N OVEL P ROPULSION C ONCEPTS O F M ANEUVER ABILITY NOVEL PROPULSION CONCEPTS OF MANEUVERABILITY & & S HIP C ONTROL -- A ORT P ERSPECTIIVE VE SHIP CONTROL A P PORT PERSPECT PERSPECTIVE SYNOPSIS {This paper primarily seeks to address the design, manufacture and operational aspects of ship maneuverability, while doing so we will also examine the various various upcoming propulsion concepts, which which have a direct bearing on the maneuverability & operations. Main aim of the paper is to advocate the induction of the advancement and research regarding the various propulsion ,ship control stabilization & steering equipment into modern shipbuilding practice so that ships of today and tomorrow are more manageable, steerable & safer as well as more economic to operate when it is entering / exiting a port limit or revisiting a channel or shallow patch. The subject of ship maneuverability or controllability is of continued importance to the shipping community as maritime traffic adapts to suit the changing needs. The commercial restrains associated with current high energy prices are shifting the balance between capital costs and running cost for marine transportation system..}
Presented By : SAPTARSHI BASU ,C/E,Fleet ,C/E,Fleet Management Ltd.(Author) Ltd.(Author) SHISHIR DUTT,2/E,Exmar DUTT,2/E,Exmar Shipmanagement India Pvt. Ltd.(Co-author) Ltd. (Co-author)
INTRODUCTION Amongst the many factors affecting the safety and the economics of ship handling , is the skill of the pilots and masters, navigation systems and vessel traffic aids, as well as ships inherent maneuverability. As for the human friability is concerned a lot of work has been devoted to improve useful navigational aids to shipmasters in the form of sophisticated collision avoidance systems, control and maneuvering data booklet available on the bridge . Additionally the training of the crews using ship handling simulators have become mandatory. An analysis has been made for large number of casualty reports, involving a total of 835 cases of CRG (C ollision ,R ,R amming & G rounding) damages, from USCG database using statistical methods. The causes of the accidents show the following distributions:-
UNA VOIDABLE = 35% HU MA N ERROR ERROR = 30% POOR POOR MAN EUVERABIL EUVERABIL ITY = 35% ________________________________________ TOTAL = 100% ____________________________________________ Referring to unavoidable casualties as to those whose causes were completely beyond operator control or vessel handling characteristics, further statistical studies on the influence of several controllability factors on collision, ramming and grounding damages (CRG) , suggest that quite a large number of CRG incidents could be potentially reduced by improving the maneuverability of the ships. Thus, claims by some quarters referring to the STCW-95 convention that some 80%to 95% of the CRG damages are due to human error may not be entirely correct.
T h i s t o p i c i s i n v e s t i g a t e d i n d e t a i l s u n d e r t h e f o l l o w i n g h e a d i ng n g :I)
MANEUVERABILITY WITH CONVENTIONAL PROPULSION STRATEGIES.
II)
PERFORMANCE ANALYSIS AND COST FUNCTION.
III)
SHIP CONTROLLABILI CONTROLLABILITY TY REQUIREMENTS , ASSESSMENTS, VALIDATION AND TRIALS.
IV)
ALTERNATIVE PROPULSION CONTROLLABILITY.
V)
CONCLUSIONS AND RECOMMENDATION RECOMMENDATIONS. S.
CONCEPTS
FOR
BETTER BETTER
DIRECTIONAL DIRECTIONAL
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(I) MANEUVERABILITY WITH CONVENTIONAL PROPULSION STRATEGIES When a ship approaches another solid underwater boundary (berth / Quay etc.) changes occur in local pressure field which can cause two ships close together to collide, or ship to hit a nearby bank / berth lying parallel to its course. Shallow water also changes the local hydrodynamic pressure field around the hull leading to bodily sinkage of the ship known as squat squat.. The total affect for ship traveling at speed in shallow water can be an increased draught by approx. 10%.Due to commercial constraints the modern ship building trends are towards larger ships with greater freeboards with slow speed engines and simpler powering arrangements, frequently using ports where operational clearances clearances in terms of available width in the locks ,depth of water under keel or room to turn are severely curtailed. For large container ships operational schedules and commercial demands force them to berth near river side or estuarial terminals, making the ship to operate in tidal conditions which tends to reduce the available safety margins & the likelihood of grounding and collision are increased .The definition of ship controllability as given by , International Tank Towing Conference (ITTC) is “Controllability is that quality of ship which determines the effectiveness of the control in producing any desired change in a specific rate, in the attitude or position of a moving ship ”. ”.
(a) Steering control:control:-The most basic control required for the ship are control of direction/ heading of the ship, and of its speed. The basic requirement for heading control is that the ship’s head is maintained within a given band of desired value which is known as steering error , which depends on the dynamic properties of ship, the effectiveness of steering arrangements, the disturbances present in form of wind and waves, and the human factor. The difference between the desired and actual course of the ship is called heading error . The helmsman or autopilot will act an this error and will alter the demand to rudder control mechanism which controls the rudder(s) to the desired angle after a time lag & within the bounds of error of the control system. Rudder at this angle acts on the slip stream of the propeller(s) and creates a turning moment on the ship. The ship will turn under the combined effect of rudder, inertial and hydrodynamic forces, about a point which will usually be some distance forward of the mid-point of the ship. In some ship,(such as VLCC), this pivot point, at which there is no component of sway velocity, velocity, is situated some distance forward of the ship.The reason for siting rudders rudders aft of the propellers at the stern of the ships is as follows : 1) It is able to exert a large lever lever arm about about the pivot point and is able to position the ship such that the hydrodynamic forces assist in the turn. 2) The rudder rudder at the stern of the ship is positioned positioned so that the the propeller propeller slipstream slipstream augments the flow of water over the rudder. The rudder forces are heavily dependent on the flow velocity of water across the rudder, the effectiveness of the rudder is enhanced by this positioning.
