Department of Mechanical Engineering Sanketika Vidya Parishad Engineering College P.M Palem
Project work on “STUDY OF MARINE PROPULSION SYSTEMS AND CONCEPTS” Submitted to
Hindustan Shipyard Limited (A govt. of India undertaking) Gandhigram Visakhapatnam – 530005 (INDIA) An ISO 9001:2000 company In partial fulfillment for the award of the Degree of Bachelor of Engineering In Mechanical Engineering Submitted by
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Ch. Manohar D.S.R Aditya G.S.R.K.N Naveen K. Surya Deepika P. Manasa T. Madhuri
It has been a great pleasure and enlightening experience to work on this prestigious project titled “STUDY OF MARINE PROPULSION SYSTEMS AND CONCEPTS”.
We express our profound gratitude to Mr. ,Engineer, Training Dept., Hindustan Shipyard Limited,Vishakapatnam for his learning council and encouragement at all stages of the project.
We owe great indebtedness to Mr. M.K Naidu, Engineer, Engineering Dept., Hindustan Shipyard Limited,Vishakapatnam for his valuable guidance and constant encouragement and who inspired us in every aspect and provided suitable environment.
We also thank all the supervisors and workforce of the engg. Dept. without whose support this project would be incomplete.
This is to certify that the following students have undertaken and successfully completed this project work entitled “STUDY OF MARINE PROPULSION SYSTEMS AND CONCEPTS” which is a bonfide record of work In partial fulfillment for the award of the Degree of Bachelor of Engineering being submitted to ANDHRA UNIVERSITY , WALTAIR, VISAKHAPATNAM under our guidance.
➢ ➢ ➢ ➢ ➢ ➢
Ch. Manohar D.S.R Aditya G.S.R.K.N Naveen K. Surya Deepika P. Manasa T. Madhuri
Internal Guide: A. Bala Raju(Asst. prof.), Dept. of Mechanical Engineering SVP Engg. College , P.m palem, Visakhapatnam.
Ship propulsion system is that part of marine engineering concerned by the design and/or selection of main propulsion plant equipments and machineries. The main role of this plant is to produce enough power to overcome the ship resistance and to generate the needed electric power for the various applications onboard the ship (lighting, control systems, pumps, navigation equipments, HVAC, etc).
The above figure shows the main two forces considered in propulsion system; the resistance of the water to the ship motion (R) and the thrust developed by the propeller (T). When considering only the engine room area, the various
TERMINOLOGY USED IN SHIPS
Aft:Towards the stern of a vessel. Articles:A paper that all the members of the crew of a ship signs to say what their position aboard will be.In modern terminology,perhaps,a contract of employment or intent. Barque:A vessel with three or more masts with square sails on the fore mast and fore and aft sails on the after mast.Generally a range of 250-700 ton capacity. Barquentine:A vessel with three or more masts with square sails on the fore mast,and fore and aft sails on the main and after masts.Generally in the 250-500 ton range. Bowsprit:A spar projecting upper end of the bow of a sailing vessel. Brig:1. A two masted vessel square rigged on both masts.Generally in the 150-200 ton range 2. A sailing vessel’s jail. Brigantine:A vessel with two or more masts,with fore and aft sails and both masts,normally in the tonnage range of 150-250 tons,but some P.E.I brigantines exceeded 400 tons. Bulkhead:A wall-like construction inside a ship. Bulwark:A guard that protects the ship from big waves. Dogwatch:A way the sailors changed places.every four bells they switched places. Dory:A small rowing vessel with a narrow,flat bottom and high sides curving outward. Figurehead:A carved figure or bust on a ship’s prow.
