(Ranger Hope © 2008, contains edits of material courtesy of A.N.T.A.  publications.)

Design Requirements

Design Features & Terminology

Principal Parts of a Vessel

Watertight Bulkheads  

Design Requirements                                      

A vessel’s design is influenced by the following factors:

Nature of Service

Area of Operation

Seaworthiness and Stability


Survey Requirements

Personnel Safety

Construction Materials

Commercial Requirements

Nature of Service

A vessel is designed to perform a specific function such as carrying passengers, cargo or fish. It’s size, layout, accommodation, machinery and equipment are all related to the type of service it is meant to provide.

Area of Operation

Generally, a vessel that is required to spend longer periods at sea without replenishment will have a correspondingly larger capacity to carry fuel and stores. Weather conditions also influence design. Frequent rough conditions may require the decks to be located sufficiently high above the water to keep them reasonably dry. The size of the vessel may have to be increased to cope with adverse conditions.

Seaworthiness and Stability

A vessel must be designed to ensure that it is capable of surviving the variety of weather and operating conditions likely to be encountered in its area of operations. This means that under normal operating conditions, the vessel should have enough stability to keep it upright and afloat. Careful attention must be paid to the hull shape, the distribution of weights and the protection of hull openings.


A vessel’s structure should be able to withstand the stresses caused by:

water pressure,

weights on board such as cargo and machinery,

action of wind and waves and

operation of machinery

when carrying out its planned operations.

Survey Requirements

A small vessel must be designed, constructed and operated in accordance with the regulations contained in the Marine Acts of each State.  Much of the detailed legislation is contained in the Uniform Shipping Laws Code (USL), published by the Australian Government Publishing Service. 
A vessel is surveyed by the marine authorities during the building stage, on completion of building, and then periodically throughout its life to ensure that it complies with the regulations.

Personnel Safety

Persons on board must have safe working areas, safe access to and from the working areas and safe accommodation. In the event of accidents and breakdowns at sea, necessary safety measures must be available as per statutory requirements.

Construction Materials

Each type of building material has its own advantages and disadvantages as we shall see later in this section. The choice of building material will influence the carrying capacity, propulsion power requirements and construction method of the vessel.

Commercial Requirements

Commercial requirements dictate that a vessel should be designed so as to keep the construction and operational costs at a level acceptable to its owner.

You should keep these requirements in mind when you study the remainder of this section.


Design Features and Terminology               

In general, vessels subject to survey by a State Authority are covered by the provision of the Uniform Shipping Laws (USL) Code.

Most of these regulations use Measured Length as a reference.

Measured Length - It is the distance from the fore part of the hull to the after part of the hull, taken at the upperside of the uppermost weathertight deck, or, in the case of open vessels, at the height of the gunwale. Figures 1.1 and 1.2 show two examples.

Figure 1.1

Figure 1.2

Length Between Perpendiculars (LBP) (Fig. 1.3) - length from the forward perpendicular to the after perpendicular.  The forward perpendicular is a vertical line drawn through the point where the load waterline cuts the stem.  The after perpendicular is a vertical line drawn at the after end of the rudder post.

Figure 1.3

Length Overall (LOA)  (Fig.  1.3) - the length from the extreme tip of the bow to the aftermost point of the stern.

Freeboard (Fig. 1.3) - the distance from deck to waterline.

Depth (Fig.  1.4) - the depth of a vessel is usually measured at the side and amidships, it is the distance from deck to keel.

Figure 1.4

Beam or Breadth (Fig.  1.4) - is measured at the widest part of the vessel.  It is the greatest width from one side of the vessel to the other.

A vessel in a normally loaded condition is said to be floating at its Load Waterline or Design Waterline (Fig.  1.3).

In that condition, the draft at which the vessel floats is called the Loaded Draught or sometimes the Service Draught. Draught is measured from the waterline to the deepest point of the vessel's hull, usually the underside of the keel.  (Fig.  1.4).

Some vessels have a keel parallel to the design waterline.  Some have a keel at an angle when compared with the design waterline called declivity of keel.

The bow is the region at the front of a vessel.  (Fig.  1.5).

When facing forward, the part of the vessel on the left hand is the port side, the part on the right hand is the starboard side.  (Fig.  1.5).

The stern is the region at the back of a vessel.  (Fig.  1.5).

Figure 1.5

A person moving towards the bow is said to be going forward.  (Fig.  1.6).

A person moving towards the stern is said to going aft.  (Fig.  1.6).

Amidships is the region in the middle of the length.  Frequently this word defines the point at the middle of LBP.

Figure 1.6

When moving along the length of a vessel a person is moving in a longitudinal direction.

When moving across a vessel a person is said to be moving in a transverse direction.

