1.

After Perpendicular (AP): A perpendicular drawn to the waterline at the point where the aft sides of the rudder
post meets the summer load line. Where no rudder post is fitted it is taken as the centre line of the rudder stock.

2. Forward Perpendicular (FP): A perpendicular drawn to the waterline at the point where the foreside of the
stem meets the summer load line.

3. Length between Perpendiculars (LBP): The length between the forward and aft perpendiculars measured along
the summer load line.

4. Length Overall (LOA): Length of vessel taken over all extremities

5. Length on waterline: The length of a vessel measured along the waterline from forward to aft.

6. Amidships: A point midway between the after and forward perpendiculars. Length Overall (LOA): Length of
vessel taken over all extremities.
7. Freeboard: The vertical distance measured at the ship’s side between the summer load line (or service draft) and
the freeboard deck. The freeboard deck is normally the uppermost complete deck exposed to weather and sea
which has permanent means of closing all openings, and below which all openings in the ship’s side have
watertight closings.

8. 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.

9. 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.

10. 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.

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

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

13. Moulded dimensions: They are often referred to; these are taken to the inside of plating on a steel ship.

14. Moulded breadth: The greatest breadth of the ship measured between the inside edges of the shell plating.

15. The moulded depth, D: It is the height above baseline of the intersection of the underside of the deck plate with
the ship side. When there are several decks, it is necessary to specify to which one refers the depth.
16. Moulded draught, T: It is the vertical distance between the top of the keel to the designed summer load line,
usually measured in the midships plane.

17. Air draft: Air Draft refers to the vertical distance between the waterline and the highest point of a vessel, typically
measured from the waterline to the top of the highest mast, antenna, or structure on board. It represents the
maximum height that a vessel can safely navigate under bridges, power lines, or other overhead obstacles without
risking collision or damage to the vessel or the structure.

18. Stem: The front-most part of the boat or ship’s bow is termed as the Stem of the ship. It is the bow frame
forming the apex of the intersection of the forward sides of a ship. At its lower end it is rigidly connected to the
keel. It and may be a heavy flat bar or of rounded plate construction. The keel itself is extended up to gunwale to
form the curved edge called the ship’s stem. These stems can be of two styles viz. raked and plumb stems, where
the former is inclined at some angle to the waterline and later is perpendicular to the waterline.

19. Stern: Rear or Aft end of a ship. The stern is designed to provide low resistance, high propulsion efficiency, and
avoid vibrations. It is the rearmost part of a ship that keeps the water out. Rudders and propellers are hanged to
the stern. The stern can be shaped flat, canoe-like, tapered, sharp to serve the purpose of cutting the water in its
way.
20. Skeg: A skeg is a vertical tapering projection or an external structural feature at the aft of the vessel, about the
centreline, directly attached to the vessel’s bottom shell plating, and mostly, but not necessarily, situated in front
of the rudder. As it is not intrinsic to the main hull structure, it can be said as an appendage for all practical
purposes.

21. Transom Stern: A transom stern is a stern shape characterized by a generally flat shape extending to the
waterline. The transom stern offers a greater deck area aft, is a simpler construction, and can also provide
improved flow around the stern. The flat surface of any transom stern may begin either at or above the vessel’s
waterline. A transom stern can be viewed as a cruiser stern whose aft-most portion is cut off.

22. Cruiser Stern: The cruiser stern was initially designed only to lower the steering gear below the armour deck. A
cruiser stern is characterized by an upward curved profile from the after perpendicular to the main deck or poop.
 Unlike the counter stern, a cruiser stern cutaway for the rudder occurs below the waterline (the rudder is fully
submerged at the design waterline). The cruiser has better resistance characteristics than the merchant stern.

23. The deck of a ship refers to the horizontal surface or floor that extends across the hull, forming a structural
platform. It is where people walk, work, or place equipment. Ships may have multiple decks, each serving
different functions. Common Types of Decks on a Ship:

 Main Deck
 The primary structural deck of the ship.
 Usually the uppermost continuous deck that runs the entire length of the vessel.
 Upper Deck
 The deck above the main deck, often partially covered.
 Usually exposed to the weather.
 Lower Deck(s)
 Deck(s) below the main deck, used for cargo, machinery, or crew quarters.
 Forecastle Deck (Fo’c’sle Deck)
 Located at the forward (front) part of the ship.
 Often used for anchor handling or storage.
 Flight Deck (on aircraft carriers)
 A flat surface used for launching and recovering aircraft.

