Question Sketch a semi balanced rudder showing: (i) Details of rudder support; (ii) Details of the hinging mechanism about which the rudder turns. State ONE advantage of the semi balanced rudder over other types. Answer
Locking pintle
Bearing pintle
Locking pintle
Bearing pintle
Rudder carrier
Advantage of the semi balanced rudder over other types.
In unbalanced rudder all rudder area has aft of pivot point CE position results in high torque on stock s tock and pintle
In semi balanced rudder up to 20% of rudder area has forward of pivot point hence torque on stock and pintle is reduced r educed Thus relatively smaller diameter stock and smaller capacity steering gear can be used
Rudder stops - Three ways Limit on telemotor - 35deg each way from fro m midship Arrangement such that actuators are put to stop by hunting gear control
In steering gear 37deg each way from fro m midship There are stop set to limit the angle to which the rudder can be moved by gear Stops are provided on steering gear itself at 37deg in order that the rudder is not forced against external stops in case of mal adjusted control gear
External stops 39deg each way from mid ship They are provided at 39deg each side with the aim of preventing unlimited movement of rudder in case of disconnection d isconnection Specially in view of protecting the propeller
Question Work is being carried out in dry dock on a large sea s ea water inlet chest and valves Describe the inspection As work has started During and after work Before starting work Check condition of grating how much the grating is chocked with sea g rowth Check condition of corrosion this will indicate whether MGAS is working properly
Check galvanic protection is OK Check condition of zinc block As work commences and is progressing At regular interval sea chest has to be examined whether surface is properly cleaned of barnacles and sea growth Corrosion – entire sea chest is scrapped check for corrosion pitting etc If ship is more than 5years old with consultation with surveyor [class] carry out thickness gauging of sea chest New zinc block are properly fitted and quality of zinc is good they should have good contact with sea chest metal If MGPS is provided fit new copper rods and zinc rods Painting of sea chest should be properly painted two or three coat minimum While painting paint should not be sprayed on zinc anodes and MGPS anodes Air and steam valves to be checked for corrosion crack condition of sitting valve seat etc
After work Check no foreign material as paint brush jute is left inside Vent line and air line should be blown through to make sure that they are clear Air vent blowing valve and main sea chest valve are properly boxed up with new joint packing etc Sea chest valve can be checked for leakage If everything is satisfactory put the grating from outside grating is cleaned Fitted with stainless steel nut and nut to be welded and secured by split pin
Write short notes on FREE SURFACE EFFECT
When a tank on board a ship is not completely full of liquid, and the vessel heels, the liquid moves across the tank in the same direction as the heel. The centre of gravity of the ship moves away from the centreline, reducing the righting lever and increasing the angle of heel. The movement of the centre of gravity from G to Gx has been caused by the transfer of a wedge of liquid across the tank. Thus if
CAVITATION The thrust of a propeller varies approximately as the square of the revolutions. Thus as the speed of rotation is increased there is a considerable increase in thrust. The distribution of pressure due to thrust over the blade section is approximately as shown in Fig
The net pressure at any point on the back of the blade is the algebraic sum of the atmospheric pressure, water pressure and negative pressure or suction caused by the thrust. When this suction is high at any point, the nett pressure may fall below the vapour pressure of the water at water temperature, causing a cavity or bubble to form on the blade. This cavity is filled with water vapour and with air which disassociates from the sea water. As the blade turns, the bubble moves across the
blade to a point where the nett pressure is higher, causing the cavity to collapse. The forming and collapsing of these cavities is known as cavitation. When the cavity collapses, the water pounds the blade material, and since the breakdown occurs at the same position each time, causes severe erosion of the blades and may produce holes in the blade material several inches deep. Cavitation also causes reduction in thrust and efficiency, vibration and noise. It may be reduced or avoided by reducing the revolutions and by increasing the blade area for constant thrust, thus reducing the negative pressure. Since cavitation is affected by pressure and temperature, it is more likely to occur in propellers operating near the surface than in those deeply submerged, and will occur more readily in the tropics than in cold regions.