Forecastle:1. A superstructure at or immidiately aft of the bow of a vessel 2. The quarters for the crew of a merchant ship. Fore or Forward:Towards the bow of the vessel. Forerunner:Maritime legends are ripe with stories of forerunners.They are a harbinger or herald of impending disaster often felt by the family of those lost at sea. Galley:1. A seagoing vesel propelled mainly by oars used in ancient times. 2. A kitchen in a ship or airplane. Harbourmaster:A man in charge of a harbour,authorizing entry, and arranging for pilots into difficult harbour,past under water obstructions,etc. Hatchway:A large covered,usually rectangular opening in a ship’s deck for putting and removing cargo from under the deck. Helm:A wheel or tiller by which a ship is steered. HMS:A prefix used before a vessel’s name to denote that it is owned by the crown, or is His/Her Majesty Ship. Ice boat:A small boat typically used on P.E.I. for transportation across northumberland strait prior to 1918 during the winter months.Typically, they were 5 meters long and 1.2 meters wide,and were covered with tin to protect the vessel from the ice.Often they had metal runners on the side of the keel.They were outfitted with sails and oars,and could be rowed or paddle across the strait.When the ice grew too thick, the crew and male passengers in exchange for a lower rate,would pull the boat across the ice.The vessel was equipped with leather harnesses to attach the crew to the vessel while pulling,also protecting them from drowning when the ice gave away. Jack:A sailor or seaman. Jibs:A triangular sail set forward of a fore mast. Keel: The central member on the bottom of the hull,extending from bow to stern. Keelhauling:A way of punishing sailors by tying them to a rope and dragging them across the botom of the ship.This was accomplished by lashing together the legs at the ankles by a significant amount of line,then dropping the unfortunate soul head first from the bow,then he was allowed to drift back to the vessel’s beam(the widest section) where the officers or deck crew would take the line from one side of the deck to the other .This would submerge the poor soul in order to bring him up under the ship’s keel surfacing on the other side. Knot:1.An interlacing of rope,cord,etc,drawn tight into a lump or end . 2.(Naut)A measure of nautical speed about 1.125 miles per hour . Log :1.A device for measuring the speed of the ship. 2. To enter in a log book. 3.A ships written record. Mast:A spar or structure raising above the hull and upper portions of a ship holding sails,rigging,etc. Master:The captain of a merchant ship. Mate:An officer of a merchant vessel ranking below the captain. Mutiny:Rebellion against the ship’s constituted authority. Pilot:A trained captain in the employ of a Harbour master who’s job is to giude vessels into harbours,past underwater obstructions. Punt:A small squre ended rowing vessel. Quarterdeck:The rear part of the upper most deck on a ship. Range Lights:A set of two small lighthouses,aligned so that if a vessel lines them up,they will be guided safely into a harbour,through narrow channels.
Ratline:Any of the small ropes that join the shrouds of a ship horizantally and serve as ships for going aloft. Rigging:The ropes,chains,etc employed to support and work the masts,sails,etc on a ship. Rudder: A hinged or pivoted vertical blade or flat that is turned to steer a boat. Rum Runner:A term applied to a person or vessel employed during prohibition to import alclholic spirits into P.E.I. from 1923 to 1938.One of the islands more notorious rum runners was the Nellie j.Banks, who eluded authorities until August 9, 1938.She was the last rum runner seized off Atlantic Canada. Schooner:A vessel with two or more masts,with fore and aft sails on both masts,normally less than 150 tons,but some of the triple masted schooners built on P.E.I. in the 1880’s exceeded 700 tons. Scallops:A locally used name refering to a vessel witrh one mast carrying fore and aft sails,normally of less than 25 tons. Ship: A vessel with three or more masts with square sails on each,often exceeding 500 tons. Shrouds:Any taut ropes running from a masthead to the side of a ship. Slope:A vessel with a single mast,fore and aft rigged,of less than 25 tons. Snow:The largest type of two-masted sailing vessel of the era, the snow,carrried square sails on both masts ,with a trysail on a jaconet known as a snow mast – which was a spar set on the deck about a foot behind the mainmast and attached at the top to the mainmast. Also:a “Snow Rigged Vessel “ had similar rigging. Souls :Number of persons aboard a vessel used as in”The schooner went down with 120 souls aboard”. Spark:A stout pole forming a ships mast from a yard,gaff,boom,etc. Starboard: The right hand side of a vessel facing forward . Stern:The rear part of a ship. Swinging from the yardarm :A sailor under punishment would be lashed high on the mast.Due to the height ,as the vessel rolled,the swing could easily be 50 to 75 feet depending upon the degree of roll.This could last for days on end without food or water.But then who could keep food down under this situation !Capt.Chris states that this,perhaps was one of the worst punishments. Tidewater:A Customs officer that watched for ship arrivals so the vessels could be boarded and inspected. Tonnage:A measurement of the carrying capacity of a vessel.it is what wins in a collision between two yachts.This little foot note from Paul Curtis! Warfinger:Basically,a man in charge of a docking facility.He authorizes ships to dock,and collects fees from them for docking,and is responsible for arranging for repairs of the dock.
Propulsion is the act or an instance of driving or pushing forward of a body, i.e. ship, by a propeller (in our case a screw propeller).
