Naval Architecture And Ship Construction

Q. defines tonnage, gross tonnage and net tonnage.

   

Tonnage:   Tonnage is a measure of cubic capacity, where one ton represents  100 ft³   or  2.83 m³. It is a measure of the ship’s internal capacity. 

Certain closed-in spaces, on or above the upper deck are not included in gross tonnage, and these are known as Exempted Spaces.

Gross Register Tonnage' (GRT) was a measure of the total internal capacity of the ship consisting of: under-deck volume excluding double-bottoms, volume of tween deck spaces, volume of superstructures, volume of deck-houses etc. Exemptions included: navigational spaces, galleys, stairways, light and air spaces. The total volume in cubic feet was divided by 100, i.e. 1 gross ton = 100 cubic feet. This was the Gross Tonnage entered in the ship's Register.

'Net Register Tonnage' (NRT) was a measure of the capacity available for the carriage of cargo and passengers. Deductions from GRT included: Master and crew accommodation, safety and storage spaces, water ballast tanks, allowance for propelling machinery. The resulting volume in cubic feet was divided by 100, i.e. 1 net ton = 100 cubic feet. This was the Net Tonnage entered in the ship's Register.

'Modified Tonnage' and ' Alternative Tonnage' were related to the gross and net tonnages where a ship was assigned greater than minimum freeboards under the Load Line Rules, i.e. where the second deck was treated as the freeboard deck and a 'Tonnage Mark' was placed on the ship's side at a tabulated distance below the level of the second deck. '

Measurement or Shipping Ton' which was equivalent to 40 cubic feet. 

Present Under the IMO International Convention on Tonnage Measurement of Ships (1969), which initially entered into force on 18th July 1982 for new ships and which fully entered into force on 18th July 1994 for all ships except warships, ships of less than 24 metres in length and ships solely navigating: the Great Lakes of North America and River St. Lawrence; the Caspian Sea; the Plate, Parana and Uruguay Rivers, the following definitions apply:

'Gross Tonnage' means the measure of the overall size of a ship.

'Net Tonnage' means the measure of the useful capacity of a ship.

The method of determining the Gross and Net Tonnages is prescribed by formula as follows:-

Gross Tonnage (GT) = K1V

where, V = total volume of all enclosed spaces in cubic metres

K1 = 0.2 + 0.02 log10 V (or as tabulated in Appendix 2 of the Convention)

Net Tonnage (NT) = K2Vc(4d/3D)2 + K3(N1+N2/10)

where, Vc = total volume of cargo spaces in cubic metres

K2 = 0.2 + 0.02 log10 Vc (or as tabulated in Appendix 2 of the Convention)

K3 = 1.25 (GT + 10,000)/10,000

D = moulded depth amidships in metres

d = moulded draught amidships in metres (Summer Load Line draught)

N1 = number of passengers in cabins with not more than 8 berths

N2 = number of other passengers

N1 + N2 = total number of passengers the ship is permitted to carry as indicated in the ship's passenger certificate; when N1 + N2 is less than 13, N1 and N2 shall be taken as zero

GT= gross tonnage of the ship as determined above

The factor (4d/3D)2 shall not be taken as greater than unity

The term K2Vc(4d/3D)2 shall not be taken as less than 0.25 GT

NT shall not be taken as less than 0.30 GT

The 'Gross Tonnage' and 'Net Tonnage' figures as determined from the above formulae are to be those quoted on the ship's International Tonnage Certificate (1969).

It should be noted that the word 'tons' is no longer to be applied since the gross and net tonnages are dimensionless, i.e. there are no physical units of tonnage. Hence the tonnage will be expressed as, e.g. the ship has 'Gross Tonnage of 12,345' without the addition of any units. Also the word Register is to be omitted, hence the correct terminology is now 'Gross Tonnage (GT)' and 'Net Tonnage (NT)'.

There are also some other forms of tonnage in everyday use which are summarised as follows:-

'English or Long Ton' = 2240 lb (1016.05 kg).

'American or Short Ton' = 2000 lb (907.18 kg).

'Tonne or Metric Ton' = 1000 kg (2204.62 lb).

'Measurement or Shipping Ton' = 1 cubic metre.

'Bill of Lading or Freight Ton' = 1000 kg or 1 cubic metre of goods, whichever is the greater.

'Displacement Ton' is the unit for the total weight of a ship and her contents, equivalent to the weight of water displaced, under any particular condition of loading given in terms of the defined weight system, i.e. Metric or Long Tons. The maximum 'Displacement Tonnage' is that determined applicable under the International Load Line Regulations.

