Back to Nautical

Ship ConstructionDefinitions:Camber: The purpose of rounding the beam is to ensure a good drainage of the water and also to strengthen the upper deck and the upper flange of the ship girder against longitudinal bending stresses- especially the compression stresses.Rise Of Floor: This is the distance from the line of floor to the horizontal, measured at the ship side. Purpose basically is to allow drainage of the double bottom water/ oil to the centre line suctions.

Tumblehome: This is the inward slope of the side plating from the water line to the upper deck today ships generally do not have a tumblehome.

Flare: This is the curvature of the side plating at the forward and gives additional buoyancy and thus helps to prevent the bows from diving too deeply into the water when pitching.

The anchors are also clear when lowered from the flare of a ship.

Sheer: This is the rise of ships deck fore and aft. This again adds buoyancy to the ends where it is needed during pitching. For calculating the freeboard a correction is applied for the sheer. In modern ship the after sheer has been greatly reduced.

Rake: This is the slope, which the forward end has with between the bottom plating and the upper deck. The length between perpendiculars and the length overall difference is mostly due to the rake forward. It helps to cut the water and thus adds to the ships form.

Parallel Middle Body: This is the part of the main body of the ship and it is a box like structure enabling maximum cargo carrying capacity. It also helps in the pushing when tugs are used to assist the vessel in berthing. Cargo stowage is also greatly facilitated.

Entrance: This part is the fore end of the ship and helps give the box like mid length a ship shaped structure.

Described as the forward underwater portion of the vessel at or near the bow.

The angle formed between the centerline of the ship and the tangent to the designed water line

Is called the angle of entrance.

Run: The after part similarly to the fore part entrance helps in giving the box like mid length a ship shaped structure and thus the handling of the vessel is enhanced.

Length means 96 per cent of the total length on a waterline at 85 per cent of the least moulded depth measured from the top of the keel, or the length from the fore side of the stem to the axis of the rudder stock on that waterline, if that be greater. In ships designed with a rake of keel the waterline on which this length is measured shall be parallel to the designed waterline.

Moulded breadth: is the greatest moulded breadth measured inside plating.

Breadth (B) is the greatest moulded breadth of the ship at or below the deepest subdivision load line.

Draught (d) is the vertical distance from the moulded baseline at midlength to the waterline in question.

Depth and the draught both are measured from the top of the keel. The depth is measure from the top of the deck beam. If there is a camber then allowance is given as 1/3 rd of the camber.

The rest of the meanings are all self-explanatory.

Forward perpendicular: This is represented by a line, which is perpendicular to the intersection of the designed load water-line with the forward side of the stem.

After perpendicular: A line represents this, which is perpendicular to the intersection of the after edge of the rudderpost with the designed load water line. This is the case for both single and twin-screw ships. For some ships having no rudderpost, the after perpendicular is taken as the centre-line of the rudderstock.

Length between perpendiculars: This is the horizontal distance between the forward and after perpendiculars.

Length on the designed load waterline: This is the length, as measured on the water-line of the ship when floating in still water in the loaded, or designed, condition.

Length overall: This is the length measured from the extreme point forward to the extreme point aft.

Base line: This represents the lowest extremity of the moulded surface of the ship. At the point where the moulded base line cuts the midship section a horizontal line is drawn, and it is this line, which acts as the datum, or base line, for all hydrostatic calculations. This line may, or may not, be parallel to the load water line depending on the type of ship.

Moulded depth: This is the vertical distance between the moulded base line and the top of the beams of the uppermost continuous deck measured at the side amidships.

Moulded beam: This is the maximum beam, or breadth, of the ship measured inside the inner shell strakes of plating, and usually occurs amidships.

Moulded draught: This is the draught measured to any water-line, either forward or aft, using the moulded base line as a datum.

Extreme beam: This is the maximum breadth including all side plating, permanent fenders etc.

Extreme draught: This is obtained by adding to the draught moulded the distance between the moulded base line and a line touching the lowest point of the underside of the keel. This line is continued to the FP and AP, where it is used as the datum for the sets of draught marks.

Longitudinal, transverse and combined systems of framing on transverse sections of the ships

Load DeadweightThe total weight of cargo, stores, bunkers etc. when the vessel is at her loaded draught (summer load line). It is equivalent to the difference between her load displacement and her light displacement

Collision Bulkhead: A heavy duty bulkhead in the forepart of the vessel to withstanddamage after impact from collision.

Floor: A vertical athwartships member in way of the double bottom. A floor will run from the centre girder out to the margin plate on either side of the vessel. Floors may be in steel plate, solid or framed bracket form.

