Seakeeptng Constderations in a Total Destgn Metholology 
II. 3. Example 
The two ships and propellers whose characteristics are given 
in Table 3 are the two cases analysed in the present study. Ships A 
and B are geometrically similar and ship B has twice the dis- 
placement of ship A. The calculations performed in this section are 
the calculations that the designer would have to perform in a typical 
sampling cycle of the solution of a problem where the unknowns are 
the vessel size and speed of a fleet of ships that will satisfy a pres- 
pecified transport capability and optimization criterion. 
Figures 3 and 4 give the EHP vs. speed and sea state curves 
for ships A and B. The EHP vs. speed curve with the traditional 
25% allowance is also shown dashed for comparison purposes. It is 
of interest to note that in the speed range of practical interest the 
EHP curve with the 25% allowance is almost identical with the EHP 
curve for H'3 = 7,94 ft. 
The SHP, assumed for this study is determined from the 
value of EHP with 25% margin using a propulsive efficiency equal 
to 0.75 as suggested in Reference 5. This was done in order to be 
able to compare the results of the present study with the results that 
would have been obtained if seakeeping considerations were not in- 
cluded in the analysis. The values of SHP, used in the present study 
were 18000 for ship A and 37400 for ship B, both of which cor- 
respond to a speed of 20 knots. The results of the speed calculations 
are shown in Figures 5 and 6. 
From resistance considerations alone, the average speed for 
ship A is 18.85 and for ship B is 18.81 knots. When motion con- 
siderations are included the average speed of ship A is reduced to 
18.62 knots while the speed of ship B remains unaffected. 
The speed reduction for ship A was primarily due tothe wet- 
nesscriterion, Slamming considerations yielded restrictions which were 
slightly less binding than wetness while vertical acceleration consider- 
ations were not binding. For ship B the motion considerations were not 
atall binding andif the shiphad more power available it could goata higher 
speed. : 
The ship speed at the low sea states was computed to be 
lower than 20 knots even though the EHP value computed with the 
method suggested in the previous section is about the same as the 
value of EHP computed using the method suggested in Reference 5 
This is due to the fact that the propulsive efficiency computed in the 
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