• History and Development of Screw Propeller Time period Inventor
• • 287-212 BC Archimedes invented his “Archimedean Screw Pumps” to irrigate the field of Syracuse in Sicily. 1452-1519 Leonardo da Vinci had sketches of screw principle to use as a helicopter rotor.
1661 Toogood and Hayes of Britain claimed patent for using helical surfaces (Archimedean screws) as a propeller 1680 Hooke the English physicist suggested to use Archimedean screw for ship propulsion
• • • • • • • • • •
1802/04 C. Steves the American used a kind of screw propeller similar to today’s screws to propel a 7.5 m twin screw steamer. 1828 R. Wilson the Scottish farmer successfully demonstrated the first principles 1836 P. Smith, the English farmer achieved the first practical application. He used single bladed screw of two turns made by wood. 1836 J. Ericsson, the Swedish engineer developed fore runner of contrarotating propeller(i.e. two wheels of three helicoidal blades rotating in opposite direction) 1839 Smith equipped 237 ton of ship Archimedes with screw props having a great success and this led to Paddle propulsion systems to screw propulsion system 1840-1850 Development of steam engines contributed to effective use of screw propellers 1845 Great Britain was the first screw propeller acrossed the Atlantic 1880 Thornycroft designed propellers similar today’s propellers 1880-1970 Basic shape of props remained unchanged 1970-1990’s Fuel crisis and environmental effects (low noise and vibrations) had an impact on propeller shape and stern configurations as well as the developments of unconventional propellers.
earliest representation of a ship under sail appears on a painted disc found in Kuwait dating to the late 5th millennium BC.
• Modern Propulsion Systems
➢ Fixed pitch propulsion system
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This type of system is mainly used for bulk carriers, tankers where the main criterion for design is payload capacity and simplicity of design. This system is housed in line with the propeller, which is connected by means of shaft to the prime-mover or gearbox. The main requirement for this system is the allotment of a reversible engine or a gearbox to counter-rotate the propeller for astern operations. The propeller used in this system is of fixed pitch type made of alloyed materials for greater reliability and performance throughout its service life.
➢ Waterjet propulsion system • This system has found considerable application on a wide variety of small high speed craft, although it is also used for larger ships. • The operation principle of waterjet is that water is drawn through a ducting system by an internal pump adding energy and the water is expelled aft at high velocity. The unit’s thrust is primarily generated as a result of momentum increase given in the water. • The system is preferred to a conventional propeller. Because a conventional propeller experiences cavitation at very high speeds (45 knots), but in the waterjet unit the pump should not cavitate. • It has a good maneuverability.
➢ Cycloidal (Voight-Schneider) propulsion system
• The system is also called vertical axis propellers which comprise a set of vertically mounted vanes, six or eight in number, rotating on a disc mounted in a horizontal or near horizontal plane. • The system has considerable advantages when maneuverability or station keeping is an important factor in the ship design. • A separate rudder installation on the vessel is not required. • Vertical axis propellers are fitted in tugs or other cases where low speed Maneuverability is desired.
➢ Paddle wheels • It is a predator of screw propulsion system. • Used largely on lakes and river services or where limited draughts are encountered.
➢ Azimuth podded propulsion system • It provides propellers with high manueverability, low fuel consumption, high efficiency, low noise and less cavitation • Today, the major users of pod units have been cruise liner operators. • The introduction of pod propulsion, which will allow the propulsion unit to be placed without considering any shaft arrangements or space for machinery will, of course, give the naval architect many new opportunities to design the 'ultimate hullform'.
Future of marine propulsion
➢ Nuclear Marine Propulsion • • Nuclear power for propulsion has several appealing operating and logistic characteristics to the designers of ships for both civil and military purposes. A small amount of nuclear fuel can provide energy equivalent to millions of times its weight in coal or oil.
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It was quite practical to build a reactor with enough fuel to operate a vessel for years, and refuelling was limited to major refits every few years. Although the cost to manufacture fuel elements was high compared to the purchase price of uranium, overall the cost due to fuel was much lower than the comparable fossil fuel. Like sailing ships, nuclear vessels are independent of the vagaries of procurement of fuel at every port (for at least a few years at a time, anyway). The laborious and costly process of loading and firing fuel was largely eliminated for most of the vessel's operating life.
