Ship Construction Flashcards
Explain the meaning and purpose of EACH of the following ship terms:
a) Hawse pipe; (4)
b) Chain stopper; (4)
c) Fairlead; (4)
d) Bollard. (4)
a.) an iron or steel pipe in the stern or bow of the ship through which the anchor chain passes from the cable lifter, through the forecastle deck and the ship’s side. Purpose - to provide an easy lead for the cable from the windlass to the anchors,
b.) The chain stopper holds the anchor while the ship is underway and guides the chain during anchor manoeuvring. During anchoring, the chain stopper
withstands the forces from the anchor, so the anchor winch is protected. fitted on the deck between the cable lifter and the hawse pipe, which automatically prevents the anchor chain running out. When letting go of the anchor the
stopper bar of the chain stopper is swung out of the way its main purpose is take load from anchor and transmit it into ships hull.
c.) Fairleads guide the
mooring lines/cables through the ships side and into the winch or
windlass assembly. enables the line to be passed through the bulwark and allows for a change in direction without snagging or fouling.
d.) A rectangular base welded to the deck of the ship, upon which two vertical parts are welded. Bollards are used to secure the mooring lines.
a) Describe, with the aid of a sketch, a freeing-port. (8)
b) Explain how freeing-ports assist in maintaining the stability of a ship. (8)
a) A freeing port is an open hole cut into a bulkward that allows rapid draining of green seas and collected rain water from the weather deck. Its sealed by a hinge plate that will open when a weight of water acts on the inside surface. These can be to the hull side plating, such as on Ro-Ro ferries in order to drain the water from internal car decks. if water was left on the main deck and kept continuously accumulating the mass of the vessel would increase, which would increase displacement, raise the centre of gravity and decrease GM and righting lever.
sketch freeing port (check notes)
b) The freeing port maintains stability by quickly allowing large volumes of water to drain away which would otherwise be able to gather on the main ship deck. The effect of this water would increase the mass of the vessel which would increase displacement and draft. Having this water collecting on the upper deck would raise the centre of gravity therefore decreasing metacentric height (GM), consequently the righting lever (GZ) would decrease and this will have a negative impact on the ships stability. Free surface effect could further decrease stability due to the large volume of water accumulating on deck, which could cause a permanent list.
a) Describe how the thrust of the propeller is transmitted to the hull of the ship. (4)
b) Describe, with the aid of a sketch, how the surfaces, within the thrust block, transmitting the thrust are separated by the lubricating oil. (8)
c) Explain the importance of the oil temperatures within the thrust block. (4)
a) thrust is created axially by the ships propeller, when the propeller spins axially through the water acting as a hydrofoil that cuts through the water.
suction is created on the back of the propeller blades and pressure on the front part of the blades. high pressure side of propeller generates thrust force that propels the vessel through the water. thrust force is passed down the propeller shaft until it hits the thrust collar on the thrust block. The thrust of the propeller is transmitted axially through a shaft to a heavily reinforced point on the ship’s hull, where a thrust block will then transmit thrust force from the propeller shaft to the ships hull.
draw propeller diagram. (check notes)
b.) Hydrodynamic fluid film wedges separate the thrust pads and thrust collar. The trust pad transmits trust to the lower half of the casing. The lower half of the casing is connected to the ships Hull. The ship moves when the trust is transferred to the ship’s Hull. Oil from the top cover cascades over the kidney pads, bearings and fall back to the sump. The top cover acts as a pad stop. The thrust shaft transmits the thrust onto the thrust collar.
draw Mitchell thrust bock
c.) Maintaining this oil wedge is essential to the performance of the block, it the wedge was to fail the Kidney pads would contact the collar cause sever damage. The oil temperature is critical to maintaining this pressure, to hot the pressure will be low, to cold the pressure will be high.
a) Describe the actions that the EOOW should take on finding that the
temperature of the thrust block is rising above normal acceptable range. (6)
b) Explain why the thrust block temperature is critical. (10
a.) firstly contact the bridge and chief engineer to ask to reduce the engine load when its safe to do so.
if thrust block bearing temperature exceeds 75degrees Celsius you should stop the engine when safe to do so to reduce damage dealt to the components.
monitor the lube oil and water level for thrust bearing and fill if necessary.
investigate the surrounding pipes and equipment if the thrust block has jacket pumps incorporated into the system then use them to increase cooling to thrust block.
b.) thrust bearing is very highly loaded and relies on hydrodynamic lubrication there is no forced pressure lubrication. Hydrodynamic lubrication is where lube oil is taken from a lube oil reservoir and the lube oil forms a barrier between running surfaces its formed by motion of moving parts and self generated pressure. For this, hydrodynamic lubrication to be effective temperature needs to maintained in order to maintain barrier between the running surfaces. This barrier will be affected if the temperature is increased because this will reduce viscosity and increase flow rate meaning the barrier will collapse and the running surfaces will come into contact and create friction and cause and wear tear which will damage the thrust block. This will also occur if the temperature decreases because then viscosity increases and flow rate reduces. Additionally thrust block is made from white metal. this metal loses its tensile strength at 120degrees and melts at 180degrees. therefore thrust block temperature must be monitored and controlled by controlling lube oil temperature through the use of LO cooler or using water cooling or there will be severe damage caused.
Similar to sleeve bearings, the performance of fluid film thrust bearings substantially deteriorates as operating temperatures rise. Oil viscosity in the bearing clearance drops drastically due to viscous heating in the tight film gaps. As a result, the oil film thins and is less able to cool the bearing. These factors, in turn, increase the likelihood temperatures will be high enough to soften bearing material and even result in bearing wipe, smearing of the bearing material along the contact surface.
In thrust block, the two basic ways to cool the unit and cut effective running temperature is to increase oil-flow rate and reduce bearing friction.
Some friction within a bearing comes from the viscosity of the lubricant itself. Lowering lubricant viscosity can cut viscous friction and corresponding power loss. But any benefit is usually negated by a drop in oil-film thickness and a higher oil-film shear rate. Still, less-viscous lubricants can improve oil-cooler efficiency and lower friction.
Increasing the volume of oil passing over each pad segment is the most-effective way to lower temperature. incorporate higher oil volume by increasing the taper over a segment, pressurizing oil feed in oil-distributing grooves, and minimizing the bearing outside diameter to keep down net surface velocity. Finally, engineers can directly cool the oil feed or the bearing housing from the outside. Minimizing carryover of hot oil from the trailing edge of one segment into the next segment’s leading edge can be helpful. External cooling fins on the bearing housing lower overall bearing temperature, as does blowing a stream of cooling air at the housing.
what are thrust block used for?
Thrust Blocks are used to transmit the ahead and astern thrust from the propeller into vessel motion.
The axial thrust from propeller is transmitted along the rotating shaft into the stationary
thrust block which in turn is strongly connected to the ships hull using hydraulic holding
down bolts.
This bearing uses a form of hydrodynamic lubrication using the motion of the shaft
collar and multiple stationary plates to create an oil wedge.
in the event that stern tube bearing temperatures are high what are your actions?
and what are your actions If stern tube temperature does not decrease or rise above 85°C?
In case that high temperatures occur in the stern tube bearing:
Reduce shaft revolutions immediately to Dead Slow. In case protection system is only set up to give an alarm or manual Slow Down, it is of high importance that duty officer immediately reduce rpm on the ME telegraph;
Keep rudder position at mid-ship position as far as possible;
Monitor stern tube bearing temperatures rise, if temperature is stabilizing keep RPM and monitor that temperature is gradually going down.
In case temperature is continuously decreasing, continue with Dead Slow RPM until temperature is stabilized below sea water temperature + 30 ºC;
At above stages never stop the Main Engine, as this could result in the tail shaft being bent due to spot heating of the propeller shaft.
If stern tube temperature does not decrease or rises above 85°C with above procedures, then:
Stop the Main Engine;
Engage the turning gear immediately and start turning of shaft to avoid spot heating of the propeller shaft;
Monitor cooling down of stern Tube;
Turning gear must not be stopped during this process.
