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   The greatest merit of the pusher-barge system, or dividing a ship into an unmanned cargo-stowing part and a manned propelling power part, consists in the fact that a larger number of unmanned barges can be operated by a smaller number of manned pusherboats and, as the result, a great saving of the building cost, manning cost, maintenance cost, etc. can be realized.
   In rivers, this merit has long been enjoyed in a form of large convoy consisting of many barges connected with ropes and pushed by a powerful pusherboat.
   In wavy sea, however, this form of multi-barge convoy cannot be applied because of its poor seaworthiness and the combination of one barge and one pusher is the only applicable form, and, so, the so-called cyclic operation of the drop-and-swap style is the only method of operation of the multi-barge fleet. In this method, the fleet must be operated according to a prefixed schedule, or "cycle", constructed so that all the pushers and barges of the fleet will be at work incessantly. Each pusher of the fleet is running together with a connected barge and other barges are at cargo-handling work, and no vessel remains at rest, at least as the ideal. When a pusher, together with a barge, arrives at a terminal, the barge is disconnected from the pusher and moored at the quay for cargo-handling, and the pusher is connected to the barge which has been staying at the same terminal for cargo-handling and departs for the next voyage to another terminal. The periodically prearranged repetition of "running with barge" and "exchange of barge" in a route connecting two or three terminals is the so-called cyclic operation and, when this manner of operation is applied to a case to make a constant flow of goods in a route, the following economical merits can be expected.

(a) The initial investment for building necessary vessels is saved, because the number of ships with propelling engines is smaller and the building cost of barges is generally low.
(b) The manning cost is low, because the number of manned ships is smaller and, in addition, smaller pusherboats need smaller number of crew.
(c) The maintenance cost is low, because the total number of propelling engines is smaller and the size of pusherboats having engines is smaller. The maintenance cost of barge relates to hull part only.
(d) As barges can stay long at terminals for cargo-handling and the on-shore cargo-handling facilities can be operated practically without interruption, their loading and unloading rates can be smaller.
(e) It is sometimes possible to reduce the running speed without affecting the total transporting capacity of the fleet to save fuel consumption.
For realizing the above merits, it is important to keep in mind the following points.
(1) The pusher arriving at a terminal must exchange the barge and depart immediately and, accordingly, the crew of a pusher is an extremely busy post. So, frequent shift of crew is indispensable.
(2) As the pusher is operated practically without interruption and, accordingly, the operating time of main engines is very long --- over 7,000 hours per year or even attaining 8,000 hours. Therefore, special care for the maintenance cost of main engines is needed. A smaller number of engines does not mean lower costs of fuel and maintenance.

2-Pusher / 4-Barge Fleet
   This is the most typical cyclic operation for realizing a constant flow of certain cargo between two terminals. A barge is loading at the loading terminal A and another barge unloading at the unloading terminal B while a fully loaded barge with pusher runs from the terminal A to B and another empty barge with pusher runs fro, the terminal B to A.:
   When a pusher and a barge are locked, the fleet is transformed into 1-Pusher / 3-Barge fleet, stated later, and the transportation capacity becomes half. When a barge is docked, the fleet becomes 2-Pusher / 3-Barge fleet having a transportation capacity equal to 2/3 of the original fleet.
   2-Pusher / 4-Barge fleets are being successfully operated as follows :
Shimbishi Kaiun K.K. Japan, operating 2,500 DWT barges for transporting coke from Sakaide to Kure over 85 nautical miles for more than 30 years.
ILVA Servici Marittimi, Italy, operating 30,000 DWT barges for transporting hot-rolled steel coils from Taranto to Genova over 710 nautical miles.
Companhia de Navegacao NORSUL, Brazil, operating 5,000 DWT deck-loading barges for transporting pulp wool from Caravelas to Portocel over 150 nautical miles.
Companhia de Navegacao NORSUL, Brazil, will operate 10,000 DWT covered deck-loading barges for transporting hot-rolled steel coils from Vitoria to Sao Francisco do Sul over 650 nautical miles,.