'Lightweight Ton' is the unit for the fixed weight of the empty as-built ship, equivalent to the weight of water displaced, given in terms of the defined weight system, i.e. Metric or Long Tons. The 'Lightweight Tonnage' is the weight commonly used as the basis for determining the scrap value of ships.

'Deadweight Ton' is the unit for the variable weight of the total contents of a ship under any particular condition of loading given in terms of the defined weight system, i.e. Metric or Long Tons, and is the difference between the Displacement Tons and the Lightweight Tons of the ship. The ship's 'Deadweight Tonnage' is typically quoted as being the maximum deadweight applicable under the International Load Line Regulations when floating at her Summer Load Line draught. The 'Deadweight Cargo Capacity Tonnage' is the Deadweight Tonnage less bunkers, water and constant weights.

'Panama Canal Tonnage' as of 1st October 1994 is comparable to the tonnage as determined by the IMO International Convention on Tonnage Measurement of Ships (1969). The tonnages stated on the Panama Canal Tonnage Certificate are therefore identical to those quoted on the International Tonnage Certificate (1969).

'Suez Canal Tonnage' is different from all other tonnage remaining based on the old Moorsom System of tonnage measurement, i.e. with gross and net tons being equivalent to 100 cubic feet or 2.83 cubic metres. There is apparently no immediate intention to change the basis of measurement under the Suez Canal Authority rules. The tonnages stated on the Suez Canal Special Tonnage Certificate are therefore different from those quoted on the International Tonnage Certificate (1969).

'Limitation Tonnage' is the tonnage that is used to determine the Limit of Liability of a shipowner (which includes the owner, charterer, manager or operator) or salvor (including any person for whose act, neglect or default the shipowner or salvor is responsible) and an insurer, in respect of loss of life or personal injury, or loss of or damage to property, occurring in direct connection with the operation of a ship or with salvage operations, including consequential losses. Under the IMO Convention on Limitation of Liability for Maritime Claims (1976) ("the LLMC Convention"), which entered into force on 1st December 1986, the 'Limitation Tonnage' is the ship's gross tonnage as determined by the IMO International Convention on Tonnage Measurement of Ships (1969) for those countries which have ratified the LLMC Convention. However, for those countries which have not ratified the LLMC Convention, the previous International Convention Relating to the Limitation of the Liability of Owners of Sea-Going Ships (Brussels, 10th October 1957), done under the auspices of the Diplomatic Conference on Maritime Law, still applies. Under this Convention the 'Limitation Tonnage' is the ship's net tonnage plus the allowance for propelling machinery that was deducted from the gross tonnage in order to obtain the net tonnage under the former Moorsom System of tonnage measurement.

(Click here for article on 1976 Limitation Convention)

The applications of Tonnage Measurement are many and varied and are used in the assessment of the following:-

Harbour Dues - which can be based on either Gross or Net Tonnage.

Pilotage Dues - which can be based on either Gross or Net Tonnage.

Light Dues - usually based on Net Tonnage.

Canal Dues - usually based on Net Tonnage.

Miscellaneous Fees - e.g. Agency, Towage, Dry Docking, P&I, Registration and Statutory Surveys.

Q. Where Tonnage value is used?

To determine port and canal dues.

To determine Safety Equipment.

To measure the size of fleet.

  

Hogging: When buoyancy amidships exceeds the weight due to loading, or when the wave crest is amidships, the ship will hog.

Sagging:   When the weight amidships exceeds the buoyancy, or when the wave trough is amidships the ship will sag.

Heel: Amount of temporary inclination of the ship, in transverse direction, due to turning or centrifugal force. It is measured in degree.

List: Amount of permanent inclination of the ship, in transverse direction, due to distribution of cargoes, ballast or any other storage condition. It is measured in degree.  

Function of port hole:    1) For light    2) For ventilation    3) For escape for emergency.

Q. Bulbous Bow: 

It is a bulb shaped underwater bow.

Reduce wave making resistance, and pitching motion of the ship

Increase buoyancy forward, and hence reduce pitching of the ship

Outer plating of bulbous bow is thicker than normal shell plating, to resist high water pressure and possible damage cause by anchor and cables.

Due to reduction in wave making resistance, it can reduce SFOC under full speed and loaded condition.

Q. How bulbous bow reduce wave making resistance?

 The bulbous bow is designed in such a ways to reduce the wave making resistance by producing its own wave, out of phase with the incoming wave system.