Reserve buoyancy A floating vessel must displace its own weight of water. Therefore, it is the submerged portion of a floating vessel which provides the buoyancy. The volume of the enclosed spaces above the waterline are not providing buoyancy but are being held in reserve. If extra weights are loaded to increase the displacement, these spaces above the waterline are there to provide the extra buoyancy required. Thus, reserve buoyancy may be defined as the volume of the enclosed spaces above the waterline. It may be expressed as a volume or as a percentage of the total volume of the vessel. Question a) Sketch the fwd section of a ship to show the position of the following component parts of the ships anchorage system; hawse pipe, cable s topper, windlass and cable lifter, spurling pipe and chain locker.
b) Describe a chain stopper and state it's purpose. c) Show by means of a sketch how an anchor cable is attached to the ship. d) Describe how a chain locker is drained of water, sand and mud. ANS - a) SKETCH
b)The chain stopper is used to hold the anchor chain in place once the anchor has been lowered and it takes the strain on the anchor when the anchor is raised and in place. c)The final link of the anchor chain is secured to a half shackle which is welded to a secure point of the ships structure in the chain locker, bulkhead or deck and is known as the " bitter end d)A chain locker is drained of water, sand and mud by means of a bilge eductor which is normally driven by water from the fire main or general service main. The reason for using a bilge eductor is that sand and mud mixed with water would damage a normal pump.
Question Explain with the aid of sketch Raise floor Water tight bulkhead Tumble home Shelter deck Free board
Tumble-Home - is the fall in of the sides amid ships. It flattens out the stability curve and allows the ship to roll more easily. Rise of Floor - is height from the base line to the bottom of the bilge radius. It helps to drain the double bottom tanks. Shelter deck - upper deck having no overhead protection from the weather, but sheltering the deck below Freeboard is the difference between the depth at side and the draught, that is it is the height of the deck above the waterline. The freeboard is usually greater at the bow and stern than at amidships. This helps create a drier ship in waves. Freeboard is important in determining stability at large angles. Freeboard is the distance from the water load line up to the main deck.
The main deck is the highest deck that is water sealed. Water falling on upper decks may run down companion ways, but it cannot go any farther down into the ship than the main deck. When the ship is in bad weather and ploughing her way through the waves, she will at times displace a greater weight of water than her own-weight, which will tend to push her up again. This extra weight of water tending to push the ship up when in the above condition, is represented by the freeboard. This extra volume of water which the ship can displace in bad.weather and is in reserve for that purpose, is measured by the water plane area multiplied by the freeboard and is called the reserve buoyancy. Every ship must have reserve buoyancy, as in case of heavy leakages into holds below the main deck, or bilge pumps becoming defective or in case of collision, the hold would gradually fill up with water. The reserve buoyancy, therefore, keeps the ship from immediately sinking and allows time for repairs to be effected and thus save the ship and incidentally the lives in the ship. Question Purpose of load line Historical note In 1876, Samuel Plimsoll introduced a law into Parliament that meant that ships were assigned certain freeboard markings above which, in particular conditions, they were not allowed to load beyond. Prior to this law a great many ships were lost at sea mainly due to over loading. In 1930 and in 1966, international conferences modified and expanded these statutory regulations dealing with the safety of ships. These r egulations have been further improved over the years by conference meetings every 3 or 4 years up to the present day. One such organisation was the Safety of Life at Sea organisation (SOLAS).
In recent years, the IMO has become another important maritime regulatory body.
The purpose of a 'load line' is to ensure that a ship has sufficient freeboard and thus sufficient reserve buoyancy. The freeboard on commercial vessels is measured between the lowest point of the uppermost continuous deck at side and the waterline and this must not be less than the freeboard marked on the Load Line Certificate issued to that ship. All commercial ships, other than in exceptional circumstances, have a load line symbol painted amidships on each side of the ship. This symbol must also be permanently marked, so that if the paint wears off it remains visible. The load line makes it easy for anyone to determine if a ship has been overloaded. The exact location of the Load Line is calculated and/or verified by a Classification Society and that society issues the relevant certificates.
