TPC (Tons per Centimeter)
In the shipping business, one of the most crucial points is the amount of cargo that can be lifted. The quantity of cargo that will be written in the bill of lading (B/L) is the quantity that will be transported. However, to arrive at the amount, the ship operator must consider the properties of the cargo against the summer deadweight (DWT) of the ship. Summer Deadweight (DWT) of the ship indicates the maximum quantity of cargo that can be lifted. At this quantity, the ship will be down to its summer marks.
One important determinant of the quantity of cargo that can be lifted is the density of the cargo, which is indicated by the stowage factor (SF) of the commodity. Stowage factor (SF) indicates how many cubic meters of space is occupied by one metric ton of a particular type of cargo in a ship’s hold. Stowage factor (SF) is therefore the ratio of space (volume) to the weight of the cargo. Certain heavy cargoes, such as iron ore, have a low stowage factor because they occupy less space per metric ton compared to lighter commodities such as woodchips. Hence, the stowage factor of iron ore is 0.40 (m3/mt), much lower than heavy grains (1.30 m3/mt) and woodchips (2.50 m3/mt). Stowage factors are available in relevant databases.
The next step is to consider is whether the ship has the space to contain a given weight of cargo. For this purpose, shipowners use the grain or bale capacity of the ship, depending on whether the cargo is in bulk or in pallet form, respectively, and divide this figure by the stowage factor of the cargo to be loaded. This is the maximum amount of cargo that can be carried within the ship’s holds. From this figure, other adjustments need to be made to ensure those draft limitations are not violated. Thus, from the available tonnage, deductions are made for
- Constants (freshwater, stores, lubricants, spare parts, even the weight of the crew) and
- Bunkers Remaining on Board (ROB)
It is generally assumed that the ship’s constants will range from 250 to 500 tons depending on size, with smaller vessels of up to 15,000 DWT assuming the lowest end of the range and larger vessels of 350,000 DWT the higher end of this range. If there are draft limitations during the voyage, either because the ship sails into a different load-line zone or because it enters shallow ports or waterways, then the cargo intake calculated must be adjusted accordingly.
TPC (Tons per Centimeter): the number of tons required to sink the ship one centimeter. For instance, if the vessel has a TPC of 60 tons and uses 30 tons of fuel and water a day, then it will reduce its draft by half a centimeter a day.
A ship cannot enter a winter load-line when loaded down to its summer marks, as this will submerge the winter freeboard. Where draft restrictions will come into effect, the summer draft is compared with the available draft, and the difference in centimeters multiplied by the TPC (tons per centimeter). TPC (tons per centimeter) multiplied by the difference in deadweight gives the available deadweight. After making allowance for bunkers, freshwater, and constants, the ship’s DWCC (Deadweight Cargo Capacity) is found. Draft allowances may be made when a ship sails into areas where the density of the water is different from that of saltwater (1.025 gr/cm3). This may occur when a ship enters inland waterways or berths and the water may be a mix of saltwater and freshwater. The allowance is referred to as Brackish Water Allowance (BWA) or Dock Water Allowance (DWA) and the density relative to that of freshwater (1.000 gr/cm3) is calculated using the following formula:
DWA = FWA (1025 – density of dock water)/25
Brackish Water Allowance (BWA) or Dock Water Allowance (DWA) Example:
Assuming density (d) = 1.020 and FWA = 300 mm
BWA mm = FWA (1025 – d)/25
Therefore BWA mm = 300 mm (1025 – 1020)/25
= 300 mm(5)/25 = 60 mm
This is the allowance for brackish water; therefore, the increased draft the ship can load to is 60 mm.