2-Pusher / 4-Barge Fleet
3-Pusher / 5-Barge Fleet
   When the distance of transport is very long, the vessels of the 2-pusher/4-barge fleet must be big. In such a case, however, it may often be more economical to reduce the vessels' sizes and increase the number of voyages. The 3-Pusher/5-Barge fleet can be adopted in such a case and, as compared with 2-Pusher/4-Barge fleet, the barge deadweight is reduced to 2/3 and the engine power 75% - 80% if the speed is same. It may be also possible to increase the barge deadweight slightly and reduce engine power to lower the speed slightly.
   When a pusher and a barge are added to the 2-Pusher/4-Barge fleet to form a 3-Pusher/5-Barge fleet, the transportation capacity is increased by 50%, but, in this case, the cargo-handling rates at terminals must also be augmented by 50%.
   There may be similar manner of increasing transportation capacity, say 4-Pusher/6-Barge fleet, and all such types are based on the principle that 2 barges are at terminals for cargo-handling and other barges are running together with pushers.

3-Pusher / 5-Barge Fleet
1-Pusher / 3-Barge Fleet
   This is also considered one of typical operation cycles. A barge is loading at the loading terminal A and another barge unloading at the unloading terminal B. The pusher brings a fully loaded barge from the terminal A to the terminal B and exchanges barge at the terminal B. Then, the empty barge is brought by the pusher from the terminal B to the terminal A, and, then, the exchange of barge takes place again. The duration of stay of a barge is equal to the time needed for a double-run of the pusher-barge combination. Even in this style, all the vessels and the cargo-handling facilities at the terminals are working incessantly and the overall efficiency is high.
   The following 1-pusher/3-barge fleet is being operated :
  • Companhia de Navegacao NORSUL, Brazil, operating 7,300 DWT covered deck-loading barges for transporting pulp from Belmonte to Portocel over 280 nautical miles.
    1-Pusher / 3-Barge Fleet
  • 2-Pusher / 3-Barge Fleet
       This style is suitable particularly when the capacities of loading facility and unloading facility are much different. Assuming that the loading rate at the loading terminal A is much greater than the unloading rate at the unloading terminal B, a pusher pushing an empty barge from the terminal B arrives at the terminal A and accompany the barge during its loading, and, then, brings the fully loaded barge to the terminal B where the exchange of barge takes place. This is a complete cycle of a pusher and the cycle of another pusher has a phase-lag equal to half a cycle.
       If the loading rate at the terminal A is much smaller than the unloading rate at the terminal B, the style of the cycle is inverted. This style of operation may be probable , for example, in crude oil transport from an offshore oil station.

    2-Pusher / 3-Barge Fleet
    1-Pusher / 4-Barge Fleet
       In addition to the transport between two terminals stated above, there are cases of loading at one terminal and unloading at two or several terminals --- steel materials, cement, coal, limestone, etc. In this type of transport from the loading terminal A to the unloading terminals B and C, the pusher will run A - B - A - C - A, and two barges are operated between A and B and the remaining two barges between A and C.
       If the direction of flow of cargo is inverted, the transport is for cargo flow from the loading terminals B and C to the unloading terminal A. This type of cycle is realized by :
    Bukser of Bjergning A/S, Norway, operating 1,600 DWT deck-loading barges for transporting pulp wood from two sources to Tofte in Oslofjord for more than 25 years.

    1-Pusher / 4-Barge Fleet
    2-Pusher / 5-Barge Fleet
       Adding a pusher and a barge to the above-mentioned 1-Pusher/4-Barge fleet, 2-pusher/5-barge fleet is formed to double the transportation capacity. Or, conversely, when the total quantity of transport is same, the barge of 2-pusher/5-barge fleet can be reduced to half deadweight and the engine power of two pushers may be of 70% order if same speed is to be maintained. Then, it may further be possible to enlarge barges slightly and reduce the pusher's engine output to lower speed without reducing the total transport.
       In 2-Pusher/5-Barge fleet trading among the loading terminal A and unloading terminals B and C, each barge will run A - B - A - C - A, while one pusher will run between A and B only and the other between A and C only. The 2-Pusher/5-Barge fleet can likewise be applied to the case of two loading terminals and one unloading terminal.

    2-Pusher / 5-Barge Fleet
  • Other Types of Fleet
       The above-mentioned fleets are so-to-speak model cases which can be realized on respective assumptions. Each of them is organized so that all the vessels and cargo-handling facilities at the terminals may be working incessantly, and there is a relation :
       [Number of pushers] + [Number of terminals] = [Number of barges]
       In practice, there may be cases where the pure application of any of the above-mentioned cycles would not give a solution, such as one loading terminal to three unloading terminals, one loading terminal A to two unloading terminals B and C, transport to B being twice larger than to C, etc., etc., and, in such a case, it may be useful to try combination or partial modification of some of these cycles.