In some vessels the deck line curves forward and aft, this is called sheer.  It aids water run off and contributes to reserve buoyancy.  (Fig.  1.6).

Flat of Bottom - in some vessels, the area of the hull near to the keel is flat, this is called Flat of Bottom.  (Fig.  1.7).

Rise of Floor (Deadrise) - if the bottom of a vessel rises from the centre line to the turn of bilge, there is said to be a Rise of Floor.  (Fig.  1.7).

Bilge - the rounded part of the hull, where the side meets the bottom is called the bilge.  A rounded bilge provides strength and reduces hull stresses.  (Fig.  1.7).

Camber - if the deck of a vessel has an upward curve, it is said to have a camber.  This helps water to run off the deck and reduces deck stresses.  (Fig.  1.7).

Tumblehome - if the sides of a vessel, 'fall-in' towards the centre line as they rise to the deck-edge, the vessel is said to have tumblehome.  This is not very common these days. 

Figure 1.7

Flare - the outward flowing of the bow (sometimes called Flare) forces water outwards and away, promoting deck dryness and assisting the bow to lift over waves.  (See Fig.  1.8).

Figure 1.8

Bow Rake is similar to flare in that it promotes deck dryness by forcing water forward when the bow strikes waves.  See Fig.  1.9.


Principal Parts of a Vessel                            


The main body of a vessel is called the hull.

Regardless of the material used in construction, the layout of the hull is similar in each case. Every vessel has a shell of material which keeps the vessel watertight.  The shell is supported and obtains its strength from a series of internal stiffeners.  In small vessels it is common for the main stiffeners to be in the form of frames, sometimes described as ribs. 

If the main stiffeners of the hull run from side to side, the arrangement is called a transversely framed structure. If the main stiffeners of the hull run fore and aft, the arrangement is called a longitudinally framed structure.

Let us now look at some typical stiffening arrangements of metal and timber hulls.

Metal Hulls

Figures 1.10 and 1.11 show typical sections through steel hulls.

Figure 1.10 Flat Chine Hull

Figure 1.11 Bottom Construction

Study figures 1.10 and 1.11.  Note the shape of the hull.  Figure 1.10 shows a “hard chine” (or Vee shaped) hull.  It is called “hard” because the topside meets the bottom at an angle as opposed to a “soft chine” hull where the topside meets the bottom in a curve.  Figure 1.11 shows the bottom construction of a “round bilge” type hull.

Note how the bottom shell plating is stiffened.  Strength is provided by fitting vertical plates to the bottom shell.  Those fitted transversely are called floors and those fitted longitudinally are called side girders.

Note the location of a centre girder on the keel plate.  This helps in resisting bending of the hull in a longitudinal direction.  Remember that the keel forms the backbone of a vessel’s hull.

Beams support the deck.  Brackets connecting frames and beams help resist the distortion of hull when a vessel rolls.

Figure 1.12 Fore End Construction

Timber Vessels

Study figure 1.13 which shows typical chine hulls, and figure 1.14 which shows typical round bilge type hulls.

Note how various parts are arranged to resist the stresses that we had identified earlier when considering design requirements.  In this respect, the arrangement of structures is similar to a metal hull.

Figure 1.13 Typical Chine Hulls

Figure 1.14 Typical Round Bilge Type Hulls

Figure 1.15 Typical Deadwood Aft

Figure 1.16 Typical Stem Assembly

Watertight Bulkheads                                  


All vessels, except the very smallest of craft, are subdivided internally into watertight compartments by means of vertical partitions called watertight bulkheads.  In vessels which have a measured length of 16 metres or over, a special watertight bulkhead called a Collision Bulkhead is fitted near the bow.  The number and place­ment of other watertight bulkheads is dependent upon the measured length of the vessel.  Vessels 12.5 metres and over in measured length must have a watertight bulkhead at each end of the machinery space except where the machinery space is located at one end of the vessel.

Look at the profile plan of KFV Albatross in fig 1.17.  The dotted lines rising vertically from the keel represent the watertight bulkheads.  The first collision bulkhead is located at frame no. 4.  It rises from the keel to the underside of the foredeck.

The second watertight bulkhead is located at frame no. 6.  It rises from the keel to the underside of the main or freeboard deck.  The space between the two bulkheads is called a cofferdam.  A cofferdam is a void space that separates two tanks.  Normally a cofferdam is only required to separate oil and water tanks.

The space enclosed between the second and third water tight bulkheads is the refrigerated hold.  The engine room space is located between frames 25 and 35.  You will notice that there is a watertight bulkhead at each end of the engine room, making a total of four watertight bulkheads in all.  The bulkhead at the after end of the engine compartment is known as the after peak bulkhead.

Figure 1. 17