Q. Reserved buoyancy: 

 

Watertight volume of a ship above the water line is called the reserved buoyancy.

It can be defined as the buoyancy, a ship can call upon, to meet losses of buoyancy in case of damage to main hull. [Water plane area, multiplied by freeboard.] 

Purpose:

To meet loss of buoyancy, in case of hull damage.

To provide sufficiency of freeboard, to make the vessel seaworthy.

Freeboard:   

Vertical distance from water load line, up to the main deck [freeboard deck], measured at the shipside amidships. 

Main deck is the highest deck that is water sealed. Water falling on upper decks may run down companion ways, but it cannot go any further down into the ship than the main deck.  

Freeboard has considerable influence on seaworthiness of the ship. The greater the freeboard, the larger is the above water volume of the ship and this provides reserved buoyancy, assisting the ship to remain afloat in the event of damage.  

Freeboard deck:  (Superstructure deck):   The uppermost complete deck, exposed to weather. It must have permanent means of closure of all opening on and below it.

Q. Marking of freeboard: 

 Marking of minimum allowable freeboard, in conjunction with an overall seaworthiness evaluation, is to ascertain that the vessel:

is structurally adequate for its intended voyages,

has adequate stability for its intended voyages,

has a hull that is essentially watertight from keel to freeboard deck, and watertight above these decks,

has a working platform that is high enough from water surface, to allow safe movement on exposed deck, in the heavy seas,

has enough reserved buoyancy above the water line, so that vessel will not be in danger of foundering and plunging when in heavy seas. 

Ordinary Load Line: A vertical line is placed 540x25 mm fwd from the disc centre. On this are marked load lines, which show the drafts to which the ship may be loaded.

S - The summer load line: level with the centre of the load line disc. 230x25 mm.                                                                                               

W- The winter load line: placed at a distance of 148 mm of the summer draft below the summer load line. 230x25 mm

T- The tropical load line: placed above the summer load line at a distance of 148 mm of the summer draft. 230x25 mm

WNA- The winter North Atlantic Load Line: placed 50 mm below the winter load line. It is only marked on ships which are 100 meters or less in length. 230x25 mm

F- The fresh water summer load line: indicates the draft to which the ship can load in fresh water.230x25 mm

TF- The tropical fresh water Load Line: indicates the draft to which the ship can load in order to her tropical load line on reaching the sea. 230x25 mm 

                                                                                                                                                   

Q. What is transom post?

A transom is the surface that forms the flat stern of the vessel. The vertical post of flat stern frame to which the rudder is attached  is known as transom post.

Q. Free surface effect?

Free surface effect:

Liquid that only partially fills a compartment is said to have a free surface that tends to remain horizontal (parallel to the waterline). When the ship is inclined, the liquid flows to the lower side (in the direction of inclination), increasing the inclining moment. This is known as free surface effect.

Q. Explain six degree on motion of ship?

*Surging*: The forward and aft linear motion (along x) of a ship is called surging.

*Heaving:* The vertical up and down linear motion (along y) of a ship is called heaving.

*Swaying: *The side to side linear motion (along z) of a ship is called swaying.

*Rolling:* The rotational motion of a ship about longitudinal axis is called rolling

*Yawing:* The rotational motion of a ship about vertical axis is called yawing.

*Pitching:* The rotational motion of a ship about transverse axis is called pitching.

Q. What are the Stress on Ships?

The modern ship is made up steel plating, section and builds up girders so connected as to provide adequate strength in all parts to withstand the forces acting on the ship under all condition of service.

The forces acting on a ship may be static or dynamic. The static forces are due to the *difference 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 wind and 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 the ship, causing longitudinal bending.

*Longitudinal Stress*

The forces are two in number, the weight of the ship and all that it carries acting downwards and the vertical component of the hydrostatic pressure. Depending upon the direction in which the bending moment acts the ship will Hog or Sag.

*Transverse Stress*

A transverse section of amidships is subjected to static pressure due to the surrounding water as well as  internal loading due to the weight of the  structure, cargo, etc.

The parts of the structure, which resist transverses, are Transverse bulkhead. Floor in the double bottom. Bracket between deck beam and side frame, together with bracket between side frame and tank top plating, or margin plate. The pillars in hole and twin 

deck.

*Local Stress*

These are created by such item:

Heavy concentrated load like boiler, engine etc. Dead cargo such as timber Hull ,vibration, Ship resting on block on a dry dock (Static Stress)

*Dynamic Forces*(6 degree motion)

The dynamic effects arise from *the motion of the ship* itself. A ship among waves as three linear motions.