Obviously vessel must liaise with the superintendent to coordinate on further actions in case of reduction in shaft revolutions due to abnormal conditions in the stern tube system and above checks are to be initiated subject to safe navigational conditions. If high temperatures have occurred, check the filter in the oil system for impurities from the bearings
what alarms are provided for stern tubes?
On some vessels additional alarms and checks are available in order to ensure stern tube safety and proper functioning.
they are;
Temperature rise max. 5 ºC/min ( m/e Slow Down)
ΔT Max differential temp. between SW and S/T temp. ( m/e Slow Down)
Increased monitoring of stern tube bearing temperatures, stern tube seal drains and LO water content during the entire low draft operation.
In general, temperature alarms for stern tube bearings are recommended to be set at:
High Alarm setting 62 ºC
High High Alarm and Slow Down 65 ºC
Other settings may have been applied originally and should only be changed in agreement with the superintendent.
how is propeller immersion stopped?
Under normal circumstances in order to avoid propeller immersion issues, the minimum draft aft must be:
Draft required for min. 100% propeller immersion (as per Trim & Stability book) + 0.6 meters.
During navigation in stormy conditions, a ship can think about postponing or eliminating trim optimization altogether, bringing the ship to an even keel instead, or adjusting the trim by the stern as necessary depending on the severity of the weather. If the propulsion shaft system is experiencing an abnormally high amount of vibration, you may want to consider increasing the aft draft in order to reduce the level of vibration.
When the propeller is only partially submerged during operation, this can result in an excessively eccentric force on the propeller and, as a consequence, a downward bending moment on the shaft. Because of this, there is a possibility that the aft bearing will experience increased localized loads (edge loading), as well as surface pressure, as a consequence of the increased relative slope and lower bearing contact area.
In exceptional cases it may not be possible to achieve 100% propeller immersion + 0.6m, for example:
Vessel going in/out of dry-dock
Phasing in/out of a certain trade
Low cargo load
Vessel trading in areas with limiting factor e.g. minimum water depth and/or port restrictions on maximum vessel draft.
In such cases vessel superintendent is to be informed to ensure that appropriate measures are planned, and following risk mitigation measures are put in place:
All options to increase propeller immersion to greater than or min. 100% must be considered, and cargo planner may be contacted if any concerns with ballast intake and/or stress & stability limits.
At propeller immersions between 87% to 100%, the maximum load on main engine should not exceed ME power corresponding to “Half Ahead”.
It must be ensured that all stern tube and intermediate bearing temperature alarms are checked and slow down functions (Manual or Automatic) are tested.
Vessels equipped with ‘Manual Slow Down’ require immediate attention during a high temperature alarm.
how does propeller immersion occur?
what is meant by immersion of propeller
and what are the effects of propeller immersion
vessel propeller immersion issues occur due to uneven load of cargo, lack of cargo or impossibility of ballasting/de-ballasting the vessel due to shear forces or bending moments. This is a very serious issue as propeller immersion less than 100% will result in loss of vessel performance, main engine over speeding and stress or damage to vessel machinery.
The immersion of propeller is defined as the ratio of the distance between free surface and propeller blade tip to propeller diameter.
If propeller is not completely immersed, it will result in:
excessive eccentric thrust
increased downward bending moment at the aft end of propeller shaft, leading to higher edge loading of stern tube bearing.
breakage of oil film and ineffective hydrodynamic lubrication in the aft stern tube bearing.
increased shaft system vibrations
increased cavitation of propeller
When propeller and shaft lines are operated outside the design criteria there is a risk of:
Stern tube seal leakage
Increased wear of stern tube bearing
Fatigue failure and subsequent damage of stern tube bearings.
Wear and damage to shaft line bearings
Cavitation and wear of propeller
how is stern tube cooling achieved?
Vessel crew must ensure efficient stern tube cooling by always keeping the cooling water tank around the stern tube filled with fresh water. As mentioned above LO water content should be checked regularly due the entire low draft operation, as in case of stern tubes with white metal bearings, water in the lubricating oil can cause severe damage with considerable repair expense and time loss. On the other hand, Wartsila Railko stern tube bearings can work with a limited amount of sea water in the lubrication oil without damage to the bearings.
what is the purpose of the stern tube?
The stern tube is a hollow tube which accommodates the bearings, the seal boxes and the propeller shaft. The stern tube is filled with oil, grease or water and forms a barrier between the water outside and the engine room inside the vessel.
explain the operation of the OWS
The bilge separator operates automatically and discharges water overboard or back to the bilge water holding tank depending on the oil content of the discharged liquid and separated oil to the waste oil drain tank. Bilge water is drawn from the bilge main by the attached pump and into the bilge separator where it passes, usually through a two-stage separation process. The separator uses the difference in density and surface tension between oil and water in usually two stages that are housed separately or in the same compartment. The separator is initially filled with clean water before admitting bilge water. The pump supplies the oil water mixture to the first stage where most of the oil is retained. Oil droplets are attracted to the coalescer surface or gravity plates, forming into increasingly larger drops until they float. The coalescer has a very large open pore surface area and a very low pressure loss and is stable against suspended matter found in bilge water, hence these particles have no detrimental effect on the coalescer. This means that the coalescer will still continue to operate effectively even with considerable fouling.
Following separation in the first stage, the water, now with a very low oil content, is passed into the second stage chamber, which contains, usually, a second coalescer filter to separate out any remaining oil particles, leaving water that may now be discharged overboard.
A conductive oil/air sensing probe at the top of the first stage (HEC) chamber constantly monitors the oil level in the separator, the length of the probe’s electrode determining the operating range. When oil (or air) is detected, the valve to the oil drain tank opens and the valve to the second stage chamber closes and the oil is discharged to the oil drain tank. The supply pump remains
running during the oil discharge. When most of the oil has been displaced, the oil sensing probe is again immersed in water and activates the control system to resume the separating operation.
The separator works automatically and will operate as long as there is water in the bilge water holding tank. Heating may be applied to improve separation, but the heater will only operate when the separator is full of liquid. The separator is fitted with sampling valves which allow oil samples to be drawn and enable the oil/water interface level to be determined. The Oil Content Discharge monitor samples the bilge water as it passes out of the separator. Should the oil content exceed maximum 15ppm, the three-way valve changes the output flow from the overboard discharge to discharge to the bilge water holding tank. An audible alarm sounds to warn the operator of the alarm condition. The 15ppm device setting can be adjusted from 1ppm up to the maximum 15ppm, but cannot be set higher. The monitor sensing element may be, normally flushed through with fresh water when in operation by moving the supply lever from the SAMPLING to the FLUSHING position.
This action automatically operates the three-way valve on the discharge line and returns the water to the bilge holding tank. Nowadays, the monitor contains a memory card recording the monitor readings for a period of 18 months, after which the data is automatically overwritten. The card is not to be removed from the instrument as it records the following information:
Time;
Date;
Oil content greater than 15 ppm;
Separator status
The oil content monitor must be checked each month and must be flushed through in order to remove any debris which could influence the reading. The maximum flow capacity should not be exceeded, as excess flow will prevent effective separation. The bilge pump suction strainer should be kept clean in order to avoid large solid particles entering the separator, as these will have a detrimental effect on the separation process. It is important to notice that the oily water separator is designed to separate oil from water, not water from oil. Therefore, if the bilge water supply to the separator contains excessive amounts of oil it will render the equipment inoperable and result in unnecessary maintenance. Same, if the separator uses flocculation chemicals, great care must be taken when handling the treatment chemicals, as these substances are caustic and can cause chemical burning on contact with skin and will cause severe damage to eyes. The appropriate protective clothing, including eye protection, must be worn when handling the chemicals.
When operating the oily bilge water separator and the overboard oil monitoring
system, the date, quantity and location of the discharge overboard is to be recorded in the Oil Record Book. All pumping operations and discharges are also to be in accordance with the latest MARPOL Regulations, Annex I, Regulations 9, 10, 11 and 16. The date, operational code and item number needs to be written in appropriate columns and the required particulars should be recorded chronologically in the blank space.For discharges overboard, the ship’s position at the start and end of the discharge should be entered. Each completed operation shall be signed for and dated by the officer or officers in charge of the operation and each completed page must be countersigned by the master of the vessel.
what mooring equipment is used on ship?
