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

SHIP CONTRACTS

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The successful tendering shipbuilder will prepare a building specification

for approval by the owner or his representative which will form part of the

contract between the two parties and thus have legal status. This technical

specification will normally include the following information:

Brief description and essential qualities and characteristics of ship.

Principal dimensions.

Deadweight, cargo and tank capacities, etc.

Speed and power requirements.

Stability requirements.

Quality and standard of workmanship.

Survey and certificates.

Accommodation details.

Trial conditions.

Equipment and fittings.

Machinery details, including the electrical installation, will normally be

produced as a separate section of the specification.

Most shipbuilding contracts are based on one of a number of standard

forms of contract which have been established to obtain some uniformity in

the contract relationships between builders and purchasers. Three of the

most common standard forms of contract have been established by:

1. AWES—Association of West European Shipbuilders.

2. MARAD Maritime Administration, USA.

3. SAJ Shipowners Association of Japan.

The AWES standard form of contract includes:

1. Subject of contract (vessel details, etc.).

2. Inspection and approval.

3. Modifications.

4. Trials.

5. Guarantee (speed, capacity, fuel consumption).

6. Delivery of vessel.

7. Price.

8. Property (rights to specification, plans, etc.).

9. Insurance.

10. Defaults by the purchaser.

11. Defaults by the contractor.

12. Guarantee (after delivery).

13. Contract expenses.

14. Patents.

15. Reference to expert and arbitration.

16. Conditions for contract to become effective.

17. Legal domicile (of purchaser).

18. Assignment (transfer of purchaser’s rights to third party).

Irrespective of the source of the owner’s funds for purchasing the ship

payment to the shipbuilder is usually made as progress payments which are

stipulated in the contract under item 7 above. A typical payment schedule

may have been as follows:

10 per cent on signing contract.

10 per cent on arrival of materials on site.

10 per cent on keel laying.

20 per cent on launching.

50 per cent on delivery.

Given modern construction techniques, where the shipbuilder’s cash

flow during the building cycle can be very different from that indicated

above with traditional building methods, the shipbuilder will probably

prefer payments to be tied to different key events. Also of concern to the

shipbuilder employing modern building procedures is item 3 in the standard

form of contract where modifications called for at a late date by the owner

can have a dramatic effect on costs and delivery date given the detail now

introduced at an early stage of the fabrication process.

BASIC DESIGN OF THE SHIP

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The dimensions are primarily influenced by the cargo carrying capacity

of the vessel. In the case of the passenger vessel, dimensions are influenced

by the height and length of superstructure containing the accommodation.

Length where not specified as a maximum should be a minimum consistent

with the required speed and hull form. Increase of length produces higher

longitudinal bending stresses requiring additional strengthening and a

greater displacement for the same cargo weight. Breadth may be such as to

provide adequate transverse stability. A minimum depth is controlled by

the draft plus a statutory freeboard; but an increase in depth will result in

a reduction of the longitudinal bending stresses, providing an increase in

strength, or allowing a reduction in scantlings. Increased depth is therefore

preferred to increased length. Draft is often limited by area of operation

but if it can be increased to give a greater depth this can be an advantage.

Many vessels are required to make passages through various canals and

this will place a limitation on the dimensions. The Suez Canal has a draft

limit, locks in the Panama Canal and St. Lawrence Seaway limit length,

beam and draft. In the Manchester Ship Canal locks place limitations on

the main dimensions and there is also a limitation on the height above the

water-line because of bridges.

2. Displacement is made up of lightweight plus deadweight. The light-

weight is the weight of vessel as built, including boiler water, lubricating oil,

and cooling water system. Deadweight is the difference between the light-

weight and loaded displacement, i.e. it is the weight of cargo plus weights of

fuel, stores, water ballast, fresh water, crew and passengers, and baggage.

When carrying weight cargoes (e.g. ore) it is desirable to keep the lightweight

as small as possible consistent with adequate strength. Since only cargo

weight of the total deadweight is earning capital, other items should be kept

to a minimum as long as the vessel fulfils its commitments.

3. In determining the dimensions statical stability is kept in mind in order

to ensure that this is sufficient in all possible conditions of loading. Beam and

depth are the main influences. Statutory freeboard and sheer are important

together with the weight distribution in arranging the vessel’s layout.

4. Propulsive performance involves ensuring that the vessel attains the

required speeds. The hull form is such that it economically offers a minimum

resistance to motion so that a minimum power with economically lightest

machinery is installed without losing the specified cargo capacity.

A service speed is the average speed at sea with normal service power

and loading under average weather conditions. A trial speed is the average

speed obtained using the maximum power over a measured course in calm

weather with a clean hull and specified load condition. This speed may be a

knot or so more than the service speed.

Unless a hull form similar to that of a known performance vessel is used,

tank tests of a model hull are generally specified nowadays. These provide

the designer with a range of speeds and corresponding powers for the hull

form, and may suggest modifications to the form. Published data from

accumulated ship records and hull tests may be used to prepare the hull

form initially.

The owner may often specify the type and make of main propulsion

machinery installation with which their operating personnel are familiar.