24. Bridge Deck: Bridge deck is the deck on which the navigational equipments of the ships are housed.

25. Poop Deck: Originating from the Latin term for a vessel’s stern-side – Puppis – the poop deck is located on the
vessel’s stern. The poop deck is basically used by the vessel’s commanding superiors to observe the work and
navigational proceedings. Technically, it is the deck that forms the roof of a cabin built in the aft part of the
superstructure of the ship.

26. Tween Deck: ‘’tween’ is a colloquial abridging of the word ‘between.’ In a ship, the tween deck actually means
an empty space separating or between (tween) two other decks in the hull of a vessel.

27. Weather Deck: A deck that is not roofed and thus is open to the ever-changing weather conditions of the sea is
referred to as the weather deck. It is the uppermost deck on the ship which is exposed to the environment.
28. Quarter Deck: The deck located near to the chief mast of a vessel on its stern is referred to as the quarter deck.
The quarter deck is a part of the upper deck and is inclusive of the poop deck. The quarter deck is generally found
on navy ships and accessible only to the most senior naval officers on the vessel. When in port, all the activities of
the ships are controlled from the quarter deck.

29. Superstructure: The superstructure of a ship refers to all parts of the ship that are built above the main deck. It
includes structures like the bridge, wheelhouse, accommodation blocks, masts, funnels, and other elevated
platforms. The superstructure provides space for navigation, communication, crew living quarters, and other
operational equipment.

30. Bilge radius – The radius of the plating joining the side shell to the bottom shell. It is measured at midships.

31. Parallel Middle Body: The length over which the midship section remains constant in area and shape.
32. Entrance: The immersed body of the vessel forward of the parallel middle body.

33. Run: The immersed body of the vessel aft of the parallel middle body

34. Angle of Entrance: The angle formed by the centerline of the vessel and the tangent to the design
waterline at the forward perpendicular.

35. Dead Rise Angle: The deadrise angle of a ship, also known as the deadrise, is the angle at which the hull
rises from the keel towards the sides. It's essentially how much the hull "V"s as it moves away from the
centerline. This angle is a key factor in a boat's performance, particularly in how it handles rough water
and its overall ride quality.

36. Knuckle: An abrupt change in direction of plating, frames, keel, deck, or other structure of a ship.
37. Bulbous Bow: A bulbous bow is a protruding hydrodynamic feature located at the foremost part of a
ship’s hull, just below the waterline. Its primary function is to alter the flow of water around the hull,
thereby reducing hydrodynamic resistance and improving propulsion efficiency. The design is widely
applied to large, ocean-going vessels such as cargo ships, tankers, cruise liners, and some naval vessels.
It is generally not used on smaller or high-speed craft, as these do not typically operate at speeds or
conditions where a bulbous bow would offer significant performance benefits.

Designing a bulbous bow isn't a one-size-fits-all approach. Its effectiveness depends on ship-specific
parameters, especially the operational speed and hull shape. An incorrect design can actually increase
resistance, so careful hydrodynamic analysis is essential.

The operational principle of the bulbous bow is based on the reduction of wave-making resistance - a
component of total resistance encountered by a ship moving through water. The bulbous bow reduces a
ship’s wave-making resistance by generating a wave that is out of phase with the natural bow wave,
causing partial cancellation. This flattens the wave pattern, lowers resistance, improves propulsion
efficiency, reduces fuel consumption, and can enhance sea-keeping.
38.
39. Weight:
The weight of a ship is the total force due to gravity acting on the entire mass of the ship, including its structure, machinery,
cargo, fuel, crew, and provisions.

In naval architecture, this is referred to as the displacement of the ship, which is equal to the weight of the volume of water
the ship displaces when floating, according to Archimedes' Principle.

Key Points:
Ship’s weight = Displacement
Measured in tonnes (metric tons) or kilonewtons (kN)
Important for stability, buoyancy, and draft calculations

Lightweight: This is the weight of the ship itself when completely empty, with boilers topped up to working level. It is
made up of steel weight, wood and outfit weight, and the machinery weight. This lightweight is evaluated by conducting an
inclining experiment normally just prior to delivery of the new vessel. Over the years, this value will change

Deadweight: This is the weight that a ship carries. It can be made up of oil fuel, fresh water, stores, lubricating oil, water
ballast, crew and effects, cargo and passengers. This deadweight will vary, depending on how much the ship is loaded
between light ballast and fully-loaded departure conditions.