Frame: Internal support member for the shell plating . Vessels may be framed transversely or longitudinally.

Garboard Strake: The first strake out from the keel.

Gusset Plate: Triangular plate often used for joining angle bar to a plate.

Intercostal: A side girder in the fore and aft line sited either side of the keel. Integral connection with the tank top and the ships bottom plating and rigidly connected by the floors.

Duct Keel: The duct keel is a plated box/tunnelled keel allowing passage right forward. It provides additional buoyancy, together with a through passageway for cables and pipelines running in the fore and aft direction.

Lightening Holes: Holes cut into floors, or intercostals to reduce the weight content of the ships build and to provide access to tank areas.

Longitudinal: A fore and aft strength member connecting the athwartships floors. Some vessels are longitudinally strengthened by having the frames run in a fore and aft direction as opposed to transverse framing. Additional longitudinals are to be found in areas where pounding canbe anticipated when the vessel is at sea.

Panting Beams: Athwartships members in the forepart introduced to reduce the in/out tendency of the shell plating, caused by varying water pressure on the bow when the vessel is pitching.

Panting Stringers: Internal horizontal plates secured to the shell plating and braced athwartships by the panting beams.

Sheer Strake: The continuous row of shell plates on a level with the uppermost continuous deck .

Stealer Plate: A plate found at the extremities of the vessel in the shell or deck plating. Its purpose is to reduce the width of the plating by merging, say, three strakes into two. The single plate producing this effect is known as a stealer plate.

MAIN STRUCTURAL MEMBERS COMPENSATING STRESS FACTORS AFFECTING VESSEL

Beam Knees: Resist racking, heavy weights and localised stresses.

Beams: Resist racking, water pressure, and local stresses due to weights.

Bulkheads : Resist racking stresses, water pressure, drydocking, heavy weights, hogging and sagging, and shear forces.

Decks: Resist hogging and sagging, shearing, bending, heavy weights, and water pressure.

Floors: Resist water pressure, drydocking stresses, heavy weights, local stresses, racking, vibration and pounding.

Frames: Resist water pressure, panting, drydocking and racking stresses. May be compared to the ribs of the body, which stiffen the body of the vessel. May be longitudinally or transversely constructed.

Longitudinal Girders: Resist hogging and sagging, water pressure, drydocking and poundingstresses, and localised shearing stresses. Examples: keel, keelsons, fore and aft members, intercostals.

Pillars: Resist stresses caused by heavy weights, racking, dry docking and ; water pressure. Extensively found in general cargo vessels in lower hold structure.

HOGGING: The length of the vessel may be considered to act like a long girderpivoted on a wave about its centre. In this position the fore and after ends of the vessel will bend downwards, causing compression forces in the keel area and tension forces at the upper deck level .The condition is brought about by increased buoyancy forces being created at and around the midships point of the vessel. Increased gravitational force, due to the metal structure of the vessel acting vertically downward, occurs at the extremities of the ship. When both forces exist at the same time, e.g. as the vessel is pivoted by a wave midships, a hogging conditionis present.This can be accentuated in a vessel of an all-aft design, where the additional weight of the machinery space would produce high loading in the aft part of the vessel. The condition may also be unnecessarily increased by bad cargo loading in the fore and after parts of the vessel,leaving the midships area comparatively lightly loaded.

SAGGING : Sagging is the direct opposite of hogging. When a vessel is supported at bow and stern by wave crests, she will tend to sag in the middle. High buoyancy forces occur at the extremities of the ship. High gravitational forces, from the weight of the ships structure, act vertically down about the midships point, in opposition to the buoyancy forces. In comparisonwith the condition of hogging, the vessel has a tendency to bend in the opposite direction .

Incorrect loading of the vessel or design characteristics may accentuate the condition of sagging. Watch keeping officers should be aware of the frequency of the waves and the likelihood of this condition developing and, if necessary, take action to relieve any sagging or hogging conditionsby altering the ships course.

Due consideration at the time of loading, with regard to weight distribution may alleviate either hogging or sagging. With shipbuilding producing larger and longer ships either condition is mostundesirable, as the prospect of breaking the ships back in a heavy seaway or swell becomes a frightening reality. Prudent ballast arrangements, together with increased scantlings at the time of building, coupled with efficient ship and cargo loading, will help minimise any structural damage at a later stage due to hogging or sagging.

