from the midships (i.e. 25% of the Length Overall from the midships). Rather than discussing much on why the cracks and split-offs developed, this article will discuss more on why do they develop in the midships region only? We will directly delve into the theory that governs the analysis of our question, and then you can read some rare and interesting articles and videos on case studies of such accidents, that I have recommended at the end of this article.

So, why midships only? Why don't hulls generally crack at the aft and for'd regions? Why didn't any ship ever split off from a region closer to the aft and for'd ends? Courtesy to Euler's Simple Beam Bending Theory. In this theory, the boundary conditions are generally fixed-fixed, simply supported, hinged-hinged or fixed at one end. Now compare your ship to one of those beams with any of the above boundary conditions. Which boundary condition do you think will fit this case? The answer is none. So how do Naval Architects use the Euler's Theory to analyse the structure of ships? 

In every engineering problem related to structures, the key deciding factor of the analysis is the boundary condition that is to be determined before the analysis started. If the designer is not efficient enough to choose the most suitable boundary condition for a structural analysis, they design is often prone to be more of an engineering disaster. Naval Architects have been very careful in deciding the boundary conditions for a floating ship and over the years, designers have considered the hull to be a beam supported by an elastic foundation. In other words, the water provides support to the hull but in a continuously varying amount. In order to understand the variation of this support or reaction force on the hull (which is basically the buoyancy force) along the length of the ship, lets start with a sample hull form in Figure 1.

Fig. 1: Hull of a RO-PAX ship.

The upward reaction (buoyancy) exerted by the elastic foundation (water) longitudinally varies in magnitude. The nature of variation will vary according to the longitudinal distribution of the submerged volume of the hull. The buoyancy force will be more at the midships as the submerged volume in this region is larger and it gradually decreases at the aft and for'd ends, as the submerged volume reduces. This variation of submerged volume can be well visualized from Figure 2. As a result, the longitudinal distribution of the magnitude of reaction force (buoyancy) is somewhat as shown in Figure 3.

Fig. 2: The submerged portion of the hull (looking from under the keel)

Fig. 3: Buoyancy per unit length - Buoyancy Curve

The ship's hull is also subjected to the weight (acting vertically downwards) of components like main engine and machinery, propulsion system, superstructure, ballast, fluids (Lube oil, Fuel Oil, Fresh water, Bilge, etc), mooring and anchoring equipment, piping, cargo (distribution of cargo weight depends on what kind of a ship it is) and the hull's own steel weight. Some of these components are almost point weights (example: anchor weight, windlass weight) and most are distributed weights. All these weights per unit the individual lengths of their distribution are plotted to scale, with the magnitude on the vertical axis and the longitudinal position on the horizontal axis, and what is obtained, generally looks like Figure 4.

Fig. 4: Longitudinal distribution of weight per unit length - Weight Curve

If we superimpose one graph on the other, and subtract the buoyancy from the weight at every single point, we will obtain the net load (often called only load) distribution along the entire hull. Since this is a continuous variation of weight and buoyancy, we use the fundamentals of calculus and express the load in the following way:

Total Load on Hull Girder (beam) = ∫w.dx - ∫w.dx

where,

• w = value of weight per unit length (from weight curve)
• b = value of buoyancy per unit length (from buoyancy curve)
• dx = length of infinitesimally small element of hull girder

What is interesting in case of ships, unlike most beams used in civil structures is that, the net load on the beam (here, the hull) is not always acting downwards. In the areas where the buoyancy exceeds the value of the weight, the net load on the hull is vertically upwards and vice versa. The load curve that is generally obtained is as shown in Figure 5. 

Fig. 5: Load curve obtained from weight and buoyancy curve

What you are going to know now, is a concept that a Naval Architect must never afford to forget in his entire career. The load curve of any ship is random and changes with almost every voyage. So we obviously cannot represent it by a particular function to obtain the Shear Force and Bending Moment Diagrams (recall your concepts from Strength of Materials). What we do, apply simple calculus knowing the significance of the following expressions:

Shear Force = ∫(w-b).dx

Bending Moment = ∫∫(w-b).dx.dx

In easier terms,

• Shear Force at a point is the area under the load curve up to that point from the aft end.
• Bending Moment at a point is the area under the Shear Force curve/diagram up to that point.