This symbol, also called an international load line or Plimsoll line, indicates the maximum safe draft, and therefore the minimum freeboard for the vessel in various operating conditions. Question State with reasons, the purpose of each of the following with regard to ship hull forms. a) bulbous bow b) flare c) bilge keels d) sheer e) camber ANS
a)The bulbous bow allows the ship to cut through the water better due to it's shape, this give s the ship more speed and improves fuel economy, also it gives the ship more buoyancy. It does this by reducing the bow wave. b)Flare is the shape of the ships bow coming up from the water line, it helps keep the bow above the water in heavy weather, especially on smaller ships when pitching. It also allows the anchor to be lowered without hitting the ships side. c)Bilge keels have the prime function of damping the rolling motion of the ship, they also give added longitudinal strength at the bilge. d)Sheer is the curvature of the ships deck from forward to aft, it gives increased strength and keeps water off the deck. e)Camber is the curvature of the ships deck from port to starboard, it keeps water off the deck, it also gives added strength to the ships hull.
Question W.R.T. hull protection against corrosion describe how each of the following operate. SACRIFICIAL ANODES IMPRESSED CURRENT Means of preventing marine growth and fouling. Answer. Cathodic protection is a system of inhibiting hull corrosion by eliminating the anodic area: on the steel surface. There are two systems in use, sacrificial anodes and impressed current SACRIFICIAL ANODES Sacrificial anodes consist of streamlined blocks of zinc, magnesium or aluminium alloy attached at strategic position around the hull, especially necessary in the vicinity of propellers. The presence of the sacrificial anode causes the previously anodic hull to become a cathode thus being protected. The anodes will of course need to be regularly inspected and periodically replaced. This is the usual protection system for smaller vessels, particularly around the after end where a concentrated cell occurs.
The anode material is cast around a steel strip to allow welding of the anode to the hull. Alternatively, the anode is attached to studs welded to the hull, allowing inwater renewal as well as renewal in way of non-gas free spaces.
IMPRESSED CURRENT The impressed current system is generally used for larger vessels or luxury craft since it is more expensive to install. Electrical current from a power unit to a number of hull anodes which, unlike the sacrificial anodes, are insulated from the hull, and are classed as non-consumable
(actually having a life of about 15 years). The electrons from the d.c. power source travel into the hull at the same rate that the anions are being formed at the cathode, thus preventing a loss of electrons from the hull. Therefore the iron ions are not formed and the hull does not anodically degrade. A reference electrode is used to measure cell potential difference gives a signal to a controller that regulates the d.c. power source which supplies electrons to the hull at the required rate. The circuit is completed by the current passed through the anodes which are consumed at a very slow rate (approximate life of 10 years). With lead-silver anode: (2% Ag) the negatively charged hydroxyl ions are attracted to the anode causing the surf ace to form a skin of lead-peroxide, after which the reaction reduces. The following diagram shows the principle of the impressed current system.
The control equipment operates from reference electrodes port and starboard, whose signal to the amplifier of the control equipment can operate separately, or in parallel. In operation, the current varies with the ships hull condition, speed, water salinity and temperature, automatically adjusting for the different potentials occurring.
Means of preventing marine growth and fouling In the initial stages, fouling is in the form of a slime composed of bacteria and single cell plants. Spores and larvae then become trapped in the slime and develop into marine plants and animals. Some marine plants require sunlight and may only be found on the sides of the vessel, e.g. green grasses, whilst others such as red and brown grasses may be found on the lower sides and bottom. Most marine animals do not require sunlight and may be found mainly on the bottom of the vessel. The extent of fouling depends upon time in port, location and time of the year. Generally, fouling does not occur at speeds greater than 4 knots. Fouling can increase hull resistance which results in a speed loss for a given power or an increase in power requirement (and hence fuel consumption) for the same speed. ANTI-FOULING COAT1NGS Two anti fouling paints are in use; conventional and self polishing. Conventional anti-fouling These rely upon the physical dispersion of toxins incorporated in the paint binder. As they leach out, a coarse insoluble matrix is left behind. This matrix hinders leaching from the inner coatings, thus reducing the effective life of the system as well as increasing the hull resistance. Some systems can be re-activated by scrubbing the spent anti-fouling from the surface. Self polishing anti-fouling coatings (SPC) In this system the toxin is chemically incorporated into the binder. The coating is water soluble at its surface, thus allowing the toxin to be released as the binder dissolves. The water flow over the hull surface erodes the paint and 'rough' peaks are smoothed to a surface that has less resistance than in the new condition, hence the name SPC. The life of the paint coating is proportional to the applied thickness but there is an environmental problem with the toxins used.