(b) Effects of disturbances: disturbances: The most common disturbances acting on the ship effecting course keeping requirements requirements are winds and waves but other effects which effect the course control behavior will include:-
i)
presence of seabed:- The ships behave differently in shallow water compared to the way they do in deep water. Turning ability is reduced and the diameter of turn is increased.
ii)
presence of bank and other ships:-Ships tend to turn away from banks and there are complex interactions between the two ships passing close to each other. The way in which the steering ability of the ship is affected by the presence of wind depends on the shape of both the above water and underwater hull, and on the strength and direction of the wind. The effect of wave will generally be to reduce the effectiveness of control mechanisms, and to make steering within a given course margin more difficult.
(c) Speed control and fuel economy requirements:requirements:- Much effort is expended in ensuring that a ship is designed to operate as economically as possible. Fuel cost is one of most significant of ship’s operating costs .Most of the merchant ships are designed to operate efficiently at a single speed, which may be the maximum/optimum speed of the ship. Other ships, such as tugs, trawlers, OSV’s etc have to be capable of operating efficiently over a range of speed and may not have very complex engine control arrangements to enable this to be achieved. This need for range of speeds is frequently achieved by equipping the ship with more than one engine per shaft, so different powers and speeds can be achieved by using either, or sometimes both, of the available engines on each shaft. Ships like offshore supply vessel during it’s operations may be required to achieve effectively a zero speed in
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The basic cost cutting measures are crew reductions, fuel consumption reduction and simplified overall propulsion system design. The more significant development towards great fuel economy have in recent years been almost universal adoption by deep sea trades of the slow speed diesel engines as a propulsion plant, with greater use of automated engine being able to both start and run on heavy residual fuels. While the propulsion arrangement consists of a simply single large engine directly coupled to a single fixed pitch propeller. The disadvantage of such single engine with single fixed pitch propeller installation from a ship controllability point of view are concerned with the difficulties of slow speed operation, the problems of close quarter maneuvering and the inherent unreliability of having only one main power source. A slow speed diesel engine will rotate, at its slowest ,at about 22 rpm without stalling. This will correspond to in some ships, to a speed of 5 knots, approaching the berth. In order to go slower, the engine will have to stopped, which will result in a loss of directional control, because of the reduction of water flow across the rudder. Once the engine is stopped, it is to be started again using air from starting air bottles, which are being charged by air compressors, hence there are a finite number of starts available to the master, depending on the charge level in the air bottles. In order to slow the ship, or reverse it, the engine has to be stopped and restarted in the reverse direction. The available air supply may not be able to start the engine in the reverse direction above a certain speed, so that the ship cannot be always relied upon to achieve its f ull stopping performance.
(II) PERFORMANCE ANALYSIS AND COST FUNCTIONS The performance of a given system may be assessed by obtaining from a number of measurements, a set of relevant data, and then preparing a combined figure which represents a meaningful measure of that performance. An attempt is made to combine a number of desirable criteria into a single function, the maximization or minimization of which will give a clear indication of the best solution. A similar process is used to assess the performance of a system under the action of a controller . A number of relevant measurements of the performance of the system is made and a combination of these measures are used to define a single quantity, which may then be used to produce the best or optimum performance of the system. This measure is known as the cost function or performance or performance criterion. criterion. A cost function is a single quantity which is used to assess the performance of a system. A cost function is typically built up of a number of factors, each of which has to be given a quantitative measure. In some cases the factors can be expressed in common units which may or may not be expressed in money terms, while in other systems some form of weighing factor / criterion has to be prepared for each other. The cost functions for an auto pilot may be expressed in terms of two most important performance requirements, the course error and error and the rudder activity. activity. The cost function will be made up of both these component parts, so that it may be written as : CF = f (course error) + W x f(rudder f (rudder angle) The total cost function will be made up of sum of two components because the components will not be of equal importance, both of them is weighed by different factors f for course error and W for rudder angle. The cost function will thus indicate by its value how well the autopilot is functioning . If both course error and rudder angle were zero for the required period of time, the value of the cost function would be zero indicating perfect performance. In practice this will not be feasible, because of the influence of disturbances and the ships directional instability. The aim, however, is to adjust the auto pilot to produce a minimum cost function. Changing the value of the weighing factor will give a different emphasis to each of the two components of cost functions, so that a minimum value of cost function, and hence an optimum performance, will be for a different type of ship behavior. Increasing W will give a greater emphasis to the degree of the rudder angle, and less to the amount of course error. This type of performance would be suited to operations away from the confines of the port area, where it is more important to minimize the rudder activity than to steer a very precise course. For close quarter operations, it is more important to steer accurately on course and large rudder angle are more acceptable, so that a small value of W would be appropriate. Thus varying W will change the type of performance, considering optimal performance.