Winches with various arrangements of barrels are the usual mooring equipment used on board ships. The winch barrel or drum is used for hauling in or letting out the wires or ropes which will fasten the ship to the shore. The warp end is used when moving the ship using ropes or wires fastened to bollards ashore and wrapped around the warp end of the winch. Modern mooring winches are arranged as automatic self-tensioning units. The flow of the tides or changes in draught due to cargo operations may result in tensioning or slackening of mooring wires. To avoid constant attention to the mooring wires the automatic self tensioning arrangement provides for paying out (releasing) or recovering wire when a pre-set tension is not present.
what anchor handling equipment is used on ships
The windlass is the usual anchor handling device where one machine may be used to handle both anchors. A more recent development, particularly on larger vessels, is the split windlass where one machine is used for each anchor. The rotating units consist of a cable lifter with shaped snugs to grip the anchor cable, a mooring drum for paying out or letting go of mooring wires and a warp end for warping duties. Each of these units may be separately engaged or disengaged by means of a dog clutch, although the warp end is often driven in association with the mooring drum. A spur gear assembly transmits the motor drive to the shaft where the various dog clutches enable the power take-off. Separate band brakes are fitted to hold the cable lifter and the mooring drum when the power is switched off. The cable lifter unit, is mounted so as to raise and lower the cable from the spurling pipe, which is at the top and centre of the chain or cable locker. Anchor capstans are used in some installations where the cable lifter rotates about a vertical axis. Only the cable lifter unit is located on deck, the driving machinery being on the deck below. A warping end or barrel may be driven by the same unit and is positioned near the cable lifter.
what is a stabilising system used for?
There are two basic stabilising systems used on ships—the fin and the tank. A stabilising system is fitted to a ship in order to reduce the rolling motion. This is achieved by providing an opposite force to that attempting to roll the ship.
what is a bow thruster?
The bow thruster is a propulsion device fitted to certain types of ships to improve manoeuvrability. The thrust unit consists of a propeller mounted in an athwartships tunnel and provided with some auxiliary drive such as an electric or hydraulic motor. During operation water is forced through the tunnel to push the ship sideways either to port or starboard as required. The unit is normally bridge controlled and is most effective when the vessel is stationary. A servo motor located in the gear housing enables the propeller blade pitch to be altered, to provide water flow in either direction. With this arrangement any non-reversing prime mover, like a single-speed electric motor, may be used. The prime mover need not be stopped during manoeuvring operations since the blades can be placed at zero pitch when no thrust is desired. The drive is obtained through a flexible drive shaft, couplings and bevel gears. Special seals prevent any sea water leakage into the unit. The complete assembly includes part of the athwartships tunnel through which water is directed to provide the thrust.
define the term LOA
Length overall (LOA)
The distance from the extreme fore part of the ship to a similar point aft and is the greatest length of the ship. This length is important when docking.
define the term LBP
Length between perpendiculars (LBP)
The forward perpendicular is the point at which the summer load waterline crosses the stem. The aft perpendicular is the after side of the rudder post or the centre of the rudder stock if there is no rudder post. The distance between these two points is known as the length between perpendiculars, and is used for some ship calculations.
define the term breadth
Breadth
The greatest breadth of the ship, measured to the outside of the shell plating.
define the term BMld
Breadth moulded (BMld)
The greatest breadth of the ship, measured to the inside of the inside strakes of shell plating.
define the term bulwark
Bulwark
Vertical plating that extends upwards and is fitted around the perimeter of the main deck or weather deck.
define the term coaming?
Coaming
Vertical side of the hatch extending from the main deck and forming a structure for the hatch lid to sit upon.
define the term DExt
Depth extreme (DExt)
The depth of the ship measured from the underside of the keel to the top of the deck beam at the side of the uppermost continuous deck amidships.
define the term DMld
Depth moulded (DMld)
The depth measured from the top of the keel.
define the term double bottom
Double bottom
Name given to the area that includes the outer hull, girders and stiffeners inside the vessel and a layer of plating to form in effect a double skin. This should not be mixed up with the new form of ‘twin hulled’ tankers. This is due to the double bottom being used as a space for liquid storage and the twin hull arrangement being an empty space.
define the term dExt
Draught extreme (dExt)
The distance from the bottom of the keel to the waterline. The load draught is the maximum draught to which a vessel may be loaded. This will vary depending upon the service and type of water. (See Chapter 10 for more details.)
define the dMld
Draught moulded (dMld)
The draught measured from the top of the keel to the waterline.
define the term freeboard
Freeboard
The distance from the waterline to the top of the deck plating at the side of the deck amidships.
define the term camber or round of beam?
Camber or round of beam
The transverse curvature of the deck from the centreline down to the sides. This camber is used on exposed decks to drive water to the sides of the ship. Other decks are often cambered. Most modern ships have decks which are flat transversely over the width of the hatch or centre tanks and slope down towards the side of the ship.
define the term rise of floor
Rise of floor
The bottom shell of a ship is sometimes sloped up from the keel to the bilge to facilitate drainage. This rise of floor is small, 150 mm being usual.
define the term sheer
Sheer
The curvature of the deck in a fore and aft direction, rising from midships to a maximum at the ends. The sheer forward is usually twice that aft. Sheer on exposed decks makes a ship more seaworthy by raising the deck at the fore and after ends further from the water and by reducing the volume of water coming on the deck.
define the term bilge radius
Bilge radius
The radius of the arc connecting the side of the ship to the bottom at the midship portion of the ship.
define the term bilge keel
Bilge keel
A section of plating fixed to the outside of the hull running for the length of the ship protruding at right angles to the bilge radius.
what is tumble home?
Tumble home
In some ships the midship side shell in the region of the upper deck is curved slightly towards the centre line, thus reducing the width of the upper deck and decks above. Such tumble home improves the appearance of the ship.
what is displacement?
Displacement
This is a measurement of the mass of the ship and everything it contains when the measurement is taken. The term comes from the amount of water that a ship will ‘displace’ when it is fully floating. Please note that there will be a difference between a ship floating in fresh water and the same ship, loaded exactly the same, floating in salt water. This is due to the difference in density between the salt water and fresh water.
Displacement can be calculated as the underwater volume times by the density of the water that the vessel is floating in, times by the value of gravity.
define the term lightweight
Lightweight
This is a measure of the mass of the empty ship, without stores, fuel, water, crew or their effects. The hull and machinery and all the fixtures and fittings are also included in this measurement.
define the term deadweight
Deadweight
The deadweight is a measure of the mass that a ship is carrying at a given time. It is the sum of the weight of cargo, fuel, water, stores and people that a ship has on board when the measurement is taken.
The deadweight is therefore the difference between the displacement and the lightweight:
Displacement = Lightweight + Deadweight
It is usual to categorise a vessel by reference to its deadweight. Thus a 10 000 tonne ship is one which is capable of carrying a deadweight of 10 000 tonne.
what is uppermost continuous deck or bulkhead deck?
Uppermost continuous deck or bulkhead deck
This is one of the most important features of a ship as it makes a watertight seal with the vertical watertight bulkheads. With cargo ships, this deck could also be the same as the freeboard deck. This deck should be provided with the means to close all openings that could be accessed by the sea, thus making a sealed watertight box with the watertight bulkheads. The uppermost continuous deck or bulkhead deck is also taken as the ‘strength’ deck when calculating the girder strength of the vessel
what is registered tonnage?
Registered tonnage
It is necessary to have an official measurement for ships and in the past the value of the gross registered tonnes (grt) has been used. However there was never a universally agreed standard definition of grt. IMO’s International Convention on Tonnage Measurement of Ships entered into force in July 1982 and as a consequence the two measurements of gross tonnage (gt) and net tonnage (nt) have been agreed upon and are now in universal use for all ships. However, they are not straight forward mathematical calculations. IMO describe ‘gross tonnage’ as a function of the volume of all the internal spaces within the ship. These include the volume of appendages but not volumes that are open to the sea.
what is ballast water used for
Ballast water
Ballast water has been used since the introduction of steel hulls in order to stabilise the vessel especially when it has little or no cargo. When ballast water treatment system was developed so was the ballast water management convention, which requires every ship to manage their ballast water and sediments to a predetermined standard in accordance with the ballast water management plan. In addition to this all ships are required to carry a ballast water record book.
explain what is meant by condition of assignment load line survey?
