In-depth | SUPERYACHTS
pure loss of stability and parametric roll. None of the vessels has a tumblehome hull form. Calculations are performed for even keel design draught only and ‘solid’ GM values have been considered (no free surface corrections applied).
Results Vulnerability for pure loss of stability is to be investigated for all vessels in the sample since their Froude numbers are in excess of 0.3. Te results of the Level 1 analysis are presented graphically in figure 2 where GMmin
values are plotted
against the waterline length of the vessels. The data series indicated by ‘Italy’,
‘Japan’ and ‘USA’ represent estimated values of GMmin
(1) when applying the various wave steepnesses SW
=f1 (L) as proposed to the
ISCG by Italy, Japan and the USA. Te three data points indicated by SLF_54- 3-8 represent the GMmin
values as derived
by direct GM calculation. The big difference in the estimated values based on the Italian and
GMmin
USA wave proposals on the one hand, and the Japanese wave proposal on the other hand is striking. Te results derived by direct GM calculations seem to agree well with estimated GMmin
values based
on the wave steepnesses as proposed by Italy and the USA. The area marked in red represents <0.05, the proposed vulnerability
GMmin
standard for Pure Loss of Stability without allowance for Fn
dependent
heeling moments. The data points indicated by RPLA represent for each vessel a vulnerability standard Fn
including
proposed to [ GMmin
< RPLA allowance ISCG by , where RPLA = 1.83*Fn for
dependent heeling moments as the
Japan: 2
*d ]. Based on the estimated GMmin values
when tested for the vulnerability standard without allowance for Fn heeling moments (GMmin
dependent <0.05),
all
vessels are found vulnerable when using the Japanese wave proposal, whilst the Italian wave proposal results in three out of 13 vessels, and the USA wave proposal in five out of 13 vessels being vulnerable for pure loss of stability. Noteworthy is the high level of the required GMmin
values according to the 42 according to equation
above proposed RPLA
standard. When
tested against this standard, most vessels are to be considered vulnerable for pure loss of stability for all three proposed wave steepnesses.
Parametric roll - results The results of the Level 1 analysis for parametric roll are presented graphically in figure 3 where ΔGM/GM values are plotted against the waterline length of the vessels. The data series indicated by ‘Italy’,
‘Japan’ and ‘USA’ represent estimated values of ΔGM/GM according to equation
(2) when various wave steepness SW
applying =f2
the (L) as
proposed to the ISCG by Italy, Japan and the USA. The three data points indicated by SLF_54-3-8 represent the ΔGM/GM values as derived by direct GM calculation. Analogous to pure loss of stability,
there is a big difference in the estimated ΔGM/GM values for parametric roll based on the Italian and USA wave proposals on the one hand, and the Japanese wave proposal on the other. Here too, results derived with direct GM calculations seem to agree well with the estimated ΔGM/GM values based on the wave steepnesses as proposed by Italy and the USA. The area marked in red represents ΔGM/GM>0.5, the vulnerabi lity standard proposed for parametric roll. Japan has proposed to the ISCG an
alternative standard not allowing a Level 1 assessment of parametric roll in case the relative area of bilge keels, abk
= 100*ABK
wave proposal in three out of 13 vessels considered vulnerable for parametric roll. When applying the ΔGM/GM>0.34 standard, the number of vessels considered vulnerable increases by approximately 15%.
Observations Following observations are of interest for
the superyachts of the
sample
considered in this article, but perhaps also for merchant vessels with comparable characteristics. SYBAss believes that the Level 1
methodologies should be sufficiently simple for application in preliminary design optimisation by in-house naval architects using their standard naval architecture software. This is especially important in tender stages when time is limited and little design information is available. Commissioning work to expert institutes would then be impracticable in connection with confidentiality, availability, turnaround time and cost. The current Level 1 methodologies for pure loss of stability and parametric roll seem to meet a desired degree of simplicity. However, the Level 1 results should
/(L.B), is less than a
limit value depending on the midship section coefficient. The relative bilge keel area of all yachts of this sample largely exceed the bilge keel limit value, and in that case the Level 1 standard ΔGM/GM>0.34 should be applied. The red dotted line in figure 3 marks this standard. Based on the estimated ΔGM/GM
values when tested for the vulnerability standard ΔGM/GM>0.5, all vessels are found vulnerable for parametric roll when using the Japanese wave proposal. The Italian wave proposal results in one out of 13 vessels, and the USA
also be sufficiently accurate to be consistent with existing operational experience, including known accidents, and to avoid too dramatic design changes in the detailed design stages. In addition, applying overly conservative methodologies in preliminary design optimisation could lead to designs with unrealistic high GM values, there by causing conflicts with other criteria such as excessive stability. The results of pure loss of stability
analysis based on the wave proposals by Italy and the USA seem to yield an acceptable degree of accuracy when testing the vulnerability on the basis of the proposed GMmin
an allowance for the Fn
standard excluding dependent
heeling moment. The results of parametric roll analysis
based on the wave proposals by Italy and the USA also seem to yield an acceptable degree of accuracy when testing the vulnerability by applying the proposed ΔGM/GM standard without allowance for bilge keels. NA
The Naval Architect September 2012
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