It is the required tonnes to cause the ship sink or rise by one centimeter (1cm) from any mean Draught.

As per the definition of TPC Required Weight = w tonnes

Hence;

Factors affecting the TPC:

 1. Water Plane Area (A) : (WPA)

WPA ∝ TPC

As WPA increases, the corresponding magnitude of thrust as Buoyancy force increases i.e. in other sense we can say that opposing force increases, hence the magnitude of the TPC must be increased

2. Draught:

When a vessel is floating in the water the distance from the underside of the hull to the water surface is called “Draught”.

  Draught ∝ TPC

As, Draught increases, corresponding value of Buoyancy have increased. Hence, “TPC” must be increased.3. Under Water Volume (UMV):

                                                       TPC ∝ UWV4. Buoyancy Force (Up thrust Force):

5. Deadweight (DWT):

i.e. Total weight of the items, for example, cargo, Fresh water, Ballast water, passengers, bunkers, provisions, spare parts for Crew effects is called DWT.

DWT ∝ TPC

6. Density:

A material’s density is defined as its mass per unit volume.

7. Temperature (T) :

It is a physical quantity which measures the hotness or coldness of the body.

i.e.   

GRAPHS FOR DIFFERENT TYPES OF SHIPS

1. The area of the water plane of a box-shaped vessel is the same for all draughts if the trim is constant and so the TPC will also be the same for all draught.

2. In the case of the ship the area of the water-plane is not constant for all draughts and therefore the TPC will reduce at lower draughts.

FWA

(FRESH WATER ALLOWANCE)

It is the difference in draught or change in the draught (▲ D = D₁ – D₂), when the ship is going from FW (Fresh water) to SW (Sea Water) i.e. in this case ship must “Rise”. Due to change in density or change in magnitude in Buoyancy Force or thrust force.

If the ship is going from SW (Sea Water) to FW (Fresh water) i.e. in this case ship must “Sink” due to change in magnitude of Buoyancy force or thrust force (change in density).

UNIT: mm / cm

Case 1) In case of FW:

Total volume = (V + v) m³

Factors affecting the FWA

(Fresh Water Allowance)

1) Total Displacement (W)

2) TPC

• WPA (Water Plane Area)
• Draught
• UMV (Under Water Volume)
• Buoyancy force or Thrust Force
• Density
• Temperature