(III) SHIP CONTROLLABILITY REQUIREMENTS , ASSESSMENTS, VALIDATION AND TRIALS
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and/or stern thruster, adequate ahead and astern power, reliable bridge equipment, a rate of turn indicator, good bridge equipment layout, and finally a good visibility from bridge. The correlation between a ship’s hull form and its maneuvering is not clearly defined as turning ability of the ship is governed by the balance of a large number of hydrodynamic forces.It should not be surprising that a small change in hull shape will cause large difference in behavior. A bluffer bow will cause hull hydrodynamic turning forces to be both larger and situated further forward. Some bulk carrier with higher block coefficient enter turn quickly and have relatively small turning circles diameter of under two ship lengths, but because of high hydrodynamic forces such ships may be slow to come out of a turn. This inability of ships with higher block coefficients to come out of a turn is reflected in a 10/10 zig-zag trial. trial. Conversely many ships with finer lines are stable directionally but cannot be said to be adequately controllable, as they do not turn readily enough in response to the helm. This type of hull shape will have smaller hull hydrodynamic turning forces acting further and so will not turn well . Hence turning circle diameter of six ship lengths are not exceptional. Such ships , although being much more stable dynamically in turn than a bluffer bulk carriers , still cannot necessarily be considered adequately adequately controllable, as far as their directional behavior is concerned. In case of turn taken by a dynamically unstable ship, when the rudder is thrown over to one side it deflects the slipstreams in the vicinity and cause the stern to move the ship initially, the bow will tend to continue on the same direction so that the ship will be inclined at an angle to the original directions. Although the ship may have turned through several degrees to the original course but it will not have moved significantly, in other words the ship is still going in its original direction, although it has started to turn. This situation produces large hydrodynamic forces caused due to deflection of the incoming water stream by the hull. The hydrodynamic forces act in the direction to increase the angle of the ship to the streamline and hence to further increase the hull forces. It is these hydrodynamic forces which largely turn the ship, the rudder producing little turning effect once the turn has been established. In a ship with smaller block coefficient, the initial action of the rudder is still to move the stream of the ship to the other side. Because of the shape of the hull the hydrodynamic forces are both smaller and centered further aft, so that the turning moment is much less and the rudder will continue to assist in the turn. Rarely and unusually, some ship designs are such that this is not the case and the ship might continue to turn in the original direction, until the speed eventually falls so that the forces are to oppose the rudder, or power is removed to allow the ship to slow. Where there is enough sea room, the use of turning maneuver to stop the ship is extremely effective with full form ships, as the ship will adopt a high drift angle, presenting a large area to the flow of water. There are no national or international conventions or standards which require maneuvering or stopping ability of tank vessels to be considered in design process. It can be inferred that assessing controllability is a matter of achieving balance between conflicting conflicting requirements. The problems facing a ship designer is that of ensuring adequate balance between these conflicting requirements.
(a) Effectors and Control surfaces:surfaces :-
Control of ship motions is exercised in two main ways , by imposing and including plane/surface to the stream line flow of water around the ship’s hull to provide mainly a force perpendicular to the motion of the ship in the required direction , or by causing a surface to move in such a way so
1. 2. 3
4
as to provide a reaction force driving the ship in the required direction. All such surfaces are known as effectors. A range of rudder types has been developed which will enhance the turning ability of the ships, and the siting of rudder in the slip streams of propellers will in general assist in turning response of the ships, as the rudder forces will be increased by the faster wake they are in . Devices to enhance the lift forces of a rudder include:Flapped rudder , which operates by increasing the stall angle of the control surface, thus allowing effective rudder sideways at higher angles. Rudder with a rotating cylinders in front of them which enable the increasing stream to adhere the control surface at higher rudder angles similarly increasing the effective lift of the rudder. Rudders with special shapes designed to operate at large angles for example a shilling rudder which operates by 0 deflecting propeller slipstream to angle up to 90 . The rudder is shaped so as to minimize staking effect at rudder 0 angles up to 75 . Flanking rudders :- In ships where rudder performance is of particular importance, multiple rudders might be provided alongwith flanking rudders as provided in some towboats ,situated immediately forward of propellers and are used when going astern. The two steering rudders and flanking rudders are coupled together. Alternatively methods of enhancing directional controllability of a ship include the use of auxiliary turning device such as bow/stern thrusters, which provide a direct thrust athwart ships. These are usually situated at tunnels, thus limiting their effectiveness to ship speeds below 4 knots, as hydrodynamic flow past the tunnel increases limiting
i)
their effectiveness at higher speeds. On ships which have high need for good maneuverability such as tugs, offshore supply vessels and drill ships, a range of auxiliary devices are used including: 0 Azimuthing thrusters, thrusters, where an auxiliary propeller is designed to be capable of rotating through 360 . These
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v)
Triple screw and single rudder :-Some cross channel ferries which have to maneuver while going both ahead and astern have, sometimes triple screw arrangements and quite sophisticated steering arrangements for going ahead; triple screws being fitted with a single rudder abaft the center screw. This enables the ship to maneuver well by putting two outer engine astern and the center one ahead. This provides a net zero thrust, but allows for the deflection of the center propeller slipstream, giving a str ong directional thrust. For going astern a single low powered propeller and rudder are fitted at the bow of the ship giving additional additional directional control with engines astern.
