Page 118 - Practical Design Ships and Floating Structures

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diameter, We assume a new afterbody which is defined as tunneled combined with twin-stem.
2.2 The Determination of Main Parameters of New Ship Types

As a result of limitations of Wujiang waterways, length overall(L0A) and breadth are not greater than
40m and 7m apart. While draught is ought to be alterable so as to fit the great changes of water depth
from low water to flood all the year round and to enhance the transport benefit. As far as block
coefficient(CB) is concerned, it effects velocity and cargo capacity by a long way. Because waters is
quick and disorder and depth of waterways alters consumedly in different seasons in Wujiang , the
velocity in deep and calm water is required to be high and not lower than 19 km/h. So CB must be
selected to be smaller on condition that the capacity is enough to carry cargo. Longitudinal prismatic
coefficient (Cp) is determined considering the appropriate matching with CB and good resistance
performance. Furthermore we must choose longitudinal buoyancy center (X,) from layout and floating
state of ship.
On the basis of above consideration, ship dimensions is optimized by tech-economic evaluation in
order to ensure safety of shipping and improve ship’s tech-economic performance. Then new lines plan
having tunneled combined with twin-stem is drawn. The main particulars of ship is shown in table1 and
the new afterbody plan is given in figure 2.
30 + -~ inon I 1000
1000
I

n BUTT I BUTT

Figure2: Afterbody plan of tunneled combined with twin-stern

3 MODEL TEST AND ANALYSIS

In order to know if the performance of new afterbody is better than tunneled stem, model experiment
is tested in tank. By self-propelled model test, the propulsion factor of actual ship with two afterbodys in
deep water are listed in table 2 in which the propellers are outwards. At one time, their propulsion factor
in shallow corresponding to different speed are shown in table 3, where the ratio of water depth to ship
draft is 1.5.

From table1 we can see that the propulsive performance of tunneled combined with twin-stern is better
than one of tunneled stem in deep water. While seeing table 3 shows that, two afterbodys have similar
propulsive performance in shallow water.
By resistance test , the curves of R/D to Fn in deep and shallow water are given in fig.3 and fig. 4
respectively, where R/D is the resistance in kilogram over displacement in ton. and Fn is Froude number.