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 Density If 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 vessels New mass of water displaced = Old mass of water displaced New 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 displacement New 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 a quantity known as the ‘Fresh Water Allowance’ must be known.

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, fresh water allowancea quantity known as the fresh water allowance must be known

The 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 TPC TPC is the mass to be loaded or discharged to change ship’s mean draft by one cm. TPC = x WPA /100

The Fresh Water Allowance

300 mm 25 mm 540 mm 230 mm 300 mm 450 mm FWA S W WNA T F TF 1000 Kg/m Kg/m 3 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

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 : Draft When displacement is constant Displacement When draft is constant Ship shaped FWA (mm.) = displacement 4 TPC Box shaped New draft = Old density Old draft New density New disp. = New density Old disp. Old density