➢ Fish Robot As An Alternative Marine Propulsion System Of The Future • The team of Darmstadt researchers analyzed videos of fish’s motions and then developed a prototype fish robot that duplicated them, and are now testing it using the locomotional patterns of various species of fish in order to refine it and improve its efficiency. Their fish robot, dubbed “Smoky,” consists of a “skeleton” composed of ten segments enshrouded in an elastic skin that are free to move relative to one another and caused to undergo snaking motions similar to those of fish by
waterproof actuators. Including its tail fin, the fish robot, which is a 5:1 scale model of a gilt-head sea bream, is 1.50 meters long. • The researchers hope that use of their fish robot for ship propulsion will help prevent shoreline erosion and the underminings of submarine installations caused by ships’ screws. The fish robot’s “soft” drive action should also prevent the churning up of seabeds and riverbeds and its effects on marine plants and aquatic-animal populations.
Components of a propulsion system
• • • Prime mover(Power plant) Transmission Propeller
➢ Prime mover:
The function of the prime mover is to deliver mechanical energy to the Propeller. The prime mover may be one of the following:
• Diesel engine
○ The diesel engine is reciprocating internal combustion engine. ○ Diesel engines are used to drive cars, trains ships and other marine structures, electric generators, pumps, compressors, etc. ○ The diesel engine is still the most frequently used prime mover in the merchant marine field. Power ranges between 0.25 MW for the smallest high speed engines to 90 MW for the for the biggest lowspeed engines. ○ The main advantages of diesel engines are: • It is relatively insensitive to fuel quality; it can be operated by light fuel as well as the heaviest residual fuels. • High reliability • High maintainability due to simple technology • High efficiency, can reach more than 50% • Low cost, in terms of initial and operational costs. Ship propulsion systems 7 ○ While the main disadvantages of diesel engines are: • Pollutant emissions • Low power to weight ratio if compared with gas turbine • Vibration and noise ○ From the application viewpoint, three main types of diesel engines are available: • Low speed diesel engines (rpm<250) • Medium speed diesel engines (250<rpm<1000) • High speed diesel engines (rpm>1000)
○ From the construction viewpoint, two types can be distinguished: • 2 stroke engines (the majority of them low speed and few medium speed) • 4 stroke engines (medium or high speed)
Arrangement options When the diesel engine used is of the low speed type it is directly coupled to the propeller without gearboxes since the engine speed can be in the range suitable for the efficient running of the propeller. In this case the propeller can be of the fixed pith (FPP) or the controllable pitch propeller (CPP). When FPP is used, the engine chosen has to be able to reverse its rotation direction for the astern operation of the ship, but when the engine is not able to do so, the propeller has to be of the CPP type for performing the astern operation by changing the pitch angle of the propeller blades.
• Gas turbine
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The gas turbine first superseded the steam installation as the propulsion power for naval ships for the following advantages: - Better efficiency - Fast starting-up time - Modular construction - Easy automation - High reliability and maintainability When compared to the diesel engine, the gas turbine has a high power density, so it is a light compact piece of machinery. This major advantage for vessels where space and weight are precious, has to be weighed against the following disadvantages: - It has a low efficiency and high fuel consumption - It needs higher quality fuel than diesel engines (recent developments have started to solve this issue) ○ It is more difficult to repair in situ because it has been designed for repair by replacements
Working principle ○ The above picture shows the basic components of a marine gas turbine. ○ The energy conversion process in a basic gas turbine is a simple Brayton cycle: compression, combustion (heat addition), expansion and exhaust (heat rejection). ○ In the rotating compressor, air is compressed, in one or two compressor sections, from atmospheric pressure to the combustion pressure, which is in order of 10 to 30 bar ○ . Fuel is injected in the combustion chamber and, after combustion at almost constant pressure, the hot gases
expands to atmospheric pressure in the turbine. ○ The turbine delivers power to drive the compressor and load. The output speed Is high; between 3000 and 7000 rpm, so if a gas turbine is used to drive a propeller, a reduction gearbox is required.
Applications ○ Many types of marine vehicles can benefit from the developments of gas turbines, as stated before the navy ships are the main client of this type of power plants, however and due to the level of technology gas turbines reached, many commercial vessels have now gas turbines installed onboard, one of the examples is the huge cruise liner Queen Mary II as gensets and many other luxury passenger ships adopted gas turbine gensets in the past few years. ○ Beside the electric generation applications, gas turbines found their way to the propulsion of commercial vessels especially in the fast ferries market.