Longitudinal framing Open floors

Longitudinal framing Plate floors

Transverse framing Open floors

Transverse framing Plate floors

Duct keel

Deckfreeing arrangements - scuppers, freeing ports, and open rails

Construction of the corrugated bulkhead

INCLUDEPICTURE "http://www.thenauticalsite.com/NauticalNotes/Const/MyConst-Lesson03-HullStruct_files/image051.jpg" \* MERGEFORMATINET A fitted corrugated bulkhead

Stress relieving while fitting the bilge keel

Hold drainage systems

The hold drainage system of older cargo vessels had limber board covered upper side of the tank side bracket areas. The drainage conduit was these areas and the pipelines were connected to the after ends, which passed through the lightening holes in the DBs.

The limber boards were removable for cleaning as they were frequently damaged (edges) leaving gaps through which cargo residue would accumulate.

Modern ships do not have the side bilges and have only a strum box at the after end of the holds and these are connected in the similar way to pipelines, which run through the DBs.

Bow and SternPounding and the additional provisions to withstand such pounding:Heavy pitching assisted by heaving as the whole vessel is lifted in a seaway may subject the forepart to severe blows from the sea. The greatest effect is experienced in the light ship condition. To compensate for this the bottom is strengthened from 0.5L to between 0.25L and 0.3L from forward depending on the block coefficient, unless the ballast draught forward is over 0.04L.

Bottom framed LongitudinallyLongitudinals are to be spaced 1000mm apart between 0.2L and 0.3L from forward and 700mm apart between 0.2L from forward and the collision bulkhead. Plate floors are to be fitted alternate frames, side girders not more than 2.1m apart.

Bottom framed TransverselyFrame spacing abaft 0.2L from forward is not to exceed 1000mm and between 0.2L and the collision bulkhead 700mm. Forward of the collision bulkhead 610mm. Plate floors are to be fitted at every frame. Intercostal side girders are to be not more than 2.2m apart with half height side girders not more than 1.1m apart, the girders extending as far as is practicable.Panting: This is a stress, which occurs at the ends of a vessel due to variations in water pressure on the shell plating as the vessel pitches in a seaway. The effect is accentuated at the bow when making headway.

Panting arrangements are to extend 0.15L from forward and abaft the after peak bulkhead.Tiers of beams spaced not more than 2000mm apart vertically are to be fitted at alternate frames in the fore peak or below the lower deck above the water line if the forepeak is small. Alternatively perforated flats may be fitted in lieu of panting beams 2.5m apart vertically.

Tiers of beams are to be supported at the centreline by a partial wash bulkhead or pillars. Beams are to be bracketed to frames and the frames to which no beams are attached are to be bracketed to the stringer. Stringer plates attached to the shell are to be fitted at each tier of beams.Abaft the collision bulkhead intercostals side stringers having the same depth as the frames are to be fitted in line with those forward of the collision bulkhead and are to extend aft for 0.15L from the fore end. Stringers may be omitted if the shell plating is of increased thickness.Abaft the after peak bulkhead the structure is to be efficiently stiffened by deep floors and tiers of beams in association with stringers spaced 2500mm apart vertically.

Stern FrameStern frames may be cast/ forged or fabricated from steel plate. In the case of cast or forged steel frames they may be in one piece or in two or more sections riveted or welded together (thermit welding).Where a riveted connection is used the two sections of the bar are scarphed together and the class rules for the scarph are 3D and the depth as one and one third D, where D is the depth of the bar used in the construction of the frame.

A scarph fitted in a rudder post should not be above the highest gudgeon.Cast steel and fabricated stern frames are to be strengthened at intervals by transverse webs. All stern frames are to be efficiently attached to the adjoining structure and the lower part of the stern frame is to be extended forward to provide an efficient connection to the flat plate keel.With larger stern frames there is a tendency for the whole stern or propeller post and adjacent sections to be fabricated.FittingsMechanical Hatch covers

The figures shown below illustrate the various parts of a mechanical hatch cover. These hatch covers may be made up of several individual pontoons (so named because prior to the MacGregor type of rolling hatch covers the pontoons had to be individually lifted and battened down).

The pontoons (individual parts of the hatch covers) are connected to one another and can easily and quickly be rolled into or out of position leaving clear hatchways and decks. The normal practice for the lengthwise opening of hatches but sideways opening hatchways are found on large bulk carriers and OBOs.The smaller versions are mainly operated either manually (using wire and winch) or electrically. The larger ones are nearly all operated hydraulically.The wheels on the side on which the pontoons rollere are eccentric in their construction thus when in the battened (lowered) position the clearance between the wheel and the trackway is minimum and the pontoon sits on the trackway, the rubber gaskets being compressed by the compression bar.