Question Define the following with respect to propeller Boss— is the part of the propeller post or stern post to which the stern tube is secured in single screw ships. Blade Tip
Maximum reach of the blade from the center of the hub.
Radius
Distance from the axis of rotation to the blade tip.
Rake The fore or aft slant of a blade with respect to a line perpendicular to the propeller axis of rotation Skew The transverse sweeping of a blade such that viewing the blade from fore or aft shows an asymmetrical shape. Pitch The distance (usually expressed in inches) a propeller would move in one revolution in a solid medium with no slippage
Question. With regard to ship construction details for watertight bulkheads a) State the purpose of this type of bulkhead. b) State how the bulkhead's are tested for water tightness. If it is necessary to penetrate the bulkhead, precautions must be taken to ensure that the watertight integrity and strength of the bulkhead are maintained. With this in mind, sketch how the following pass through bulkheads: (i) Main transmission shaft; (ii) Electric cable. Answer. a) The purpose of a transverse watertight bulkhead is to divide the ship into watertight compartments, thus restrict the volume of water which may enter the ship due to damage to the shell plating. They also serve to separate different types of cargo. In the event of a fire, they greatly prevent the spread. They also increase the strength of the ship transversely.
Male and female halves of a gland around the cable are screwed together. Neoprene seal and felt washer are compressed to make a good seal. (ii)
Over size hole in the bulkhead has a stuffing box fitted, fastened by studs to the bulkhead. Tightening of the bolts compresses hemp packing onto the shaft. May also use a mechanical seal as shown in the right hand sketch.
Question Describe suitable methods for testing each of the following for watertight integrity; a) a hollow rudder b) a bulkhead c) a double bottom tank d) a watertight door e) shell plating ANS a)A hollow rudder would be tested in drydock by filling the rudder with water and checking it for leaks, a small pressure may be applied to aid this, ie a head of pressure from a stand pipe of 2metres or 0.2 BAR air pressure.
b)Bulkheads would be tested by means of a simple hose test. On construction, bulkheads are hydrostatically tested.
c)Double bottom tanks are tested by filling them completely with water, including the sounding pipe, by doing this, a small head of pressure is applied to the tank, adjacent tanks, cofferdams around the tank being tested would be checked.
d)A watertight door is checked for watertight integrity by means of a simple hose test, but on construction are hydrostatically tested.
e)Shell plating can be tested for watertight integrity by a vacuum test, where in drydock, a sealed box is placed over welds and evacuated of air, loss of vacuum showing leaks, welds on ships plating may also be x-rayed to check the integrity of the weld.
Question a)describe how anodes prevent corrosion; b) i)state three suitable materials that can be used for anodes; ii)state any restriction on the use of these materials as anodes in oil tankers.
ANS a)Anodes prevent corrosion by becoming a sacrificial metal. Anodes are less noble metals in the periodic table than the cathode, which is steel in this case. The saltwater sets up a galvanic couple, galvanic current attacking the sacrificial anodes, which conveys a protective coating around the steel work in the t ank (the cathode). The anodes eventually waste away and have to be replaced.
b) i) These three metals are suitable fore anodes as they are lower in the galvanic series than iron; Zinc, Magnesium, Aluminum
ii) Restriction on these materials as use as an anode in oil tankers is that Magnesium is banned as there is a risk of sparks, fires and explosion with magnesium . Question With regard to the roll motion of the ship in seaway; a)explain why roll motion is considered more serious than pitch motion b)discuss the basic principles of roll damping utilised in the following anti-roll systems;
i)bilge keels ii) activated fins iii)passive roll damping tanks ANS a)Roll motion is considered more serious than pitch motion because with roll motion, the ship can get into synchronization with the wave and while the wave gets bigger, so will the roll, also slack tanks can be a problem with roll motion, or can be a shift in cargo. b) i)Bilge keels offer a damping effect against roll motion by their surface area acting against the roll. ii)Activated fins work by offering a damping effect against the roll motion by creating an opposite force to the roll, plus using the ships forward velocity to achieve this. Ie, if the ship were to roll to port, the port fin would be sent a signal by a gyroscope to have it's leading edge above the axis of tilt, while the stbd fin is sent a signal to have it's fin below the axis of tilt. See sketch
iii)Passive rolling tanks dampen the roll motion of the ship by using the effect of water movement in a tank. The tank contains perforated baffles to slow the motion in the tank from side to side, thus using the force of the water in the tank to act against the motion of roll.
