ship construction

Definitions and Ship’s Dimensions

Hull:
The structural body of a ship including shell plating, framing, decks and bulkheads.
 Afterbody :
That portion of a ship’s hull abaft midships.
 Forebody:
That portion of a ship’s hull forward midships.
 Bow :
The forward of the ship
 Stern :
The after end of the ship
Port :
The left side of the ship when looking forward
Starboard :
The right side of the ship when looking forward
Amidships: 
point midway between the after and forward perpendiculars

Length Overall (L.O.A.):
Length of the vessel taken over all extremities.

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/ Draught: 
The distance from the bottom of the keel to the waterline. The load draft is the maximum draft to which a vessel may be loaded
Moulded Depth: 
Measured from the base line to the heel of the upper deck beam at the ship’s side amidships.
 Sheer: 
Curvature of decks in the longitudinal direction. Measured as the height of deck at side at any point above the height of deck at side amidships
Camber / Round of Beam: 
Curvature of decks in the transverse direction. Measured as the height of deck above the height of deck at side
Rise of floor / Deadrise: 
The rise of the bottom shell plating line above the base line. This rise is measured at the line of moulded beam
Half siding of keel: 
The horizontal flat portion of the bottom shell measured to port or starboard of the ship’s longitudinal centre line. This is useful dimension to know when dry-docking.
Tumble home: 
The inward curvature of the side shell above the summer load line.
Freeboard:
The vertical distance measured  from the waterline to the top of the deck plating at the side of the deck amidships. Normally exposed to weather and sea.
Flare:
The outward curvature of the side shell above the waterline. It promotes dryness and is therefore associated with the fore end of ship
Extreme Beam: 
The maximum  beam  taken over all extremities.
Extreme Draft: 
Taken  from the lowest point of keel to the summer load line. Draft  marks  represent extreme drafts.
Extreme  Depth:
Depth of vessel  at  ship’s side  from  upper deck  to  lowest point  of keel.
Half  Breadth:  
Since  a  ship’s  hull  is  symmetrical about  the  longitudinal centre line, often  only the half beam  or half breadth at any section  is given.
SCANTLING
The dimensions of the structural items of a ship, e.g. frames, girders, plating , etc.

strong>INTERCOSTAL
Composed of separate parts, non-continuous
CENTER OF FLOATATION
It is the center of the waterplane area and is the axis about which a ship changes trim.

CENTER OF BUOYANCY
It is the center of the underwater volume of the ship where the force of buoyancy acts.

CENTER OF GRAVITY
It is the point at which the whole weight of the object may be regarded as acting. If  the object is suspended from this point, it will remain balanced and not tilt.

TONNAGE MEASUREMENT

• This is often referred to when the size of the vessel is discussed, and the gross tonnage is quoted from Lloyd’s register.
• Tonnage is a measure of the enclosed internal volume of the vessel, 100 cubic feet representing one ton
• Its normally divided into categories as follow:

1. DISPLACEMENT TONNAGE

• A ship’s displacement is the sum of the ship’s actual weight (lightweight) and it’s contents (deadweight).
• The metric unit of measurement is 1 tonne (= 1000 Kg).
• The displacement represents the amount of water displaced by the ship expressed in tonnes.
• The weight of water displaced therefore equals the weight of the ship

TONNE PER CENTIMETRE (TPC)
It is the mass required to increase the mean draught by 1 centimetre.

LOAD DISPLACEMENT
The weight of the ship and its content, measured in tonne. The value will vary according to the ship’s draught.

DEADWEIGHT SCALE
It is a scale diagram indicating the deadweight of the ship at various draughts.

FORM COEFFICIENT
It is devised to show the relationship between the form of  the ship and the dimension of the ship.

2. Lightweight Tonnage (LWT)

• The lightweight is the weight of the ship as built (hull, machinery) including boiler water, lubricating oil and the cooling water system.
• Lightweight like displacement is expressed in units of tones.
• It assumes importance in a commercial sense only when considering the value of the vessel which is to be broken up for scrape.

3. Deadweight tonnage (DWT)

• Deadweight is the weight of the cargo which a ship carries plus weights of fuel, stores, water ballast, fresh water, crew and passengers and baggage.
• It is the difference between the loaded ship displacement and the lightweight.