The cross wedges are used to ensure the pontoon rubber gaskets compress against the compression bars of the forward pontoons.The side cleats ensure that the pontoons stay compressed to the trackway compression bar and the ship motion is effectively compensated or removed.These hatch cover systems consist of various parts:

The pontoons, eccentric wheels, trackway wheels, cross wedges, and the side cleats.

Battening down a hatch is to be done after reading the operations manual.

A hatch cover should not be battened with cargo on top.

The Channels are to be swept prior battening so that the packing do not rest on dirt.

The drain channel on the front of the hatch pontoons are to be cleaned prior closing the hatch.

Once the wheels are turned the next item to be engaged are the cross wedges and the side cleats are to be fitted last.

Prior proceeding to sea (long voyage) the hatch cover sealing should be tested with chalk marks made on all the compression bars on the hatch coaming as well as on the pontoons. The hatch is to be battened and then opened to see if all the rubber gaskets have got chalk mark on them or not if not hen rectification to be done.

Oil tight hatchcover:

These hatch covers are small in size and may have butterfly nut locking arrangement. The sealing is done by Hi-nitrile rubber which is not affected by oil.

Manhole covers do not vary much in design, their shape however are sometimes different for different places.

When fitted outside a tank they may be either circular or elliptical. But when fitted inside they are almost always elliptical to facilitate their removal.

Usual size openings vary between 450mm to about 600mm.

Roller, Multiangle, Pedestal and Panama fairleads

A roller is to be found on the forward and after stations area generally at the leads to the mooring ropes as well as on top of old man pedestals.

These facilitate the hauling of ropes since they reduce the friction when the rope is hauled through a panama fairlead which has no rollers.

A panama fairlead is o named since they were mostly used in the Panama Canal. The ship is hauled by small locomotives and the wires are sent out through these leads they are of adequate strength to prevent the metal being cut open by the wires.

A multi angle fairlead again is a fairlead used due necessity when in the great Lakes. The ship moves through numerous locks as the ship is made to climb a great height the Welland Canal system itself uses about 13 lock gates to cross the Niagara falls. The movement of the ship being fast and the difference in height being enormous the ship steadies itself with 2 wires forward and 2 wires aft, when in the locks. These wires are passed through the multi angle fairleads to reduce the enormous friction generated.

Mooring bitts are prefabricated and then are welded onto the deck. The size of the bitts are dependent on their use. Thus a small set may be fitted next to an occasional winch while the larger ones are fitted at the mooring stations.

The bitts are hollow and as such require care to ensure that the sides do not corroded and holed.

A typical forecastle mooring and anchoring arrangement, showing the leads of moorings

Securing anchors and making spurling pipes watertight in preparation for a sea passageOnce the anchor has been washed the anchor is hove right up into the hawse pipe, the bow stopper is lowered and the locking pin inserted.

The winch is reversed a little to make the chain sit properly into the slot of the bow stopper and then the brake is tightened and the windlass gear removed.

The anchor chain at the deck level (hawse pipe) is lashed with extra lashings as provided by the shipyard, if none are present or if expecting heavy weather, then extra wire rope lashings are taken, The wire rope to be used should be tested one, if an old (good condition) life boat falls are available then this makes a very good extra lashing wire. This wire is flexible and can be used by hand. A number of turns (figure of eight) are taken around two sets of bitts. The free ends being fastened by bull dog clips at least two fixed in opposite directions.

Generally the shipyard would have provide lashing point as well as short length of wire attached to a bottle screw. These should be well oiled and are the most efficient for lashing the anchor. The wire should be tight.

Once the anchor is lashed the hawse pipe covers are not placed but stowed under deck or in their stowage positions.

The spurling pipe area is chipped to remove any residual remains of earlier cement.

The metal spurling pipe covers are placed around the chain and over the spurling pile. The clips provided at the edges of the covers should be hooked to the lips of the spurling pipe.

A new canvas cover is then placed over the metal covers just fitted and is tied around the lips of the spurling pipe as well as the chain. No empty spaces should be found.

Cement mixture is prepared and the entire cover is covered with this mixture.

Cable stopper: A chain stopper as shown below may be of various designs, but all serve the same purpose to hold the cable.

The cable is passed through the stopper with the holding bar lifted up by the counterweight on top. There is a pin to hold the bar in this position.

Once the decision has been taken to hold the cable, the safety locking pin is removed and the bar is eased down on top of the cable. Note that the default position of the holding bar is to arrest the cable, only a effort is required to keep it up.

Once the bar is placed over the cable the cable may have to be adjusted a little to ensure that the flat part of the cable falls in the holding area and not the vertical section, the safety locking pin is now introduced to prevent the bar from jumping u[ in case the cable slip from the brake.

