Topic: BusinessIndustry

Last updated: April 12, 2019

Optimum choice of propulsion systems in merchant ships.

Student Full Name Ioannis NannosStudent ID 150534347A dissertation submitted in fulfilment of the partial requirementsfor the Bachelor Degree of Engineering in Marine Technology with Naval ArchitectureSchool of EngineeringNewcastle UniversityMay 2018Supervisor: Prof/Dr Xiangyin MengDeclarationNo part of the work presented in this thesis has been submitted in support of another degree or qualification of this or any other university or other institute of learning.ConfidentialityIf your work is confidential, you should declare here.Table of Contents TOC o “1-3” h z u Abstract PAGEREF _Toc513175719 h 3Introduction PAGEREF _Toc513175720 h 3Propellers PAGEREF _Toc513175721 h 3Main Engines PAGEREF _Toc513175722 h 4Investment and operating costs PAGEREF _Toc513175723 h 5Investment costs PAGEREF _Toc513175724 h 5Operating costs PAGEREF _Toc513175725 h 5Ship speed PAGEREF _Toc513175726 h 5Route planning PAGEREF _Toc513175727 h 5Vibration criteria PAGEREF _Toc513175728 h 5Machinery vibration. PAGEREF _Toc513175729 h 5Axial or longitudinal vibration PAGEREF _Toc513175730 h 6Torsional vibration PAGEREF _Toc513175731 h 6Lateral or transverse vibration PAGEREF _Toc513175732 h 6Power estimation criteria PAGEREF _Toc513175733 h 7Vessel’s resistance PAGEREF _Toc513175734 h 7Estimation of the mechanical and electric alternator ratings. PAGEREF _Toc513175735 h 7Mechanical propulsion PAGEREF _Toc513175736 h 7Electric propulsion PAGEREF _Toc513175737 h 7Manoeuvrability criteria PAGEREF _Toc513175738 h 8Turning circle: PAGEREF _Toc513175739 h 8Zig-zag test PAGEREF _Toc513175740 h 8Spiral test PAGEREF _Toc513175741 h 8Pull-out test PAGEREF _Toc513175742 h 9Stopping test PAGEREF _Toc513175743 h 9Rudders PAGEREF _Toc513175744 h 9Conclusions PAGEREF _Toc513175745 h 9AbstractThis research is made in order to analyse the right criteria for the right selection of a propulsion system on a merchant ship.

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The procedure that was followed in this dissertation paper is the analysis of the following criteria: investment and operations costs, vibration, power estimation and manoeuvrability and their importance at the right selection. Throughout the research, it can be realized that all these steps must be followed if the company is going to make profit out of their investment. Moreover, these steps must be followed due to safety reasons, because each vessel is designed for calm weather conditions, and not the weather conditions that might or will face during its lifetime.IntroductionAlthough that ships are sailing the seas for thousands of years, mechanical propulsion systems were firstly introduced to the shipping industry during the 19th century. These ships were using steam engines and screw propellers at their propulsion system. One of the most famous vessels of this type was named Selandia in 1912 and was using this type of propulsion system during the maiden voyage to the Bangkok and was the largest and most advanced ship in her time (I. Nannos, 2015). The 20th century was important for the industry because that’s where the most technological achievements for the propulsion systems took place.

In the middle of the 20th century marine steam turbines were introduced to the industry but only by the end of the second world was were completed (A. Tzifakis, 1978). In our days’ diesel engines are the most commonly used propellers prime mover in the marine industry because of their high efficiency and slow speed engines can be connected directly to the propeller without the use of a gearbox. A large engine room is required for this component of the propulsion system.

Most of the merchant ships use diesel engines connected directly to fixed pitch propeller (I. Martinez).PropellersThe main component of the propulsion system of a ship is the propeller, and it has a major role in a propulsion system (I.Martinez). In addition, they produce a thrust to move the ship through water. They are at the aft of the ship in order to have the maximum efficiency (MAN, 2013). It is not necessary for ship to have only one propeller, sometimes ships can use two propellers and in rare occasions more than two.

Also, the number of the blades may be different. Propellers might be constructed with two to six propeller blades. Thrust (T) is required to move a ship at a speed (V) and at a certain direction. The ship’s speed is normally greater than the total resistance (RT) and the propeller’s speed depends on the number of the blades (G Politis, G. Tzampiras, 2016). Propellers may be divided in two sub groups: fixed pitch propellers and controllable pitch propellers. 457202062480Figure 1: Fixed pitch Propeller with 6 blades.

