Seakeeptng Constderattons in a Total Destgn Methodology 
reduction is allowed in the model proposed in the present study. When 
this occurs it is advisable to augment the optimization criterion by 
outputing the seakeeping qualities of the ship at this reduced speed in 
order to provide the decision maker with all the information neces- 
sary for the selection of the ''best" ship. 
In the present analysis since only head seas are considered it 
is only meaningful to satisfy the motion limitations by speed reduc- 
tions. This is not always what happens in actual operation where in 
heavy seas the operator might elect to change course. It is actually 
common practice to take heavy seas at 30°-35° off the bow in order 
to ease the pitching motion. However this limitation in the model is 
not considered important because in other headings the speed in- 
creases, see Reference 13 , which partly compensates for the lost 
time due to extra distance traveled. 
The average ship speed in a seaway can be computed using the 
same formula given in the previous section the only difference being 
that now V,; is the ship speed in the ith sea states including motion 
considerations. The results of such a computation can be found in 
Figure 5, 
Fuel Consumption 
Steam Turbine is the main propulsion unit adopted in this study. 
The specific fuel consumption at powers other than 100% power can 
be obtained from 
SEC) 904, 1100% 
150% 
Typical values for power, RPM and efficiency 7 are given in 
Table 2. 
It is also assumed that for a given power setting if the value of 
RPM is less than the one shown in Table 2, it will not affect the ef- 
ficiency of the steam turbine. This is a reasonable assumption since 
steam turbines are constant power machines, 
1601 