Once the lacking pin is in position the brake can be released and the stopper would do the work of holding the cable.

Masts and Sampson posts

Bilge and ballast piping system of a cargo ship

The following shows a bilge and ballast line diagram of a general cargo ship.

The bilges are all fitted with non return valves so that not water may be inadvertently be pumped into the holds.

The bilges are serviced by a bilge pump which incorporates a strainer and this should be checked before starting the pump.

The strum box fitted in the holds is to be kept clean and the perforations are to be checked that they are not closed due to muck and rust.

Same with the mud boxes in the ER fitted into the system.

Arrangement of a fire main

Capacity of fire pumpsThe capacity of the fire pumps is stated in SOLAS but need not exceed 25m3 per hour

Arrangements of fire pumps and of fire mains. Ships shall be provided with independently driven fire pumps as follows: Passenger ships of 4,000 tons gross tonnage and upwards at least three

Passenger ships of less than 4,000 gross tonnage and cargo ships of 1,000 tons gross tonnage and upwards at least two Cargo ships of less than 1,000 tons gross tonnage to the satisfaction of the Administration

Sanitary, ballast, bilge or general service pumps may be accepted as fire pumps, provided that they are not normally used for pumping oil and that if they are subject to occasional duty for the transfer or pumping of oil fuel, suitable change-over arrangements are fitted.

The arrangement of sea connections, fire pumps and their sources of power shall be such as to ensure that:

In passenger ships of 1,000 gross tonnage and upwards, in the event of a fire in any one compartment all the fire pumps will not be put out of action.

In cargo ships of 2,000 gross tonnage and upwards, if a fire in any one compartment could put all the pumps out of action there shall be an alternative means consisting of a fixed independently driven emergency pump which shall be capable of supplying two jets of water to the satisfaction of the Administration. The pump and its location shall comply with the following requirements:

The capacity of the pump shall not be less than 40% of the total capacity of the fire pumps required by this regulation and in any case not less than 25 m3/h.

Number and position of hydrants

The number and position of hydrants shall be such that at least two jets of water not emanating from the same hydrant, one of which shall be from a single length of hose, may reach any part of the ship normally accessible to the passengers or crew while the ship is being navigated and any part of any cargo space when empty, any ro-ro cargo space or any special category space in which latter case the two jets shall reach any part of such space, each from a single length of hose. Furthermore, such hydrants shall be positioned near the accesses to the protected spaces.

Pipes and hydrants

Mainly galvanised steel pipes are used and during repairs no doublers or such part renewals are allowed change is flange to flange renewal.

The arrangement of pipes and hydrants are to be such as to avoid the possibility of freezing.

On cargo ships where deck cargo may be carried, the positions of the hydrants are to be such that they are always readily accessible and the pipes are to be arranged, as far as practicable, to avoid risk of damage by such cargo.

A valve is to be fitted at each fire hydrant so that any fire-hose may be removed while the fire pump is at work.

The above figure shows a typical fire mains line. Note that the emergency fire pump is located away from the machinery space as per rules.

Isolation valves are provided so that any system being damaged the other system may be used for example the port system and the starboard system.

In the machinery space a separate pump (Fire and GS pump) is also coupled, this is generally used when washing decks, and as an emergency measure while the fire pump is being overhauled.

Sounding pipes

Sounding pipes covers come with varied designs. That shown below is a sunken cap type generally the cap is made of brass. The justification being that of the two thread and cap assembly the thread of the brass is to wear out first and that of the deck pad. The renewal of the brass cap being inexpensive and convenient rather than the deck pad which entails hot work.The metal cap (not sunken) type of covers have a chain attached to them to prevent their being washed overboard.

Air pipes to ballast tanks or fuel oil tanks

The above figure shows a design of air pipe cover.

In normal condition the ball remains at the bottom of the air pipe head and the tank breathes in and out through the vent.

However in the event that the air pipe is submerged then the ball floats up and closes the opening at the top thus preventing any water from entering the tank.

Sea spray and rain is prevented from entering the tank by the design of the head. It is totally enclosed and a rectangular plate, which leaves a small gap between the mesh and itself, allowing the breathing of the tank.

Fittings and lashings for the carriage of containers on deck

In the figure above the containers on deck are loaded on top of shoes which are welded on top of the deck as well on top of the hatch covers.Twistlocks are fitted on the shoes and the containers placed on the twistlocks. Hinged eyes are welded on deck to secure the container rod lashings.Load Lines and Draught MarksDeck line

The deck line is a horizontal line 300 millimetres in length and 25 millimetres in breadth. It shall be marked amidships on each side of the ship, and its upper edge shall normally pass through the point where the continuation outwards of the upper surface of the freeboard deck intersects the outer surface of the shell, provided that the deck line may be placed with reference to another fixed point on the ship on condition that the freeboard is correspondingly corrected. The location of the reference point and the identification of the freeboard deck shall in all cases be indicated on the International Load Line Certificate (1966).