*Surging*: The forward and aft linear motion (along x) of a ship is called surging.

*Heaving:* The vertical up and down linear motion (along y) of a ship is called heaving.

*Swaying: *The side to side linear motion (along z) of a ship is called swaying.

*Rolling:* The rotational motion of a ship about longitudinal axis is called rolling

*Yawing:* The rotational motion of a ship about vertical axis is called yawing.

*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 dynamic forces can be created in the structure.

Panting: As wave passes along the ship they cause fluctuation in water pressure which tends to create in and out movement of the shell plating.

Q. What is meaning of seaworthy ship/when a ship is called seaworthy?

A vessel is seaworthy if the vessel and all of its parts and equipment are reasonably fit for their intended purpose [and it is operated by a crew reasonably adequate and competent for the work assigned].legally defined, a seaworthy ship is one that is fit for any normal perils of the sea, including the fitness of the vessel itself as well as any equipment on it and the skills and health of its crew. 

Note that this only includes the perils of the sea, as opposed to the perils on the sea, and so does not include piracy, severe storms or other such hazards that may occasionally be encountered.

A number of other considerations have to be taken into account.

The vessel stability.

Whether the vessel has good handling in rough weather.

The vessel buoyancy even when it is carrying heavily loaded.

Propulsion system reliability.

Speed and agility of the vessel given its age and size.

Solid construction that is able to withstand the danger of the sea.

Fire resistance and suppression capabilities

Sufficient safety equipments that is appropriate for emergency situation.

Q. What is meaning of seaworthy ship/when a ship is called seaworthy?

 A ship is sea worthy if it fulfill two important stability criteria:

Intact stability

Damaged stability

Intact stability:

Initial GM or metacentric height should not be lass than 0.15m

Righting lever, GZ should be atleast 0.2m and angle of heel > 300

Maximum righting lever should occur > 300 practicably  but not less than 250

Damaged stability:

Damaged stability varies from ship to ship . it can be single compartment flooding, multi compartment flooding, engine room flooding etc.

A seaworthy ship is one that fit for any normal perils of the sea. Including the fitness of the vessel itself as well as any equipments on it and the skill and health of the crew.

Q. Inclining Experiment- Determining Metacentric height of the ship

The Metacentric height of the ship plays an important role in setting the loading capacity and stability of the ship. The Initial metacentric height of the ship is determined by an inclining experiment after the ship is completely built.

Metacentre and Metacentric height

 let’s consider a ship which was in equilibrium is now inclined by an angle Ѳ.

When a ship is heeled by an angle, the center of buoyancy is shifted from B to B1.

When a vertical line is drawn from B and B1, they intersect at a point known as metacentre of the ship.

The metacentric height is the distance between the centre of gravity and metacentre of the ship i.e. GM and it is used to calculate the stability of the ship.

Inclining Experiment

Requirements

The experiment is carried out when the ship is built completely or when major structural changes have been done.

The experiment is carried out with empty ship or as near to empty ship as possible.

The ship must be in upright position.

The ship should be sheltered and in calm waters.

Mooring ropes should be slackened and gangway lifted.

Draught and density of water are to be correctly noted.

All tanks in the ship must be empty or pressed up tight to reduce free surface effect.

Only those people responsible for conducting the experiment must go onboard.

The Experiment

To conduct this experiment, a special tool known as stabilograph is required. The tool consists of a heavy metal pendulum balanced on a knife edge and connected to a pointer to record the heel angle readings.

Normally minimum of two stabilographs are used and are placed at maximum distance from each other i.e. one in forward and one at aft.

Four masses are placed on the ships deck, two on each side of the mid ship, placed away from the centre line.

In the next step, the masses are moved one at a time until all four are on the same side, then all four on the other side, and lastly two on each side.

The deflection on both the stabilographs is recorded for all the movement of mass and an average of these readings are used to determine metacentric height.