(b) Assessing Controllability and ship trials:trials :-It would be most desirable if controllability were able to be assessed before a ship is built, and some indications of methods of predicting it from ship plans are described. The International Maritime Organization (IMO) has a sub – committee on ship design and equipment which has developed draft guidelines for considering maneuvering performance in ‘ship design’. These guidelines define specific maneuvering characteristics which quantify maneuverability and recommend estimation of these characteristics during design stage as well as full scale tests to confirm the maneuvering performance estimation. These IMO guidelines are intended to be applied to ships greater than 10000 GRT, it was also felt by the IMO group that it is of importance to perform full scale test to validate design estimation. The most appropriate speed at which the test should be conducted is the maximum maneuvering speed rather that the full speed, to compare with navigation in restricted waters, however deep water is preferred to shallow water .A lot of information for formulation of guidelines was received from European Research Community project COST-301, which includes investigations in several maneuvering areas, principally related to ship handling in confined water and vessel traffic systems. The recommendation are given in the 1975 ITTC Maneuvering trial code as a guide to the performance of the trials.
(i) Assessing Controllability :-
The usual approach is to device trials which are persistent to each of the perceived desirable qualities, to measure the results and to device a formula to relax the trial results to presumed performance needs. At each step in this chain of events requires a decision to be made on what is suitable and relevant. It is perhaps not surprising that there is a little uniformity in evaluating suitable measures of controllability. Some agreement is however possible on what might constitute a suitable range of trials to determine some of the performance characteristics characteristics of the ship.
(ii) Steerability cost function w.r.t. course keeping :-
For a ship under automatic steering control, under a auto pilot, the most commonly used cost function for “good steering” is :
J = 1/T∫0T(Ψ2e + λδ2)dt
where
Ψe = Heading error , δ = rudder angle and λ = weighing factor depending on the ship type , loading condition etc The heading error may be used to describe the effect of an increase of path length as well as increase of resistance a more generalized cost function will be of :
J = 1/T∫0T(aΨ2e + bφ2e + cδ2)dt 2
2
2
This may be used to minimize fuel consumption, where Ψ e is concerned with increased path where φ e and δ terms are concerned with increased resistance the main draw back of these cost functions is that the sway velocity is 2
eliminated by taking it to be proportional to φ e since long period oscillations are considered. This is done because it is seldom possible to obtain an accurate measurement of sway velocity. The above cost function are usually associated with steering behavior in calm water, therefore 1. the first order wave motions and and measurement measurement noise are , in many applications applications removed, by filtering filtering 2. different values of parameter parameter of loss functions are chosen chosen depending depending on weather weather conditions.
(iii) Ships trials – purpose and conduct:conduct :-
Ship trials can be inconvenient ,expensive and time consuming to carry out as they could require full availability of ship in good condition, under predetermined environmental conditions often with extensive trial equipments and personnel embarked. Despite this, ship-trials are normally carried out for one of the several purposes:1. As a contractual check at the end end of a ship building building contract, such such trials are normally normally short in duration duration and limited to small number of contractual points. 2. First of class trials, when several several ships of the same same type and structural structural similarity (sister ships) are being being built, it is worth while testing first ship to be built more extensively, as a check on overall effectiveness of design on the hull and machinery. 3. As a check check on maneuverability, maneuverability, same will be detailed later. 4. To provide data for ship maneuvering mathematical models. These trials, as they need to examine the ship behavior in all maneuvering regimes, will be very extensive in scope. The desirable features of particular ship trials are :1. the starting conditions and tests inputs inputs are easily reproducible. reproducible. 2. It should be conducted and recorded with a minimum of added equipment. 3. The trial should should bear bear a close relationship to actual ship ship operating operating conditions. conditions. The USCG has proposed the development of new regulations concerning the maneuvering and stopping
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3. 4. 5. 6. 7. 8. 9.
1)
The ability to slow down down while maintenance maintenance of steerage steerage in Y knots of wind wind speed and Z knots knots of current current speed. The ability to maintain the heading of the ship as it is affected by ship / bank / bottom suction / sheer at X knots of ship speed in Y knots of wind speed and Z knots of current speed. The ability to control the heading of the vessel at X knots of ship speed in Y knots of wind speed and Z knots of current speed. The ability to turn the vessel more more sharply with X knots of ship speed in Y knots of wind speed and Z knots of current speed. The ability of the master to obtain additional power in Y knots of wind speed and Z knots of current speed with W number of tugs already assisting. The ability to control the heading heading of the vessel backing backing at X knots knots of ship speed speed in Y knots of wind speed and and Z knots knots of current speed The ability ability to stop the the vessel from from X knots of ship speed speed in Y knots of wind wind speed and Z knots knots of current current speed in W minutes. The requirements 4 ,5 and 8 refer to course keeping , turning and stopping abilities while introducing external disturbance and 7 refers to astern course keeping. The remainder are typical needs of controllability in restricted waters and a source of potential casual risk. A data bank of maneuvering trial results of some 600 ships have been created. The data corresponding to different maneuvers have been analyzed statistically and graduated in terms of indices. With the objective of facilitating the task of the regulatory bodies concerning the establishment of maneuvering standards a set of representative representative indices have been proposed as follows.:Turning and course keeping keeping::A) tactical diameter / length at 35 rudder angle. B) Overshoot angle from 20/20 zig-zag maneuver. C) K-t relationship relationship from 20/20 zigzag maneuver. ˚
2)
Stopping:- Head reach / length from a crash stop maneuver. Stopping:-
3)
Crash-stop maneuver ::- Head reach / length from a crash-stop maneuver. maneuver.