Reference: CITATION Shi17 l 2057 (Ship Technology, n.d.)00Figure 1: Fixed pitch Propeller with 6 blades.Reference: CITATION Shi17 l 2057 (Ship Technology, n.d.

)476252245360Figure 2: Controllable pitch propellerReference: CITATION Nak17 l 2057 (Nakashima Propeller Co., LTD. , n.d.

)00Figure 2: Controllable pitch propellerReference: CITATION Nak17 l 2057 (Nakashima Propeller Co., LTD. , n.

d.)The less blades in a propeller gives higher efficiency, but because the propellers are subjected to heavy loads they must have more than two or three blades. For merchant ships propellers usually have four to six propeller blades and in some occasions five to six blades (Container ships). Fixed pitch propellers are usually constructed of copper alloy and the position of the blades is fixed and so the pitch cannot be changed in operation. Which means when operating in heavy weather conditions, the combination of speed and power, will change by physical laws (MAN). These types of propellers are usually used by ships that do not require good manoeuvrability like Container ships, Bulk Carriers and Crude Oil Tankers.

Controllable pitch propellers have a larger centre part (hub) than fixed pitch propellers, because they have a hydraulic mechanism for the control of the pitch of blades. Controllable pitch propellers have lower efficiency than fixed pitch because the centre part is larger. These types of propellers are used for shuttle tankers, ferries and RO-RO ships.

Moreover, controllable pitch propellers are more complicated and have a high risk of damage during service. In order to obtain the maximum propulsive efficiency (nD) the biggest diameter of the propeller is used(MAN). For types of vessels that are sailing in ballast condition for a big amount of their lives (bulk carriers and tankers) in this condition the propeller must be fully immerged and there are limits about its size. Main EnginesDiesel engines are the main movers of a vessel and they use marine diesel or heavy fuel oil. Diesel engines have high efficiency compared to turbines. Furthermore, because of their relatively low fuel consumption are used widely in marine industry in most of the ships since 1930s. The most common manufacturers of these type of engines are MAN/B;W and Wartsila-Sultzer (I.

Martinez).514352141220Figure 3: MAN Diesel Engine.Reference: CITATION MAN171 l 2057 (MAN Marine Engines and Systems, n.

d.)00Figure 3: MAN Diesel Engine.Reference: CITATION MAN171 l 2057 (MAN Marine Engines and Systems, n.d.

)50800170815Figure 4: Wartsila-Sultzer Diesel EngineReference: CITATION ATu13 l 2057 (Turpen, 2013)00Figure 4: Wartsila-Sultzer Diesel EngineReference: CITATION ATu13 l 2057 (Turpen, 2013)Diesel engines are internal combustion engines and have a function of converting the chemical energy of the fuel into mechanical energy (MAN) and some of them can be seen in the figures 3 and 4. They deliver the mechanical power to the shaft. First, they deliver thermal energy with combustion, by the gas’s expansion in the cylinders they move the pistons.

This work is rotational work on the crankshaft. Moreover, this shaft might be connected to a propeller or to a reduction gearbox before the propeller depending on the vessel’s speed. A two-stroke engine has two processes, compression and expansion and they have air inlet and gas exhaust at the beginning of compression and at the end of expansion these are called piston’s two strokes. A four-stroke engine has the same processes but air inlet and gas exhaust are separated so, compression, expansion, air inlet and gas exhaust. These are the called piston’s four strokes (A. Tzifakis, 1978).

A ship can have more than one main engines and the number of the main engines is the same of the number of the propellers. The ship’s main engine is in the engine room which is the largest, most complex ship’s compartment and the noisiest. In most of the ships, the engine room is located at the bottom aft of the ship, in order to have the minimum shaft length of the propeller (or propellers). However, in large ships there are several engine rooms and auxiliary engine rooms. Their efficiency depends on the losses of the exhaust gas, cooling water and friction losses between the parts of the main engine (MAN).

