Chapter five

Group weights, water draft, air draft and density

Lightweight• This is the weight of the ship itself when

completely empty, with boilers topped up to working level.

• It is made up of steel weight, wood and outfit weight, and the machinery weight.

• This lightweight is evaluated by conducting an inclining experiment normally just prior to delivery of the new vessel.

• Over the years, this value will change

Deadweight• This is the weight that a ship carries.

• It can be made up of oil fuel, fresh water, stores, lubricating oil, water ballast, crew and effects, cargo and passengers.

• This deadweight will vary, depending on how much the ship is loaded between light ballast and fully-loaded departure conditions.

Displacement

This is the weight of the volume of water that the ship displaces.

Displacement = Lightweight + Deadweight

Displacement curves

A displacement curve is one from which the displacement of the ship at any particular draft can be found, and vice versa.

The draft scale is plotted on the vertical axis and the scale of displacements on a horizontal axis.

Water draft• This is the vertical distance from the waterline

down to the keel. • If it is to the top of the keel, then it is draft

moulded. • If it is to the bottom of the keel, then it is draft

extreme. • Draft moulded is used mainly by Naval

Architects. • Draft extreme is used mainly by masters, mates,

port authorities and dry-dock personnel.

Water draft

Air draft• This is the quoted vertical distance from

the waterline to the highest point on the ship when at zero forward speed.

• It indicates the ability of a ship to pass under a bridge spanning a waterway that forms part of the intended route.

Effect of change of density when the displacement is constant

• When a ship moves from water of one density to water of another density, without there being a change in her mass, the draft will change.

• This will happen because the ship must displace the same mass of water in each case. Since the density of the water has changed, the volume of water displaced must also change.

Mass = Volume X DensityIf the density of the water increases, then the volume of water displaced must decrease to keep the mass of water displaced constant, and vice versa.

Effect of change of density when the displacement is constant

The effect on box-shaped vesselsNew mass of water displaced = Old mass of water displacedNew volume X New density = Old volume X Old density

New volume Old density =Old volume New density

But volume = L x B x Draft

L x B x New draft Old density = L x B x Old draft New density

New draft Old density

= Old draft New density

The effect on box-shaped vessels

The effect on ship-shaped vessels

New displacement = Old displacementNew volume X New density = Old volume X Old density

New volume Old density =Old volume New density

With ship shapes this formula should not be simplified further as it was in the case of a box shape because the underwater volume is not rectangular.

To find the change in draft of a ship shape due to change of density aquantity known as the ‘Fresh Water Allowance’ must be known.

On a ship shaped vessel:On a ship shaped vessel:

• For a ship shaped vessel the formula is not applicable

• But to find the change in draft due to the change in density,

• a quantity known as the fresh water fresh water allowanceallowance must be known

The Fresh water allowanceThe Fresh water allowance

• It is the number of millimeters by which the mean draft changes when the ship passes from salt water to fresh water and vice versa.

• It is found by the formula:

• FWA mm = Displacement tons

4 TPCTPC is the mass to be loaded or discharged to change

ship’s mean draft by one cm.

TPC = 1.025 x WPA /100

The Fresh Water Allowance

300 mm

25 mm

540 mm

230 mm

300 mm

450 mm

FWA

S

WWNA

T

F

TF

1000 Kg/m3

1025 Kg/m3

ship’s load line marks

The centre of the disc is at a distance below the deck line equal to the ship’s

statutory freeboard.

Then 540 mm forward of the disc is a vertical line 25 mm thick, with horizontal

lines measuring 230 x 25 mm on each side of it.

The upper edge of the one marked ‘S’ is in line with the horizontal line through

the disc and indicates the draft to which the ship may be loaded when floating in

salt water in a Summer Zone.

Above this line and pointing aft is another line marked ‘F’, the upper edge of

which indicates the draft to which the ship may be loaded when floating in fresh

water in a Summer Zone.

If loaded to this draft in fresh water the ship will automatically rise to ‘S’ when

she passes into salt water.

The perpendicular distance in millimeters between the upper edges of these

two lines is therefore the ship’s Fresh Water Allowance.

The Dock Water Allowance

The Dock Water Allowance

Effect of density on displacement when the draft is constant

Should the density of the water in which a ship floats be changed without the ship altering her draft, then the mass of water displaced must have changed. The change in the mass of water displaced may have been brought about by bunkers and stores being loaded or consumed during a sea passage, or by cargo being loaded or discharged.

Effect of density on displacement when the draft is constant

Effect of density on displacement when the draft is constant

Effect of density on :

DraftWhen displacement is constant

Displacement When draft is constant

Ship shapedFWA (mm.) = displacement 4 TPC

Box shaped

New draft = Old density

Old draft New density

New disp. = New density

Old disp. Old density