Question Define the purpose of cofferdams. State where cofferdams are most likely to be found on : (i) Dry cargo ships; (ii) Oil tankers. (c) (i) State what information is available about danger of entering void spaces. (ii) Identify, with reasons, the precaution to be observed before and during entry to cofferdams. ANS
a) The purpose of cofferdams are to prevent leakage of one tank or double bottom to another ie, there would be a cofferdam between fuel tanks and fresh water tanks. They also allow for inspection of tanks, cofferdams should also be sounded regularly to check for leaks.
b) i) On dry cargo vessels, cofferdams would be found between fuel tanks and cargo, ballast tanks and cargo, also found around the main engine LO drain tank.
ii) On oil tankers, cofferdams would be found between the oil cargo tanks and the accommodation or engine room, in some cases this can be the pump room, also there will be a cofferdam around the main engine LO drain tank.
c) Precautions taken before entry into an enclosed space are as follows; An entry permit must be obtained from a responsible officer, you may be the issuing officer. The space is to be well ventilated and oxygen levels tested by an 02 meter. Safety gear, such as self contained breathing apparatus sets must be at hand, rescue lines, communications. Also while anyone is in the enclosed space, someone must be on standby at the entrance who must be in communication with the people in the space and also must be able to raise the alarm if they are to get into difficulties. Official guidance on entry into enclosed spaces can be found in "M" notices and also in the "Code of Safe working practices".
Question DEC 2006 Describe the arrangement of tank top and double bottom in the machinery space making particular reference to the structure and scantlings below the main engine. Show the method adopted in the arrangement of D.B. tanks to avoid contamination of fresh water, fuel oil and lube oil stored in D.B. tanks. Answer Double bottom in the machinery space In the machinery space other factors must be taken into account. Forces of pulsating nature are transmitted through the structure due to the general out of balance forces of the machinery parts. The machinery seats must be extremely well supported to prevent any movement of the machinery. Additional girders are fitted in the double bottom and the thickness of the tank top increased under the engine in an attempt to reduce the possibility of movement which could cause severe vibration in the ship. For similar reasons the shaft and propeller must be well supported.
Great care must be taken in the machinery space to ensure that the main and auxiliary machinery are efficiently supported. Weak supports may cause damage to the machinery, while large unsupported panels of plating may lead to vibration of the structure. The main engine bedplate is bolted through a tank top plate which is about 40 mm thick and is continuous to the thrust block seating. A girder is fitted on each side of the bedplate in such a way that the holding down bolts pass through the top angle of the girder. In welded ships a horizontal flat is sometimes fitted to the top of the girder in way of the holding-down bolts.
MAIN - ENGINE HOLDING
DOWN BOLTS
In motor ships where a drain tank is required under the machinery, a cofferdam is fitted giving access to the holding down bolts and isolating the drain f rom the remainder of the double bottom tanks. Additional longitudinal girders are fitted in way of heavy auxiliary machinery such as generators. Access to these D\B tanks and cofferdam is provided by watertight manhole door. Docking plugs are fitted in all D\B yanks for complete draining in drydock for purpose of inspection Air pipes and sounding pipes are also fitted to D\B tank and these tanks are tested by maximum service pressure head
[The construction of the double bottom under the machinery space regardless of framing system has solid plate floors at every frame space under the main engine Besides the continuous center girder of greater thickness, additional side girders arc-fitted outboard of the main engine seating. The double bottom height is usually increased to allow for the alignment of the shafting system, as well as to provide fuel oil, diesel oil lubricating oil and fr. Water tank of suitable capacities. Where the double bottom u, increased in depth above the normal, continuity of strength is
maintained by the gradual sloping of the tank top and intercostals girders between the machinery space and the adjacent compartments. Heavy plate (40mm) support with, bar stiffener is provided on the tank top for the seating of main engine bedplate and thrust block, which are bolted direct to heavy plates incorporated in the tank top, the bolts penetrating into cofferdam. The lubricating oil drain tank, fuel oil tank, diesel oil tank, fresh water tanks in the: double bottom space are separated by cofferdams to avoid contamination. Access to these double bottom tanks and cofferdam is provided by water-tight manhole doors-Docking plugs are fitted in all double bottom tanks for complete draining in dry dock for inspection purpose. Air pipes and sounding pipes are also fitted to these double bottom tanks and the tanks are tested by the maximum service pressure head.]