A turbocharger can be used in order to reduce the exhaust gas losses. As a result, it increases the engine’s output power, reduces the thermal losses from the exhaust gasses and it reduces the fuel oil consumption. Investment and operating costsInvestment costsThe total cost of the investment of a newbuilding vessel or a second-hand vessel is a matter of the ship owner of how much money he wants to invest. Throughout the years the market has changed, the market is related to the global economy and the needs of the people. For example after the second World War the need for more ships lead to an increase in the shipbuilding market.

From the year 2005 till 2015, the market showed an increase from 2005 to 2007 where it reached a peak and it dropped till 2009 due to the economic crisis and since it shows increases and decreases roughly every two years. So it is obvious that the market is not stable. For the ship owners point of view, the best time to make an investment is when the market is at the lowest, but in order to accomplice such a thing is difficult cause the market is not stable.

When the market shows a decrease in orders for a specific type of vessel, it might show an increase in another type. So, when the market has a decrease in one type it will show an increase to another type. Between 2014 and 2015, the orders for bulk carriers decreased, while the orders for tankers and containers increased.

When some major news are taking place in the world, for example a decrease in currencies the market is expected to decrease as well. On the other hand, when the currencies increase especially USD ($) because all the exchanges in the shipbuilding market are made in dollars the market will show an increase.Operating costsShip speedlifetime the ship will encounter different weather conditions and furthermore in some cases these conditions have an impact on the propulsion features.

The ability of a vessel to maintain a certain speed during a voyage is the main goal in the stage of the design. The additional resistance on a ship during voyage is of great importance cause of a demand in increase in speed and reduction in voyage duration. Moreover, especially after 2009 there is an increase need to reduce the ships emissions.

When the wavelength of the wave is less than the half of the length of the vessel, the waves motions are minimum and the additional resistance is caused mainly by the ship. When the ships motions are increased, they influence the additional resistance.Route planningOne of the most important factors to calculate the total cost of the vessel’s voyage, is the route planning. With the selectin of a vessel’s route and taking into consideration the weather conditions, will not only improve the efficiency of the shipping, in addition it reduces the risks, also allows a better prediction in the vessel’s arrival time at its destination. If the best route for a vessel is to be selected the vessel’s performance at the weather conditions that the vessel may encounter during that route. The selection of the route can be influenced by some factors. First of all, the weather conditions that the vessel most likely will face at a specific part of the route. Furthermore, the calculation of the vessel’s dynamic response cause of the effect of these weather conditions.

The sea state can have an effect to the vessel’s speed and the vessel’s operation, so these must be taken into consideration. When the weight height and the wind’s speed increase, the propulsion system operates in different conditions from the designed ones, as a result the efficiency of the engine and the propeller will reduce. In order to reduce these effects the route might slightly change and/or the vessel’s speed will be reduced for the vessel’s safety.Weather conditions can have an impact on the vessel’s fuel consumption and the CO2 emissions, so the optimum speed of the vessel and its heading course have to be calculated in advance for the fuel consumption to minimize.

Vibration criteriaVibration can be categorized in machinery vibration and hull vibration depending basically on the source for the propulsion system the main concern is machinery vibration.Machinery vibration.The vibration that occurs due to the operation of the main engine or the propulsion shaft or any other machinery (propellers, gearbox, generators) and it happens because of the rotation of their moving parts at a specific frequency.So, at the design level it must be known how these machinery operate and cause vibration so it can be kept at a safe level and they can be either axial or longitudinal vibration or torsional vibration or lateral vibration.Axial or longitudinal vibrationIs the most important category of machinery vibration and is the one that it will probably cause vibrations is the axial vibration. In order to analyse a category, the most important thing is the identification of excitation of each type. The axial vibration causes the propulsion system to act as a horizontal spring mass system of multiple degrees.

The thrust that it is generated by the ship’s propeller depends in the water’s velocity that acts on the propeller blades. The wake of the propeller is not uniform. The wake at the bottom of the propeller is different than the wake at the top of the propeller, and that happens because of the curvature of the aft of the vessel. The value of the wake is different for each angle of the propeller. The wake is different for each distance from the centre of the propeller and for each angle as well.

What can be understood from this is that for a specific propeller blade at a certain angle the velocity of water will be different than if the propeller blade had a different position (angle). This difference along half of the propeller’s rotation is continuous. (for example, the trust will be different for a specific blade at zero degrees’ angle position and 90 degrees and 180 degrees.