Freeboard. The freeboard assigned is the distance measured vertically downwards amidships from the upper edge of the deck line to the upper edge of the related load line.

Freeboard deck. The freeboard deck is normally the uppermost complete deck exposed to weather and sea, which has permanent means of closing all openings in the weather part thereof, and below which all the openings in the sides of the ship are fitted with permanent means of watertight closing. In a ship having a discontinuous freeboard deck, the lowest line of the exposed deck and the continuation of that line parallel to the upper part of the deck is taken as the freeboard deck. At the option of the owner and subject to the approval of the Administration, a lower deck may be designated as the freeboard deck, provided it is a complete and permanent deck continuous in a fore and aft direction at least between the machinery space and peak bulkheads and continuous athwartships. When this lower deck is stepped the lowest line of the deck and the continuation of that line parallel to the upper part of the deck is taken as the freeboard deck. When a lower deck is designated as the freeboard deck, that part of the hull which extends above the freeboard deck is treated as a superstructure so far as concerns the application of the conditions of assignment and the calculation of freeboard. It is from this deck that the freeboard is calculated.

Load Line Mark: The Load Line Mark shall consist of a ring 300 millimetres in outside diameter and 25 millimetres wide which is intersected by a horizontal line 450 millimetres in length and 25 millimetres in breadth, the upper edge of which passes through the centre of the ring. The centre of the ring shall be placed amidships and at a distance equal to the assigned summer freeboard measured vertically below the upper edge of the deck line.

The Load line rules which were brought in were due to the fact that the ships were being loaded in such a way that the ships were foundering.

Thus the important fact to remember is that it was the freeboard that was being restricted, from very low to a safe figure.

Depending on this freeboard the load line circle was marked as well as the other marks were made for different zones and densities.

Thus the chapter on CONDITIONS OF ASSIGNMENT OF FREEBOARD is very important as it determines as to how much would be the distance between the deck line and the load line circle.

Once this is determined the load line marks are painted, keeping the above in reference.

The calculations give rise to the assigned summer freeboard.

Lines to be used with the Load Line Mark

The lines which indicate the load line assigned in accordance with these Regulations shall be horizontal lines 230 millimetres in length and 25 millimetres in breadth which extend forward of, unless expressly provided otherwise, and at right angles to, a vertical line 25 millimetres in breadth marked at a distance 540 millimetres forward of the centre of the ring.

The following load lines shall be used:

(a) The Summer Load Line indicated by the upper edge of the line which passes through the centre of the ring and also by a line marked S.

(b) The Winter Load Line indicated by the upper edge of a line marked W.

(c) The Winter North Atlantic Load Line indicated by the upper edge of a line marked WNA.

(d) The Tropical Load Line indicated by the upper edge of a line marked T.

(e) The Fresh Water Load Line in summer indicated by the upper edge of a line marked F. The Fresh Water Load Line in summer is marked abaft the vertical line. The difference between the Fresh Water Load Line in summer and the Summer Load Line is the allowance to be made for loading in fresh water at the other load lines.

(f) The Tropical Fresh Water Load Line indicated by the upper edge of a line marked TF, and marked abaft the vertical line.

If timber freeboards are assigned in accordance with these Regulations, the timber load lines shall be marked in addition to ordinary load lines. These lines shall be horizontal lines 230 millimetres in length and 25 millimetres in breadth which extend abaft unless expressly provided otherwise, and are at right angles to, a vertical line 25 millimetres in breadth marked at a distance 540 millimetres abaft the centre of the ring.

The following timber load lines shall be used:

(a) The Summer Timber Load Line indicated by the upper edge of a line marked LS.

(b) The Winter Timber Load Line indicated by the upper edge of a line marked LW.

(c) The Winter North Atlantic Timber Load Line indicated by the upper edge of a line marked LWNA

(d) The Tropical Timber Load Line indicated by the upper edge of a line marked LT.

(e) The Fresh Water Timber Load Line in summer indicated by the upper edge of a line marked LF and marked forward of the vertical line.

The difference between the Fresh Water Timber Load Line in summer and the Summer Timber Load Line is the allowance to be made for loading in fresh water at the other timber load lines.