Question Explain why weighted cocks fitted on tank sounding pipes, Answer Cocks and valves are designed to control or interrupt flow. This is done in cocks by rotating the plug and in valves by lowering, raising or rotating a disc in relation to a seating surface or by controlling the movement of a ball Cocks
Figure Example of a sleeve-packed cock A cock may be straight-through, right-angled or open-bottomed as required by its situation in a pipe system. Its plug may be tapered or parallel with tightness achieved by lapping in or by resilient packing material (Figure 4.a) often in the form of a ready made sleeve.
In machinery spaces, the short sounding pipes for fuel or lubricating oil tanks must be fitted with cocks having parallel as opposed to tapered plugs.
This, together with the requirement for weighted handles which will automatically close the cock when released, is for safety.
Tapered plugs, when tightened to hold the cock open for sounding and then forgotten, have contributed to fires when tanks have overflowed.
Remote operated gear for bilge valve In the case of flooding of engine room it is not possible to operate the valve from local position it can be operated remotely Emergency bilge valve is operated remotely
Ventilation pipe While deballasting air should entre from outside to prevent vacuum formation in tank and during ballasting air must escape to prevent tank getting pressurised other wise tank will collapse During rolling and pitching tank level continuously chanes since mass of water movement which leads to development of air pr or vacuum
At this point air should entre to release vacuum or should go out to avoid over pressuriastion
Gauze are made of some good conducting material such as copper M.S. or S.S. They act as flame arrestor because of expanded surface of gauze flame front will get cooled down and there by extinguish the flame thus preventing flame or spark from outside from entering the tank and preventing fire and explosion If the mesh size is too big then efficiency of flame trap will reduce and it will not serve the purpose for which it is fitted If too small it will get chocked due too rust cargo dust hence tank will get pressurised and it will affect the functioning of vent Hence tanks may over pressuriese or go into vacuum
Question State the purpose of each of the following and describe where they are located in the ship structure; a) Duct keel; b) Stern frame aft heel; c) Forward and after sheer; d) Round of bilge ANS
a) The duct keel is situated in the double bottom of some vessels, it runs from the forward engine room bulkhead to the collision bulkhead, they are used to carry pipe work, making accessible when cargo is loaded. See Sketch.
b) Stern frame aft heel is where the heel block is placed when the vessel is in dry dock; this part of the vessel has added strength to support the after end of the ship. See Sketch.
c) Forward and after sheer is the rise of the deck at the forward end of the ship and at the after end, this shape provides added strength, plus keeps water off the deck. See Sketch.
d) Round of bilge is found where the side of the ship meets the bottom, it is rounded to give extra strength, plus it also reduces racking. (Side motion of the ship) See Sketch.
Question
Define five of the following, stating how they affect the ship structure and what component parts of the ship structure help to resist the effect of; Answer Racking. When a ship rolls there is a tendency for the ship to distort transversely in a similar way to that in which a picture frame may collapse. This is known as racking and is reduced or prevented by the beam knee and tank side bracket connections, together with the transverse bulkheads, the latter having the greatest effect.
Water pressure effect. A considerable force is exerted on the bottom and side shell by the water surrounding the ship. The double bottom floors and side frames are designed to withstand these forces, while the shell plating must be thick enough to prevent buckling between the floors and frames. Since water pressure increases with the depth of immersion, the load on the bottom shell exceeds that on t he side shell. It follows, therefore, that the bottom shell must be thicker than the s ide shell. When the ship passes through waves, these forces are of a pulsating nature and may vary considerably in high waves, while in bad weather conditions the shell plating above the waterline will receive severe hammering.
Panting As the waves pass along the ship they cause fluctuations in water pressure which tend to create an in-and-out movement of the shell plating. The effect of this is found to be greatest at the ends of the ship, particularly at the fore end, where the shell is relatively flat. Such movements are termed panting and, if unrestricted, could eventually lead to fatigue of the material and must therefore be prevented.
