IRC YEARBOOK 2009
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R SSSN, STIX AND AVS
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IRC Safety and Stability Indices (SSI)
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Since the late 1980s, IRC certificates have incorporated a SSS Numeral Responsibility for selection of which is to be used, for the minimum
for use by race committees in determining suitability of a particular boat qualifying value(s), lies with the Organising Authority for a race. It is not

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for a race. When the information is available, they now in addition carry a possible for the IRC Rating Authority, nor any other body remote from the
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STIX (STability IndeX) value and AVS (Angle of Vanishing Stability). For organisation of a race, to lay down firm recommendations or guidance.
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further information on these latter, see the Q&As opposite. Only the organisers of a race can be fully aware of the circumstances of a
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With effect from 1st January 2009, the adjustment element of SSS for particular race.
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compliance with a particular category of ISAF Offshore Special
Regulations (OSR) has been discontinued. SSS or STIX or IRC STIX?
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Prior to 1st January 2009, an Organising Authority stated in its For many races, SSS will continue to be an entirely valid method by which
Notice of Race the minimum qualifying SSS Numeral. This SSSN Organising Authorities will be able to screen the suitability of particular
was made up of the SSS Base Value plus any adjustment for boats for a race.
compliance with a category of OSR. With effect from 1st January Qualification for RORC organised races will depend on the ISAF Offshore
2009, Organising Authorities will specify the minimum qualifying Special Regulations Category of the particular race and also the age of
SSS Base Value (see Table 1 below). In overall terms, the effects will the boat as follows:
be zero except that the requirement for a boat to comply with the
conditions of any adjusted SSS Numeral at all times, irrespective of Cat 1 and 2 races: Boats with series date of 1995 and later will be
the OSR category of the race she is competing in, will disappear. categorised under STIX only.
Boats with series date before 1995 may be
SSS categorised under either STIX or SSS.
An SSS Base Value, specific to that particular boat, is printed on each Cat 3 races: Boats with series date of 2000 and later will be
boat’s IRC certificate issued by the IRC Rating Authority. A high value categorised under STIX only.
indicates that the system evaluates a boat as being more seaworthy. Boats with series date before 2000 may be
The Base Value for the hull, rig and appendages is computed from the categorised under either STIX or SSS.
boat’s dimensions and rated parameters. It is the product of several Cat 4 races: Boats may be categorised under either STIX
factors, each representing a different safety related feature. A Base Size or SSS.
factor, calculated from the boat’s principal dimensions, is modified by
each of the other specific factors to take account of variations from the Organising Authorities are therefore recommended whenever possible
norm. The modifying factors are: displacement/length, beam/ to incorporate qualification for races using STIX and AVS into their race
displacement, sail area/displacement, beam/length, self righting, rig, rules, particularly for races of a more onerous nature, or if there is some
keel, engine, and dayboat. doubt as to the suitability of a boat, or when it is wished to apply more
rigorous criteria, including in every case some direct knowledge of a
STIX and IRC STIX boat’s stability.
When the data is available, either as STIX assessed by a notified body or
as IRC STIX calculated by the IRC Rating Authority following submission SSS Base Values
of data by a boat’s owner, STIX and AVS will be printed on each boat’s IRC As examples only, the RORC typically uses the SSS Base Values shown
certificate. In a similar manner to SSS, STIX or IRC STIX is a number in Table 1 as default values. With effect from 1st January 2009, the SSS
representing the perceived ‘seaworthiness’ of the design, with again a Base Values below replace the previous SSS Numeral Values. There are
higher value reflecting a more ‘seaworthy’ boat. however circumstances when the RORC will use different minima for a
Each boat’s STIX or IRC STIX is calculated in accordance with ISO variety of reasons. These qualifying values for RORC organised races
12217 Part 2 by the combination of factors related to dynamic stability, were reviewed for 2006 onwards and in the cases of category 1 and 2
inversion recovery, knockdown recovery, displacement-length, beam- races increased slightly. Additionally, no RORC recommendation is now
displacement, wind moment and downflooding, using the following data made for category 0 races, these being considered to be so specific that
and methods. no general guidance can be offered.
IRC STIX Numbers are calculated using declared or measured IRC
data. Each boat shall have its stability assessed in accordance with IRC
Table 1
Rating Authority procedures or by an approved alternative method except
Min. SSS Typical course Example/ISAF Race
that boats of approved one-design classes may use class standard Base Value Category

stability data and true sisterships of designs already assessed may make
use of sistership data. In such cases, stability data may be reduced.
10 Short day race, usually with Typical Solent race/Cat 4
Subject to approval by the IRC Rating Authority, a lines plan from any
time limit
reasonable source may be used for the calculation of hydrostatic data.
15 Passage race along or near RORC cross Channel
Owners are responsible for the supply of righting moment data from a populated coast races/Cat 3
source acceptable to the IRC Rating Authority in the form required by ISO
28 Between 250 and 600miles Fastnet Race/Cat 2
12217, Part 2, Paragraph 6. In addition to IRC STIX, certificates will show
35 1,000 miles in all weather Plymouth to Gibraltar/Cat 1
the angle of vanishing stability as calculated by the boat’s designer or
other qualified person.
STIX and IRC STIX
Application of SSI by Organising Authorities ISO 12217 Part 2 defines four Design Categories related to ocean,
offshore, coastal and local sailing. These are analogous (but not directly
Important Notice related) to the ISAF Offshore Special Regulations Categories. The
Organising Authorities may incorporate a minimum qualifying SSS Base minimum permitted values of STIX and AVS for them is shown by Table 2,
Value and/or STIX/AVS and/or IRC STIX/AVS for entry to a race. below. IRC STIX is directly compatible with STIX.
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IRC 09 Screening P54-56_bs.indd 54 1/12/08 09:31:19
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