closed gauging
In-depth | EEDI
by 5%, reducing the block coefficient from 0.850 to 0.816 (Table 1 of
the April article). However, Kristensen has
assumed that the Panamax tanker’s lightweight increases by only 5% when the length increases by 5%. This seems to be an over-simplification, ignoring the fact that scantlings have to be increased when length increases, due to higher bending moments. Nevertheless, if we assume that a VLCC design could
behave in a similar way and maintain a 16knot service speed through an increase in length of 5% and reduction in required propulsion power of 10%, we would end up with a Lbp of 338m and a Loa of about 350m. Such an increase in length and displacement would render
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important oil terminals inaccessible. Kristensen accepts that, for Panamax tankers, a length limit is required because of certain harbour restrictions and suggests that it should be “re-evaluated”, but does not explain how this could be done! Dimensional restrictions are key design criteria for any tanker or bulk carrier and naval architects give very serious consideration to the consequences of designing a ship that cannot access a particular lock, canal, offshore facility, drydock or terminal that is essential for the ship’s profitable operation. It is not something that can be dismissed by re-evaluating it! In summary, the EEDI is not a measure of hydrodynamic
efficiency and there is clear evidence from comprehensive regression analysis that hull form design and propeller/hull interaction have slightly improved over time, resulting in improved hydrodynamic efficiency. Naval architects should not be slaves to parametric relationships that are usually developed in order to assist the preliminary design process. Our function is to design a ship for our client that can access all the major terminals required for profitable trading and, on present assumptions about future market conditions, is most likely to maximise his net income over its expected life. NA
* Te word “guidelines” is put in inverted commas since there is an issue over the basis of the “guidance”. A relationship between block coefficient and Froude number can be useful as a preliminary design tool. Such a relationship can be derived simply from historical data using a least squares (or other) curve fit. It is interesting to note that the most recent of the three lines reproduced in Figure 2 of the April article (Watson, 1998) suggests a higher block coefficient (for the same Froude number) than earlier guidelines, confirming the fact that improving hydrodynamic efficiency enables the naval architect to use a fuller hull shape.
Such “guidelines” do not necessarily represent the relationship For more information visit our website
www.tanksystem.com or call +41 26 919 15 00 © 2012 Honeywell International, Inc. All rights reserved
that results in the design generating the highest earnings since they cannot take into account current or future trends in revenue and costs. Tere is no law of naval architecture which states that the relationships between any two design parameters MUST obey certain historical criteria, but the naval architect must obviously be aware of the consequences of straying too far from the norms established by successful operational experience.
22 The Naval Architect May 2012
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