4. Gross Tonnage (GT)

• Measurement of total internal volume of a vessel and includes all under deck tonnage and all enclosed spaces above tonnage deck.
• 100 cubic feet of space being considered as 1 ton

5. Nett Tonnage (NT)

• Ship measurement derived from gross tonnage by deducting spaces allowed for crew and propelling power.
• 100 cubic feet of space being reckoned as 1 ton

LOAD LINE

The marking on the ship side that relate to the loading condition of the ship termed as the load line mark.

Load line mark

• consists of a ring 300 mm in outside diameter and 25 mm thick which is
• intersected by a horizontal line 450 mm in length and 25 mm thick, the upper edgeof which passes
• through the centre of the ring. The centre of the ring is placed amidships and at a distance equal to the assigned summer freeboard measured vertically below theupper edge of the deck line.

Margin Plate: 

1. The outboard strake of the inner bottom.
2. Knuckle down to the shell by means of Margin Plate at angle of 45°to tank top, meeting the shell almost at right angle.
3. It can form a bilge space.

Keel plate:  

Keel is a horizontal plating of increased thickness, which runs along the centre line, for complete length of bottom shell plating.

Types of keel:   (1) Bar keel  (2) Flat plate keel  (3) Duct keel.

 Bar keel:  

• The first type, used from wood to iron ship building.
• Do not provide sufficient strength for larger ship.
• No direct connection between the keel and floor.

Flat plate keel: 

• A keel of welded ship. The centre girder is attached to the keel and inner bottom plating by continuous welds.
• Keel plate width is about 1 to 2 meter
• It must be full thickness, for 3/5 of length amidship and then thickness may reduce towards the ends of ship.

Duct keel:  

1. An internal passage of watertight construction, running same distance along the length of ship, often from fore peak to forward machinery space bulkhead.
2. It is to carry pipeworks, and entrance is at forward machinery space bulkhead through a watertight manhole.

Bulkhead

Class A bulkhead

• Constructed to prevent passage of flame for 1 hour standard fire test at 927°C
• It must be insulated so that the unexposed sides will not rise more than 139°C above the original temperature within the time, as follows:

Class A- 60 ,  1 hour:       Class A- 30 ,  30 minutes.

Class B bulkhead:

• Constructed to prevent passage of flame for ½ hour standard fire test
• It must be insulated so that the unexposed sides will not rise more than 139°C above the original temperature within the time, as follows.

Class B- 15 ,  15 minutes:       Class B- 0 ,  0 minute.

Class C bulkhead:      

• They are constructed of non-combustible material.

 Standard fire test:     

• The exposure of a material specimen in a test furnace, to a particular temperature for a certain period of time.

Collision Bulkhead:

• Foremost major watertight bulkhead, which extends from bottom to main deck(upper deck).
• It is at a distance of L/20  from forward perpendicular.

Corrugated bulkhead:   

• Used on transverse bulkhead, thus improves transverse strength.

Non-watertight bulkhead:   

• Any bulkhead, which does not form, part of a tank or part of a watertight subdivision of a ship, may be non-watertight.

Wash bulkhead: 

• A perforated bulkhead fitted into a cargo tank or deep tank, to reduce sloshing or movement of liquid through the tank.

After peak bulkhead:

• Provided to enclose the stern tube in watertight compartment.
• Aft peak bulkhead needs only to extend to first deck above load water line.
• Plating must be doubled to resist vibration around stern tube.

Minimum required bulkhead:

1. One collision bulkhead.
2. An after peak bulkhead.
3. One bulkhead at each end of machinery space.
4. Total no: of bulkheads depends upon the ship and position of machinery space

Functions of bulkhead:

1. To increase transverse strength of ship, particularly against racking
2. To divide the ship into watertight compartments.
3. To give protection against fire.
4. To prevent undue distortion of side shell.
5. To restrict volume of water, which may enter the ship, if shell plating is damaged.

Construction of bulkhead:

• Collision bulkhead must extend from bottom to upper deck.
• Aft peak bulkhead needs only extend to first deck above load water line.
• All others must extend to uppermost continuous deck.
• Plating usually fitted vertically, and thickness gradually increases from the top downward.
• Stiffeners are fitted at 750mm apart, but collision bulkhead and deep tanks have 600mm spacing.

Why Collision Bulkhead kept at L/20 of the ship?

