EFFECT OF DENSITY ON DRAFT AND DISPLACEMENT
When a ship goes from SW to FW, her draft would increase and vice versa. This can be illustrated by a simple example. Consider a ship of 10000 tonnes displacement.
W = u/w volume x density of water displaced.
In salt water: 10000 = V sw x 1.025
or Vsw = 10000 = 9756 m3
1.025
Underwater volume in SW = 9756 m3
In fresh water: 10000 = V FW x 1
or VFW = 10000 m3
Underwater volume in FW = 10000 m3
From the foregoing example it is clear that when a ship goes from SW to FW her underwater volume (and hence her draft) increases, and vice versa, though her displacement is constant.
FRESH WATER ALLOWANCE
FWA is the increase in draft when a ship goes from SW to FW and vice versa.
FWA = W
40 TPC
W is the displacement of the ship in salt water, expressed in tonnes.
TPC is the tonnes per centimetre immersion in salt water
FWA is the fresh water allowance in centimetres.
DWA is the increase in draft when a ship goes from saltwater to dockwater, and vice versa, where the dockwater is neither fresh not salt i.e., RD between 1 and 1.025. When loading in a dock, the ship can immerse her loadline by the DWA so that when she goes to sea, she would rise to her appropriate loadline.
FWA of a ship usually increases as draft increases. This is because W depends on underwater volume whereas TPC depends on water plane area. As draft increases, both Wand TPC increase but W increases at a faster rate. Hence FW A, as calculated by the foregoing formula, also increases as draft increases. The FWA calculated, by the foregoing formula, for the summer load condition is called the FWA of the ship.This FWA is mentioned in the loadline certificate and is considered constant for those loadlines marked on the ship's sides - T, S, W and WNA. When a ship is loading down to her marks in FW, she can immerse her loadline by the FWA of the ship so that when she goes to SW, she would rise to her appropriate loadline.
If it is desired to find the FW draft of the ship when she is not immersed upto the loadline marked on the ship's sides, the FW A must be calculated by the formula and added to the SW draft of the ship at that time.
When a ship goes from SW to FW (change of RD of .0 25) she increases her draft by FWA. So for any change of RD between 1.025 and 1.000, linear interpolation may be done.
Change of draft
= change of RD x FWA
.025
The change of draft, so obtained, would be in the same units as the FWA - mm, cm or m.
This formula holds good for any change of RD. However, when the change of draft is calculated between SW and DW, it is called DWA. The term dock water is used here only symbolically to represent water whose RD is between 1.000 and 1.025 and, for stability purposes, includes the water of rivers, harbours, etc., even though they may not have enclosed docks.
Part II: When draft is constant
When a ship floats at the same draft, on different occasions, in water of different RD, her displacement each time would be different. This is illustrated by a simple example.
Suppose the underwater volume of a certain ship at 7 m draft is 14000 m3.
In SW, at 7 m draft, W=14000 x 1.025=14350 t.
In FW, at 7 m draft, W=14000 x 1.000=14000 t.
RD 1.01, at 7 m draft, W= 14000 x 1.010 = 14140 t.
RD 1.02, at 7 m draft, W= 14000 x 1.020 = 14280 t.
The
Vertical distance between the upper edges of S and T and also between S and W
is 1/48 of the summer draft of the vessel. The WNA mark, if applicable is
exactly 50 mm below the W mark.
When a ship goes from SW to FW, her draft would increase and vice versa. This can be illustrated by a simple example. Consider a ship of 10000 tonnes displacement.
W = u/w volume x density of water displaced.
In salt water: 10000 = V sw x 1.025
or Vsw = 10000 = 9756 m3
1.025
Underwater volume in SW = 9756 m3
In fresh water: 10000 = V FW x 1
or VFW = 10000 m3
Underwater volume in FW = 10000 m3
From the foregoing example it is clear that when a ship goes from SW to FW her underwater volume (and hence her draft) increases, and vice versa, though her displacement is constant.
FRESH WATER ALLOWANCE
FWA is the increase in draft when a ship goes from SW to FW and vice versa.
FWA = W
40 TPC
W is the displacement of the ship in salt water, expressed in tonnes.
TPC is the tonnes per centimetre immersion in salt water
FWA is the fresh water allowance in centimetres.
DWA is the increase in draft when a ship goes from saltwater to dockwater, and vice versa, where the dockwater is neither fresh not salt i.e., RD between 1 and 1.025. When loading in a dock, the ship can immerse her loadline by the DWA so that when she goes to sea, she would rise to her appropriate loadline.
FWA of a ship usually increases as draft increases. This is because W depends on underwater volume whereas TPC depends on water plane area. As draft increases, both Wand TPC increase but W increases at a faster rate. Hence FW A, as calculated by the foregoing formula, also increases as draft increases. The FWA calculated, by the foregoing formula, for the summer load condition is called the FWA of the ship.This FWA is mentioned in the loadline certificate and is considered constant for those loadlines marked on the ship's sides - T, S, W and WNA. When a ship is loading down to her marks in FW, she can immerse her loadline by the FWA of the ship so that when she goes to SW, she would rise to her appropriate loadline.
If it is desired to find the FW draft of the ship when she is not immersed upto the loadline marked on the ship's sides, the FW A must be calculated by the formula and added to the SW draft of the ship at that time.
When a ship goes from SW to FW (change of RD of .0 25) she increases her draft by FWA. So for any change of RD between 1.025 and 1.000, linear interpolation may be done.
Change of draft
= change of RD x FWA
.025
The change of draft, so obtained, would be in the same units as the FWA - mm, cm or m.
This formula holds good for any change of RD. However, when the change of draft is calculated between SW and DW, it is called DWA. The term dock water is used here only symbolically to represent water whose RD is between 1.000 and 1.025 and, for stability purposes, includes the water of rivers, harbours, etc., even though they may not have enclosed docks.
Part II: When draft is constant
When a ship floats at the same draft, on different occasions, in water of different RD, her displacement each time would be different. This is illustrated by a simple example.
Suppose the underwater volume of a certain ship at 7 m draft is 14000 m3.
In SW, at 7 m draft, W=14000 x 1.025=14350 t.
In FW, at 7 m draft, W=14000 x 1.000=14000 t.
RD 1.01, at 7 m draft, W= 14000 x 1.010 = 14140 t.
RD 1.02, at 7 m draft, W= 14000 x 1.020 = 14280 t.
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