These are the conditions which must be met before free board is assigned to a ship and load line certificate is issued following a load line survey. Free boards are computed assuming ship to be a completely enclosed and water tight / weather tight envelop. The convention then goes onto recognize the practical need for opening in the ship and prescribes means of protection and closure of such openings. These are called condition of assignment since the assignment of computed free board is conditional upon the prescribed means of protection and closure of openings such as hatchways, doorways, ventilation, air pipes, scuppers, etc.
Following are the conditions which must be met before assigning the load line.
1.Enough structural strength should be possessed.
2.Enough reserve buoyancy should be possessed.
3.Safety and protection of crew.
4.Prevent entry of water through hull.
Ships to be surveyed annually to ensure that they fulfill the condition of assignment. Most of the condition of assignment are concerned with the water tight integrity of the ship. Hull construction should meet the highest standard laid down by the classification society. This ensures protection against flooding of the ship. The superstructure and bulkheads must be strengthened sufficiently. Some of the condition of assignment which contribute towards water tight integrity are:
1.Hatchways
2.Machinery space openings
3.Details of opening in free board
4.Details of opening in superstructure deck
5.Ventilators
6.Cargo ports
7.Air pipes
8.Scuppers
9.Side scuttles
10.Inlet and discharges
All the above parameters ensures water tight integrity and protection against flooding of compartment. If above are not water tight then during rough weather water can enter into the areas below main deck causing to reduce the free board. So, condition of assignment very much contributes towards water integrity of the ship. Also if green sea effect is not reduced and water is being accumulated on the deck, it can cause free board to reduce and add free surface effect. In rough weather if any longitudinal or transverse girder give way it can cause structural failure and water can enter area below main deck.
what force act on a ships structure?
A number of forces act on a ship’s structure; some are static forces while others are dynamic. The static forces are set up due to the differences in weight and support along the length of the ship, while the dynamic forces are created by the force of the wind pushing on the ship, as well as the water interacting with the ship, by the passage of waves along the ship and by the moving propulsion parts.
explain the overall design concept of a ship
A ship may be regarded as a non-uniform beam, carrying both uniformly distributed and non-uniformly distributed loads and having varying degrees of support along its length. Some of the load will be distributed evenly over a section of the ship while some will be more concentrated. The overall strength of the beam is referred to as the ‘girder strength’ and the overall bending moment envelope curves are used to calculate the required girder strength for all circumstances that the vessel is likely to encounter. Then the internal structures and the hull plating are sized and arranged to meet the minimum girder strength required for the design and duty of the vessel. Values for the still water bending moments, under all conditions of operation, are calculated. The results of all the bending moments give the overall bending moment for the hull girder, and this divided by the maximum design stress allowed by the classification society, will give the strength modulus required for the hull girder steel sections.
The components designed to resist the buckling of the girder and contributing to the ultimate hull strength are the:
*size, number and strength of longitudinal beams
*thickness and strength of the shell plating
*bilge keels
*quality of welding
*number and strength of the transverse sections.
The ultimate hull girder strength for tankers, bulk carriers and now container ships is determined using a system known as the iterative-incremental method, which is where the hull girder is divided into a set of transverse elements. The forces are calculated in one element and the resultant strain (for any one given condition) is used to modify the stress calculations on the next connecting transverse element. This is repeated until the final strength profile for the hull girder has been calculated. For the hull girder, to keep the ‘box section’ in shape there needs to be a combination of beams running along the length of the box and a series of shapes designed to maintain the square shape of the box. The way that these two are combined will determine the overall design of the vessel.
what are the different system of framing used for a ship
There are two different systems of framing, called longitudinal framing and transverse framing. As the two names suggest, longitudinal framing is where the main load carrying sections of steel run parallel to the sides of the vessel. Transverse framing on the other hand, has the framing arranged at right angles to the sides of the ship. These frames are then supported by girders, or stringers, running in parallel with the sides of the vessel. Not only can the overall vessel be of one or the other or indeed a combination of both, but so can smaller parts of the overall vessel, with respect to hatch openings for example.
with the aid of a cross section of a ship drawing explain how and why the transverse structure is designed and
with reference to transverse bending?
see figure 2.7
The transverse structure may be subjected to different types of loading, such as the weight of the ship’s structure, machinery, fuel, water and cargo due to the water pressure causing longitudinal bending.
The decks must be designed to support the weight of accommodation, winches and cargo, while exposed decks may also have to withstand a tremendous weight of water that might be taken on board during heavy weather. The deck plating is connected to beams which transmit the loads to the longitudinal girders and to the side frames. In the area of heavy local loads such as cranes and windlasses and so on, additional stiffening will be required. The shell plating and frames form pillars which support the additional weights that are situated on the deck. Tank tops are required to be strong enough to keep the cargo in place or resist the upthrust exerted by the liquid in the tanks.
In the machinery space other factors must be taken into account. Fluctuating forces transmitted from the reciprocating machinery through to the structure need to be accommodated. Modern resilient mountings reduce the magnitude of the forces and the strength of the fixings means that the machinery is well supported to prevent any excess movement. Under the position of the engine additional girders are fitted in the double bottom and the thickness of the tank top increased to ensure that the main propulsion remains fixed despite the additional stresses caused by rough weather acting upon the vessel.
Special consideration must be given to the thrust block, the propeller shaft and the propeller. Thrust to push the ship along is generated by the propeller and must be carefully transmitted to the hull of the vessel. This is a difficult process as the propeller shaft is relatively small in diameter when compared with the area of the hull. The thrust block is first in line to take the force from the propeller shaft. The important issue is for the thrust block to be connected to as large an area of the hull as possible. This will transmit the force generated by the thrust to the hull evenly. The weight of the propeller shaft is supported by intermediate bearings which in turn must be supported by the vessel’s hull structure. The arrangement of the stern of the vessel needs to counter the forces transmitted through the stern tube bearing. These forces will mostly be the weight of the propeller which will be acting at the end of the shaft. Any out of balance forces will have a significant effect as will any vibration caused by cavitation.
A considerable force is exerted on the bottom and side shell by the water surrounding the ship and the double bottom floors and side frames are designed to withstand these forces, while the shell plating must be thick enough to prevent buckling as it spans the distance between the floors and frames. Since water pressure increases with the depth of immersion, the load on the bottom shell plating will be greater than the load on the side shell, therefore the bottom shell must be thicker than the side shell to withstand the increased force. 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.
with the aid of a sketch explain what is meant by racking and how its prevented
see figure 2.8
When a ship rolls there is a tendency for the ship to distort transversely due to the fluctuating forces. Due to the fluctuating forces the vessel structure distorts transversely and this causes deformation, the deck moves laterally relative to the bottom structure and shell on one side moves vertically relative to the other side, this action 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. Transverse bulkheads divide the ship from side to side and are habitually used to create watertight compartments on the vessel. Additionally, they stiffen the structure of the hull, preventing deformation and racking stresses The stress mainly affects the corners of the ship, i.e., on the tank side brackets and the beam knees, which must be made strong enough to resist it. Transverse bulkheads, frames and web frames provide very great strength to resist racking.
what are deck air vents?
explain their purpose
Deck air vents are devices that allow the passage of air in and out of the tanks onboard vessels, such as cargo holds, ballast tanks, fuel tanks, and fresh water tanks. They are essential for maintaining the pressure balance, the quality of the cargo or fluid, and the safety of the vessel and crew.
The Purpose of Deck Air Vents
Deck air vents are strategically placed openings on the deck of a vessel, each serving a specific purpose for various onboard tanks. Their primary functions include:
Preventing Overpressure:One of the most critical roles of deck air vents is to prevent the overpressure of tanks. When a tank is loaded or unloaded, it undergoes changes in volume due to temperature fluctuations and the addition or removal of liquid cargo. Without proper venting, pressure imbalances can develop within the tank, leading to structural damage or even catastrophic failure. Deck air vents provide a controlled release of excess pressure to ensure the tank’s integrity.