If the process is followed according to the two point above, the so called SF and BM diagrams of the ship is obtained for the given loading condition as shown in Figure 6 (remember, this is subject to change during its next voyage depending on whether it returns only on ballast or partially loaded).

Fig. 6: SF and BM Diagrams of a ship (Remember, these graphs will be different for different loading conditions, but their natures always remain same. Always.)

Now that we have obtained the bending moment diagram for the ship, its time to note a few of the most important aspects of structural design of a ship. As you read each point below, make sure you never let them out of your brains!

• The shear force in the hull girder is always zero at the aft, ford and midships. 
• The bending moment in the hull girder is always maximum at the midships.
• Due to the the maximum bending moment occurring at midships, if we can design the hull with a longitudinal strength sufficient enough to sustain the bending moment at the midships, our design is safe! (even then a check is always conducted for every frame of the ship. Prevention as you see, is always better than cure!)
• The three above points will remain the same irrespective of the kind of loading on the ship.

All what we saw till now, was a mere application of Euler's beam bending theory in the way used by hull designers. Now, a ship's hull can bend in two ways depending upon the distribution of loading on the hull:

When the concentration of weight is more at the for'd and aft ends or when the crest of a passing wave is at the midships with the troughs at the for'd and aft ends (therefore providing more buoyancy at midships than at the ends), the ship is said to Hog, as shown in Figure 7. Similarly when more weight is concentrated at the midships or when the trough of a wave is at the midships and the crests at the ends of the ship (therefore more buoyancy is now being exerted on the for'd and aft ends), the ship is said to Sag.

Fig. 8: Container ship "Fowairet" in a hogged condition due to grounding. Can you guess the position of the grounding impact on the ship, going by the fact that it has hogged?  

(Courtesy: Google Images)

Fig. 9: Oil Tanker "Prestige" that split off due to excessive sagging.
(Courtesy: Google Images)

What actually happens within the hull girder due to the developed bending moment is that, a bending stress is developed at every transverse section of the hull. The universal expression for bending stress is :

Bending Stress = (Bending Moment)/(Section Modulus)

This brings us to the two most important observations that are kept in mind during the structural design of a ship's hull:

• Maximum Bending Moment at the midship means that the bending stress at the midships will be the maximum, and hence the deciding factor for the design.
• The bending stress at the midship is kept within safe limits by designing the midship section with a sufficient section modulus. (Again assuming you are thorough with the basics of Strength of Materials!)

So, if we look at a midship section, and study the bending stress on it due to the loading on the ship, we will be closer to the answer of the question we are looking for an answer to. 

Fig. 10: Bending Stress distribution at the midship section of a tanker in Sagging condition.

Observe the above figure and tally them with the points which will follow:

• The neutral axis of the section is generally closer to the keel (due to more construction material at the bottom of the ship)
• Since the ship is sagging, the bottom plate is subjected to tension and the main deck plating is under compression (Visualize!). The opposite happens during hogging.
• The deck plate being further away from the neutral axis, compared to the bottom plate, experiences more magnitude of bending stress than the bottom plate. (Tally with the formula of bending stress!)

If the tension in the bottom plate in this condition exceeds the maximum tensile strength of the hull girder material (Mild Steel: Yield Strength = 490 MPa), the plate fails or in other word, the bottom plating cracks. Whereas, if the ship was exposed to a condition of hogging such that the tensile stress at the deck exceeded the ultimate strength of the deck plating and shear strake material, they fail or develop cracks. When the crack propagates through the side shell, the ship is prone to split off into two. 

What you read,  was just the entire theory behind why ships have generally been seen to crack or in some cases, split off due improper loading or inefficient design. Go back to the name of this article, and you will now be able to answer the question yourself! LSD

Article By: Soumya Chakraborty

Recommended Readings and Visuals of Accident Cases:

1. MSC Carla Sinking Case: How lengthening of the original ship, led to the disaster. (Courtesy: Ship Structure Committee)
2. Titanic Disaster (Video): How and why Titanic actually broke into two? (Courtesy: Titanic Movies)
3. Analysis and Design of ships subjected to Collision and Grounding. (Lin Hong, Thesis for the degree of doctor philosophiae)

Dry Docks

Dry docks are structures which are used for the repair, maintenance, construction and launching of vessels.They are termed the ''workhorses'' of ship repair facilities and sometimes used in place of traditional shipbuilding methods.

There are four types of such docks in use based on how they provide ''dry'' access for work.