       For realizing such a cyclic operation as is explained above, the minimum indispensable condition is that all the pushers and barges belonging to the group are interchangeably connectable. The sizes, powers and designs of these vessels may be different from one another as far as connectability is maintained. But, if such differences are too large, the running time in a same course may be uneven and the maintenance of cycle may become somewhat difficult and, so, the size and performance of the vessels should preferably be unified as far as practicable.
       The economical merit of the cyclic operation is widely known, but, in practice, there seems a problem that many people have a fixed idea that such a cyclic operation would need a group of brand-new purpose-built pushers and barges and, even when the contents of the plan are satisfactory and reasonable, they often hesitate before the amount of the initial investment. But, in the practical case, the cyclic operation can be started even with a group of existing second-hand vessels (tugs and barges) converted properly to meet the above-mentioned "minimum indispensable condition". In such a case, it is important to consider replacement with and/or addition of such new vessels in the future as should preferably lead to where to go.
       The operation cycle should be constructed logically on the basis of all the related factors and conditions with due consideration for time margin to be contained. The basic factors to be taken into account are as follows :
    --- Kind of cargoes.
    --- Required quantity to be transported - daily, weekly, monthly or yearly.
    --- Effective running days - per month or per year.
    --- Distance of transport.
    --- Type and capacity of loading facility - which should move, loader or barge ?_
    --- Type and capacity of unloading facility - which should move, unloader or barge ?
    --- Conditions of terminal ports - quay, dolphin, sea-berth, etc., position of moored barge - incoming or outgoing ?
    --- Motion and time needed from arrival to commencement of cargo-handling.
    --- Motion and time needed from finish of cargo-handling to departure.
    --- Influence of tide.
    --- Method and time of exchange of barge.
    --- Dimensional restrictions for barge --- length, draught, air-draught, reach of cargo-handling gear, etc.
       When such data and information as are listed above are available at hand, it is possible to select the style of the cycle, sea speed and time schedule of operation, and the deadweight of the barge, to proceed to the next stage of basic design of the vessels. Here, the selection of the sea speed is very important and a good balance of deadweight and dimensions of the barge and the speed must be maintained to avoid excessive fuel consumption. If the speed is selected low, the engine power is small and fuel consumption low. But, in the cyclic operation, berthing and unberthing must take place very frequently and, in order to save service tug's cost, the pusher-barge train is required to berth and unberth by its own power in most cases. It is widely said that, in windy weather, the ultimate factor deciding the manoeuvring capability is the main engine power and, if the figure of engine power (in HP or KW) is less than a certain limit in relation to the figure of deadweight (in tones), the barge cannot be manoeuvred by the pusher's power. And, further, when the pusher has such an engine power as required for manoeuvring the barge in heavy wind, a proper running speed can be attained without particular efforts, if the barge has a fairly good hull form. Therefore, the designer should know the balance of technical figures in these aspects.
       For cyclic operation of barges, frequent shift of crew is inevitable. As the human life cycle is based on a unit of 24 hours, it may be preferable and desirable to construct the operation cycle adjusted to 24 hours' unit in any manner. A schedule constructed to realize crew's shift at a same time of a day may be the ideal one.

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       The exchange of barge at the terminals is a necessary condition for the realization of cyclic operation. Though it is very often considered a simple matter, it involves difficulties in many cases for the reason that the arriving barge is required to be moored in most cases at the same quay where another barge has been moored for cargo-handling and the latter barge, being moored, cannot move by its own power to make room for the incoming barge. If a service tug is hired in every exchange of barge, the cost is high, and the use of the incoming pusher will need a lengthy troublesome procedure.    A proposal is shown below. The barge is to have a side thruster at the stern. The moored barge B1 is moved forward by warping the bower mooring rope and, while keeping the bower mooring rope, its stern is turned by the stern side thruster to make room for the incoming barge B2. Then, after taking the bower mooring rope of the barge B2, its stern is turned by the stern side thruster to lay the barge alongside the quay. The pusher is disconnected at a proper time and brought to the barge B1 for connection. Then, the barge B1 is pulled back and turned by the pusher for departure. Though the above-mentioned method is a mere proposal, the exchange of barge is an inevitable factor of the cyclic operation, and it should be studied carefully with due consideration for the form and arrangement of the terminals, degree of congestion, etc.

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