to the seastate, does not provide practical guidance on varying levels of safety, and does not present
information in a simple format. Regulatory boundaries have driven designs in undesirable ways and this was
something to be avoided in this work.
The recommendation was for a Stability Notice; a single page of information posted prominently on the
vessel, simple enough to be understood and memorised by the crew. It is proposed to provide information
that stability is variable, may be inadequate and is under the control of the fishermen.
The method developed in Project 509, to relate residual stability to safety with regard to vessel size and
seastate, was ideal for application to fishing vessels, and was used as the basis for guidance. For a vessel
with full stability data, it is quite simple to determine the maximum load, or the maximum lift on a particular
towing block or derrick, at which the residual stability is reduced to some particular level. Two levels were
proposed; amber, at which the vessel may be described as having a low level of safety, and red, at which
there is danger of capsize. In each case the situation is associated with a maximum recommended seastate.
The amber level was aligned with the safety offered by the minimum criteria, and the red level at 50% of that
in terms of the maximum seastate. The guidance cannot be precise, but is intended to raise the safety
awareness of the crew, assisting them in their decisions on loading and lifting, having regard to the prevailing
conditions.
For the smaller vessels the source of guidance information was more problematic, because the expense of a
full stability assessment would not be acceptable politically. The research involved a study of fleet
characteristics, and the stability of sample vessels, together with calculations on systematic variations of hull
form and loading. The proposed method is for the same information to be provided as for the larger vessels,
Feature 3 | cAd/cAm upd
but basing
Ate
the guidance on the residual freeboard. Heavily loaded vessels, or those lifting heavy weights
over the side, are far more vulnerable to capsize, and this is reflected on the Stability Notice, Figure 4. An
additional recommendation was for a freeboard guidance mark to be placed on the vessel’s side, indicating
the residual freeboards corresponding to the safety zone boundaries. These would have no regulatory
purpose, but would enable the crew to relate the stability notice directly to the vessel operation. They would
Pragmatic solu
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the safety awareness of a fishing
community. The information for the notice can be derived in a few minutes with a calculator, using only the
length and beam of the vessel to determine the guidance freeboards and seastates, and might be a valuable
tool with which to improve the very low safety levels that occur in some parts of the world.
Wolfson Unit has been undertaking stability research for the MCA.
S
tability is an aspect of naval
architecture that continues to attract
widespread research, justified
because stability accidents carry a relatively
high fatality rate. The modern trend is
towards increasingly complex numerical
simulations or probabilistic methods
of assessment, targeting naval or large
commercial vessels because that is where
the interest behind the research funding
lies. Unfortunately, a familiar result of much
research is a recommendation for further
work to improve predictions or validate
them. Another sad fact is that most lives
are lost on very small boats, in sectors of the
industry where such approaches will never Figure Figure 1. A catamaran during tests with 1. A catamaran during tests with reefed sails
be applicable. The loss of 24,000 fishermen reefed sails.
every year, predominantly from the small
craft fleets of developing countries, is
evidence of that. of the principal findings were that the
The Wolfson Unit has been conducting conventional methods of calculating heeling
stability research for the UK Government for moment and gust response were invalid. It
20 years. Much of the work has been aimed was determined that there is virtually no
at small craft, where simple, pragmatic, dynamic response to a gust, because the rise
solutions to safety are essential, but this time of a severe gust is similar to the natural
approach has also been developed for period of a quarter of a vessel’s roll cycle,
application in those projects relating to larger and the aerodynamic damping of a sailing
vessels. Throughout the projects, the Unit rig minimises any dynamic effects of shorter
engineers have consistently found that many rise times. The Wolfson Unit proposed
of the conventional stability criteria address an entirely new method of assessment,
parameters which are not the best measures recommended new criteria, and developed
of safety. Simple alternative formulae a graphical presentation to advise the crew
have been provided to facilitate safety of their level of safety from downflooding
assessment, regulation, and the provision of or capsizing in a gust or squall. Heeling
information to enhance operational safety, moments are so dependent on sail shape
and the projects’ objectives have been met and sheeting that estimation of maximum
without concluding that further research moments is unreliable. The method avoids Figure Figure 2. An intact monohull capsizes in 2. An intact monohull capsizes in moderate waves
was required, except where a project was any restrictive regulation of the sail area, and moderate waves.
specified as the first phase of a study. instead provides information to assist the
A thorough research study was crew in deciding when to shorten sail, using
commissioned in 1988. It included collating guidance on maximum safe heel angles. The and wave height, smaller vessels are more
stability data for yachts and sailing ships, methods and criteria have been adopted by likely to encounter dangerous waves, and
studying documented casualties, wind several other national authorities, and the the minimum range criterion therefore
tunnel testing a variety of rig types to guidance is widely used and respected by was dependent on size. Externally ballasted
quantify heeling moments and their crews. yachts have a high level of stability to carry
variation with heel angle, creating a new The earlier capsizing tests had shown sail effectively, and this gives them adequate
facility in a wind tunnel where floating that all vessels are vulnerable to capsize in safety with regard to other hazards.
models could be subjected to gusts, and breaking waves of height similar to the vessel
installing data acquisition systems on beam, and that a large range of stability was Adjusting methods
two vessels to gather wind speed and heel the essential characteristic to ensure recovery In 2006, the MCA commissioned Research
angle data over a full sailing season. Two from capsize. Given the link between beam Project 534, to determine whether the
The Naval Architect January 2008 59
NA Jan - p59+60+61.indd 59 04/01/2008 15:52:08
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