This repeats by each rotation, so the thrust that is generated by the propeller is periodic and is also called alternating thrust. And is the force of the axial vibration of the ship’s propulsion system.The excitation’s frequency is given by (RPM x Number of blades), for the resonance to be avoided the thing that must be ensured is that the first frequencies of the axial vibration of the vessel’s propulsion system has a variance of at least 5% from the frequency of the propeller’s excitation. The frequency of the propulsion system depends on the system’s mass and stiffness.

The most common approach is the change of the system’s stiffness by changing the thrust bearing and its foundation. The foundation is the component that deflects in response to the thrust of the shaft that is transmitted by the bearing. So, it operates as a spring. The bearing must be changed in order to obtain a stiffness that would change the frequency of the propulsion system to a preferred value.The steps that must be followed if the frequency of the propeller is within the range of frequency of the propulsion system. The propeller frequency must change or the frequency of the propulsion system should change.

If the propeller frequency is going to change then either the RPM or the number of the blades must change. Changing the number of the propellers is not a suitable option cause it has an impact on the propeller’s efficiency.If the RPM will change is not suitable option either cause in order to provide the effective power to the vessel the RPM have been decided, so this option can’t be applied.The only way to accomplice that is by changing the frequency of the propulsion system because it doesn’t affect anything on the vessel.

Torsional vibrationA ship’s propulsion system consists of a propeller a shaft and an engine. The shaft and the engine are not single components of the system. From the propeller to the engine there is a propulsion shaft an intermediate shaft a coupling flange and an engine coupling flange. The pistons of the engine with their rotational movement, create an excitation.At the stage of the design it is important to select the optimum engine so its frequency at MCR is not in the frequency range of 5% of the entire propulsion system.Lateral or transverse vibrationThis type of vibration happens at the direction of the axis of the rotation of the shaft.

Cause of the shafts bending, its centre of gravity is not on the shaft’s centreline and when it rotates the force causes the shaft to move further from the shaft’s centreline. The number of bearings is a factor of this type of vibration.It should be taken into consideration that the frequency of the transverse vibration is not the same with the engine’s frequency. Because it can lead to a shaft to snap and cause damage.Furthermore, when the engine starts to operate, the speed increases. The is one point when the maximum vibrations of the vessel can be felt for a few seconds. This happens because there is one point at which the RPM of the engine is the same with the frequency of the shaft. This speed must be avoided.

It is called also critical speed. No vessel should operate at this speed range and it should be passed quickly to prevent shaft vibrations.At the design level the sources of vibration must be considered, in order to take protective measures: manufacturing defects of gearing system cause high frequency and has to be considered at the manufacturing process.

Main engine excitation, the moving parts of the engine are the primary source of excitation and they’re responsible for whirling and torsional vibration. propeller loads and shaft alignment errors, at the manufacturing process if the centrelines of the coupling flanges are not.carefully chosen then as a result it can lead to a loss of the shaft’s rotation axis.Power estimation criteriaWhen a vessel needs a high amount of power, needs also high amount of fuel for every voyage.

Which means high expenses for the shipping company. In recent days’ vessels are designed with low power requirements, so low fuel consumption.In order to decide which is the optimum selection of the propulsion system for a vessel the power requirements for each type must be calculated by completing the following steps: calculation of ship’s resistance, type of the propulsion system and the estimation of the ratings.Vessel’s resistanceThe first step in the calculation of the vessel’s resistance is the towing tank test.

At the towing tank test the resistance of the model is obtained through the computer and then scaled up to the vessels dimensions. The towing tank is necessary only if the vessel’s hullform hasn’t been tested before, otherwise only the scaling method is used.With the towing tank test only the resistance of the bare hull is obtained. In order to obtain the vessel’s total resistance, the air resistance and the correlation allowance has to be added. For the calculation of the vessel’s effective power the velocity of the vessel has to be multiplied with the total resistance.Decision of the type of the vessel’s propulsion systemThis is the most important step in this criterion, because the wrong type selection might lead the company to an economic catastrophe. Through research it is known in our days which type of propulsion system is preferred for every type of vessel.

The mechanical is used in the majority of the cargo vessels (motor oil tankers, bulk carriers, containers) the need low speed and low operation costs. The mechanical propulsion is preferred also due to the times of recession in the industry.The electric propulsion is used in vessels that need high amount of electric power for the operations for their voyage such as cruise ships and also ships that need varying torque.The difference between the mechanical and the electric propulsion system is that the electric provides high efficiency at all torques where the mechanical propulsion does not provide high efficiency at all torques.