1. Basin dry docks which are basically excavations with a gate at one end opening to the waterway which when flooded buoys the ship structure from the keel blocks.
2. Marine railways consisting of cradles on rail running on inclines which launch and retrieve vessels.
3. Vertical Lifts which are ship elevators designed to be lowered into water and then lift it clear from the water surface in the basin.
4. Floating dry docks are "vessel" structures having their own stability which are capable of sinking and receive a vessel to lift it out of water.

SOME FLOATING DRY DOCKS BASED ON SHAPE AND MODULAR STRUCTURE

Although each of these docks have their own uniqueness in terms of being a facility for construction and repair, some factors needing consideration for the desirable operation of the dry dock regardless of the type are:

• Adequate space in and around the dry dock ease of movement of materials and people.
• Fast access to and from the vessel  and dry dock especially for vehicular traffic is a an advantage.Some of these arrangements are known as travelling stagings and 'Dock Arms'.
•  Good lighting and ventilation.
• Electrical facilities(outlets) and machinery.
• For heavy items, cranes and material handling systems need to be present.
• Efficient system for moving vessel in and out of dry dock.(tensioning winches,capstans,other line handling hardware.)
• Proper Block Arrangement.(Use of keel and bilge blocks).

Shipbuilding dock with an intermediate gate

Most of these blocks are made of composite construction having concrete or steel bases and 'cap pieces' made of rubber or timber for providing cushioning against ship hull.

 Removal of these blocks provide access to hull areas in case repair.This is accomplished by use of sand frames set on the top of the block's concrete base.

Sand in the frame supports the top pieces of wood and access is obtained by using water jets to remove sand which allows the wood to drop so that block can be removed.Although other sophisticated methods exist today for the removal of blocks but this simple arrangement is the most preferred and perhaps quite successful.

SELECTING A DRY DOCK:

Selection of a dry dock is influenced by many factors like:

• Dimensions, weight and characteristics and general features of vessels to be serviced by the dry dock.
• Available land area,available area in water,proximity to navigable channels or open water.
• Purpose of the dock(new building/long term vessel repairs/short term repairs or a combination of all of these).
• Near and far term goals and the possible future extension of facilities.
• Financing(Generally depends upon the type of dock and its features).

We shall discuss some basic features of basin and floating dry docks.

BASIN DRY DOCKS:

Basic features of a basin dry dock as shown in the photo include floor, sidewalls,head wall and dock gate.The structures may be gravity structures,ground anchored, and under drained (pressure relieved).

Basin dry docks are suited for large vessels. Advances in material sciences have continually expanded the dimensional limitations of dry docks.

The operation of this kind of dry dock is very simple.Once, the ship and docking blocks have been properly prepared, the basin is filled with water until the level of water reaches the depth with keel blocks above/below sill.

These docks are prone to silting and scouring at dock entrance.This factor needs to be considered as excessive silting would mean high costs from maintenance dredging.

Tides and waves at the site have a direct impact on construction,as they influence the elevation at the top of the gate and along the coping.

Soil types and groundwater levels also influence the structure suitable for the site,the depth to which it can be economically built and the structural design of floors and walls.

Also, supporting shop facilities should be in close proximity and sometimes crane services are essential for moving material in and out of basin.

The key dimensions of the basin depend on the type of ships to be docked and whether the basin is to be used strictly for construction,repair or a combo.There are allowances in these dimensions for providing working space .Most basin dry docks today have an effective length-to-width ratio between 5:1 and 7:1,in approximate proportion to large contemporary vessels.

A Basin Dry Dock

FLOATING DRY DOCKS:

As vessels grew larger and heavier, floating dry docks followed suit with a variety of designs.Maintenance of dry dock by virtue of a self docking capability was an important feature.Today floating dry docks continue to be important parts of many ship repair facilities.Modern floating dry docks are most often built as non-self-docking,one-piece steel units.

Floating dry dock works on the principle of Archimedes principle,by displacing a volume of water equal to its own weight.The dimensions and arrangement of structure,presence of flooding/pumping system permit the weight of the dry dock to vary which allow the dock's pontoon deck to be submerged or undock a vessel.Ballast tanks are present which allow sinking of the dock to desired water depth.The vessel is then placed/or removed from the dry dock. 

Capacity of such dry docks are determined by their buoyancy,stability and structural strength.