4)
Ability to operate at moderate speed speed::- Continuous operation at a speed between four to six knots for all ship load conditions.
5)
Operation under severe environmenta environmentall conditions :- Satisfactory operation for any loading at the respective severe wind condition. As a consequence of this characteristic , the maximum above water surface would be limited ,and will be function of the submerged area , as well as ship and wind speeds.
6)
Course change test :- Though not included in the ITTC maneuvering code this gives invaluable data regarding the steerability of the ship and its fundamental fundamental design of the auto pilot. For any trials it is necessary for the ships position to be fixed relative to either a shore or to the water at intervals of every few seconds. This may be done using shore based tracking equipments viz geodimeter. geodimeter. It is also possible to measure ship position using ship-borne set of equipment incorporating a inbuilt navigation package (Gyro pilot) which will give the acceleration of the ship. To assess maneuverability/controllability ,one or more trials will be necessary for each of the desired qualities referred above. The trials most commonly associated with measuring controllability are the Crash- stop, the turning test or circle maneuver and the zigzag or Z maneuver carried out at full speed and full maneuver. The crash stop is maneuver to test the vessels stopping ability. The ship proceeds at a straight course at a meaningful speed for the maneuver concerned, and the engine is put to full astern. The distance traveled before the ship stops is measured along with the time taken to stop. The circle maneuver is of particular interest in that it gives information about transit and steady state maneuverability of the ship. With ship running on steady course & speed ,the rudder is pulled over an appropriate predetermined angle and held there. The ship then starts to turn and eventually reach turning condition. The measure of interest will include the advance, transfer and turning / tactical diameter, which gives information of the ships turning ability. If the helm is put to mid-ships after the ship has been reduced to a steady turn a simple assessment can be made of its course stability. This maneuver is called pull out maneuver . Unstable ships will continue turning while stable ships will return to an approximately straight path. As far as controllability is concerned a degree of instability can be advantageous, as turning and checking ability can be faster than with a similar size of ship with a directionally stable hull. The zigzag maneuver is perhaps closest to the actual ship operations. The ship again starts straight line at a constant speed and the rudder placed over by specified amount, typically 5 ,10° or 20°, depending on the size and maneuverability of the ship. After the ship has turned through a determined number of degrees,called the check angle;; the helm is put over the other way by an amount equal to the rudder angle and the ship will then turn in the angle ˚
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than to find that the ship has undesirable control characteristics at trial time or even at service, which cannot be considered as sound design practice. It can be seen though, that the controllability of ship is made up of a number of aspects some of which such as the swing control, depend on the interrelationship of both the hull and effectors. Captive model trials are unlikely to give more than a indication of the controllability , as they rarely are realistic representative of the hull and the rudder interaction in maneuvering conditions. Some progress is being made with the ability of researchers to model the maneuvering ability of the ships before they are built from a combination of free running model tests and mathematical models.
(v) Improving controllability:controllability:-If a ship is found to have an adequate controllability a number of measures can be taken to improve the controllability situations in some cases. Where the rudder performance is inadequate as could be shown in the excessive first over shoot angle in a zigzag trial, a rudder with greater lift can be retrofitted. Similarly stopping ability and slow speed maneuvering ability maybe able to be improved by fitting a controllable pitch propeller. In some case, however, the basic hull form is at fault, leading to excessive overshoot and virtually uncontrollable behavior. In these cases some degree of palliative action may be taken by increasing the size of the skeg after, increasing the size of the rudder or by fitting additional control surfaces. These attempts to improve controllability on an otherwise uncontrollable hull are not in themselves a satisfactory solution, but merely a way of making the best of poor design. A hull should be inherently controllable, without relying on imposed devices to make it so. Of particular interest is the precise shape of the forward part of the hull, as the hydrodynamic lift and drag forces are to a large extent generated at the fore body of the hull. If the fore part is too bluff, generating an excessive amount of lift at the forward extremity of the hull, it could be that hull will become totally uncontrollable in YAW with normal control surface.