Minimizing Vacuum Conditions:During the discharge of liquid cargo, especially in tanks like ballast or cargo tanks, a vacuum can form as the liquid is pumped out. This vacuum can potentially collapse the tank structure if not relieved. Deck air vents allow air to enter the tank, equalizing the pressure and preventing collapse.
Reducing Gas Build-up:Certain tanks, like fuel oil or sewage tanks, may produce gases or vapours that need to be vented to prevent the build-up of hazardous conditions or explosions. Deck air vents allow these gases to dissipate safely into the atmosphere.
Maintaining Tank Integrity:Proper ventilation helps to reduce the corrosion of tank internals caused by moisture accumulation. It also minimizes the risk of contamination, ensuring the quality and safety of stored liquids.
what are some common issues that occur on deck air vents and how to fix them?
deck air vents common issues and rectifcations :
Blockages:If you suspect a blockage, inspect the vent for debris or obstructions. Remove any foreign materials and clean the vent thoroughly.
Leakage:If you notice leaks, inspect the sealing gaskets and connections. Replace any damaged components and ensure a tight seal.
Inoperative Valves:If pressure relief or vacuum-breaking valves fail to operate, consult the manufacturer’s guidelines for maintenance or replacement instructions.
what kind of maintenance is carried out on deck air vents
To ensure the effectiveness of deck air vents, regular maintenance and inspections are essential.
Maintenance tasks:
Clean and Clear Vents:Deck air vents should be cleaned periodically to remove any dirt, dust, salt, rust, or debris that may accumulate inside them. This can be done by using compressed air, water jets, brushes, or solvents.Cleaning should be done more frequently for cargo hold vents that handle dusty or dirty cargoes
Functional Valves:If equipped with pressure/vacuum relief valves, make sure they are in good working condition. Replace any malfunctioning valves promptly.
Leak Checks:Inspect for leaks around the vent openings. Leaking vents can compromise the tank’s integrity and should be repaired immediately. Deck air vents that are damaged, corroded, leaking, blocked, or malfunctioning should be repaired as soon as possible to restore their normal operation and prevent further deterioration. This can be done by replacing worn-out parts, welding cracks, sealing leaks, clearing obstructions, or adjusting settings. Repairing should be done by qualified personnel following the manufacturer’s instructions and safety precautions.
Corrosion Prevention:Apply appropriate anti-corrosion coatings to vent openings and surrounding areas to protect against corrosion.Deck air vents that have moving parts, such as valves, springs, hinges, or flaps, should be lubricated regularly to ensure smooth operation and prevent seizing or jamming. This can be done by using grease, oil, or spray lubricants. Lubricating should be done more frequently for deck air vents that are exposed to salt water spray or humid conditions.
Operational Testing:Deck air vents should be tested periodically to check their performance and functionality. Testing should be done more frequently for deck air vents that handle hazardous cargoes or fluids. Periodically test the pressure relief and vacuum-breaking functions of the vents to ensure they are functioning as designed.
why is it it important that deck air vents operate properly
Deck air vents are important for ensuring the safety and efficiency of the vessel and its cargo.
They help to:
Prevent damage to the cargo:By ventilating the cargo holds properly, deck air vents prevent moisture condensation, heating, gas emission, odour generation, or tainting that can affect the quality and integrity of the cargo.
Prevent damage to the vessel:By maintaining the pressure balance in the tanks, deck air vents prevent overpressure or vacuum that can cause structural damage to the tank walls or hull deformation.
Prevent fire or explosion:By removing hazardous gases from the cargo holds or fuel tanks, deck air vents prevent fire or explosion that can endanger the vesseland crew.
Prevent asphyxiation or poisoning:By removing hazardous gases from the cargo holds or fresh water tanks, deck air vents prevent asphyxiation or poisoning of the crew or other personnel who may enter the tanks.
Prevent contamination:By preventing seawater, fuel, dust, insects, or bacteria from entering the tanks through the vent pipes, deck air vents prevent contamination of the ballast water, fuel, or fresh water.
On the other hand, deck air vents also pose some risks if they are not operated or maintained properly. Some common risks include:
Cargo damage:If deck air vents are not opened or closed at the right time or frequency, they may cause cargo damage due to excessive or insufficient ventilation. For example, if deck air vents are opened too frequently or for too long, they may cause cargo sweat or ship sweat by introducing moist air into the cargo holds.If deck air vents are closed too early or for too long, they may cause cargo heating or gas accumulation by trapping warm air or gas inside the cargo holds.
Vessel damage:If deck air vents are not opened or closed properly during ballasting or de-ballasting operations, they may cause vessel damage due to overpressure or vacuum in the ballast tanks. For example, if deck air vents are not opened sufficiently during deballasting, they may cause vacuum in the ballast tanks that can suck in seawater through the vent pipes. If deck air vents are not closed sufficiently during ballasting, they may cause overpressure in the ballast tanks that can blow out seawater through the vent pipes.
Fire or explosion:If deck air vents are not closed properly when handling hazardous cargoes or fluids, they may cause fire or explosion due to ignition of flammable gases. For example, if deck air vents are not closed tightly when loading or unloading coal, they may allow oxygen to enter the cargo holds and ignite the coal dust.If deck air vents are not closed securely when re-fuelling or transferring fuel, they may allow fuel vapour to escape and ignite by sparks or static electricity.
Asphyxiation or poisoning:If deck air vents are not opened properly when entering confined spaces, they may cause asphyxiation or poisoning due to lack of oxygen or presence of toxic gases. For example, if deck air vents are not opened sufficiently before entering a cargo hold that contains carbon dioxide, carbon monoxide, methane, or hydrogen, they may cause asphyxiation or poisoning of the personnel who enter the hold.If deck air vents are not opened adequately before entering a fresh water tank that contains bacteria, they may cause poisoning of the personnel who enter the tank.
Contamination:If deck air vents are not fitted with proper filters, screens, covers, or caps, they may cause contamination of the tanks due to ingress of foreign substances. For example, if deck air vents are not fitted with filters that can remove dust particles from the air, they may cause contamination of the fresh water tanks by introducing dust into the water.If deck air vents are not fitted with screens that can prevent insects from entering the vent pipes, they may cause contamination of the fresh water tanks by introducing insects into the water.
with the aid of sketches explain how a ship enters dry dock
see figure 2.9 and 2.10
Dry-docking
A ship usually enters dry dock with a slight trim aft. This means that as the water is pumped out, the after end touches the blocks first. As more water is pumped out an upthrust is exerted by the blocks on the after end, causing the ship to change trim until the whole keel from forward to aft rests on the centre blocks. At the instant before this occurs the upthrust aft is at maximum. If the design of the ship results in this thrust being excessive, it may be necessary to strengthen the after blocks and the after end of the ship. Such a problem arises if it is necessary to dock a ship when fully loaded or when trimming severely by the stern. As the pumping continues the load on the keel blocks is increased until the whole weight of the ship is taken by the blocks in the dry dock. The ship structure must be strong enough to withstand this unevenly distributed load. The ‘docking’ plan, carefully worked out before the ship arrives, ensures that the blocks are all placed in the correct position. The strength of the hull is carefully considered during the design of the ship and there could be up to three different docking arrangements specified for each vessel. Different systems can be used on subsequent dry docks so that the spaces not examined at the last docking can be covered during the current one. It will also be obvious to students that the hull cannot be prepared and painted in the area in contact with the blocks. In most ships the normal arrangement of keel and centre girder, together with the transverse floors, is quite sufficient for the purpose. If a duct keel is fitted, however, care must be taken to ensure that the width of the duct does not exceed the width of the keel blocks (Figure 2.9). The keel structure of a longitudinally stressed vessel such as an oil tanker, bulker or container ship is strengthened by fitting docking brackets and tying the centre girder to the adjacent longitudinal frames at intervals of about 1.5 m.