• In the events of collision and grounding, standard of subdivision has to give good chance, that the ship remains afloat under such emergencies.
• Longitudinal Bulkheads are avoided, as far as possible, as they might cause dangerous angles of heel, in the event of flooding of large compartment through damage.
• Transverse Bulkheads are reliable in this case, and Classification Society requires a watertight Collision Bulkhead within reasonable distance from forward.
• If the ship is supposed to have wave trough amidships, there will be excess weight amidships and excess buoyancy at the ends, hence the ship will be (Assuming wave length = length of ship)
• If the ship is supposed to have wave crest amidships, there will be excess weightat the ends, and excess buoyancy amidships; hence the ship will be
• By “Trochoidal Theory”, wave height from trough to crest is 1/20 of the wave length, therefore maximum shearing force usually occurs at about L/20 of ship from each end.
• For this reason, Collision Bulkhead is located at L/20 of the ship, so that it is not so far forward, as to be damaged on impact. Neither should it be too far aft, so that the compartment flooded forward causes excessive trim by bow.

Panting:

• As wave passes along the ship, they cause water pressure fluctuation, which tends to create in and out movement of the shell plating, especially at forward end.
• This in and out movement is called panting.
• Resisting structures against panting are beams, brackets, stringer plates, etc.

Racking:

• When a ship rolls, there is a tendency for the ship to distort transversely.
• This is known as racking.
• Resisting structures are beam knee, tank side bracket, and especially transverse

Slamming or Pounding:

• When ship is heaving and pitching, the fore end emerges from water and re-enter with a slamming effect.
• It is called pounding.
• Resisting structure: extra stiffening at the fore end.

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.

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

Transverse stresses:

• Transverse section of a ship is subjected to transverse stresses, i.e. static pressure due to surrounding water, as well as internal loading due to weight of structure, cargo, etc.
• Structures or parts, that resist transverse stresses:
• Transverse bulkhead
• Floors in double bottom
• Brackets between deck beams and side frame
• Brackets between side frame and tank top plating
• Margin plates
• Pillars in holds and tween deck.

Local stresses:

Causes:

• Heavy concentrated loads like engine, boiler.
• Deck cargo such as timber.
• Hull vibration.
• Ship, resting on blocks in dry dock.

Dynamic forces:

• Caused by the motion of the ship itself
• A ship among waves has three linear motions:

1. Vertical movement: heaving 
2. Horizontal transverse movement: swaying
3. Fore and aft movement: surging And

• three rotational motions:

1. Rolling about longitudinal axis
2. Pitching about transverse axis
3. Yawing about vertical axis.

• A ship among waves has three linear motions:
  1. Vertical movement: heaving
  2. Horizontal transverse movement: swaying
  3. Fore and aft movement: surging  AND 
• three rotational motions:
  1. Rolling about longitudinal axis
  2. Pitching about transverse axis
  3. Yawing about vertical axis.

The difference between Timber Load Line and Load Line:

• When ship is carrying timber, the deck cargo gives additional buoyancy and a greater degree of protection against the sea.
• The ship has smaller freeboard than normal (type-B) vessel.

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.

Bow Thruster:

• Lateral Bow Thrusters are particularly useful, for manoeuvring in confined water at low speed.
• For large vessel, used at channel crossing, and docking.
• For research vessels and drilling platform, etc. very accurate positioning
• Bow Thruster consists of: (As a Rule)
• A controllable pitch or reversible impeller, in athwartship watertight tunnels.
• Bridge controlled and driven by
• Thrust provided is a low thrust, about 16 tons.
• Greatest thrust is obtained, when ship speed is zero.
• Less effective, when ship gets underway.
• Athwartship tunnels appreciably increases hull resistance.
• Close the tunnels at either end, when not in use, by butterfly valve or hydraulic valve.

Cofferdam:

• A narrow void space between two bulkheads or floors that prevents leakage between the adjoining compartments.
• In tankers, between cargo tanks: In ER, between DB LO tank (sump tank) and adjacent tanks. Maximum width =  760 mm.

Double Bottom:

• The double bottom consists of outer shell and inner skin, 1m and 1.5 m above thekeel and internally supported by

Double Bottom Tank:

• Double bottom space is subdivided longitudinally and transversely, into large tank, by means of watertight structures. Its functions are:

1. Protection of shell in the events of damage to bottom shell.
2. Tank top being continuous increases the longitudinal strength.
3. To act as platform for cargo and machinery.
4. Can be used for storage of fuel, fresh water, ballast, and for correcting list, trimand draught.
5. Diminish oil pollution, in the event of collision.

Wing Tank:

Purpose:

• To carry water ballast or liquid cargo.
• Protection of shell in the events of damage to side shell.
• To locate oil cargo tank
• To correct list of the ship.

