The Naval Architect January 2008

cAd/cAm updAte
methods should be adjusted for the largest
“The MCA
yachts, now approaching 100m. It was
Intact Monohulls
concluded that the existing methods and
commissioned
Damaged Monohulls
Intact Catamarans
criteria remained valid, but that there may
be instances where a yacht has a very high
the Wolfson Unit
Damaged Catamarans
Damaged Trimaran
maximum righting moment in relation
to determine the A
0.12
to the potential heeling moment, perhaps
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because of extreme beam or a very small sail validity of the roll
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s
i
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e
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plan. In such cases an alternative approach
was proposed, which harmonised with the
prediction within the
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requirements for sailing multihulls.
criterion, in particular
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The safety of sailing multihulls was 0.04
addressed in 1995 because a number were for wide, shallow
a
v
e

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W
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operating commercially and the methods
used to assess them had not been developed
draught passenger
i
n
.

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0.00
0.0 0.2 0.4 0.6 0.8 1.0 1.2
in the previous studies. The work identified
vessels”
Range(RMmax)^0.5
aspects that were poorly understood, and
LB
Figure 3. The strong relationship between minimum wave height to capsize, and a proposed stability
was followed in 1998/9 by Research Project Figure 3. parameter. the strong relationship between
427, with tests conducted in the towing and its variation with heel angle, based on minimum wave height to capsize, and a
tank to investigate vulnerability to capsizing the profile area, but also dependent on the proposed stability parameter. STABILITY NOTICE
in breaking waves, and pitchpoling. An beam to height ratio.
Name: Jolly Polly SAFETY GUIDANCE
No: AB789
interesting result was that the range of The IMO weather criterion has been
LOA: 10.6m
LOADING &
mum
Maximum
stability again was important, this time problematic for some ship types, and has ano
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o address the
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el
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ti
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e
USE
Zone
Mini
Freeboard
recommended
seastate
in determining whether capsize occurred been the subject of much research. With leve
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by the monohull
following an encounter with a large breaking the introduction of EC Directive 98/18, all and multihull stability criteriaG.o Thod mea rfgoinr omf resr idual
freeboard
Good margin At least
wave. Wind tunnel tests were conducted to European sea-going domestic passenger were widely accepted conventional criteria
of safety 47cm
quantify the heeling moments, Figure 1, and vessels will be required to comply, but some while the latter had no sound technical basis.
a new formula for calculating the heeling UK ferries cannot. In 2007, with Research In Research Project 509, the WLoaodlinfgs oorn h aUulingi t reduces
minimum freeboard to less Low level of
moment was developed. This included the Project 571, the MCA commissioned the tested two monohulls, three cattahamn 4a7rcamn s, and safety
24 to 47 cm 1.3 metres
deck area, which is neglected in conventional Wolfson Unit to determine the validity a trimaran, in a range of loading conditions,
formulae, but has a significant influence of the roll prediction within the criterion, upright and heeled, intact and damaged,
Excessive loading or hauling
on the lift force and its contribution to the in particular for wide, shallow draught with damage openings towar
re
d
d
s
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c
a
es
n
m
d
in

i
a
m
w
um
a
f
y
reeboard to Danger of Less than
less than 24cm capsize 24cm
0.6 metres
heeling moment. passenger vessels. Five vessels were from the waves. The tests were unusual in
Further work on the wind heeling modelled and subjected to rolling tests in that the models were ballasted to conditions
moments of catamarans was conducted waves representative of those assumed in the deFsiiggunree 4d. E xtaom ptlest St atbhilitey Nmoticne ifmor au small m cfishiriteng riboat a, with no stability data.
as part of a coordinated group of research weather criterion. A total of 19 configurations rather than to test a particular vessel. The
projects in support of the IMO review of the were tested and it was clear from the results models were stationary, but unrestrained, in
2000 High Speed Craft Code. The work was that the existing criterion does not provide waves. Tests were conducted at a matrix of
conducted in phases, as Research Projects a reliable prediction of the roll angle. The frequencies and heights, and at all headings,
503 and 537. Models of monohull and method generally gives an under prediction to determine the minimum wave height
catamaran ferries, and a systematic series of of roll for vessels of low beam/draught ratio, required to capsize the model in each case.
rectangular blocks, were tested in the wind and an over prediction for those of high The large number of variables resulted in a
tunnel to develop improved formulae for beam/draught ratio. Wide, shallow draught test programme of 800 cases, each of which
wind heeling moment, and its variation with vessels therefore are at a disadvantage in the was tested at all headings.
heel angle. As with the sailing multihulls, the assessment. Researchers in Russia and Italy
deck area and beam of the vessel were found had proposed adjustments to some factors Rescaling for larger sizes
to have a strong influence on the heeling in the criterion and, while these do not Whilst a model at a particular scale can be
moment, because the vertical lift is a major provide reliable estimates, they offer a more ballasted such that it just complies with the
component. This had been found during consistent correlation with the model test criteria, if it is re-scaled to represent a larger
tests conducted in 1988 by the Wolfson data. It appears that the criterion is flawed size vessel it will comply with ease because
Unit, acting as sub-contractors in a previous in several ways, but it was appreciated that the GZ values will be greater. If the model
MCA project, and the principal contractors its elimination is unlikely to be acceptable capsizes in a certain wave height, this has
had made some recommendations for in the short term, and it was recommended obvious implications for the validity of the
adjustment of the method, but they were not that the existing proposals be supported by fixed criteria in terms of immunity from
adopted. Resulting from the Projects were the UK. capsize. The tests therefore verified the
proposed formulae for heeling moment, Returning to the HSC Code review, simple fact that safety is dependent on vessel
60 The Naval Architect January 2008
NA Jan - p59+60+61.indd 60 04/01/2008 15:52:08
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