Steam turbine Components of the plant ○ A steam turbine plant consists of one or two boilers and a number of turbines. ○ Steam is generated in the boilers and then it expands in the turbines. ○ The boilers can be powered by any of a variety of fuels (poor quality oil, coal, LNG) or by a nuclear reactor (naval
applications). ○ Steam turbines rotate at high speed (~6000 rpm), so it cannot drive a propeller directly, only geared. ○ The steam turbine plant consists of: - Boiler(s) in which steam is generated by burning fuel or nuclear reactor - Turbine(s) in which steam expands delivering power to an output shaft, they may be connected to an alternator for electric power generation (turbo-generator) or to a gearbox for propulsion - A sea water-cooled condenser in which steam condenses to water that can re-enter the power cycle ○ In large installations the boilers are water tube units. ○ The walls of these boilers consist of tubes in which water is vaporized. ○ The boiler also contains oil-burners, so it requires inlet and exhaust ducts for the air and exhaust gases. ○ Marine boilers have a pressure of about 40 bar, which corresponds to a vapor temperature of 250°C, and in case of superheating the temperature reaches 450°C. ○ The expansion may take place in several stages: a high, a medium and a low pressure turbine, each connected to a gearbox. ○ Also a separate turbine for astern operation may be provided.
Basic steam turbine plant Combined power plants ○ Sometimes due to the ship functions a single prime mover may not be suitable for the ship services, and the combined plant may be a preferred choice. ○ In combined plants, two or more prime movers are usually connected to the propulsor through a common transmission system to take advantage of the desirable features of each prime mover. ○ Many combined plants configurations have been in use in several applications, e.g. CODAG, CODOG, COGAS, CODLAG, etc. COmbined Diesel And Gas (CODAG) The figure shows the CODAG concept for a fast ship. In this concept, the diesel engines and gas turbines each drive a waterjet. The jets driven by the diesel engines may be steerable and will be used for manoeuvring and low speed sailing. The jets driven by the gas turbines may be fixed and will be used to boost the ship to maximum speed together with the diesel driven jets.
COmbined Diesel Or Gas (CODOG) The above figure is a typical power plant for a navy ship. The two diesel engines are usually high speed engines and they are used for the low speed operation. The gas turbines are the main machinery and they are used for full speed operation. Gas turbines and high speed diesel engines are not reversible, thus CPPs are used.
COmbined Gas And Steam (COGAS) This system is suggested to be used as an upgrade for ships powered by gas turbines, the steam is generated by using the heat in the exhaust of the gas turbine thus recovering some of the lost heat. This recovered heat can provide the plant by up to 25% of its total power with overall efficiency up to 55%. Ship propulsion systems 31
COmbined Diesel eLectric And Gas (CODLAG) This system is considered a hybrid system since it consists of both mechanical and electrical drives. Mechanical propulsion power is developed by two gas turbines, each through a gearbox connected to FPP. Additionally electric motors, fed by diesel gensets, are delivering propulsion power.
• Electric motor Electric motors found their way as prime mover in the 90’s; they are used with electric generation plant combined of an engine (one of the above types) and an electric generator. They are mainly found in advanced passenger ships, some new designs of offshore support vessels (OSV) are intended to use electric motors especially for dynamic positioning applications. Prime Mover
The transmission system is located between the prime mover and the propulsor. Its main function is to convert or transmit mechanical energy. The transmission system transmits (1) the torque generated by the prime mover to the propulsor, and (2) the thrust generated by the propulsor to the hull.