Important terms in the ship-dock stability calculations.

The dimensions of the wing walls and compartmentalization of the pontoon provide necessary stability of the ship-dock system.Intact stability for all phases during docking operation.Until the pontoon deck breaks the water's surface, the stability is at minimum.The dimensions of wing walls and ballasting should be co-ordinated with the dock's design vessel to ensure positive stability characteristics.

Compartmentalization of the pontoon provides for more precise control of the dry dock, in addition to enhancing the stability.Floating dry docks have the initial advantage that they can be built and fully equipped in shipyard and factory conditions. LSD

A Floating Dry Dock

Article By: Sudripto Khasnabis

Author's Note: This article is intended to let the readers know about dry docks and how they function in the shipbuilding process .The figures and photos do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at learnshipdesign@gmail.com

MV Sewol - What Possibly Happened?

The case of the sinking of the South Korean Ferry MV Sewol surely has a lot of untold mysteries hidden underneath. She started heeling rapidly to her port side sometime before 08:58 AM (Korea Standard Time), then distress calls were made to Jindo Vessel Traffic Services Center (VTS). Not less than two hours later, the first ship had approached for rescue operations. But the first question that must be pinging your mind now is "Where were the lifeboats? Liferafts? Why didn't the Captain of the ship give an evacuation order with ample time in hand? "

According to the possibilities that have been taken into consideration by our team of Naval Architecture students, we have discussed possible scenarios that may have led to the tragedy which has been reported to take away 185 lives as to today (25 / 04 / 2014)

Rather than directly answering to the questions (which would be vague at this stage of the investigation), lets make this much easier with a little bit of background. The picture below shows the standard route followed by Sweol in its voyages from Incheon to Cheju do island. 

Route of MV Sewol from Incheon to Cheju do and the accident site (Courtsey: Google Earth)

If you notice carefully in and around the area of the accident site, the colour of the sea water shown changes to blackish. It is because the region is full of underwater rocks. So you must be thinking she hit a rock and got grounded. As reported by a state broadcaster, she was off her usual course on the day of accident. But possibility of grounding can still be easily ruled out. Wondering why? Understand the figures below:

Weight, Buoyancy and Vertical Reaction acting on a ship's hull when grounded.

Clear from the above diagram, the side of the hull hitting the rock will eventually tend to emerge from the water and the other side will tend to sink below. That is, the ship heels by the side which has not faced grounding impact. If one considers that Sewol's hull hit an underwater rock, then going by the capsizing pattern (she capsized by the Port Side), she must have hit the rock by her starboard side. At higher angles of heel as shown below, the opening created by the rock on her steel hull would be clearly visible. Her starboard side was intact. Grounding ruled out!

Starboard side of MV Sewol's hull didnot have any marks of grounding impact.

MV Sewol was "Ferry Naminoue" before it was renovated in 2013. What happened during the renovations is actually a question raising issue. In 2012, extra passenger cabins were added to her third, fourth and fifth decks, increasing the passanger capacity by 181. This caused a rise in the center of gravity of the ship by 0.51 meters. Passanger ships generally operate in moderate GM (metacentric height). A high GM would make the ship too stiff and uncomfortable for passengers. On the other side, an unsafely low GM is also not preferred so as to maintain the required stability margin. But the rise in center of gravity of the ship eventually caused a decrease in its GM after the renovation. It is even unknown if the owner had made any other changes on the ship after regulatory approvals were completed. 

Not only this, the ship carried more than three times of cargo weight on the day of accident. She was designed to carry cargo load of 987 tons but she had 3608 tons of cargo that day. Obviously, another cause supporting the rise of center of gravity and even decreased metacentric height.  If a ship with marginal metacentric height turns at high speed, it is likely to heel by significant angle, and in worst cases even capsize.

This scenario matches pretty well with MV Sewol's case. The Captain was possibly aware of the marginal metacentric height and this is why he may not have ordered the passangers to move to the upper decks during the first one hour, expecting that the ship's heel could be controlled, if weight of the passangers (64.2 tons approximately, if an average passanger has weight of 65 kg) were limited to the lower decks thus preventing further rise in center of gravity.

