Cargo Calculation Work

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Loading cargoes on board involves ensuring that the space available is used most efficiently. It is therefore first necessary to work out the spaces available, tonnage that can be loaded and the limitations that are paced by the cargo itself. The module takes into account the mathematics involved in the process.

Grain and Bale Spaces/Capacities

The internal volume of a vessel is measured in two ways. 

  1. The total internal volume, measured from shell plating to shell plating and from tank top to under deck is considered grain space. This space is not only associated with the carriage of grain, as such, but with any form of bulk cargo which would stow similarly, that is to say, completely filling the space.
  2. The internal volume of a compartment measured from the inner edges of the frames and from tank top to the lower edge of the beams is considered as ‘Bale Space’. 

It is obvious that a solid cargo can be stowed only up to the limit of the.frames and beams; therefore, when measuring for general cargo, it is the bale capacity, which is taken into consideration.

bale capacity

Stowage Factor

For successful loading, a vessel must utilise every cubic foot of space to the best advantage, with due regard to the necessary care and attention to the condition of stowage. Thus freight earning capability of the vessel is kept at maximum. 

All vessels are provided with capacity plans, which set out the volume content of each compartment and it is important to arrive at a computation of the space which any particular consignment will require and thus be able to determine the remaining space which is available for further cargo. 

To do this it is necessary to know the amount of space, which each ton of a commodity will require. 

This will differ widely with different goods according to their ‘light’ or ‘heavy’ nature. 

The above figure, which expresses the amount of space in cubic feet, which a ton of any particular cargo will require, is known as its ‘Stowage Factor’. 

The above figure does not express the actual measurement of a ton of cargo, but takes into consideration the necessity for dunnage and the form and design of the packages. It is, in fact, the gross amount of space needed for the actual stowage of a ton of the particular cargo in question. 

It is unreasonable to expect the stowage factor to apply with equal success to subsequent voyages. Latitude must be allowed in order to cope up with the variable factors always possible and generally present. These include the structural design of the compartments, the fact that some cargoes are irregular or nonuniform in size with various shipments, the manner in which a commodity is packed and, in case of bagged cargo, whether the bags are slack or tightly filled. 

Experience is the best guide as to the stowage factor of a commodity and on general trades it is the policy of a cargo of to check the spaces required for particular goods and so be able to tabulate a reliable list of stowage factors. Most lists given in books are based upon experience and this explains the reason why a commodity may be given a variable stowage factor, for example, from 69 to 75 cubic feet per ton. 

A good knowledge of the use of the stowage factors is necessary in order to make economic use of each available space unit. 

On Board Distribution

The distribution of cargo covers not only its movements to and from a ship. but also its place within the vessel, this aspect of involving the safety of the ship and crew, the effective utilisation of space, the assessments of standards desirable in the use of equipment and the conformity to regulations. 

It involves four basics in management practice viz. 

  • Planning, 
  • Forecasting 
  • Co-ordinating and 
  • Controlling. 

Cargo Plans

Cargo plans are known as stowage plans and may be described as a plan of the stowage of the cargo in the vessel. The outline of a cargo plan shows the vessel in profile and the various decks in plan formation, so that it is possible to show the general distribution of the cargo within the vessel and, also by means of the deck plans, its actual disposition. 

Cargo plans are compiled from the data which each cargo officer should enter in his cargo note book and from the detailed information contained in the mate’s receipts or boat notes. 

The method and manner in which a cargo plan is compiled depends, to a large extent, upon the individual ideas of the responsible officer. There is no standard profile, or arrangement; much will depend upon the ingenuity of the officer . preparing the plan. ‘Different ships, different long splices’ is a very true expression and although different shipping companies may stress their own ideas, the essential feature common to all, is that of making it possible to discharge the cargo with as little delay as possible. 

 A cargo plan, in its simplest term, is a diagrammatic picture of the distributions of cargo within the holds/spaces of a ship- picture being the operative term. it serves relationship/proximity to other loads. It has, however, a useful ancillary purpose… the compiling of a ‘Master Plan’ whilst the loading is proceeding. Such that the Cargo Officer can supervise the loading having regard to space available and the type of cargo ‘in situ’ available for loading or further consignments yet to arrive ex ship, or to be picked up at a further port of loading. 

The cargo plan need not necessarily be to scale. On the other hand it should not be put together carelessly, for if this were so the ‘picture’ to be aimed for would be difficult of interpretation. There should be a degree of accuracy in showing the  place of loads, preferably with weights and capacities if possible …relative to holds/spaces, structure and any obstructions or restrictions therein. 

The main purpose of the cargo plan is to assist discharging of cargo, to prevent delay and to influence correct and appropriate: 

  1. Labour and equipment for discharging. 
  2. Suitable and adequate distribution facilities (warehouse etc.) 
  3. Any particular requirements for special cargoes. 

Systems of stowage relative to the different consignments should be indicated, as for example, in bulk; unitised; fork lift trucks; palletised, pre-slung, optional discharge and containers their sizes and contents. 

It is usual to adopt a distinctive colour for each port of discharge so that the disposition of the cargo may be easily distinguished in relation to its respective port of discharge. In this manner different consignments may be seen at a glance and the possibility of over carriage thereby lessened and short delivery avoided. Over carriage of cargo results in a loss of freight to the ship, for when it is subsequently found, Probably at a later port, the ship is responsible for its return to its proper destination.

A copy or copies of the cargo plan are sent on ahead of the ship, to each port of discharge, so that previous arrangements may be mad as to the type of discharging equipment required, the number of gangs to handle the cargo and proper methods of disposal arranged. 

Micro Computerisation

Every owner expects a vessel’s capacity to be used to the fullest every time she is loaded. It is also expected that loading and discharging shall be carried out as quickly and rationally as possible (port speed and quick turn around) without endangering the structural strength and stability of the ship. 