– The following components can be distinguished from the above figure: ○ One or multiple line shafts transmit the torque generated by the engine, and they transmit thrust if located behind the thrust bearing. The shaft sections are connected to each other with flange couplings. ○ The thrust bearing and the thrust shaft (with thrust collar) transmit the thrust, generated by the propeller, to the hull. The thrust bearing may be independent of the engine, but mostly is integrated in the engine. ○ The shaft bearings support the weight of the shafts. ○ The propeller shaft connects the shafting system inside the ship with the propeller. ○ The stern tube guides the propeller shaft through the hull. In the stern tube, the shaft is supported by one or two oillubricated bearings: the aft and forward bearing. These bearings carry the shaft and propeller weight, and also the transverse hydrodynamic load acting on the propeller. ○ The forward stern tube seal assures that the lubrication oil stays within the stern tube. ○ The aft stern tube seal has two functions: to keep the lubrication oil in and to keep sea water out. ○ Where the shaft line passes through a bulkhead, a bulkhead stuffing box assures that the bulkhead stays watertight. In more complex power plant configuration such as in geared drive with one or more prime mover, some additional components may be encountered. ○ The gearbox is installed in order to reduce the speed of the engine to the speed required for efficient operation of the propeller. Reduction can be achieved in one or two steps: in one step for medium and high speed diesel engines (1:2 to 1:6) and in two steps for gas turbines and high speed diesel engines (1:10 to 1:35). The thrust bearing is usually integrated with the gearbox or installed close to the gearbox. ○ A clutch is used to connect or disconnect the engines to the
shaft line. It is often included in the gearbox, but sometimes it is integrated with an elastic coupling. ○ The elastic coupling has two functions: (1) it improves the torsional behavior of the installation, and (2) it accommodates inaccuracies of shaft alignment and movements of the engine relative to the gearbox. ○ The stern tube bearing may be water lubricated instead of oil lubricated. In that case, only one stern tube seal will be necessary to prevent sea water from entering the ship. ○ The propeller shaft is situated behind the ship in the water. It is supported by the strut and water lubricated strut bearing just before the propeller. Due to its shape this strut is often referred to as an A-bracket. ○ A muff coupling connects the propeller shaft and the stern tube shaft. This coupling does not require flanges at the end of the shaft, so it enables removal of the shafts through the strut bearing or the stern tube.
Transmission components Propeller shaft In general shaft are made of forged (mild steel). Sometimes high tensile steel, or alloys such as stainless steel are used and may be of composite materials. Most often shafts are solid, but they may also be hollow for example when light shafts are required in passenger vessels or naval vessels or when CPPs are used. ○ The Approximate composition is 0.2 - 0.5 % C 0.4 – 0.9 % Mn <0.05 % S and P 1.1 – 0.45 % Si
The Approximate material properties are Tensile strength 400-800 MPa Yield stress 200-700 MPa Bending stress 180-400 MPa Elongation App. 20 % Modulus of elasticity ~ 205 GPa Shear modulus of elasticity ~83 GPa
Shaft bearings Shaft bearings support the shafts. In general sleeve bearings are used. In sleeve bearings the shaft is supported in a lubricating film in a bearing that is usually lined with white metal (babbit). The oil is added to the bearing through a ring that is mounted on the shaft, and distributed by the rotation of the shaft. The bearing capacity of these bearings lies in the range of 0.3 to 0.5 N/mm2 on the projected bearing surface. The length-diameter ratio of sleeve bearings lies in the range of 0.8 to 1. The roller bearing may be used as an alternative for the sleeve bearing. It is sometimes applied in shaft and thrust bearings. In relation to the sleeve bearing it has the following advantages: - It is smaller (lower weight) - Friction losses are less - There is no clearance - It is well suited for low shaft speeds However, the disadvantages need to be considered as well: - More sensitive to dirt and impulse loads - It offers hardly any to no damping for vibrations in the shaft - Less reliable - Higher maintenance costs - Applicable for shaft diameters up to 600 mm
Self aligning sleeve bearing
Roller bearing in a line shaft bearing (left)
and in a thrust bearing (right)
Thrust bearing The thrust bearing converts the mechanical energy in the rotating shaft into translating mechanical energy to propel the ship. The thrust bearing has to transfer thrust to the hull while sailing both forward and astern.
Michell type thrust bearing In a Michell type thrust bearing, the thrust is transferred through the thrust collar on the thrust shaft to tilting pads that are supported by an oil film. The bearing capacity of this type lies in the range of 2 to 3.5 N/mm2 on the pads. Stern tube In general, two types of stern tube can be distinguished: - Stern tube with oil lubricated bearings - Stern tube with water lubricated bearings Water lubricated bearings are rarely applied in merchant vessels. In naval vessels the stern tube bearings and the A-bracket bearing are sometimes water lubricated. In that case, the shaft is fitted with a bronze sleeve for protection against corrosion by the sea water. The bearings will consist of a bronze bearing bush on which the bearing material, rubber or synthetic material, will be mounted. In case of oil lubricated stern tube bearings, the shaft does not need to be protected against corrosion because the stern tube is filled with oil from a tank. This tank is located 3 to 5 m above the waterline and ensures a slight overpressure relative to the sea water pressure. The bearing bush is often of cast iron and the
inner surface of the bush is centrifugally cast with white metal.
Oil lubricated stern tube seals and bearings The stern tube will require two seals: the aft seal and the forward seal. The aft seal shown in the next figure includes three lip seals: two water repellent lip seals to keep water out and one oil repellent lip seal to keep oil in. The forward seal has two lip seals, both to keep oil in the stern tube.
