SHIP DIMENSIONS AND FORMS

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The hull form of a ship may be defined by a number of dimensions and

terms which are often referred to during and after building the vessel. An

explanation of the principal terms is given below:

After Perpendicular (AP)

: A perpendicular drawn to the waterline at the point

where the aft side 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.

Forward Perpendicular (FP)

: A perpendicular drawn to the waterline at the

point where the foreside of the stem meets the summer load line.

Length Between Perpendiculars (LBP)

: The length between the forward and aft

perpendiculars measured along the summer load line.

Amidships

: A point midway between the after and forward perpendiculars.

Length Overall (LOA)

: Length of vessel taken over all extremities.

Lloyd’s Length

: Used for obtaining scantlings if the vessel is classed with

Lloyd’s Register. It is the same as length between perpendiculars except that

it must not be less than 96 per cent and need not be more than 97 per cent of

the extreme length on the summer load line. If the ship has an unusual stem

or stern arrangement the length is given special consideration.

Register Length

is the length of ship measured from the fore-side of

the head of the stem to the aft side of the head of the stern post or, in the

case of a ship not having a stern post, to the fore-side of the rudder stock. If

the ship does not have a stern post or a rudder stock, the after terminal is

taken to be the aftermost part of the transom or stern of the ship. This

length is the official length in the register of ships maintained by the flag

state and appears on official documents relating to ownership and other

matters concerning the business of the ship. Another important length

measurement is what might be referred to as the

IMO Length

. This length is

found in various international conventions such as the Load Line, Tonnage

and SOLAS conventions and determines the application of requirements of

those conventions to a ship. It is defined as 96 per cent of the total length on

a waterline at 85 per cent of the least moulded depth measured from the

top of keel, or the length from the fore-side of stem to the axis of rudder

stock on that waterline, if that is greater. In ships designed with a rake of keel

the waterline on which this length is measured is taken parallel to the design

waterline.

Moulded dimensions are often referred to; these are taken to the inside

of plating on a steel ship.

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

: Measured from the base line to the summer load line at the

midship section.

Moulded Depth

: Measured from the base line to the heel of the upper deck

beam at the ship’s side amidships.

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.

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.

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 amid-

ships.

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.

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.

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 a useful

dimension to know when dry-docking.

Tumblehome

: The inward curvature of the side shell above the summer

load line.

Flare

: The outward curvature of the side shell above the waterline. It promotes

dryness and is therefore associated with the fore end of ship.

Stem Rake

: Inclination of the stem line from the vertical.

Keel Rake

: Inclination of the keel line from the horizontal. Trawlers and

tugs often have keels raked aft to give greater depth aft where the propeller

diameter is proportionately larger in this type of vessel. Small craft occa-

sionally have forward rake of keel to bring propellers above the line of keel.

Tween Deck Height

: Vertical distance between adjacent decks measured from

the tops of deck beams at ship side