DIMENSIONING A SHIP

Preface

Before we get deeper into the vast field of naval architecture let us first learn the language of naval architecture. Language of naval architecture means the basic terminologies of naval architecture that we will use frequently in our future topics and it is of vital importance that we know them like the back of our hand. This will not only help for a faster and easier learning but will also make a topic more interesting.

Whenever we want to construct or design something we initiate it from the base or bottom of the structure. For example, if we want to draw a lay out of a building we first start with the base or foundation of the building. In the same way, for understanding and designing a ship we start from the base of the ship, the hull. The hull is the foundation and the most essential part of the ship.

The ship’s hull form determines almost all of its main attributes; its water displacement capacity, its load carrying capacity, its resistance in water, the power needed to propel the ship, its ability to maneuver smoothly and also its seaworthiness. Thus it is of utmost importance that the hull shape is defined with extreme precision and without any ambiguity.

Dissecting a ship’s hull

Let’s assume a symmetrical hull form. A hull form which is symmetrical about fore and aft plane. The hull plane is divided into three sets of mutually orthogonal planes. The horizontal planes are known as the waterplanes and the lines intersecting the planes are known as waterlines. The planes which are at right angle to waterplanes are known as transverse planes or transverse sections. These planes form the buttock lines and also the profile of the hull.

Definitions and terminologies

First let’s learn the definitions of some very important terminologies which are essential from the dimensioning point of view.

Summer load waterline or design waterline: The line that runs along the waterplanes, from aft to the fore of the ship is known as summer load waterline. It is the line at which the ship is generally floating with its load.

Fore perpendicular: It is the vertical line passing through the point of intersection of the summer load line with forward side of the stem.

Aft perpendicular: It is the line perpendicular to the waterplane and passing through the centerline of the rudder pintles .

Length between perpendiculars(lbp): The distance measured along the summer load water plane from fore to aft perpendicular is know as length between perpendiculars.

Length overall (loa): It is the distance measured parallel to the summer load waterline between the extreme points at the forward and the aft. The extreme point at the forward can be taken on the bulbous bow.

Length on the waterline (lwl): It is the distance measured between the intersection points of the bow and the after end with the summer load waterline. In case it is not mentioned, the length of the summer load waterline is taken into consideration as the length of the waterline.

Midship or amidship : The midpoint between the aft and the forward perpendicular is known as midship or amidship. The section passing through this point and which is normal to the waterplane is called midship section. The term ‘beam’ is quoted at amidship. The breath extreme is measured at amidship. It is also known as moulded breadth line.

Few more

Depth: The depth of the ship varies along the length. But the depth of the ship is taken as the distance between the undersides of the deck amidship to the bottom of the keel. You might not get an exact value of depth as the hull is symmetrical and the depth varies thorough out the ship. For example, the depth is greater at stern than at midship.

Width or beam or extreme breadth: The width of the ship is also known as the extreme breadth. It is generally measured at the amidship. It is the greatest distance between the two sides of the ship at the greatest width.

Sheer : It is measured as the rise of the deck towards the stem or stern. It is the height of the deck at the side above the deck at sides amidships.

Camber or round of beam: It can be defined as the side of the deck going from side to the centre of the ship.

Rise of floor or dead rise : At the amidships region, the bottom of the ship is extended out to intersect the moulded breadth line . The rise or the height of this intersection above the keel is known as rise of floor or dead rise.

Tumble home : If the sides of the ship at the amidships is not vertical and if the upper deck beam is less than the waterline, it is said to have tumble home.

Draught: It is the distance between the keel and the waterline at any point along the length of the ship. Moulded draught is measured from the inside of the keel plating.

Trim : The difference between the draughts at forward and aft is known as the trim. Trim is mentioned as by the bow or by the stern depending upon which one is greater.

Freeboard: It is essential in determining the stability of the ship. It is the difference between the depth at side and the draught, i.e. it is the height of the deck above the waterline. Freeboard is generally less at amidships than at bow and stern.