Trans RINA, Vol 154, Part C1, Intl J Marine Design, Jan - Jun 2012
When a vessel is optimised for a single performance measure, there is generally a trade-off in the other performance measures. This is shown in Figure 9. Here the normalised performance of each of the five vessels is shown. The performance in each of the areas of interest are the four points of the diamond. Remembering that a larger value for stability, but lower values for the other performance measures are desired, it can be seen that the vessel optimised for stability shows poor performance in the other areas. (A clue to this can be seen in Figure 8 since the design parameters of the vessel optimised for stability are somewhat opposed to those of the vessels optimised for the other performance measures.) Figure 9 also indicates that all the remaining vessels out-perform the baseline in all areas (other than stability) despite each being optimised for a different performance measure.
then done for a range of vessel lengths, generating a series of vessels, each optimised for minimum power. The performance of each of these vessels can also be calculated for the other areas of interest. Figure 10(a) shows the required delivered power against length and Figure 10(b) shows the stability performance for the same vessels. As has already been observed, the stability tends to reduce with increasing vessel length.
Figures 10(a), (b): Performance of Vessels of Different Lengths Optimised for Minimum Required Delivered Power.
The data presented in Figure 10(a) can also be thought of as the benchmark for delivered power as a function of vessel length (because at each length the design was optimised for minimum power). Thus any other design can be compared with the corresponding benchmark value at the same vessel
curves could also be developed for the other performance measures of interest and/or for the
length. Similar benchmark other design
Figure 9: Normalised Performance for Vessels Optimised for Different Performance Measures.
4.2 (b) Design Exploitation and Refinement Under other
parameters. 5.
CONCLUSIONS circumstances, certain design parameters
may be absolutely fixed, for instance length (for classification, or other reasons), or have a narrow range of allowable variation. In this case the design team may wish to fine-tune the vessel design, allowing narrower ranges of variation for some parameters and possibly tighter constraints. This type of data can be useful when answering questions of the type: “How can I improve the performance if I have to have the length fixed at 71m?” for example.
Depending on the level of detail of the design, it may actually be appropriate to repeat the
design-space
exploration exercise but over a much narrower range of parameter values. This will provide a greater level of detail to the response surfaces in the area of interest. These data may also be added to the original data set so as to improve the accuracy of the response surfaces covering the original, broader design-space.
Figure 10 shows the effect of fixing the vessel length and optimising all other design parameters to achieve the minimum required delivered power. This optimisation is
C-24
One of the most challenging tasks for the ship designer is to gain an insight into the non-linear relationships between competing objectives, constraints, free- and dependent-variables so as to be able to obtain a suitable final design that meets the customer’s requirements.
A methodology for automated design-space exploration and subsequent
semi-automated utilisation has been
described and demonstrated by means of an example application to the design of a luxury motor-yacht. The methodology comprises six main steps: the first four perform the set-up of the design-space; the last two capture its subsequent utilisation, Figure 11.
5.1 DESIGN-SPACE SET-UP
In order to benefit from the high-performance, low-cost computers and established numerical tools for ship- performance prediction currently available, automated methods are used to predict
identified performance
measures over the relevant design-space. The key tasks are summarised below:
identification of the performance measures of interest;
©2012: The Royal Institution of Naval Architects
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