Naval Architecture Terminology and Coefficients of Forms

The ship is defined in terms of its size and the shape. This is required to find out it’s characteristics for stability, resistance, power needed to run the ship at a particular speed, her seaworthiness, it’s maneuverability, load carrying capacity etc.

Read about the terminology that is used in the naval architecture and ship construction. Also know about the coefficients of forms.

Naval Architecture Terminology

These are mainly length, breadth, depth & the draught.
1       Perpendiculars: these are the imaginary vertical lines drawn on a ship to define the points on the hull forward & aft.
2       Forward perpendicular: A line drawn perpendicular to the water line at the point where the forward edge of the stem intersects the summer load line
3       After perpendicular: A line drawn perpendicular to the water line, where the after
edge of the rudder post/centerline of rudder pintle, meets the summer load line
Ship Terminology
4       Length Between Perpendiculars (LPP) : This is the distance between the forward and aft perpendiculars measured at the height of summer load line.
5       Amidships/Midship: This is the point midway between the forward and aft perpendiculars.
6      Length Overall (LOA): The distance between the extreme points of the ship forward and aft.
7       Extreme Breadth: This is the maximum measurement between the extreme points, port and starboard.
8        Moulded Breadth: It is the distance from port to starboard measured from the inside edges of the plating.
9       Extreme Depth: This is defined as the distance from the upper deck to the underside of the keel.
10    Extreme Draught: it is the distance from water line to underside of the keel.
11    Base Line: it is the horizontal line drawn along the top edge of the keel from midships.
12    Moulded Depth: it is the distance from the upper deck to the base line, measured at the midship section.
13    Moulded Draught: it is the distance from the summer load line to the base line, measured at midship section.
14    Draught of Ship: it is the distance of the lowest point of the keel below the water line Special shapes have been given to some portion of the ship from practical point of view to serve some purposes
   1. Camber is provided to drain the water from deck.
   2. Flare is provided at the fore end to direct the waves away from the fore castle.
Naval Architecture Terms


1.       Camber: it is the curvature of the deck in a transverse direction. It is measured between the deck height at the centre and deck height at the ship’s side.
2.       Sheer: This is defined as the curvature of deck, in a longitudinal fore and aft direction. It is measured between the deck heights at midships and the perpendicular point along the deck.
3.       Rise of floor: it is the height of bottom shell plating above the base line. This is moulded base line.
4.       Bilge radius: it is the radius of plating joining the side shell to the bottom shell. It is measured at midships.

5.       Trim: it is the longitudinal inclination of ship, determined by the difference between the forward and the aft draughts. When forward & aft draughts are same, the ship is said to be on an even keel. Down by the head means draught forward is greater. Down by the stern means draught aft is greater.

Coefficient of Forms for Ship

The number of coefficients are used which describe the relation between the ship’s hull form and a surrounding regular shape.

Water plane area coefficient (cw)

It is the ratio of the actual area of the waterplane to the product of the length and breadth of the ship.
              Actual Water Plane Area
   CW = ………………………………………………………..
                    Length X Breadth
Block Coefficient
   CW = …………… (1)
               L x B

Midship Section Area Coefficient (Cm)

It is the ratio of the actual area of the immersed portion of the ship’s midship section to the product of the breadth and the draught.
          Area of Immersed Midship Section
 Cm =
                  Breadth   X
  CM = ———   ————————————— (2)
               B x d

Block Coefficient (Cb)

Block Coefficient defined as the ratio of the volume of displacement (Actual) to the product of the length, breadth and draught. See the figure above
          Actual Volume Of Displacement
  Cb =………………………………………………….
           Length  X Breadth
X  Draught
x  B  x  d

Prismatic Coefficient (Cp)

Prismatic Coefficient is the volume of displacement to the product of the length and the actual area of the immersed portion of the midship section.
                      Volume of Displacement (Actual)
   Cp = ………………………………………………………………….
           Length X Area of Immersed Midship
Prismatic Coefficient, Naval Architecture Terminology