(vi)Controllability (vi) Controllability as a Port-ship problem:problem :-
The controllability of a ship doesn’t only affect the ship itself. If a ship has poor level of controllability, it is likely to require greater sea room for maneuver or will be operating closer to margin of safety in port. For example, if a high sided container ship with a single speed diesel engine with fixed pith propeller may have too high Speed Over Ground (SOG) in a strong favoring current, due to minimum stalling speed requirement of diesel engine, besides it will also lack directional controllability at very low speed, while crossing very narrow passages or height restriction such as bridge pier, especially if a transverse thrust by a strong wind or under current is imposed by such area. Under these conditions safety conditions are minimal. Techniques are developed to assess the suitability of such ships to transit a port area safely in the range of environmental conditions. The simulation requires a mathematical model of the ship under consideration, with its controllability adequately modeled. This can be achieved by using a range of techniques, including carrying out special ship trials, performing free running model trials,or by modeling the ship directly from the knowledge of its lines, plans and effectors design. A number of such models have been produced which have been tested against actual ship results, and a measure of confidence is able to be built up in the techniques. The use of ship simulators for developing suitability of a port for particular ship operations is now becoming a routine part of port design process.
(IV) ALTERNATIVE PROPULSION CONCEPTS FOR BETTER DIRECTIONAL CONTROLLABILITY The most popular arrangement for the commercial ships is a large diameter as slow turning as possible ,single fixed pitch main propeller in stern coupled with a large slow speed cross-head type diesel engine and fixed speed side thrusters fitted at the bow, this is for reason of simplicity, efficiency and the need to minimize initial costs, with a balanced type of spade rudder aft. On this type of arrangement a maximum propeller efficiency of about 70% can be obtained, the 30% loss can be split into 3
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engine, the margin of safety available in certain cases to ensure that the failure of one engine will not disable the vessel, economy of the running costs, initial expenses and efficiency of propulsion. The use of twin screw propulsion is an adaptation of utilizing the largest possible diameter of the propeller, but in case where the diameter is limited by draught of hull multiple propeller allow greater mass of water to be utilized, than a single screw alternative. In twin screw the benefit form the energy from the wake is lost due to the off-centre position of the propellers but this can be obtained with triple screw where the centre-screw absorbs most of the power. The loss in efficiency is compensated by increased maneuverability as each propeller can be rotated ahead and astern independently. The propulsive coefficient of a triple screw is slightly better than twin screw, which may be ascribed to the better efficiency of the centre screw working in the frictional wake of the ship and propulsive coefficient is increased by the rudder behind the centre screw and the resistance is less than the corresponding twin screw owing to the smaller dimension of the bossing. One advantage of a triple screw arrangement is that it makes it possible the stopping of one of the three engines during the voyage for the repair without any undue loss to the propulsive efficiency. If the engines have clutching mechanisms, the engine and the propeller can be disconnected so that the latter can be freely turned in the water and need only overcome the frictional loss at the shaft bearings. In multiple screw ships the direction of turning of the propeller is such that the blade tips turn outwards when in upper positions. With outward turning screw both ahead and astern motion, the resultant is shifted outwards with respect to the centre of the screw shaft owing to the unequal distribution of the intake velocities at the screws. A twin Screw ship with outward turning propeller can therefore be steered better by means of the propellers than they if were inward turning, as in latter case, the point of application of the thrust is moved inwards with respect to the centre of
T r i p l e Sc S c r e w A r r a ng ng e m e n t
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the screw-shaft so that the turning moment becomes smaller. There are other t ypes of two propeller arrangements arrangements which are non-conventional non-conventional and have their own advantage:Tandem Propellers Overlapping or interlocking propeller, Two propeller placed vertically above one another Contra-rotating propeller arrangem arrangement ent The propellers on the same shaft and turning on the same direction are called Tandem propeller .As aft propeller works in the same race of the forward ,forward one requires a higher pitch to give same power absorption.With both the propeller rotating in the same direction, the rotational energy in the race is augmented by the working of the after-one. In the interlocking or overlapping propeller arrangement the propeller disc overlap each other, the two propellers of the normal twin screw can be moved aft to a longitudinal position of normal single screw propeller and inwards until the distance between the shafts is less than the diameter of the propellers , which in this ways interlocks in the centerline zone. This
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• •
1. 2. 3. 4. 5.
Recovery of rotational energy with both propeller turning in the same direction. The resistance of open-shaft bracket and shafts placed obliquely in the flow is lower than the conventional single screw arrangement. arrangement. The decrease in jet area and the possibility of utilizing the concentrated wake mutually influence efficiency. The overall propulsion efficiency attained is higher than conventional design. A reduction in resistance of struts and shafts brings about a decrease in power required, corresponding to one –third of this part of resistance. Interaction effects can cause vibration and cavitation, both of these can be overcome by the setting of the blades . It is advisable to have different number of blades on port and starboard propellers. The following are the variants:Direction of of rotation of the propellers. Distance between the shafts. shafts. Clearance in in the longitudinal longitudinal direction Stern shape. Ship block-coefficient. block-coefficient.
2. Rudder Propeller :- It is an auxiliary propulsion and steering device. The rudder mounted propeller have the 1. 2. 3. 4. 1. 2. 3. 4.
following advantages:advantages:Good control, control, especially especially when starting starting from rest. rest. Unlike lateral thrust thrust propellers, control effect improves improves with speed. speed. If necessary, a lift mechanism mechanism can adjust the unit unit to ships draught draught . To reverse the vessel the propeller propeller is swiveled 180 degrees degrees for astern action. action. The disadvantages are:propeller has to be fitted to the support support mechanism, mechanism, which increases resistance. resistance. Complicated Z- Gearing. Limited power power in current design. design. Influence of the jet with hull hull or the adverse effect caused by interaction interaction with main propeller propeller race.
