Understanding ship terminologies and their definitions is important for effective communication and technical understandings.
Table of Contents
Ship Terminologies
Understanding basic ship terminology is essential for anyone involved in ship design, operation, or regulation. Let’s explain into some fundamental terms:
- Length Between Perpendiculars (LBP or LPP): This measurement is the distance along the summer load waterline from the aft to the fore perpendicular of the hull. The aft perpendicular typically passes through the rudder stock, while the fore perpendicular intersects the forward side of the stem with the summer load waterline.
- Length Overall (LOA): LOA is the total distance between the extreme points forward and aft, measured parallel to the summer (or design) waterline. This includes the entire length of the ship, from the furthest point forward, which may be the raked stem or a bulbous bow, to the furthest point aft.
- Design Waterline Length (IWL): This refers to the length on the summer load waterline, where the ship floats between the intersections of the bow and aft end with the waterline.
Empirical formulas are often used for estimating ship length based on statistical analyses of existing ships. These formulas provide approximations for various ship dimensions. For instance, Papanikolaou (2014) and Tupper (2013) offer formulas derived from data analysis. These formulas aid in estimating ship length accurately for design and planning purposes.
Ayre’s formula
Schneekluth Formula is Ayre’s formula. Ayre’s formula is a method used in naval architecture to estimate the length of a ship based on its displacement.
where, V is the ship speed in Knots, ∇ is the displacement in ݉ଷ and L is the length in meter.
Posdunine and V. Lammeren formula:
The Posdunine and V. Lammeren formula is another method used in naval architecture to estimate the length of a ship’s waterline.
C=7.62 all types
= 7.16 cargo ships
= 7.32 fast twin-screw ships
= 7.92 fast passenger ships
Völker’s formula
Völker’s formula provides a method for estimating the length-to-displacement ratio (L/∇1/3) of a ship, where L is the ship’s length in meters, ∇ is the displacement in cubic meters, and V is the ship’s speed in meters per second. The formula is given by:
Schneekluth Formula
One of ship terminologies is Schneekluth Formula. The Schneekluth Formula is utilized to estimate the “length of minimum building cost” for a ship. The formula is expressed as:
Ship Breadth (or beam, B)
Commonly used definitions for Ship Breadth (or beam, ( B )) of the ship are:
- Moulded Beam: This is the greatest distance between the inside of plating on the two sides of the ship at the greatest width at the chosen section.
- Breadth Extreme: Measured to the outside of plating, this definition also takes into account any over-hangs or flare.
Variations in breadth for a particular ship design may result in:
- Changes in ship resistance, necessitating an optimal ratio of length to breadth B/T and L/B.
- Potential increases in production costs for greater breadth (( B )).
- Changes in stability criteria, including roll amplitude and acceleration.
- Draught changes – for instance, smaller draft (( T )) if B x T ܶ = ܿConstant while B is increased.
Ship Depth (or moulded depth, ( D )) varies along the length but is usually quoted for amidships. It is measured from the underside of the deck plating at the ship’s side to the top of the inner flat keel plate. Unless otherwise specified, the depth is to the uppermost continuous deck. ( D ) is used to determine the ship’s volume and freeboard, and it also influences longitudinal strength.
Ship Draught
One of ship terminologies is Ship Draught (or molded draft, ( T )) is the distance from the keel to the surface of the water, also known as the waterline. Mean draft Tm is the average of the bow Tfwd and stern drafts Taft at the perpendiculars. Mathematically, mean draft is typically calculated as:
Given that the draught must relate to the displacement equation, a large draught benefits from low resistance and offers greater freedom in propeller design and the selection of large propeller clearances.
Camber refers to the rise of the deck as one moves from the side to the center (see Figure 2-3). Decks are cambered to facilitate water runoff, typically seen on weather decks. To streamline construction and reduce costs, camber is usually applied only to weather decks, and a straight-line camber often replaces the older parabolic curve.
The rise of floor, also known as deadrise, refers to the bottom of a ship in the midships region. It is generally flat but not necessarily horizontal. If we extend the line of the bottom to intersect the molded breadth line, the height of this intersection above the keel is termed the rise of floor or deadrise. Most ships have a flat keel, and the extent to which this extends athwartships is called the flat of keel or bottom.
Flare is a characteristic of the forward sections of many ships’ bows. On a flared bow, the half-breadths increase as the distance above the keel increases. Flare enhances a ship’s wave-piercing performance, resistance to roll, and increases the available deck space.
Conversely, tumblehome is the opposite of flare, where the beam at the deck is smaller than the beam at the waterline. In some ships, particularly those with unconventional designs, the sides are not vertical at amidships. If the upper deck beam is less than that at the waterline, it is said to have tumblehome.
Freeboard is the height of the deck at the side above the waterline and is determined by the difference between the depth at the side and the draught. It plays a crucial role in intact stability, especially at large angles, and is also important for damage stability situations where reserve buoyancy matters. Sheer, which refers to the curvature of the deck along the length of the ship, helps distribute freeboard and reduces deck wetness in rough weather conditions.