It is essential, because of maritime operational costs that a ship for efficient cargo stowage needs to have its cargo stowed in the right sequence, ensuring that the stowage confirms to rules, regulations and recommendations of authorities. 

The demand for safer and more efficient cargo handling aboard ships is now a prominent requirement. it is no longer a matter of comparison between manual and instrumentation methods but a choice of which systems better suits the circumstances, bearing in mind the technical factors involved, the reliability and ease of handling and the economy of the operation. 

This can involve lengthy and detailed planning and co-ordination between ship owner/charterer, port working authorities and ship personnel. To effectively promote these functions and to meet the demands of sophisticated cargo distribution, conventional systems are now quickly being replaced by high performance micro computers- albeit some of these systems have now been in operation for quite some time. 

The systems comprise of three components: 

  1. Computer for processing information relative `to a ship’ (programming), 
  2. Video terminal for communication with the computer and 
  3. Print out that provides ‘print outs’ i.e. results and / or actions to be taken. 

 

Various ‘pictures’ within the systems provide phases of cargo planning be it for general pre planning or for more appropriate detailed planning as would be necessary for the specialised container or roll on / roll off vessels, e.g.- apart from the particular requirements of grain, liquid and dangerous cargoes transportation cargo planning and monitoring involves: 

■ Cargo types and their distribution, 

■ Loading

■ Stability and stress Considerations in all classes of vessels.

■ Container and RO/R0 compartments and, 

■ With Liquid cargoes, predictions for ongoing loading, discharging and simulation. 

These are, to name but a few major aspects. Every change in the weight and position of the cargo results in a different distribution of hull stress the computer processes are such that from appropriate progressing the magnitude of the stresses and the permissible limits can be calculated instantaneously and e shown on the video display. 

Distribution of homogenous bulk cargo

It frequently happens that a ship is chartered to carry cargoes of a bulk nature with only two commodities to fill the vessel. 

The cargo officer is therefore obliged to determine the actual space and tonnage available and distribute this cargo so that the vessel is fully loaded; in a reasonable state of trim and has satisfactory stability qualities. 

The type of problem is best appreciated by reference to the following example.

A ship of 3520 tons deadweight has on board 420 tons of stores and bunkers and 70 tons of water. Stow timber (at 35 cubic feet per ton; and Rolls of Paper (at 120 cubic feet per ton) to fill the ship. The hold capacities are: 

No. 1. 55100 cu ft, 

No. 2. 55970 cu ft, 

No. 3. 44100 cu ft, and 

No. 4. 35900 cu ft. 

It will be necessary first to find out how many tons of each commodity can be placed in the ship, and afterwards divide such amounts among the holds in proportion to their sizes. The first can be done by solving a simultaneous equation as shown below, and the distribution can be made by means of the ratio the cubic capacity of each hold bears to the cubic capacity of the whole ship. 

  • First, to find the deadweight available for cargo:  
Stores
420 tons
Total Deadweight
3520 tons
Water
70 tons
Stores and water
490 tons
Total
490 tons
Cargo
3030 tons
  • To find the amount of each commodity that can be loaded in the ship

Let x= the number of tons of timber and y = the number of ton of paper. 

Then x + y = 3030 (1st equation). 

Hold capacities 

No 1. 55 100 cu ft 

No. 2. 55 970 cu ft 

No. 3. 44 100 cu ft 

No. 4. 35 900 cu ft 

Total 191 070 cu ft 

 

Then 35x+ 120y’ = 191 070 (2nd equation). 

The equations are solved by the elimination of either x or y, and finding the other unknown. 

Suppose we decide to eliminate x. To do this multiply the 1st equation by 35. 

Then 35x + 35y = 106050 and .35x + 120y =191070 (2nd equation) 

Subtracting the smaller: 

85y = 85020 

y = 85020 / 85 = 1000 (nearly). 

The total number of tons of Paper will thus be 1000 and of Timber 3030- 1000 = 2030. 

  • To distribute the commodities pro rata amongst the holds. 

This can be done by simple proportion: 

Total cubic capacity of ship: Capacity of particular hold: 

Number of tons in the ship: Number of tons in particular hold. 

To find the number of tons of Timber in No. 1 hold. 

X = 585.4 tons

To find the number of tons of Paper in No. 1 hold. 

Y = 288.4 tons

In No. I hold therefore we have 585.4 tons Timber and 288.4 tons paper.

The amounts of other hold can be found in same way. 

Broken Stowage

Broken stowage explains space ‘lost’ in a compartment. This loss of space is due  mainly to the variable sizes of packages loaded into a compartment, thus Preventing a compact and uniform stow. This loss of space is also caused due to the various shapes of the holds of the ship and the obstructions in the holds e.g. pillars, ladders, stanchions, large angle brackets and beams though modern construction has minimised this. It is expressed in terms of percentage since it is a ratio of space lost. Every effort is to be made to see that broken space is to be kept to a minimum so that the freight earning capacity of the ship is maximised. 

The term ‘Broken Stowage Cargo’ covers these commodities, though the term ,B roken Stowage’ is more correctly associated with the ‘loss of space’. 

 

Attention is drawn to the fact that the amount of broken stowage is related to the cargo and not to the space in which it is stowed 

Example 

50 tons of general cargo is to be loaded into a space containing 6000 cubic feet with a stowage factor of 40 cubic feet per ton with a broken stowage 5 per cent. Loaded. What is the space remaining? 

50 tons at 40 cu ft 2000 cu ft 

+ 5 per cent broken stowage 100 cu ft 

Space required 2100 cu ft 

Total space 6000 cu ft 

Space remaining 3900 cu ft 

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