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