3) Grim wheel :- It is a vane wheel with larger diameter than propeller ,freely rotating like a turbine mounted on an
1. 2. 3. 4. 5.
extension to the propeller shaft aft of the main propellers.The grim wheel has more number of blades than the usual propeller and is about 20% larger in diameter. The vanes of the grim wheel are so designed that the inner section acts as water turbine to extract from the propeller slip stream a large amount of energy which would otherwise be lost. The recovered energy is converted directly into additional and useful thrust in the aft part of the vanes which acts as a propeller. This function of making use of the active propeller spin and the jet energy results in savings either in the form of increase in thrust and ship speed or reduction in the horsepower input required for a given speed. The improvement in propulsive efficiency ranges from 5% to 15% depending on the type of the vessel. The unit rotates on low friction roller bearings on a steel stub shaft flanged to the propeller boss and the radial lip seal prevent water ingress to the bearings. it can be used with either fixed or CP propeller .Compared with a conventional propeller system the addition of a grim wheel brings a following advantage:Propulsion efficiency increase increase upto upto 15% The main propeller propeller can be operated operated at a higher rpm favorably affecting affecting weight and cost cost of the prime movers. Grim wheel can be as large in diameter as possible since the large number of blades and low speed of the wheel allows small vertical clearances with the hull to be acceptable. There is less resistance from a rudder rudder fitted behind the grim grim wheel, this is shown in the relative relative rotative efficiency. Ships fitted with this this device have better stopping capabilities. capabilities.
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5. Azimuthing thruster :- It is a rudderless Z-drive propulsion system where a
1. 2. 3. 4. 5.
6.
propeller is mounted on a slew-able shaft supported by the ships structure. The normal azimuthing mechanism is a worm wheel drive mounted on the head to allow the complete unit to be turned through 360° so that the propeller thrust can be directed anywhere in the horizontal plane. The combination of steering and thrust executed by one mechanical device which gives the following major advantages:Most effective application application of thrust as a whole unit unit is turned to direct propeller propeller thrust where it is wanted , rather than relying on deflection by rudder. Very precise & effective effective steering of of vessel, through thrust thrust direction. Ease of vessel control through through single lever system or microprocessor microprocessor controllers controllers linking more than one unit. Much simpler machinery machinery installation with no shaft bearing bearing housing to be bored, bored, giving flexibility in stern design of vessel. Great flexibility in choice choice of propulsion units units such as direct drive electric motor, motor, hydraulic motors, etc. If a constant speed C.P. propeller is used , use can be made of engine or transmission system. Engine room length can be remarkably shortened short ened meaning the payload section of the vessel can be increased by 5% to 10%.
6. Controllable pitch propellers( C.P.P) :- Controllable pitch propellers have blades separately mounted on the hubs, each on an it’s own movable axis ;the blades can be turned about these axes leading to change of pitch of the blades, and even reversal of the pitch while the propeller is running by means of an internal mechanism in the hub. The forces necessary to turn the blades are relatively large and for this reason the area of the blades tends to be lower than normal. The boss diameter ratio is 50% greater than the fixed pitch propellers under the same operating condition. These propeller have also been known to cause Cartwheel Effect ,at zero/low pitch at full rpm, which causes the stern to swing in the direction of the propeller at the top of the circle. The main advantages of the propeller despite the initial first costs are as follows:1. Rapid reversal of thrust thrust from ahead to astern and practically practically unlimited number number of reversing not dependent on the air bottle pressure. 2. Full power in astern thrust, compared to steam turbine direct couple diesel engine where engine parameter doesn’t ensure optimum performance during astern maneuvering due to timing consideration. 3. Optimum utilization utilization of power at partial partial loading loading when the engine engine rpm is reduced . 4.The engine overloading can be avoided by automatic pitch control system to reduce pitch, if the engine overloading is maximum and the permitted cylinder pressures are likely to be exceeded. 5. Constant speed operation means operation in the optimum speed range leading to fuel costs savings and avoidance of the barred speed range during maneuvering. 6. Possible improvement improvement in the the propulsive propulsive efficiency, efficiency, simpler simpler engine engine plant {e.g. gas turbine}, greater maneuverability, maneuverability, less wear and tear. 7. Spare propeller blades are cheaper than a complete solid propeller of fixed pitch type. In a standardized fleet with one type of c.p.p. spare parts costs are reduced.