The calculation of freeboard is based on regulations like the International Load Line Convention (ILLC) outlined in the International Maritime Organization’s Safety of Life at Sea (SOLAS) convention. The summer freeboard is measured from the top of the freeboard deck to a line passing through the center of a circle on the ship’s side, known as the Plimsoll mark. Load line marks, representing maximum allowed draught in different conditions, are also painted on the ship’s side.
Determining the freeboard for a specific ship is complex and depends on various factors such as ship type, length, shear, superstructure, and bow height. Typically, the process involves estimating a preliminary freeboard from standardized tables, correcting it based on the ship’s characteristics, applying additional corrections for deck shear and superstructure, and ensuring compliance with minimum bow height and reserve buoyancy requirements. This results in the minimum summer freeboard required for the ship.
Displacement and tonnage
Displacement refers to the mass or weight of a ship, typically measured in tonnes or kilonewtons (KN). It varies depending on the ship’s draft, which is the depth of the ship’s submerged portion. Different loading conditions result in different displacements, with the fully loaded condition often studied for performance analysis. Key nomenclature is:
Deadweight represents a ship’s profitable cargo-carrying capacity, essentially defining its earning potential. It is the difference between the ship’s displacement and its lightship weight, which includes the weight of the hull, machinery, and outfitting. Cargo deadweight specifically refers to the weight of the cargo a ship can carry, while displacement encompasses both deadweight and lightweight.
Tonnage, on the other hand, is a volume measure. Traditionally, it referred to about 100 cubic feet (2.83 cubic meters). Two commonly used terms for tonnage are Gross Tonnage (GT) and Net Tonnage (NT). GT is based on the volume of all enclosed spaces on the ship, representing its overall size. NT, on the other hand, considers the volume of cargo and passenger spaces, adjusted by a coefficient to represent the ship’s carrying capacity.
The empirical equation used to calculate tonnage involves multiplying the volume of the ship’s enclosed spaces by a coefficient.
NT = k x GT
where ݇ k = 0.3 – 0.5 for Container Ships and 0.5 for other ships.
Fineness coefficients
In naval architecture, hull forms are compared using coefficients known as fineness coefficients. These coefficients indicate the fullness of a vessel’s hull form, which can range from very rounded ends to sharp knife-like ends. The choice of fineness coefficients depends on the ship’s design speed and the anticipated seakeeping conditions.
As a general rule, ships designed for higher speeds tend to have finer hull forms. This means that they have narrower and more streamlined shapes to reduce resistance and improve performance. On the other hand, vessels intended for lower speeds may have fuller hulls to enhance stability and carrying capacity.
Block coefficient CB
One of ship terminologies is block coefficient. CB is a fundamental parameter in naval architecture, denoting the ratio of the volume of displacement to the product of the ship’s length, breadth, and draught. This coefficient offers crucial insights into the hull’s shape and buoyancy distribution, influencing various aspects of ship design and performance.
The formula for calculating the block coefficient is as follows:
Midship Coefficient CM
One of Ship Terminologies is Midship Coefficient. CM is a key parameter in naval architecture that quantifies the shape of the ship’s midship section. It is defined as the ratio of the area of the immersed portion of the midship section to the product of the ship’s breadth and draught.
The formula to calculate the Midship Coefficient is as follows:
Prismatic Coefficient CP
One of important ship terminologies is The Prismatic Coefficient. CP is a fundamental parameter in naval architecture that characterizes the shape of a ship’s hull. It is defined as the ratio of the volume of displacement to the product of the ship’s length and the immersed portion of the midship section.
The formula to calculate the Prismatic Coefficient is as follows:
Waterplane Area Coefficient CW
One of the Ship Terminologies Waterplane Area Coefficient. CW is a significant parameter in naval architecture that characterizes the shape of a ship’s waterplane, which is the area of the cross-section of the ship’s hull at the waterline. It is defined as the ratio of the area of the waterplane to the product of the ship’s length and breadth.
The formula to calculate the Waterplane Area Coefficient is as follows:
Typical fineness coefficients for different ship types.
Deadweight coefficient CD
One of ship terminologies is The Deadweight Coefficient. CD is a significant parameter used in naval architecture to evaluate the efficiency and capacity of a ship. It is defined as the ratio of the ship’s deadweight to its displacement:
Where:
- Deadweight represents the total weight that a ship can carry, including cargo, fuel, crew, and other supplies.
- Displacement refers to the mass or weight of the ship, including all its components, when it is afloat in water.
Typical values of the Deadweight Coefficient
Speed Parameters.
Speed parameters play a crucial role in understanding a ship’s performance, particularly in terms of its resistance and propulsion characteristics. One of the fundamental speed parameters used in naval architecture is the Froude Number (Fr). The Froude Number is a dimensionless quantity that describes the ratio of a ship’s speed to its length.
Mathematically, the Froude Number (Fr) is calculated as:
Fn=V / gL
Where:
- V is the velocity of the ship.
- g is the acceleration due to gravity.
- L is the characteristic length of the ship, typically its overall length.
Slenderness coefficients
One of Ship Terminologies are Slenderness coefficients are important parameters in naval architecture as they describe the relationship between a ship’s displacement and its hull length. There are several slenderness coefficients commonly used in ship design and analysis:
In the above equations Cv and M are non-dimensional and therefore useful in general design.