Estimation of the mechanical and electric alternator ratings.Mechanical propulsionThe resistance of the vessel is increased from the value of the bare hull. During the towing tank test the effect of the propeller was not taken into consideration. The propeller has to operate at a torque value that can overcome the vessel’s resistance. Moreover due to the propeller’s losses the delivered power() has to be higher than the effective power() the ratio between these two values takes values between 0.

55 to 0.65.The engine’s output power is not the same with the shaft’s output power due to the frictional losses that happen along the shaft’s length.

These are known as shaft’s losses. They’re usually 2%. If a gearbox is used for the reduction of the shaft’s RPM then the losses are roughly 4-5%Furthermore the effect of the waves leads to an increase to the vessel’s resistance, so a margin line of a 15% is taken and the engine output power should be higher than the margin line.For diesel engines, the fuel consumption reaches a minimum value when the engine’s RPM is operating at the 85%Electric propulsion The electric propulsion system consists of the following components: electric motor diesel generators, loads where usually are bow thrusters or propulsion pods and transformers.Motors, propellers and loads will operate in all conditions.

If dynamic positioning is in present all loads and propulsion unit will be at maximum. Moreover, the total power requirement has to be calculated, before choosing how many generators are required. When the total power is estimated, then the number of generators will be chosen, based on principles.

For the calculation of the total power requirement, the designers use the load chart list which includes all the loafs on-board, three operations are taken into consideration.On ships, the electrical loads can be calculated by multiply the maximum rated power of the component: Load factor is the maximum power rating divided by the operating power of the component.Utility factor is the factor that determines the operation of a component in a particular condition.

The utility factor during the conditions of manoeuvring and sailing is 0.8 and in harbour is 0 since is not used. So the total power requirement during sailing is going to be 0.On every vessel, the load chart is prepared for every electrical component.When the load chart is prepared, the total power requirement of every condition: manoeuvring, sailing and harbour can be found by the addition of every component’s power requirement for every condition. That’s how the total number of the diesel generators on-board is decided.two rules have to be followed in the decision for the number of generators:If more than one generator is operating in any condition, both the generators should share equal amount of load.

The load on each generator in any of the three conditions should not be more than 70 percent of the rated power of the generator. One additional generator should always be included, which is for standby purpose. It has to be noted that standby generator won’t share the load in any of the three conditions unless any of the working generators are out of order. So the standby generator is not included in the calculation, but it is usually of the same rating as the other generators.This process is iterated by varying power ratings and varying number of generators until the first two conditions are satisfied.Manoeuvrability criteriaThe manoeuvrability is the ability of ships to perform the navigation of the voyage. It’s different for every type of vessels. Ferries need high manoeuvrability because is essential for operations because of the danger of collisions and is strictly related to its safety, composes turning, course keeping course change speed change and stopping ability and because they reach port really often they need increased manoeuvrability.

Other types of vessels such as tankers and bulk carriers they don’t need high manoeuvrability because they operate in lower speed. Container ships need higher manoeuvrability than tankers and bulk carriers because they operate at higher speed and their delivery contracts are very strict.Because manoeuvrability of a ship is difficult to evaluate, some standards have been set. The majority of the vessels spend most of their time in the open sea. Tug boats can be used in order to assist ships with manoeuvring. So, ships must have good directional stability. In order to evaluate the manoeuvrability of a ship, the ship has to complete during the sea trials the following tests:Turning circle:As the rudder is put over there is a force that’s pushing the vessel during the opposite direction from the one it wants to turn. As hydrodynamic forces are building up on the hull, the vessel slows down and moves in a steady circle with steady speed and a steady radius.

Firstly the ship advances at a 90 degree angle and must not exceed 4.5 times the length of the ship. Then the rudder of the vessel takes a heading of 35 degrees with a steady angle and speed and when the vessel has completed 270 degrees of the circle, the circle’s length must not exceed 5 times the length of the ship.Zig-zag testThe purpose of the zig-zag test is to assess the capability of a vessel to make a zig-zag motion which is of great importance. When the vessel is moving under full linear speed, then a quick starboard rudder operation to a predefined rudder angle of 10 degrees has to made.