The third mate Park Han-Geyol reportedly ordered the helmsman to make a 5 degree turn, which was a part of the ship's course. But tracking data show that she made a turn of 45 degrees at a speed of around 18 knots (4 knots below its design speed). Whether this was a careless move, or a failure of the steering mechanism, is unknown as of yet. But two things can be easily inferred:

• If it was not a failure of the steering mechanism, the human error involving carelessness of the helmsman and the navigation officers was the prime reason behind such a steep turn.
• But a ship is not designed to heel over and capsize even when it turns steeply at its design speed. In this case, the speed was 4 knots lower. There comes in the problem of stability which Sewol surely had. She was operating at a marginal metacentric height (already explained 2 minutes ago!). The more the metacentric height of the ship, greater is the uprighting moment of a ship when it heels to either side. A reduced GM must have caused the righting lever GZ to fall below what it should have been for safe operation of the ship. And as the ship took a steep turn of 45 degrees at 18 knots (which is not much below its maximum design speed of 22 knots), the centripetal force acting on the ship caused it to heel to an angle at which the righting lever reduced to zero, causing her to capsize. Sewol obviously made this turn to her starboard, as she capsized by heeling towards port side (apply Laws of Physics and understand the figure below or watch the video below).

Forces on a ship when it turns to Starboard Side.

Sewol was not only designed to carry passengers. It had additional capacity to carry cars and containers. Reportedly, many passangers heard loud explosions after the sudden heeling of the ship. That was possibly due to shift of unproperly lashed containers and cars in the ship's hull. This might have not only gave rise to explosions, but caused the ship to heel further to the port side due to a shift in transverse center of gravity.

Also, unlike other ships, ROPAX vessels like MV Sewol are not designed with bulkheads that divide the ship into watertight compartments. This possibly caused the entire car decks to get flooded once the ship started taking in water due to large angles of heel. The problem with ships taking in water is actually something that results in exponential rise in risk of capsizing. Why? If a ship starts taking in water, the free surface generated by the water in the ship creates a free surface effect which raises the center of gravity thus rendering the ship more unstable and prone to taking in more water. Since ROPAX ship's donot have bulkheads, this problem cannot be limited to a compartment, and the entire ship floods eventually.

What can be seen from all these evidences by the help of Naval Architectural Principles is that, the prime cause of the ship's capsizing must have been the reduced stability, coupled with the human error of the crew involved in not carrying out effective evacuation procedures at the correct time.

Official investigations are on the way and actions have already been taken against the Captain and officers. Not only this, the authenticity of Korean Register of Shipping is also under investigation as there can be every possibility of illegal approval of many such similar ship designs that are already sailing even now with hundreds of lives in danger!

Article By: Soumya Chakraborty

Author's Note: This was a report of an investigation done by the team of students who own the LSD blog. Conclusions in this article have been drawn on the basis of informations obtained from News and by application of Naval Architectural principles to estimate what might have actually caused MV Sewol to sink the way it had, on 16th April, 2014. The video used in this doesnot belong to LSD and full credit goes to the owner. Thank You for reading. In case of any queries and doubts please comment or write to learnshipdesign@gmail.com

Autonomous Underwater Vehicles-Their Design and Functioning

INTRODUCTION

An Autonomous Underwater Vehicle (AUV), also called  unmanned underwater vehicles, are robots that perform underwater survey missions such as detection and mapping of obstructions, rocks, submerged wrecks( like that of ships) without the need for input from an operator.The first ever AUV was the SPURV (Special Purpose Underwater Research Vehicle), developed at the Applied Physics Laboratory at the University of Washington as early as 1957.
The design of these vehicles is influenced by the purpose of their operation. How the individual components function as a whole is also determined largely by the circuital pathways and of course, it is necessary to factor into account, the energy management. AUV's are sold by around 10 major manufacturers on the international market, including prominent ones like Kongsberg Maritime, Bluefin Robotics, and International Submarine Engineering (ISE) Ltd.

Collections of propelled AUVs and gliding AUVs (also called gliders) are now often used for mapping and oceanographic research, for military reconnaissance and harbour protection, or for deep-sea oil-well maintenance and emergency response. Today, fleets of up to 20 such AUVs have been deployed, but in the coming years far larger fleets could come into service.

DESIGN

National Institute of Oceanography (N.I.O), Goa,India, has developed an AUV ‘Maya’ as a tool for scientific and commercial applications. 