Displacement: This is the weight of the volume of water that the ship displaces:

Displacement = Lightweight + Deadweight

40. Center of gravity (G):

 All floating bodies have a centre of gravity (G) and a centre of buoyancy (B)
 The relative positions of G and B affect:
o Trim (how the ship sits fore and aft)
o Stability (how the ship resists rolling)
o
  Improper loading or poor design can lead to:
o Poor fuel efficiency
o Excessive trim
o Capsizing risks

Ship’s Weight and Centre of Gravity (G)

 The ship's weight is the total weight of everything onboard:

o Hull + Machinery + Cargo + Fuel + Passengers + Ballast, etc.
 This weight acts vertically downward through a point: the Centre of Gravity (G)
 G has three components:
o Longitudinal (x-axis): LCG
o Vertical (z-axis): VCG
o Transverse (y-axis): TCG (not the focus today)

NOTE: G moves as weights are added or removed. Always monitor it during cargo operations!

Longitudinal Centre of Gravity (LCG)

Definition:

 LCG is the position of G along the ship’s length, measured from a reference point like the forward
perpendicular (FP).

Importance:

 LCG must align with the Longitudinal Centre of Buoyancy (LCB) to avoid unwanted trim.
 If LCG > LCB → ship trims aft
 If LCG < LCB → ship trims forward

Consequences of Poor Trim:

 Propeller efficiency reduced

 More resistance → higher fuel use
  Increased stress on hull

Vertical Centre of Gravity (VCG)

Definition:

 VCG is the height of G above the keel (usually measured in meters)

Importance:

 Directly affects stability:

o High VCG → low stability
o Low VCG → high stability

Related Concept: Metacentric Height (GM)

 GM = KM – KG
 Larger GM = more stable
 Small or negative GM = danger of capsizing

Practical Examples

Example 1: Cargo Shift Forward

 Cargo is loaded too far forward → LCG moves ahead of LCB

 Result: Bow trims down, stern trims up → increased resistance

Example 2: Heavy Cargo on Deck

 Raises the VCG

 Reduces GM, lowering stability
 Ship rolls more → dangerous in rough seas

41. Archimedes’ principle

42. Buoyancy. Buoyancy is the upward force of all the hydrostatic pressures on the hull. The horizontal
components of the water pressures on unit areas of the ship’s sides and bottom, increasing with depth, act in
opposite directions and cancel each other. The vertical components of the water pressures on unit areas combine
to form an upward force (B) equal to the weight of the water displaced by the underwater hull volume. This
weight varies slightly with the specific gravity of the water. The centre of buoyancy (B) lies at the geometric
centre of the immersed volume. The ship sinks in the water until the force B exactly equals the force W, in
accordance with Archimedes’ principle.
43. Center of Buoyancy (CB): The longitudinal position of the centre of buoyancy with respect to any reference
point on the ship is called the longitudinal centre of gravity (LCB). Usually, the reference point for locating the
LCG is either of the forward or aft perpendiculars.

The vertical distance (along the ship’s centerline) between the keel and the centre of buoyancy is expressed as
‘KB’,

44. Metacenter (M): Refer to the following figure to understand that when a ship heels to any angle, a portion of the
lower side of the ship is now submerged, and a portion of the hull from the upper side emerges out of the water.
This can be noticed by visualizing the hull when the waterline was WL (without heel), and when the waterline
was changed to W1L1 (after heel). Due to this shift of submerged volume, there is a shift of the center of
buoyancy from the centerline to the side that is lower after the heel. The new position of center of buoyancy is
illustrated as B1. If a vertical line is extended from the new center of buoyancy, then the point at which this line
meets the centerline of the ship, is called the transverse metacenter (shown as ‘M’) of the ship.
45. Center of Floatation (LCF): When the ship floats at a particular draft, any trimming moment acting on the
ship would act about a particular point on the water plane. This point is the centroid of the area of the water plane,
and is called the center of the floatation. The distance of the center of floatation is read with respect to either of
the perpendiculars or the mid-ship, and is abbreviated as LCF.

46. Heel. A ship is said to be heeled when she is inclined by an external force. For example, when the ship is inclined by the
action of the waves or wind.

List. A ship is said to be listed when she is inclined by forces within the ship. For example, when the ship is inclined by
shifting a weight transversely within the ship. This is a fixed angle of heel.

Trim: Trim is the difference between the forward draft (depth of the ship’s bow below water) and the aft draft (depth of the
stern below water). In simple terms, trim describes how much a ship is tilted forward or backward (longitudinally) in the
water.