Bilge blocks or shores could be fitted to support the sides of the ship. The arrangements of the bilge blocks vary from dock to dock. In some cases they are fitted after the water is pumped out of the dock, while other dry docks may have blocks which can be slid into place while the water is still in the dock. The latter arrangement is preferable since the sides are completely supported. At the ends of the ship, where the curvature of the shell does not permit blocks to be fitted, bilge shores are used. The structure at the bilge must prevent these shores and blocks buckling the shell.As soon as the after end touches the blocks, shores are inserted between the stern and the dock side to centralise the ship in the dock and to prevent the ship slipping off the blocks. When the ship grounds along its whole length additional shores could be fitted on both sides, holding the ship in position and preventing tipping. These shores are known as breast shores and have some slight effect in preventing the side shell bulging. They should preferably be placed with respect to transverse bulkheads or side frames as these offer more resistance to buckling than the side placed do on their own (Figure 2.10). On the larger vessels the side supports are not necessary as the ship sits safely on the blocks situated on the floor of the dock. When undocking the vessel, care must be taken to ensure that all the ballast, fresh water and fuel oil tanks are in the same condition as they were when the vessel came into the dry dock. It is usual to start filling the dock and then, when the ship’s side valves are just covered, stop the filling to check for any leaks.
what is pounding?
Pounding
When a ship meets heavy weather and commences heaving and pitching, the rise of the fore end of the ship occasionally synchronises 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 the event does not occur with great regularity, it may nevertheless cause damage to the bottom of the ship at the forward end. The designers must ensure that the shell plating is stiffened to prevent buckling due to the forces involved in the pounding. Pounding also affects the aft end section of the vessel but the effects are not nearly as great. Nevertheless, provision must be made in the design of the hull to counteract the effects of pounding at the aft end.
what is panting?
Panting
As waves pass along the length of a ship the various parts of the vessel are subjected to varying depths of water which causes fluctuations in water pressure. This tends to create an in-and-out movement of the shell plating. The knock-on 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, panting could eventually lead to fatigue of the material and must therefore be prevented as much as possible. This is achieved by the structure at the ends of the ship being stiffened to prevent any undue movement of the shell plating.
when are stresses and bending moments calculated?
Highly sophisticated computer tracking systems monitor the movement of containers as they are transported around the world. Therefore, ships will have their loading and discharging sequencing calculated before the vessel reaches port and stresses and bending moments will be calculated while the vessel is moving between ports.
with the aid of sketches explain what effect does wave bending/dynamic loading have on the bending moment?
see figure 2.5 and 2.6
When a ship passes through waves, alterations in the distribution of buoyancy cause alterations in the bending moment. The greatest differences occur when a ship passes through waves whose lengths from crest to crest are equal to the length of the ship thereby placing the greatest bending moment on the hull. When the wave crest is amidships (Figure 2.5), the buoyancy amidships is increased while at the ends it is reduced. This tends to cause the ship to hog. A few seconds later the wave trough lies amidships. The buoyancy amidships is reduced while at the ends it is increased, causing the vessel to sag (Figure 2.6). The effect of these waves is to cause fluctuations in stress, or, in extreme cases, complete reversals of stress every few seconds. The ship is designed to withstand this cycle of stressing without causing undue damage. However, if any part of the structure has already been damaged or is corroded, then the hogging and sagging will make the weakened structure worse.
with the aid of a sketch describe how a vessel withstands longitudinal bending?
see figure 2.4
The structure resisting longitudinal bending is made up of all the continuous longitudinal material. The features farthest from the axis of bending (the neutral axis) are the most important (Figure 2.4).
These features are the:
*keel
*bottom shell plating
*centre girder
*side girders
*tank top
*tank margin
*side shell
*sheer strake
*stringer plate
*deck plating alongside hatches
*and in the case of oil tankers, any longitudinal bulkheads.
Buckling and/or deformation may occur at a point in the structure that is the greatest distance from the neutral axis which will become a high stress point, such as the top of a sheer strake, such points are avoided as far as possible, since they may result in the plate cracking. The greatest stresses set up in the ship as a whole are due to the distribution of loads along the ship, causing longitudinal bending. The efficiency of the ship structure in withstanding longitudinal bending depends to a large extent on its girder strength and the ability of the transverse structure to prevent the buckling of the shell plating and decks. Some ships are required extra structure in order to withstand stresses like In many oil tankers the structure is improved by joining the sheerstrake and stringer plate to form a rounded gunwale.
with the aid of sketches explain how load is distributed for a ship with reference to still water bending/static loading explain how hogging and sagging occurs?
Still water bending – static loading
If we consider a loaded ship lying in still water, then the upthrust at any 1 m length of the ship depends upon the immersed cross-sectional area of the ship at that point. If the values of upthrust at different positions along the length of the ship are plotted on a base representing the ship’s length, a buoyancy curve is formed (Figure 2.1).This curve increases from zero to a maximum value in the midship portion, then decreases back down to zero. The area of this curve represents the total up-thrust exerted by the water on the ship. The total weight of a ship consists of a number of independent weights concentrated over short lengths of the ship. These include; cargo, machinery, accommodation, cargo handling gear, poop and forecastle sections of the hull construction, and a number of items which form continuous material over the length of the ship, such as decks, shell and tank top.
A curve of weights is shown in Figure 2.1. The difference between the weight and buoyancy at any point is the load at that point. In some cases the load is in excess of weight over buoyancy and in other cases there is an excess of buoyancy over weight. A load diagram formed by these differences is shown in the figure. Since the total weight must be equal to the total buoyancy (assuming that the vessel is still floating), the area of the load diagram above the base line must be equal to the area below the base line.
Due to the unequal loading, however, shearing forces and bending moments are set up in the ship with the maximum bending moment occurring around the midship section. The load distribution will determine the direction in which the bending moment will act, and this in turn will create the state of hogging or sagging.
Class terminology for the condition of hogging and sagging in the bending moment calculations is to go negative for sagging and positive for hogging. If, for example, the buoyancy amidships exceeds the weight, the ship will hog, and this may be likened to a beam supported at the centre and loaded at the ends. As with a simply supported beam, when a ship hogs, the deck structure is in tension while the bottom plating is in compression (Figure 2.2). If the weight amidships exceeds the buoyancy, the ship will sag, which is equivalent to a beam supported at its ends and loaded at the centre. When a ship sags, the bottom shell is in tension while the deck is in compression (Figure 2.3). Students will be able to appreciate that when a hull is continuously changing between hogging and sagging, as in a rough sea, considerable cyclical stresses happen in the deck and the bottom shell plating.Changes in bending moments also occur in a ship due to different loading conditions. This is particularly true in the case of cargoes such as iron ore which are heavy compared with the volume they occupy. When these types of cargo are loaded into a ship, especially if it is on the spot market or performing the role of a tramp ship, care must be taken to ensure a suitable distribution throughout the ship. The even distribution of stresses is calculated by using the on-board loading computer.
In the past these calculations have proved difficult especially if the ship has a machinery space and deep tanks/cargo holds amidships. These older vessels would also have had only a very basic method of calculating the bending moment. There would however be a tendency in such ships, when loading heavy cargoes, to leave the deep tank empty. This results in an excess of buoyancy by way of the deep tank. This action must be considered carefully as there could also be an excess of buoyancy by way of the engine room, since the machinery (especially if large two stroke engines are fitted) might be light when compared with the volume it occupies.
The International Association of Classification Societies (IACS) sets out guidance for the loading sequences in its rules for the ‘Strength of Ships’. Careful consideration should also be given to the sequencing of loading and using bunker fuel as well as the filling or emptying of the ballast tanks. A ship loaded carelessly, might hog considerably, creating unusually high stresses in the deck and bottom shell. This may be very dangerous and could lead to the vessel breaking in two if loaded using an incorrect sequence. If the owners intend for the ships to be regularly loaded in this manner, additional hull strength must be provided to ensure the safe operation of the vessel. In cases where there is a long transmission shaft between the main engine and the propeller, excess hogging or sagging could also lead to excessive bending of this shaft and the engineering staff would continually be checking for any overheating of the main shaft bearings.
what is minimum freeboard?