Standard SUPREME aft (left) and forward (right) seal Flexible coupling To reduce vibration in a system to an acceptable level, flexible couplings need to be fitted. In a geared drive, these couplings are fitted between the engine and gearbox to allow some misalignment and to control the torque variations within the system. An elastic coupling introduces a low stiffness, thus reducing the natural frequencies of the system. Also, they may have good damping quality thus reducing the amplitude of the torsional vibrations. Rubber elements are not the only solution to effectively damp torsional vibrations. Instead of rubber elements a coupling may also use elastic leaf springs combined with oil displacement damping (hydrodynamic damping). The springs themselves have a stiffness, and the oil, while moving from one oil chamber to another, is subjected to resistance, which retards the movement of the outer part relative to the inner part of the coupling.
Vulkan RATO-S coupling with rubber elements and a membrane
Geislinger Elastic Damping Coupling with leaf springs
and hydrodynamic damping by oil displacement Low speed engines have a rigid foundation, but it is common practice to mount medium and high speed engines resiliently. Vibration absorbing mounts, usually of rubber material, reduce the transmission of hull borne noise originating from the engine to the hull. If resilient mounting is applied, the elastic coupling should be able to absorb the displacements of the engine that result from this configuration. The engine will be moving in reaction to the engine torque and the ship’s motions. To accommodate the engine motions the above mentioned couplings are often not sufficient, so special arrangements need to be made, for example: - Two elastic couplings in series with an intermediate shaft - A coupling with flexible elements in series with an elastic coupling. With these solutions radial displacements up to 50 mm may be absorbed.
Highly flexible RATO-S coupling with 2-row element for articulated drive Shafts
Geislinger Flexible Link in series with a leaf spring elastic coupling: the flexible links are shown in section A
Clutches If a ship is equipped with one shaft line and two or more engines, the need arises to connect and disconnect engines to the shaft line in order to sail with one or more engines. This is the task of a clutch. They are either pneumatically or hydraulically actuated. The connection between input and output shaft is established by compressed air forcing the inner ring of the drum to move into contact with the drive. In a plate type clutch the input shaft has a hub with steel pressure plates at its extreme end. When the input shaft has to be connected to the drive, the pressure plates and the clutch plates are moved into contact. The clutch plates are connected to the clutch spider and the pinion.
Combination of an elastic coupling and an air actuated clutch
Diagram of a plate clutch integrated in a gearbox In configurations with a steam or gas turbine a self-shiftingsynchronous (SSS) clutch will often be used. This a teethed clutch which engages automatically when input and output speeds are synchronized. Hydraulic or fluid couplings combine the clutch function and the vibration attenuation function of a flexible coupling. In a hydraulic coupling the input shaft delivers kinetic energy to oil, and the oil will transfer the kinetic energy to the output shaft. The clutch operates smoothly and no wear will take place between the shafts. Gearboxes Basically, marine gearboxes consist of meshing teeth on pinions and wheels, which transfer power from a drive shaft (primary) to a driven shaft (secondary) and reduce speed: [i = nengine/npropeller]. Three configurations will be discussed; parallel, locked train and epicyclic. Parallel configurations consist of pinions and wheels with teeth on the periphery. Single and double stage reduction systems are used. In single gears, the diesel engine drives a pinion with a small
number of teeth. This pinion drives the main wheel that is directly coupled to the propeller shaft. The double reduction systems are more usual for turbine drives. In a double gear, the prime mover would drive the primary pinion, which drives the primary wheel. The primary wheel is connected by a shaft to the secondary pinion, which drives the main wheel. A special type of the double gear has a quill shaft with a PTO (power take-off) for the drive of, for instance, a generator. The combination of multiple disc couplings and quill shafts makes it possible to use the engine to drive only the PTO shaft or only the propeller shaft or both shafts. A quill shaft consists of a hollow shaft through which another shaft is led. Marine gears are often of the double helical type which means they have two sets of helical teeth in opposite direction on the same wheel or pinion. A single set would produce a resulting axial force, the double set balances out the axial force.