= ………………4)
             L x Am

Wetted Surface Area (Aw)

This is the area of the ship’s hull, which is in contact with the water (surface area of the immersed part of hull). This area can be found out by using the empirical formulae.
FORMULA    S = 1.7 L d + ▼/d ……………… (1)
FORMULA  S = C   Δ L ………………2)
where  s = wetted surface area
C = COEFFICIENT = 2.6 (depends on shape
of the ship)
Note: Cp X Cm = Cb

Similar Figures in Ship Construction

Two lines, planes or bodies are said to be similar when their linear dimensions are in the same ratio.
Example:  Project image of small figure on slide to that of a bigger figure on screen two circles of different diameter. The area of similar figure vary as the square of their corresponding dimensions consider two circles of diameter ‘D’ and ‘d’. Then
 Area of larger circle       D2
    —————————————- = —– —————–(1)
 Area of small circle         d2
IF ‘D’ IS twice ‘d’, then area of former is four times the area of the latter.
A1         L12       B12
—— = —— = ——- ————————————–(2)
A2          L22      B22
Volume of similar bodies varies as the cube of their corresponding dimensions.
Consider two spheres of diameter ‘D’ and ‘d’.
Volume of Larger Sphere    D3
——————————————- = ——- ———- (1)
   Volume of Small Sphere      d3
   If ‘D’ is twice the diameter‘d’, then volume of former is eight times the volume of the latter.
V1     L13       B13  D13
—– = —– =  —— =  —– ———————————(2)
V2     L2 3      B23        D23
Above rules may be applied to similar bodies, no matter what their shape is. Similarly it can be applied to ships of any shape. And is quite useful in their construction of ships, when its prototype is being made.
If L = length of ship, Δ = displacement & S = wetted surface area then
S ∝ L2         …….(1)
Δ ∝ L3         …….  (2)
S ∝ Δ2/3      ……. (3)
Δ ∝ S3/2    ……. (4)
Δ1        L13
—– ∝ —–   ……. (5)
Δ2      L23
S1          L12
—– ∝  —–   ……. (6)
S2       L22
So this was all from this article, hope you liked reading this topic of Ship Terminology and Coefficients of Form.

You Might Like to Read These Posts Too:

Harsha Sharma

I sail with one of the leading shipping companies as engine officer. Loves to brag about the places that i have been too. Love Dogs a lot. Would love to talk to different people out there. Writing is my hobby, and loves to write about what i like. Bikes drives me crazy.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Special Topics For You

Read these

Marine Engineering Practice Questions and Answers-... This is our second article in the Marine Engineering Practice Series which deals in questions related to the troubleshooting of the ship machinery e...
MEO Class 4B Exam Questions and Answer of MEP In this post we have given the list of the questions that were asked in oral examinations of 4B. In this post I have picked some of the famous questi...
Centrifugal Purifiers or Separators Theory, Workin... Learn about centrifugal purifiers working, theory related to their working and about the new ALCAP principal that is used nowadays. Purifiers are on...
Marine Engineering Practice Questions and Answers-... Find the answers to the questions related to troubleshooting of various machineries onboard like hydrophore system, and they come in Marine Engineer...
What is Center of Gravity of Ship and how it Chang... Learn about center of gravity of ship and how does it changes when there is a movement of mass on the ship. Concept of Center of Gravity Center of...
Marine Engineering Practice Question and Answer- P... This is the third article in this series. We have covered topics like Steam, Liner, Control air, Sea Water Line, Expansion tank, Sump, Spanners, O- ...
Types of Marine Boilers and Difference Between Wat... This article is to discuss about the various types of marine boilers that are being used or once used to drive the ships or to generate the steam....
Marine Sewage Treatment Plants, Regulations and Wo... Get to know about the marine sewage treatment plants working, types and regulations concerning the discharging of sewage in to the sea. The importan...
error: Content is protected !!