Deep Tank:

• When ship is underway in light condition, it is necessary to carry certain amount of water ballast.
• If DB tanks alone are used for this purpose, the ship might be unduly “stiff”.
• So it becomes a practice to arrange one of the lower holds, so that it can be filled with water when necessary.
• This permits a large amount of ballast to be carried without unduly lowering the centre of Gravity of the ship.
• Such a hold is called a Deep Tank.
• This tank is usually designed to carry dry cargo, and in some cases may carryvegetable oil or oil fuel as cargo.
• If the tank extends full breadth of the ship, a middle line bulkhead, called Wash Plate must be fitted to reduce free surface effect.
• Strength of Deep Tank structure is greater than that required for dry cargo hold bulkhead.

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 decksmay 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 providesreserved buoyancy, assisting the ship to remain afloat in the event of damage.

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 byfreeboard.]

 Purpose:

• To meet loss of buoyancy, in case of hull damage.
• To provide sufficiency of freeboard, to make the vessel seaworthy.

Marking of freeboard:

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

1. is structurally adequate for its intended voyages,
2. has adequate stability for its intended voyages,
3. has a hull that is essentially watertight from keel to freeboard deck, and watertight above these decks,
4. has a working platform that is high enough from water surface, to allow safe movement on exposed deck, in the heavy seas,
5. has enough reserved buoyancy above the water line, so that vessel will not be in danger of foundering and plunging when in heavy seas.

Hatchways: 

These constructions must be in accordance with standards, such as heights of coamings, covers, and fittings exposed. They have standard of strength and protection.

Machinery Casing:  

Machinery space openings on exposed portion of freeboard deck (superstructure deck), must be provided with Steel Casing, with any opening fitted with Steel Doors. Fiddley Opening is to have permanently attached Steel Covers.

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.

Gross Tonnage:

• Gross tonnage is the total of the Underdeck tonnage & the tonnage of the following spaces:

1. Any Tweendeck space , between second and upper deck.
2. Any excess of hatchways over ½ % of vessel’s Gross Tonnage.
3. Any permanently closed-in spaces, on or above the upper deck.
4. Any engine light and air space on or above upper deck, at shipowner’s option and with Surveyor’s approval.
5. Certain closed-in spaces, on or above the upper deck are not included in gross tonnage, and these are known as Exempted Spaces.

Exempted spaces:

• Dry cargo space.
• Space fitted with machinery or condensers.
• Wheelhouse, chartroom and radio room.
• Galley and bakery.
• Washing and sanitary spaces in crew accommodation.
• Light and air spaces.
• Water ballast tanks not appropriated for any other use.

 Net or Registered Tonnage:

• It is obtained by making “deductions” from the Gross Tonnage.
• Principal “Deducted Space”, which already have been included in Gross Tonnageare:

1. Master’s and crew accommodation.
2. Chain lockers and space for working anchor and steering gear.
3. Propelling Power Allowance.
4. Ballast tank, capacity ≯ 90%.

• Port and Harbour dues are assessed on Net Tonnage.

Where Tonnage value is used?

1. To determine port and canal dues.
2. To determine Safety Equipment.
3. To measure the size of fleet.

Propelling Power Allowance:

The largest “Deduction” and is determined according to certain criteria, as follow:

1. If machinery space tonnage is between 13% and 20% of gross tonnage, PPA is 32% of gross tonnage.
2. If machinery space tonnage is less than 13% of gross tonnage, PPA is the amount expressed as a proportion of 32% of gross tonnage.
3. If machinery space tonnage is more than 20% of gross tonnage, PPA is 1.75 times the machinery space tonnage.
4. There is a maximum deduction for propelling power of 55% of gross tonnage, remaining after all other deductions have been made.

Tonnage Deck:  The tonnage deck is the second deck, except in single deck ships.

Water tightness of steel hatch cover:

Rubber jointing is used, and the hatch being pulled down by cleats and cross joint wedges.  Cleats are placed about  2 m apart with minimum of two cleats per panel. Cross joint wedges should be 1.5 m apart.

Hose test and chalk test:

1. To check the water tightness of hatch covers and watertight doors :
   • By using water jet pressure of 2 kg/cm² and a distance of  5 m, and jet diameter  ½”.
   • If hose test cannot carried out, chalk test can be done.

2. Cover or door seals, painted with chalk powder, and close the cover or door tightly.
3.  Open the cover or door, and check whether the chalk painted is cut off or not.

Written by Marine Study