Suppose Ѳ is the angle of heel and G1 is the moved position of the centre of gravity after inclination. Then by trigonometry,

GG1= GM tanѲ

Also GG1 is = m x d/Δ

Where m= mass moved

d= distance by which the mass is moved

Δ= displacement of ship in water

Hence GM = m x d  /ΔtanѲ

and GM is metacentric height

Where tanѲ can be determined by the readings of stabilograph

a) Metacentre:  When a ship inclined with small angle of heel its CG remains in same position but centre of buoyancy shift from original position to a certain distance. If a vertical line is drown through the new of buoyancy. Then it will intersect the centre line in a  particular point. This point is known as metacentre. 

b)  Metancentric height :- The vertical distance betn the metacentre (M) and centre of gravity (G) is called metacentric height. In the figure GM is metacentric height.

c)  Righting lever :- Righting lever is the perpendicular distance between the vertical through the centre of buoyancy and the centre of gravity. The distance may be measured at any point

iv) Angle of loll : The angle of heel at whhich the Cb (Centre of buoyancy) remains constant vertically under  centre of gravity  is called the angle of loll. 

Tender ship : A ship with a small meta centric hight will have a small reghting lever at any angle and will roll easily. The ship is then said to be tender.

 Stiff ship : A ship with a large meta centric  height will have a large righting lever at any angle and will have a considerable resistance to rolling. The ship is then said to be stiff.

Stability or statical stability :-

Statical stability is a measure of the tendency of a ship to return to the upright if the ship inclined by the upright if the ship inclined by an external force.

Conditions of Stability :-

i).  The weight of the ship must be equal to the buoyoncy force and should be in opposite direction 

ii) These two forces (Weight & Buoyancy force) must lie on the same vertical line 

Metacentric height must be positive ic metocentre (M) must lie above centre of gravity (G)

Forward and aft construction component:

Margin Plate : A slopping plate which extend to the length of bilge and acts as a cover of D.B tank.

Keel Plate: A horizontal plate of increased thickness which run along the center line for the complete length of the bottom plating.

Bilge keel:   An external fin at round the bilge to reduce roiling

Breast hook: A horizontal flat plate which stiffens the stem structure.

Bulwark: A barrier fitted at the deck edge to protect passengers and crew, and avoidthe loss of items over board should the ship roll excessively

Cable stopper: A device used to hold the anchor cable in place while the ship is at anchor or the anchor is fully housed.

Hatchway: The opening in a deck or deck house which provides access to the varios tween deck and hold spaces below.

Hawse pipe: A thick section pipe through which the anchor cable passes from the forecastle deck to the ship’s deck. A doubling plate is fitted around it at the forecastle deck and a chafing ring at the ship’s side.

Strake: A continuous line of plating extending fore and aft over the length of a ship.

Stringer: A horizontal stiffener fitted along the ship’s side or longitudinal bulkhead, I order to provide  strength and rigidity.

Stringer plate: The outboard strake of plating of any deck.

Spurling pipe: A heavy plate pipe which is fitted at the entrance to the chain locker to lead the anchor cable in and out.

Bollard: A rigid base plate which is used o secure mooring rope

Samson post: A rigid vertical post used in place of a mast to support derricks

Others definition:

Base Line: A horizontal line drawn at the top of the keel plate. All vertical moulded dimensions are measured relative to this line.

Moulded Beam: Measured at the midship section is the maximum moulded breadth of the ship.

Moulded Draft: Measured from the base line to the summer load line at the midship 

section.

Moulded Depth: Measured from the base line to the heel of the upper deck beam at the ship’s side amidships.

Extreme Beam: The maximum beam taken over all extremities\.

Extreme Draft: Taken from the lowest point of keel to the summer load line. 

Extreme Depth: Depth of vessel at ship’s side from upper deck to lowest point of keel.

Camber (or Round of Beam): Curvature of decks in the transverse direction. Measured as the height of deck at centre above the height of deck at side.

Rise of Floor (or Deadrise): The rise of the bottom shell plating line above the base line. This rise is measured at the line of moulded beam.

Tumblehome: The inward curvature of the side shell above the summer  load line.

Flare: The outward curvature of the side shell above the waterline. It promotes dryness and is therefore associated with the fore end of ship.

 Freeing port: An opening in a bulwark to enable water to flow off the deck into the sea.

Garboard strake: The bottom shell plating on either side of the keel plate

BILGE RADIUS: The radius of the plating joining the side shell to the bottom shell. It is measured at mid ships

Bilge block: A large wooden block which is used to support the bilge the region of a ship when a ship in dry dock.

Sheering strake: The strake or plate of side plating nearest to the deck. It is usually increased in thickness or  a higher tensile steel is used because of the high bending stress experienced..

Fair lead: It facilitates mooring on the correct direction.

Fish plate: Lower portion of the bulwark. Fashion plate Accommodation extension plate.

Goose neck: A fitting at the end of derrick which gives motion.

Cowl: Natural ventilation trunk.