Q2 Z2,(Q2) Z2(Q2) - Institute for Water Resources - U.S. Army

oat output, or effort, is not proportional to the number of tons or
ton-miles being pushed. In particular, a towboat does not expend twice
the "effort" in pushing a barge loaded with 1200 tons of cargo as it
does with 600 tons of cargo. Yet, for each mile towed, twice as many
ton-fliiles ,are produced by towing the 1200 ton barge. This tends to
make costs rise more than in proportion to increases in output. This
is because the relationship between the draft of the barge (as deter-
mined by the weight of the cargo in the barge) and the resistance of
the flotilla is not proportional. As draft increases, the amount of
turbulence in the water also increases, thus regniring increasing mar-
ginal effort by the towboat to achieve any given speed. There is an
additional interaction with waterway depth: In general, the deeper the
water the less severe the turbulence for any draft.
In addition to this engineering phenomena, two additional factors
alter the draft, or cargo load, and output relationship: waterway con-
gestion and locking time. If, for example, a large portion of the total
time taken on any trip is spent navigating locks, an empty or partially
loaded barge becomes just as difficult to push as does a fully loaded
barge. .
All of these factors are closely related to the waterway condit-
ions which prevail for any movement (thus the insistence above on in-
cluding waterway characteristics in the EBN definition). The number of
EBN's per hour produced by a given boat will vary with the physical con-
ditions of the waterway (depth, width, stream flow), average congestion
experienced, and With the time required to pass through locks. It
should be noted that, viewed in this way, the EBM provides a way of
. dividing the river systems into uniform river districts.
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