The minimum freeboard is based on providing the vessel with a volume of reserve buoyancy which cannot be loaded with cargo and therefore may be regarded as making the ship safe and ensuring that the ship proceeds to sea in a stable condition.
define freeboard?
Freeboard is the distance from the waterline to the top of the deck plating at the side of the freeboard deck amidships.
define free board deck
The freeboard deck is the uppermost continuous deck (also known as the ’bulkhead deck’) that has the necessary equipment to close all openings to the outside weather.
the exact level of ‘reserve buoyancy’ required depends upon several factors:
What are these factors?
*conditions of service of the ship
*type of vessel
*stability of the vessel in still water
*degree of subdivision after suffering ‘prescribed damage’
*safety of the ship’s staff when out on deck
*ability of the vessel to protect the weather deck from taking on water
*fixtures and fittings used to allow any ‘shipped’ water to be removed.
what is reserve buoyancy?
Reserve buoyancy is the volume equivalent of freeboard. Its the difference between the volume of a hull below the designed waterline and the volume of the hull below the lowest opening incapable of being made watertight. Reserve buoyancy is a very crucial yardstick for the seaworthiness of a vessel. The extent of the reserve buoyancy determines the safety limit or margin before the vessel can sink. The greater the reserve buoyancy, the safer the vessel is regarding sinkage, and vice-versa. In deep sea ships, for example, sufficient reserve buoyancy must be provided to enable the vessel to rise up again when shipping the heavy seas that could be encountered in the oceans of the world, small vessels on the inland waterways will not encounter such conditions and therefore are allowed to sail with a different level of ‘reserve buoyancy’.
with reference to load line regulation why is bow height required?
One point to consider is the likelihood of water coming onto the fore deck. This is largely a function of the distance of the fore end of the deck from the waterline and for this reason a minimum bow height is stipulated. The bow height required depends upon the length of the ship and the block coefficient and may be measured to the forecastle deck if the forecastle is 7% or more of the ship’s length. Should the bow height be less than the minimum then either the freeboard is increased or the deck raised by increasing the sheer or fitting a forecastle.
function of forecastle?
the forward part of the upper deck of a ship, a superstructure at or immediately aft of the bow of a vessel,used as a shelter for stores, machinery, The function of the forecastle has been brought into sharp focus recently as some bulk carriers suffered weather damage to the forward hatch coaming due to the lack of protection offered by the inadequate forecastle.
how is freeboard calculated?
Vessels conforming to the Load Line Rules are assigned a freeboard according to a table of values, and this is termed the tabular freeboard. Its like the starting point used to calculate the actual freeboard, there are two set of tables split into TYPE A ship and TYPE B ship.
Type A ships are designed to carry only liquid cargoes and hence have a high integrity of exposed deck, together with excellent subdivision of the cargo space. The hatches are small openings and are oil/water tight, and heavy seas are unlikely to cause flooding of the cargo space or the accommodation. As a result, these vessels are allowed to load to a comparatively deep draught than the type B ships.While these ships have a high standard of watertight deck, they have a comparatively small volume of reserve buoyancy and may therefore be less safe if damaged. It is necessary, therefore, in all such vessels over 150 m in length, to investigate the effect of damaging the underwater part of the cargo space and, in longer ships, the engine room. Under such conditions the vessel must remain afloat without excessive heel and have positive stability.
Type B ships cover the remaining types of vessels and are assumed to be fitted with steel hatch covers. In older ships having wood covers the freeboard is increased.Should the hatch covers in Type B ships be sealed with efficient securing arrangements, then their improved water tight integrity is rewarded by a reduction in freeboard of up to 60% of the difference between the Type A and Type B tabular freeboards. If, in addition, the vessel satisfies the remaining conditions for a Type A ship (e.g. flooding of cargo spaces and engine room), 100% of the difference is allowed and the vessel may be regarded as a Type A ship. The tabular freeboards for Types A and B ships are given in the Rules for lengths of ship varying between 24 m and 365 m.
each table used has various values dependant upon the type of ship and its length and is based on a standard vessel having a block coefficient of 0.68, length ÷ depth of 15 and a standard sheer curve.
Once a value is chosen the freeboard, is then calculated from the tabular freeboard and Corrections are then made to this value for any variation from the standard, together with deductions for the reserve buoyancy provided by weather tight superstructures on the freeboard deck.
where are freeboard markings located?
what effect does timber have on reserve buoyancy?
The freeboard markings (Figure 10.1) are cut into the shell plating with the centre of the circle at midships.
Special provision is made in the Rules for vessels carrying timber as a deck cargo. The timber increases the reserve buoyancy and hence the vessels are allowed to float at deeper draughts. An additional set of freeboard markings is cut in aft of midships with the normal letters prefixed by L (lumber).
how stresses occur on a ship?
The modern ship is made up steel plating, section and builds up girders so connected as toprovide adequate strength in all parts to withstand the forces acting on the ship under all condition ofservice. The forces acting on a ship may be static or dynamic.The static forces are due to thedifference in the weight and buoyancy, which occur throughout the ship.The dynamic forces are cause by the motion of the ship at sea and the action of the windand wave.
These forces create:
1.Longitudinal stress
2.Transverse stress
3.Local stress
The greatest stress set in the ship as wholes are due to the distribution of load along theship, causing longitudinal bending.
what is hogging and what is sagging?
Hogging
If the buoyancy amidships exceed the weight due to loading or when the wave crestis amidships, the ship will Hog, as a beam supported at mid length and loaded at the end.
Sagging
If the weight amidships exceed the buoyancy or when the wave troughamidships the ship will sag, as a beam supported at a ends and loaded at mid length.
what is transverse stress
Occur when transverse section of amidships is subjected to static pressure due to the surroundingwater as well as internal loading due to the weight of the structure, cargo, etc.
The parts of the structure, which resist transverse, are
1.Transverse bulkhead.
2.Floor in the double bottom.
3.Bracket between deck beam and side frame, together with bracket between side frame and tanktop plating, or margin plate .
4.The pillars in hole and tween deck.
how do local stresses occur
created by:
1.Heavy concentrated load like boiler, engine etc.
2.Dead cargo such as timber
3.Hull vibration
4.Ship resting on block on a dry dock (Static Stress)
describe with the aid of a sketch how dynamic forces occur?
Dynamic Forces
The dynamic forces arise fromthe motion of the shipitself.
Due to the action of the waves on the ship the following motions occur
1.Surging: The forward and aft linear motion (along x) of a ship is called surging.
2.Heaving:The vertical up and down linear motion (along y) of a ship is called heaving.
3.Swaying:The side to side linear motion (along z) of a ship is called swaying.
4.Rolling:The rotational motion of a ship about longitudinal axis is called rolling.
5.Yawing:The rotational motion of a ship about vertical axis is called yawing.
6.Pitching:The rotational motion of a ship about transverse axis is called pitching.
When the ship motions are large particularly in pitching and heaving, considerable dynamicforces are created in the structure.
what is panting?
Panting
As wave passes along the ship they cause fluctuation in water pressure which tends to create inand out movement of the shell plating.
This is particularly the case at the fore end.
The rules of the classification societies required extra stiffening, at the end of the ship, in theformof beams, brackets, stringer plate, etc. in order to reduce the possibility of damage.
This in and outmovement is called panting.
what is slamming or pounding?
Slamming or Pounding
In heavy weather when the ship is heaving and pitching, the fore end emerges from the waterand reenters with a slamming effect which is called pounding.
Extra stiffening require at the fore end to reduce the possibility of damage.
what is apparent slip?
Apparent Slip
Since the propeller work in water, the ship speed Velocity will normally be less than thetheoretical speed.
The difference between the two speeds is known as Apparent slip and is usuallyexpress as a ratio or percentage of the theoretical speed.
what is real slip?
Real slip or true slip
This is the difference between the theoretical speed and the speed of advance, express as a ratio orpercentage of the theoretical speed.
The real slip is always positive and it dependant of current.
what is meant by load line?