When that step is done then the vessel will change its heading direction. When the heading will reach the predefined value of 10 degrees, then a second quick port rudder operation to a predefined rudder angle of -10 degrees. Due to the effect of the inertia, the vessel will continue to move towards starboard turn motion but it will be slowing down, after that the vessel will start making an adverse turn motion towards port side. When the vessel will change its heading angle to -10 degrees, then a third quick starboard rudder operation with an angle of 10 degrees. Due to the inertia effect, the ship will continue moving towards port and slowing down, after that the vessel will start make an adverse turn motion towards starboard side. When that rudder operation is repeated five times the test is completed.Spiral testThe purpose of the spiral test is to give a fell for the vessel’s directional stability. When the vessel is moving under a straight course and a steady speed, the rudder is put over to 15 degrees towards starboard.

After that, when the vessel is moving under a steady rate of turn, then the rudder angle has to be reduced to 10 degrees towards starboard side, the new steady turn rate has to be noted. This operation has to be repeated also for 5 degrees towards starboard side, also 15 degrees towards port side, 10 degrees towards port side and 5 degrees towards port side. Then the steady rates of turn have to be plotted on a diagram against the rudder angle.

Now if the vessel is not stable, the diagram will have two curves for the small rudder angles. And if the vessel is stable, the diagram will have a unique rate of turn for every rudder angle. In the non-stable state, because there is no unique response the direction that the ship is going to turn, based only on the rate of turn. The minimum rudder angle has to be known, and the range of the rudder angles is not determinate is only a mere guide.

Pull-out testThis particular test has also a purpose of the ship’s directional stability. A rudder operation has to be done at a certain angle until the vessel turns under a steady rate. The rudder then has to return to amidships and the change in the turn rate with the time has to be noted. If the ship is stable the turn rate will reduce to zero and the vessel is moving under a new steady line course.

A diagram has to be plotted with the log of the rate of turn against the time and it has to be a straight line after a short transition period. The area under the curve gives the total heading change, and the smaller it is, the more stable is the vessel.Stopping test When a large vessel is trying to stop, the vessel can travel for miles before it stops and also move from its original course. The turning under control has to be a better manoeuvre in order to avoid collision, the stopping test has to be carried out in order to find the head reach towards port or starboard direction at a crash stop and also the stopping time.

What it takes the propeller to stop, depends on the type of the propeller. Fixed pitch propellers will stop and the put in reverse. And controllable pitch propellers can have their pitch in reserve more quickly.RuddersRudders are primarily used in order to steer a vessel. Rudders can be categorized as balanced, unbalanced and semi-balanced. When the rudder is balanced it needs less torque in order to turn in compare with unbalanced and semi-balanced.

Furthermore, rudders give a good lift to drag ratio.In the more recent days new type of rudders have been developed. Flap rudders use a flap at the edge and with their shape they improve the lift.

So it can take twice the angle of the main rudder. Moreover, there is the Flettner rudder which has to flaps at the edge, and they move so they can improve the movement of the rudder with the reduce of the toque that is required by the steering gear. In addition, there is also the cycloidal rudder which has a rotor casing and rotates vertically. This type of rudder has two operations:Active: The rudder operates as a vertical axis propeller to improve the thrust in all directions.Passive: The rotor turns only in certain angles, so the blades of the rudder create steering forces.The cycloidal rudder has good shock resistance and low noise levels.Kitchen rudder is like a two-part propeller and turns at the vertical axis.

At a turning operation the two parts are moving together so they can deflect the propeller. For ahead operation are open to both aft and fore flow, and for stopping operation the two parts are moving so they can block the propeller.Conclusions In order to do the right selection of a propulsion system the criteria that were analysed in the research have to be followed.

The most important step is the operations costs and the route planning in order for the shipping company to have low operating costs and start earning profit from their investment. Furthermore, depending on the type of the propulsion mechanic or electric there are different efficiencies, velocities and torques so the right selection has to be made depending on the type of the vessel in order to for the shipping company to avoid economic losses. Moreover, the total investment cost can be reduced with the right research from the shipping company for the right offer from a shipyard. The calculation of the total power requirement will lead to the right number of generators that have to be put on-board and their right operation is necessary for terms of safety and also for less cost in operations. In addition the manoeuvrability tests are essential, due to the fact that each type of vessel should have the right results from these tests for the right behaviour during its lifetime.


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