The structure at the ends of the ship is stiffened to prevent any undue movement of the shell.
Pounding When a ship meets heavy weather and commences heaving and pitching, the rise of the fore end of the ship occasionally synchronizes with the trough of a wave. The fore end then emerges from the water and re-enters with a tremendous slamming effect, known as pounding. While this does not occur with great regularity, it may nevertheless cause damage to the bottom of the ship forward. The shell plating must be stiffened to prevent buckling. Pounding also occurs aft in way of the cruiser stern but the effects are not nearly as great.
Local weights. The deck must be designed to support the weight of accommodation, winches and cargo, while exposed decks may have to withstand a tremendous weight of water shipped in heavy weather. The deck plating is connected to beams which transmit the loads to longitudinal girders and to the side frames. In way of heavy local loads such as winches, additional stiffening is arranged. The shell plating and frames form pillars which support the weights from the decks. The tank top is required to carry the weight of the hold cargo or the up thrust exerted by the liquid in the tanks, the latter usually proving to be the most severe load.
Vibration from engine and propellers If the frequency of the main or auxiliary machinery at any given speed coincides with the natural frequency of the hull structure, then vibration may occur In the machinery space other factors must be taken into account. Forces of pulsating nature are transmitted through the structure due to the general out of
balance forces of the machinery parts. The machinery seats must be extremely well supported to prevent any movement of the machinery. Additional girders are fitted in the double bottom and the thickness of the tank top increased under the engine in an attempt to reduce the possibility of movement which could cause severe vibration in the ship. For similar reasons the shaft and propeller must be well supported.
Question Explain with the aid of sketch Raise floor Water tight bulkhead Tumble home Shelter deck Free board
Tumble-Home - is the fall in of the sides amid ships. It flattens out the stability curve and allows the ship to roll more easily. Rise of Floor - is height from the base line to the bottom of the bilge radius. It helps to drain the double bottom tanks.
Shelter deck - upper deck having no overhead protection from the weather, but sheltering the deck below Freeboard is the difference between the depth at side and the draught, that is it is the height of the deck above the waterline. The freeboard is usually greater at the bow and stern than at amidships. This helps create a drier ship in waves. Freeboard is important in determining stability at large angles. Freeboard is the distance from the water load line up to the main deck. The main deck is the highest deck that is water sealed. Water falling on upper decks may run down companion ways, but it cannot go any farther down into the ship than the main deck. When the ship is in bad weather and ploughing her way through the waves, she will at times displace a greater weight of water than her own-weight, which will tend to push her up again. This extra weight of water tending to push the ship up when in the above condition, is represented by the freeboard. This extra volume of water which the ship can displace in bad.weather and is in reserve for that purpose, is measured by the water plane area multiplied by the freeboard and is called the reserve buoyancy. Every ship must have reserve buoyancy, as in case of heavy leakages into holds below the main deck, or bilge pumps becoming defective or in case of collision, the hold would gradually fill up with water. The reserve buoyancy, therefore, keeps the ship from immediately sinking and allows time for repairs to be effected and thus save the ship and incidentally the lives in the ship.
Question Show with the aid of sketch as to how a vessel which is stable will return to the upright after being heeled by an external force. Consider a ship floating upright in still water as shown in Figure (a). The centres of gravity and buoyancy are at G and B, respectively. Figure (c)shows the righting couple. GZ is the righting lever. Now let the ship be inclined by an external force to a small angle
θ
as shown in
Figure (b). Since there has been no change in the distribution of weights the centre of gravity will remain at G and the weight of the ship (W) can be considered to act vertically downwards through this point. When heeled, the wedge of buoyancy WOW1is brought out of the water and an equal wedge LOL1becomes immersed. In this way a wedge of buoyancy having its centre of gravity at g is transferred to a position with its centre of gravity at g1. The centre of buoyancy, being the centre of gravity of the underwater volume, must shift from B to the new position B1,such that BB1is parallel to gg1, and BB1 = v x gg1 / V Where v is the volume of the transferred wedge, and V is the ship’s volume of displacement.