Types of Trim:

 Trim by the stern – Stern is deeper than the bow

 Trim by the bow – Bow is deeper than the stern

 Even keel – Draft is the same at both ends (no trim)

47. Degrees. At first glance, the ocean may appear flat, but a closer view offers a better look at the
wind, currents, and wave action that influence how a ship moves in six degrees of motion: heave, sway,
surge, roll, pitch, and yaw. These environmental conditions create a dynamic and ever-changing plan as
the vessel navigates or moves across — not just forward and reverse, but up and down, side to side, and
even heaving into the air. Regardless, all six happen in combination, and understanding how this works
helps scientists, engineers, surveyors and bridge operators make informed decisions about how and
where to operate the ship, and how to calibrate our technology systems.
Here is a quick look at the six planes of ship motion:

1. PITCH: describes the up and down motion of a vessel. This is characterized by the rising and
falling of the bow and stern in much the same way as a teeter-totter moves up and down.

2. ROLL: is how we describe the tilting motion of the ship from side to side. Wind and waves
push against the ship and cause it to rock back and forth.

3. YAW: spins the ship on an invisible middle line similar to swiveling on a chair. This can be
caused by waves moving in perpendicular to the motion of the ship and can change its heading,
or direction.

4. HEAVE: defines the up and down motion of a ship as large swells heave Nautilus vertically
on the crests and troughs of waves.

5. SWAY: this sliding motion occurs when the hull of a ship is pushed by the wind or current.
6. SURGE: occurs when Nautilus is being followed by larger swells, which can push the vessel
forward and impact the front to back motion of the ship.
48. Hull. The hull of the ship is the outer skin of the vessel which ensures there is no water ingress inside
it and ensures the watertight integrity of the ship to maintain its floating status. The outer skin known as the hull
of the ship can be of a single layer or of double-layer, i.e. there are two hulls of the ship known as double hull.
The double hull is most commonly found in oil tanker ships.

a. Double Bottom A double bottom is a space between the ship’s inner bottom plating (tank
top) and outer bottom shell plating, running along the bottom of the vessel.

Purpose: Structural strength – adds rigidity to the hull. Protection against grounding damage – if
the outer bottom is punctured, the inner bottom may prevent flooding into cargo holds.
Ballast & liquid storage – double bottom tanks often carry ballast water, fuel oil, or fresh water.

Location:
Found along the keel area, extending to a certain height on the ship’s side (often up to the turn of
the bilge).

Typical Design:
Height: usually 1–2 meters, depending on ship size. Subdivided by longitudinal and transverse
girders into tanks.
b. Double Hull A double hull is a ship construction where the sides, bottom, and sometimes deck
have two layers of watertight hull plating with a void or tank space between them.

Purpose:
o Enhanced safety – provides greater protection against oil spills if outer plating is
breached (especially in tankers, after MARPOL requirements).
o Additional strength – increases overall hull girder stiffness.
o Extra storage – the void can be used for ballast or other liquids.

Location: Includes double bottom plus double sides—extending the second layer up to or near
the main deck.

Typical Application: Required for most modern oil tankers over 5,000 DWT (per MARPOL
Annex I). Also common in bulk carriers, LNG carriers, and some passenger ships.

48. Bulkheads. Vertical partitions in a ship arranged transversely or fore and aft are referred to as
‘bulkheads’. Those bulkheads which are of greatest importance are the main hull transverse and
longitudinal bulkheads dividing the ship into a number of watertight compartments. Other lighter
bulkheads, named ‘minor bulkheads’, which act as screens further subdividing compartments into small
units of accommodation or stores, are of little structural importance. The main hull bulkheads of
sufficient strength are made watertight in order that they may contain any flooding in the event of a
compartment on one side of the bulkhead being bilged. Further they serve as a hull strength member not
only carrying some of the ship’s vertical loading but also resisting any tendency for transverse
deformation of the ship. As a rule the strength of the transverse watertight bulkheads is maintained to the
strength deck which may be above the freeboard deck. Finally each of the main hull bulkheads has often
proved a very effective barrier to the spread of a hold or machinery space fire.
49. Cofferdam. A cofferdam means an empty space provided in a ship so that compartments on each side
have no common boundary; a cofferdam may be located vertically or horizontally. As a rule, a
cofferdam shall be kept gas-tight and must be properly ventilated and of sufficient size to allow proper
inspection, maintenance and safe evacuation.