The load line is a term given to a mark located amidships on both sides of a shipto show the limiting draught to which the vessel may be loaded.
This limiting draught is obtained bymeasuring from the uppermost continuous weather tight deck (normally the freeboard deck) down tothe load line mark amidships. This distance is called the freeboard of the ship.
what is a pilgrim nut?
Pilgrim nut
The pilgrim nut provides a predetermined grip between the propeller and its shaft.
The propeller boss is fitted with a S.G cast iron internally tapered sleeve, which is secured (fixedfirmly in position) into the boss. This sleeve is bedded to the shaft cone before mounting in theboss so that better fit is achieved which, combined with the pilgrim nut push up, ensure a goodfriction grip. No key is required.
The pilgrim nut is a threaded hydraulic jack, which screwed on to the tailshaft. A steel ringreceives thrust from a hydraulically pressurised nitrile rubber tyre. This thrust is applied to thepropeller to force it onto the taper sleeve.
Propeller removal is achieved by reversing the pilgrim nut and using a withdrawal plate, which isfastened to the propeller boss by studs. When the tyre is pressurised the propeller is drawn offthe taper.
what pressure tests are done for cargo tanks?
1.A structural test by testing with water to a height of 2.45 meters above the tank crown, or
2.A leak test consisting of a soapy solution test while the tank is subjected to an air pressure of0.14 bar. It is recommended that the air pressure is initially raised to 0.21 bar and then loweredto the above test pressure before inspection is carried out.
Difference between Stiff ship and Tender ship?
Stiff Ship
1.Greater GM due to high density cargo onbottom
2.Ship rolls very fast
3.Very uncomfortable
Tender Ship
1.Small GM (but not negative ) due to loadingon top
2.Ship rolls very slowly
3.Uncomfortable but better than stiff ship
What are water tight door requirements ?
1.The door may be either vertical or horizontal sliding
2.The means of closing the door must be positive i.e.. They must not rely on gravity or a droppingweight
3.They capable for operating with a list of 15 degree and to be capable of being quickly closedform an accessible position above the bulkhead deck
4.It must be operated from the vicinity of the door in addition to a point above the bulk head deck
5.If no power is available in hydraulically operated system, the door may be closed and opened by manual operated pump
6.Must have an index at the operating position showing whether the door is opened or closed.
describe the theory of rudder for a ship?
Theory of Rudder on Ships
The rudder is used to steer the ship. Theturning actionis largely dependent on thearea ofthe rudder. The required area of the ruddervarieswith different type of vessels since desiredmaneuvering ability differs considerably and the general ship design may imposed restriction. In practice the rudder area is usually relative to the area of the immersed metal plane. The ratio of thedepth to width of a rudder is known as theaspect ratio and its value is generally 2. High aspect ratio is used in large vessels, where depth is not a constraint. Higher aspect ratio reduces the astern torque considerably.
The force on the rudder depend on:
1.Area of the rudder
2.The form of rudder
3.The speed of the ship
4.The angle of helm
Rudder may be hinged on the pintles and gudgeon, or the may turn about an axlewhich passes down through the rudder.The weight of rudder may be taken by bearing pintles, or by a bearing at the rudder head(rudder carrier), or by a combination of both.
what are the different types of rudder?
Balanced rudder
When 20% to 37% of the area is forward of the turning axis there is notorque on the rudder stock at certain angles.
At some angle of rudder, it is balanced. i.e., torque is zero, to keep rudder at that angle.
Axis of rotation lies between 0.2 L and 0.37 L.
Semi-balanced rudder
A rudder with a small part of its area, less than 20%, forward of theturning axis.
At no angle rudder is balanced.
Axis of rotation lies less than 0.2 L.
Unbalanced rudder
A rudder with all of its area aft of the turning axis.
At no angle rudder is balanced.
Axis of rotation is the leading edge.
describe the the construction of a rudder?
Modern rudders are of stream lined form and are fabricated from steel plate, the plate sizebeing stiffen by internal webs. Where the rudder is fully fabricated, one side plate is prepared andthe vertical and horizontal stiffening webs are welded to this plate.
The other plate often called the closing plate is then welded to the internal webs from theexterior only. This may be achieved by welding, flap bars to the webs prior to fitting the closing plate,and then slot welding the plate.
The upper face is formed into a usually horizontal flat palm, which acts as thecoupling point for the rudder stock.
A lifting hole is provided in the rudder to enable a vertical inline lift of a rudder when it isbeing fitted or removed. This lifting hole takes the form of a short piece of tube welded through therudder with doubling at the side and closing plate.
A drain hole is provided at the bottom of the rudder to check for water entry when the ship isexamined in dry dock.
To prevent internal corrosion the interior surfaces are suitably coated, and in some casesthe rudder may be filled with inert plastic foam.
The rudder is tested when complete under a head of water 2.45 M above the top of therudder.
Why is Rudder Angle Limited to 35 Degrees ?
Beyond 35 degree rudder efficiency is reduced due to formation of eddies on the back of rudder as the flow is no longer streamlined. This is calledstalledcondition.
The manoeuvrability does not increase beyond 35 degree, but rudder torque increases and ship’s turning circle increases.
Why Steering Test Rudder angle 35 degree to 30 degree ?
So that the point at which it is reached can be exactly judged as it crosses 30 degree.
As hunting gear puts pump stroke to zero, the rudder movement slows down progressively as it approaches 35 degree.
Why is Astern Turning Moment much less than Ahead ?
The propeller thrust adds to the force on the rudder when going ahead, but in astern that thrust is lost.
The pivoting point (point about which ship turns) shifts aft to 1/3 rd the length from aft. This reduces turning moment greatly.
What is the Pivoting Point for Ships ?
The ship turns about a point called pivoting point. This is situated about 1/3 rd to 1/6 th of the ship length from forward, depending on the ship design.
Why Rudder is situated Aft of the Ship ?
To make use of propeller wash for thrust.
The pivoting point of ship is 1/6 to 1/3 rd of length of ship from bow, the greater the perpendicular distance between point of action of force and pivoting point, the better rudder movement.
Better protected at astern from damage.
Drag is reduced if rudder is situated aft.
Why Full Astern Power is usually Less than Full Ahead Power ?
Propeller blade section is designed for maximum efficiency in ahead.
In astern direction, angle of attack is high on back of blade.
Propeller will absorb very little available power, severe eddying occurs on face. Therefore, efficiency is very low.
Hence, if 80% of full ahead power is available for astern, then boosting it to 100% will have minimal return in thrust from propeller.
function of duct keel?
Duct Keel
An internal passage of water tight construction (two longitudinal girders spaced not morethan 2.0 m apart) running same distance along the length of the ship, often from the forepeak to theforward machinery space bulkhead.
To carry the pipe work, and an entrance is at forward machinery space via a watertightmanhole.
function of bulbous bow?
Bulbous bow
A bulb shaped under water bow which is designed to reduce wave makingresistance and any pitching motion of the ship.
function of camber?
Camber
Curvature given to a deck transversely.
It is measured by the difference betweenthe heights of the deck at side and centre.
The camber amidships is frequently one fiftieth of thebreadth of the ship.
what is carving note?
Carving note
Form completed by the owner of the ship under construction.
Gives details oftonnage, name, port of registry, etc.
what is cofferdam
Cofferdam
Narrow void space between two bulkheads or floors that prevents leakagebetween the adjoining compartment.
what are coffin plates
Coffin plates
They are used to connect stern frames to the flat plate keel.
The stern frame isextended forward far enough, two or three frame spaces, to provide a good connection with a flatplate keel.
The aft most plate of the keel, coffin plate is dished around the extension.
what is CRP?
CRP
Contra – rotating propeller.
A propulsion arrangement with two propellers rotatingin opposite direction on the same shaft.
define freeboard?
Freeboard
Vertical distance from the load water line to the top of the freeboard deck.
Freeboard has considerable influence on sea worthness of the ship.
The greater the freeboardlarger is the above water volume of ship.
This provided reserve buoyancy assisting the ship toremain afloat in the event of damage.
what are the types of keel?
Types of keel
Bar keel, Duct keel, Flat plate keel.