The verticals through the centres of buoyancy at two consecutive angles of heel intersect at a point called the metacentre. For angles of heel up to about 15°the vertical through the centre of buoyancy may be considered to cut the centre line at a fixed point called the initial metacentre (M in Figure (b)). The height of the initial metacentre above the keel (KM) depends upon a ship’s underwater form. The vertical distance between G and M is referred to as the metacentric height. If G is below M the ship is said to have positive metacentric height, and if G is above M the metacentric height is said to be negative.
Stable equilibrium A ship is said to be in stable equilibrium if, when inclined, she tends to return to the initial position. For this to occur the centre of gravity must be below the metacentre, that is, the ship must have positive initial metacentric height. Figure (a) shows a ship in the upright position having a positive GM. Figure (b) shows the same ship inclined to a small angle. The position of G remains unaffected by the heel and the force of gravity is considered to act vertically downwards through this point. The centre of buoyancy moves out to the low side from B to B1to take up the new centre of gravity of the underwater volume, and the force of buoyancy is considered to act vertically upwards through B1and the metacentre M. If moments are taken about G there is a moment to return the ship to the upright. This moment is referred to as the Moment of Statical Stability and is equal to the product of the force ‘W’ and the length of the lever GZ; i.e. Moment of Statical Stability = W GZ tonnes meter
The lever GZ is referred to as the righting lever and is the perpendicular distance between the centre of gravity and the vertical through the centre of buoyancy. At a small angle of heel (less than 15°): GZ = GM sin θ and Moment of Statical Stability = W GM sin θ
[When a ship is inclined by an external force, such as wind and wave action, the centre of buoyancy moves out to the low side, parallel to the shift of the centre of
gravity of the immersed and emerged wedges, to the new centre of gravity of the underwater volume. The force of buoyancy is considered to act vertically upwards through the centre of buoyancy, whilst the weight of the ship is considered to act vertically downwards through the centre of gravity. These two equal and opposite forces produce a moment or couple which may tend to right or capsize the ship. The moment is referred to as the moment of statical stability and may be defined as the moment to return the ship to the initial position when inclined by an external force. A ship which has been inclined by an external force is shown in Figure 1
The centre of buoyancy has moved from B to B1parallel to gg1, and the force of buoyancy (W) acts vertically upwards through B1. The weight of the ship (W) acts vertically downwards through the centre of gravity (G). The perpendicular distance between the lines of action of the forces (GZ) is called the righting lever. Taking moments about the centre of gravity, the moment of statical stability is equal to the product of the righting lever and the displacement, or Moment of statical stability = W GZ]
Correcting unstable and neutral equilibrium When a ship in unstable or neutral equilibrium is to be made stable, the effective centre of gravity of the ship should be lowered. To do this one or more of the following methods may be employed: 1. Weights already in the ship may be lowered. 2. Weights may be loaded below the centre of gravity of the ship. 3. Weights may be discharged from positions above the centre of gravity. 4. Free surfaces within the ship may be removed.
Question Write short notes on: Cellular double bottom tank Transverse watertight bulkheads Keel, Bilge.
Cellular double bottom tank.
The inclusion of a cellular double bottom in the vessel of the present invention gives to the vessel a large "KM" which is the vertical distance above the keel at which the transverse metacenter occurs, and investigations have established that the "GM," the vertical distance from the center of gravity to the metacenter, which will obtain, after free surface correction is made for the liquid contained in all the cargo-oil tanks in the vessel which, in a loaded condition will be about 6.00 feet, which is more than adequate to ensure complete safety under all conditions likely to be encountered in service.
Question Transverse watertight bulkheads Transverse watertight bulkheads are some of the principal transverse strength members of a ship. They are very strong in construction and responsible for maintaining the transverse form of the hull. The forward most transverse watertight bulkhead is called the Collision Bulkhead. As its name implies, this bulkhead is the strongest bulkhead and is designed to protect the vessel in case of a collision. It is usually fitted between 0.05 and 0.075 of the length of the vessel from the forward end. The space forward of this bulkhead is the Fore peak tank. Above this bulkhead is the first hold or tank. The After Peak Bulkhead situated aft and serves to enclose the stern tube in a watertight compartment. Other bulkheads will also be fitted on the forward and after sides of the machinery spaces