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inner shaft with bearings with outer sealing arrangement and the installation of the star type planetary gear, necessary for two driven shafts to turn in opposite direction, when one main engine is used, along with additional costs involved was not considered justified and till now has inhibited the use of CRP. This problem was solved by using a much simpler methods by podded CRP concept where a contra-rotating propeller on an electric pod or Rotatable Z-drive thruster is located directly behind the main propeller in the centerline skeg. The two propellers are arranged as close as possible to each other hence the rudder blade section of the thruster is behind the propeller which can act as the pulling thruster. The pod propeller is of fixed pitch type, while the main propeller is of controllable pitch type.The podded configuration offers a better hydrodynamic efficiency compared to the conventional vessel with a single twin screw due to the following reasons:1. The resistance resistance of a single skeg skeg hull form with a single pod is lower lower than that of of a twin screw hull hull with two shaft lines. lines. 2. The aft propeller propeller takes advantage advantage of the rotational energy energy left in the slipstream slipstream of the forward forward propeller. propeller. 3. The skeg offers a more favorable wake than a shaftline, resulting in better hull efficiency ηH. The podded CRP’s offer excellent maneuvering due to the azimuthing pods giving much higher turning moment due to the directional thrust vectoring. The optimization of power split between main shaft and the pod is important as electrical transmission per kilowatt is more expensive than mechanical transmission and the z-gearing has a ceiling power rating due to current design limitation. The solution is that a 25/75 power split between pod and main propeller is the best option from the total efficiency point of view. The economical factors also favor a small pod. The cavitation performance of the pods as measured in towing tanks is on a good level for the CRP concept and better than conventional system.
8) Waterjet and pumpjet :- A water-jet is basically a marine propeller in water fed through an inlet duct at the bottom of the vessel, to an axial flow water pump which adds energy before expelling the water through the nozzle at a much higher velocity than the incoming stream. The resultant change in momentum provides the thrust to the vessel; and the jet can be directed either side by a movable nozzle and/or scoop to provide steering and reversing control. Waterjets are typically driven by diesel engines (medium or high speed) or even gas turbine and are popular for high speed craft and vessels required to operate in shallow water. A major advantage advantage for the waterjet is that for a given power loading a higher static thrust can be obtained compared to an equivalent conventional or shrouded propeller. This is because for the low speed application the pump can be operated close to the point of optimum efficiency. Another feature is the ability to absorb full driving engine power at all water speeds without cavitation. Compared with conventional propeller this results in superior accelerating characteristics as well as directional control as maximum thrust is immediately available. A typical design of waterjet has an axial flow impeller blades with very wide chord running close to the pump casing with minimum tip losses and stator blades to strengthen the water flow down stream of the impeller to avoid rotational losses. To overcome the problem of debris a grille is normally fitted at the intake .For steering, the outlet nozzle pivots about an axis set at 45° to the vertical using hydraulic rams ,are provided. When the nozzle is swung either side of the straight ahead position the whole jet stream is swung with a minimum loss for positive steering action. Selection of ahead through neutral and astern is obtained by progressive lowering of scoop or bucket which rotates about a horizontal pivot mounted on the outlet nozzle.Jet units with power upto 15,000 Hp/Units are used in military amphibious vehicles, workboats and high speed catamarans. A variation of the waterjet is the PUMPJET developed by Schottle. This is basically a centrifugal type pump in which impeller axis is mounted and works in a volute type casing. The suction port is mounted against the flat bottom of the boat and the water is energized in the pump and expelled through a nozzle back into the water down at an angle of 15°, at the bottom of the craft. The complete volute casing can be rotated through 360°.
9) Vertical axis or Cycloidal propellers :- The vertical axis propeller has a number of blades of aerofoil section connected perpendicularly to a disk whose axis of rotation is vertical. The blades of the propeller are fully immersed and all other parts such as rotor are housed inside the hull. The bottom of the rotor is flushed with the
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full power i.e. it is a controllable pitch propeller. Also by further variation in blade attitude the thrust can be directed to any direction through 360°. As far as the propeller operation is concerned, there is no difference in the ahead or astern command, or sideways. Thus, these propellers are used for normal steering and no rudder of any kind is required. This propulsion gives an abnormally high degree of maneuverability, but the thrust of the propeller is lower than with a shrouded propeller, thus larger powers are required to obtain the pull required. The propeller itself and itself and its mechanisms are complicated in construction and is commensurately costly both to install and maintain. The hull must be especially constructed to accept these propellers. These propellers may not be used in high speed vehicles.
(V) CONCLUSIONS AND RECOMMENDATION RECOMMENDATIONS S In so far we analyzed the concept of maneuverability, how it is assessed and verified by model testing. We are also aware of how maneuverability is affected right from the design stage and analyzed all the parameters effecting the maneuverability and propulsion performance. Lastly we have analyzed all the major alternatives propulsion modes which are mostly advantageous in terms of propeller efficiency, increased maneuverability or both. We have also seen in details the interplay between the various major components of a propulsion and maneuvering system. At present time, various regulatory bodies are considering formulation of rules concerning ship maneuverability, maneuverability, in so far as it influences safe navigation of ships. Proposals have been made for a number of criterion to be satisfied during design of a new ship, which would be subsequently be verified during full scale trials. In view of the proposed design requirements of the regulatory bodies, the ability to specify the hydrodynamic and aerodynamic coefficients either theoretically or semi-empirically would be of immense help. This could be achieved by continuing to examine the means of correlating the captive model data, involving the co-operative efforts amongst various bodies.The problem of ship maneuvering in extreme condition persists as many aspects have to be satisfactorily analyzed and examined, the maneuvering in transient and non-uniform condition shall receive special attention. Classification Societies are largely silent on aspects of maneuverability and controllability. While most societies have extensive rules about the construction of rudder, the guidance to their effectiveness is largely missing. It will be noted that while the need for adequate maneuverability is stressed, no attempt is made to give any quantitative and