(f) The Tropical Fresh Water Timber Load Line indicated by the upper edge of a line marked LTF and marked forward of the vertical line.

Where the characteristics of a ship or the nature of the ships service or navigational limits make any of the seasonal lines inapplicable, these lines may be omitted.

Where a ship is assigned a greater than minimum freeboard so that the load line is marked at a position corresponding to, or lower than, the lowest seasonal load line assigned at minimum freeboard in accordance with the present Convention, only the Fresh Water Load Line need be marked.

On sailing ships only the Fresh Water Load Line and the Winter North Atlantic Load Line need be marked.

Where a Winter North Atlantic Load Line is identical with the Winter Load Line corresponding to the same vertical line, this load line shall be marked W.

Additional load lines required by other international conventions in force may be marked at right angles to and abaft the vertical line specified in paragraph (1) of this Regulation.

Mark of assigning authority

The mark of the Authority by whom the load lines are assigned may be indicated alongside the load line ring above the horizontal line which passes through the centre of the ring, or above and below it. This mark shall consist of not more than four initials to identify the Authoritys name, each measuring approximately 115 millimetres in height and 75 millimetres in width.

Details of marking

The ring, lines and letters shall be painted in white or yellow on a dark ground or in black on a light ground. They shall also be permanently marked on the sides of the ships to the satisfaction of the Administration. The marks shall be plainly visible and, if necessary, special arrangements shall be made for this purpose.

ZONES, AREAS AND SEASONAL PERIODS

The zones and areas are, in general, based on the following criteria:

Summer - not more than 10 per cent winds of force 8 Beaufort (34 knots) or more.

Tropical - not more than 1 per cent winds of force 8 Beaufort (34 knots) or more. Not more than one tropical storm in 10 years in an area of 5 square in any one separate calendar month.

Tropical Zone

(1) Northern boundary of the Tropical Zone(2) Southern boundary of the Tropical Zone(3) Areas to be included in the Tropical ZoneSeasonal Tropical Areas

The following are Seasonal Tropical Areas:

(1) In the North AtlanticTROPICAL: 1 November to 15 July

SUMMER: 16 July to 31 October.

(2) In the Arabian SeaReading Draughts:The following figure shows the draught marks between 11m and 12m.

It means that the mark is submerged up to the level of the mark, measurement of draught being from the bottom up.

When the water is touching exactly the 11M mark at the bottom, only then is the draught read as 11m. anywhere above that is more than 11m.

The height of the mark being 20cm, therefore the top of the 11m mark would read a draught of 11.20 m.

The bottom of the decimal mark of 2 coincides with the top of the 11M mark and is to be read as 11.20m.

The decimal marks are each 10 cm in height.

Since the decimal marks are at 2, 4, 5 and 8, the odd numbered decimal being ignored, thus the top of this 2 would read as 30 cm above the 11m mark or 11.30m.

If the water level were at a position between the top of 2 and the bottom of 4 then the reading would be 11.35m.

For load line surveys the surveyor would mark a long baton (wooden) with the total length of the freeboard (summer) and others and then checks with the same against the deck line and the markings on the shipside (midship marks).

Rudder and Propellers

The shape of a rudder plays an important part in its efficiency. The area of the rudder is approximately 2% of the product of the length of the ship and the designed draught.

Since the vertical dimensions of the rudder are somewhat restricted due to the area constraint as mentioned above, the fore and aft dimensions are increased.

Again due to this increased dimensions the torque necessary to turn this rudder is overcome by fitting balanced or semi balanced rudders. Such a rudder has about 1/3rd of the rudder area forward of the turning axis.

An ideal rudder is one where the centre of pressure and the turning axis coincide for all angles of the helm.

An unbalanced rudder consists of a number of pintles and gudgeons, the top pintle being the locking pintle which prevents any vertical movement in the rudder and the pintle And gudgeon taking the weight of the rudder.

Principle of screw propulsion

Some people still occasionally refer to the propeller as the airscrew, a very accurate and descriptive term that reflects the basic design and function of the propeller.

Leonardo da Vinci had proposed the concept of a helical screw to power a machine vertically into the air.

The propeller uses that principle to provide propulsion through the air, much like a threaded screw advances through a solid medium, with some notable exceptions, primarily related to the loss of forward movement because the medium is not solid.

Nonetheless, the propeller is similar to a screw in some common features. First, the pitch of a propeller is the theoretical distance the propeller would move forward in one revolution (similar to a screw) and conceptually is the same as the pitch of a screw, namely the distance between threads if the propeller were a continuous helix.

The second feature that relates to its screw design is that the angle of the blade changes along the radius, so that close to the hub, the angle is very steep and at the tip of the blade it is much more shallow.