Single input, single output gear
Double input, single output gear
Double input, single output and PTO Shown are one of the input shafts and the PTO
Schematic layout of the gear transmission system (starboard side) of a CODOG propulsion plant The above figure shows the schematic layout of a gear transmission system for a CODOG propulsion plant of a naval vessel. It shows the input line for the diesel engine, which drives a pinion with double helical teeth through two clutches. The diesel engine is provided with a single stage reduction. The two clutches are installed in series. The first is a fluid coupling with oil. The second is self-shifting-synchronous (SSS) clutch. The gas turbine input line is also provided with an SSS clutch. The gas turbine needs a higher reduction ratio and consequently provided with two reduction stages. Because of the high torque to be transmitted, the gas turbine power is split over two parallel gear trains. The gas turbine input pinion meshes with two with two intermediate gear wheels, which should transmit 50% of the torque each. The intermediate gear wheels are connected by intermediate shafts to secondary pinions, which mesh with the main gear wheel. This type of gear transmission is called a locked train.
Planetary gear (epicyclic transmission gear) In an epicyclic system, one or more wheels travel around the outside or inside of another wheel whose axis is fixed. They are referred to as planetary, solar and star gears. The next figure shows an example of this type of gears. Note that the input and output shafts are in-line. The wheel on the principal axis is called the sun wheel. The wheels whose axis revolves around the principal axis are the planet wheels. The internal teeth-gear that meshes with the planet is called the annulus. The different arrangements of fixed arms and the sizing of sun and planet wheels provide a variety of different reduction ratios.
The Propeller converts the rotating mechanical power delivered by the engine into translating mechanical power to propel the ship. The most common Propeller is the propeller. In general, two types of propeller are distinguished, fixed pitch and controllable pitch propellers. Other types of Propellers are for example, waterjets and Voith-Schneider Propellers (vertical axis propeller). • Types of propellers The screw propeller is the most common propulsor, but there are other types used like the waterjet and the Voith Schneider.
The screw propeller
A propeller generates thrust by means of lift on the blades that rotate at an angle of attack relative to a flow. The geometry of blades is very important in light of efficiency and cavitation. The propeller consists of blades and a hub or boss. The connection between hub and blades is the fillet area or the blade root. If a ship is viewed from aft, the side of the blades facing aft is the face or the pressure side, whereas the side facing the ship is the back or the suction side. The propeller pitch is the distance that a propeller theoretically (without slip) advances during one revolution. The pitch angle varies with increasing radius. For calculation purposes, a nominal pitch is defined; it is the pitch at 0.7 of the radius.
Two other properties of the propeller blade are rake and skew. Rake is the distance between the blade and the propeller plane at a certain angle. A backward rake, increasing the tip clearance when fitted behind a ship, is a positive rake. A propeller is skewed when the tip of the blades is shifted in relation to the blade reference line.
Ship propellers may have from three to six similar blades, the
number being consistent with the design requirements. It is important that the propeller is adequately immersed at the service drafts and that there are good clearances between its working diameter and the surrounding hull structure. The screw propeller may be either fixed pitch or controllable pitch propeller. The pitch of the FPP, although not constant along the radius of the blades, is fixed in any point, since the blades are rigidly attached to the hub. The amount of thrust developed by the propeller is controlled by the rotational speed of the propeller. Stopping and reversing the ship require special measures: it must be possible to change the direction of rotation of the propeller in either the gearbox or the engine. A CPP consists of a hub with the blades mounted on separately, so that they can rotate, thus changing their pitch. The shaft is hollow and contains a control system, mainly hydraulic, that can adjust the pitch angle of the blades. Adjusting the position of the blades changes the angle of attack in the flow, thus changing the thrust without changing the rotational speed. This has major advantages with respect to manueverability of the ship. On the other hand, the disadvantages are a larger hub, restrictions to blade design, slightly lower efficiency, more complicated and expensive. CPP is used instead of FPP for one of the following reasons: - To improve the low speed manueverability of the ship - To adapt the load characteristics to the drive characteristic - To generate constant frequency electric power with a shaft generator
Controllable pitch propeller installed on a ship
Screw propellers can be fitted in several configurations according to the type of the ship and the area in which she is operated.
Materials used for the manufacture of propellers Minimum stress (kg/mm2) Cast steel Special propeller bronze Ni-Al-bronze 41 45 60 ultimate tensile Minimum Elongation (%) 20 20 16
Nodular cast iron, heat treated Not heat treated Special cast iron Ordinary cast iron Gun metal
40 55 24 14
15 3 8
Density Single Screw (lb/ft3) Reciprocating engines 6000 6750 2500
Design Stress (lbs/inch2) Twin Screws Turbine diesel electric 6250 7000 2600 or Reciprocating engines 6250 7000 2600 Turbine diesel electric 6500 7250 2700 or