Equipment:

AUVs operate independently of the ship and have no connecting cables. AUVs can be equipped with a wide variety of oceanographic sensors or sonar systems. NOAA’s hydrographic survey AUVs are typically equipped with side scan sonar, Conductivity-Temperature-Depth (CTD) sensors, GPS-aided Inertial Navigation Systems (INS), and an Acoustic Doppler Current Profiler (ADCP).Primarily oceanographic tools, AUVs carry sensors to navigate autonomously and map features of the ocean. Typical sensors include compasses, depth sensors, sidescan and other sonars, magnetometers, thermistors and conductivity probes. 

Propulsion:

One of the largest design considerations for autonomous underwater vehicles (AUV’s) that have specific mission scenarios is the propulsive efficiency. The propulsive efficiency affects the amount of power storage required to achieve a specific mission. As the efficiency increases the volume of energy being stored decreases. The decrease in volume allows for a smaller vehicle, which results in a vehicle that requires less thrust to attain a specific speed.

Propeller based thrusters or Kort nozzles are the most common among AUVs.These thrusters are usually powered by electric motors and corrosion of motor internals are avoided. One consideration which impacts this process of waterproofing is the decision to use brushed motors or brush-less motors which also impacts reliability, efficiency, and cost. Propellers are usually designed with a complex geometry that changes along the blades radius. The process of selecting an efficient propulsive system becomes an iterative process between motor, propeller, and battery storage. 

A Kort Nozzle is a propeller fitted with a non-rotating nozzle.

Navigation

AUVs navigate using an underwater acoustic positioning system. When some reference such as a support ship is available, baseline  positioning is used to calculate where the sub sea vehicle is relative to the known (GPS) position of the surface craft by means of acoustic range and bearing measurements which is some sort of a via mechanism.Orientation (including heading) is determined by in an alignment process where the Kalman filter uses gyros and accelerometers to determine local gravity vector and the Earths Rotation which are essential in navigation.Alignment is done both statically and while in motion.

Power

AUVs use many of the existing rechargeable battery systems in existence today, lithium ion(the one in an average mobile phone), lithium polymer, nickel metal hydride among others , and are having some sort of system for battery management. Some vehicles use primary batteries . Some larger vehicles are powered by the extremely efficient semi-fuel cells, but these require proper handling and up keep,also they are not cheap and require disposal of wastes. An emerging trend is to combine different battery and power systems with supercapacitors. High density energy sources are what designers look for today.

The battery compartment showing the 50 kWh Al/HP semi fuel cell. 

AUV BLUEFIN 21 : MH 370

Bluefin-21 AUV is currently completing mission ten in the underwater search area. Bluefin-21 has now completed more than 80 per cent of the focused underwater search area. No contacts of interest have been found to date.

                                                

As per the reports of the indianexpress ,underwater search for the crucial flight recorders of the crashed Malaysian jet can be completed , provided the weather is favorable for the AUV. The focused underwater search area is defined as a circle of 10 km-radius around the second Towed Pinger Locator detection which occurred on April 8.Don't forget to watch the video below.

Finding the black box and the wreckage are crucial to know why the Beijing-bound plane veered off from its route and mysteriously vanished after taking off from Kuala Lumpur. The mystery of the missing plane has continued to baffle aviation and security authorities who have so far failed to trace the aircraft despite deploying hi-tech radar and other gadgets.

THE CHOICE

There comes a choice in the basic design form which influences the hydrodynamic properties of flow around it and which in turn influences the performance and effiecieny. Something which helps in the analysis of the flow are computational flow techniques (CFD), now according to this approach, the design form (B) on the right has a more streamlined form and has better flow charateristics around it, reduces power consumption. 

However the design (A) on the left has better storage and larger equipment carrying capacity. The choice of design is now largely influenced by the needs of the owner or organisation. Both designs have been used, but an AUV equipped with the most routine equipments and basic power storage for a medium endurance mission generally edges in favour of the cylindrical body shape. Having said that, there are underwater semi-submersibles and robots which come in a range of shapes and sizes including the design (B). Factors worth considering before making the choice would be power requirements, equipment capacity, manoeuvring characteristics and cost. Hope this helps in understanding why most AUVs are cylindrical although there are ways to make them more streamlined. LSD

Article By: Sudripto Khasnabis

Author's Note: This article is intended to familiarize the reader about Autonomous Underwater Vehicles in light of the recent MH 370 Flight incident and how AUV's are proving to be useful in this search.The videos and the figure do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at learnshipdesign@gmail.com