From a practical standpoint, this means that unless the pitch for a given propeller is known, it requires a trigonometric calculation to determine the pitch empirically.

Thirdly, just as screws come in left hand and right hand threads, propellers have the same designation. When facing the water/ air flow if the top of the propeller moves to the right, it is designated Right Hand and if to the left it is Left Hand. (As viewed from the front a right hand propeller turns counterclockwise and a left hand propeller turns clockwise.) Propellers will frequently be stamped as RH or LH.

Propeller and some definitions

Boss or HubThe central portion of a screw propeller to which the blades are attached and through which the driving shaft is fitted.Rake

The point displacement, from the propeller plane to the generator line in the direction of the shaft axis. Aft displacement is considered positive rake (see Figure 2). The rake at the blade tip or the rake angle are generally used as measures of the rake. The strength criteria of some classification societies use other definitions for rake.

SkewThe displacement of any blade section along the pitch helix measured from the generator line to the reference point of the section (see Figure 2). Positive skew- back is opposite to the direction of ahead motion of the blade section. The skew definition pertains to midchord skew, unless specified otherwise.

Back (of blade)The side of a propeller blade which faces generally in the direction of ahead motion. This side of the blade is also known as the suction side of the blade because the average pressure there is lower than the pressure on the face of the blade during normal ahead operation.

Tip

The maximum reach of the blade from the center of the propeller hub. It separates the leading edge from the trailing edge.

Radius

Radius of any point on a propeller.Pitch

The pitch of a propeller is the theoretical distance the propeller would move forward in one revolution (similar to a screw) and conceptually is the same as the pitch of a screw, namely the distance between threads if the propeller were a screw. For this reason, propellers will frequently be stamped with a designation such as D 2550/P2610. This means that the diameter (in this case length of propeller or thickness of a screw)is 2.550 meters, and the pitch is 2.610 meters, so that in a mathematical sense, one revolution of this propeller would move it forward a distance of 2.610 meters.

Comparing fixedpitch with controllablepitch propellersAdvantages of a controllable pitch propellerAllow greater manoeuvrability

Allow engines to operate at optimum revs

Removes need for reversing engines

Reduced size of Air Start Compressors and receivers

Improves propulsion efficiency at lower loads

DisadvantagesGreater initial cost

Increased complexity and maintenance requirements

Increase stern tube loading due to increase weight of assembly, the stern tube bearing diameter is larger to accept the larger diameter shaft required to allow room for Oil Tube

Lower propulsive efficiency at maximum continuous rating

Prop shaft must be removed outboard requiring rudder to be removed for all prop maintenance.

Increased risk of pollution due to leak seals

Sketches the arrangement of an oillubricated sterntube and tailshaft

Stern tubes are fitted to provide a bearing for the tail end shaft and to enable a watertight gland to be fitted at an accessible position.

The tube is usually constructed of cast steel with a flange at its forward end and a thread at the after end. It is inserted from forward and this end is bolted over packing to the after peak bulkhead. A large nut is placed over the thread at the after end, tightened and secured to the propeller post.

In an oil lubricated stern tube the bearings are made of white metal. A gland is fitted to each end of the stern tube and since the after end gland will not be accessible during sea service it is made self adjusting. The flange shown is attached to the propeller so that it rotates with the shaft and oil tightness is obtained by a rotating gland.

States how the propeller is attached to the tailshaftThe after end of the tail end shaft is tapered to receive the propeller boss and a key is provided to transfer the torque from the shaft to the propeller. A nut fitted with a locking plate secures the propeller in position and as an additional safeguard it is fitted with a left hand thread in association with a right hand ed propeller or vice versa.

To remove the propeller and the tail end shaft the propeller should be slung on special eyes provide on the shell for this purpose the rope guards removed and the propeller nut slackened.

The propeller is then started from the shaft by driving steel wedges between the boss and the propeller post. When it is free the nut is removed.

Crosssection of a shaft tunnel

Sketc and Label:

Midship section of an oil tanker May12.

Midship section of a Bulk Carrier Jul 12.

Transversely Framed Double Bottom Tank Oct 11.

Draw and label a neat diagram of Fore Peak tank of a Merchant ship. Sep 11

Sketch and describe a mid ship of a double hull tanker.

Draw a neat sketch of the mid-ship section of longitudinally framed, self trimming bulk carrier and label its parts. May 11.

Draw and label oil cooled stern tube.

Draw suitable sketch showing After Peak & other arrangements aft. Mar 11

RGU

SOURCE: Internet

PAGE 15