irc YeArBOOK 2010
Screening
SSSn, STiX And AVS
irc Safety and Stability indices (SSi)
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
for a race. When the information is available, they now in addition carry a not possible for the irc rating Authority, nor any other body remote S
STIX (STability IndeX) value and AVS (Angle of Vanishing Stability). For from the organisation of a race, to lay down firm recommendations cheme
further information on these latter, see the Q&As opposite. or guidance. Only the organisers of a race can be fully aware of the
With effect from 1 January 2009, the adjustment element of SSS circumstances of a particular race.
for compliance with a particular category of iSAF Offshore Special
regulations (OSr) has been discontinued. SSS or STiX or irc STiX?
Prior to 1 January 2009, an Organising Authority stated in its For many races, SSS will continue to be an entirely valid method by
notice of race the minimum qualifying SSS numeral. This SSSn which Organising Authorities may 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 1 January 2009, Qualification for rOrc-organised races will depend on the iSAF
Organising Authorities now specify the minimum qualifying SSS Offshore Special regulations category of the particular race and
Base Value (see Table 1 below). in overall terms, the effects will be also the age of the boat as follows:
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 to
norm. The modifying factors are: displacement/length, beam/ incorporate qualification for races using STIX and AVS into their race rules,
displacement, sail area/displacement, beam/length, self righting, rig, particularly for races of a more onerous nature, or if there is doubt as to the
keel, engine and Dayboat. suitability of a boat, or when it is wished to apply more rigorous criteria,
including in every case some direct knowledge of a boat’s stability.
STiX and irc STiX
When the data is available, either as STIX assessed by a notified body or SSS Base Values
as IRC STIX calculated by the IRC Rating Authority following submission As examples only, the RORC typically uses the SSS Base Values shown
of data by a boat’s owner, STIX and AVS will be printed on each boat’s IRC in Table 1 as default values. With effect from 1 January 2009, the SSS
certificate. In a similar manner to SSS, STIX or IRC STIX is a number Base Values below replace the previous SSS numeral Values. There
representing the perceived ‘seaworthiness’ of the design, with again a are however circumstances when the RORC will use different minima for
higher value reflecting a more ‘seaworthy’ boat. a variety of reasons. These qualifying values for RORC-organised races
Each boat’s STIX or IRC STIX is calculated in accordance with ISO were reviewed for 2006 onwards and in the cases of Category 1 and 2
12217 Part 2 by the combination of factors related to dynamic stability, races increased slightly. Additionally, no RORC recommendation is now
inversion recovery, knockdown recovery, displacement/length, beam/ made for Category 0 races, these being considered to be so specific that
displacement, wind moment and downflooding, using the following data no general guidance can be offered.
and methods.
IRC STIX Numbers are calculated using declared or measured IRC
Table 1
data. Each boat shall have its stability assessed in accordance with IRC
min. SSS Typical course example/OSr race
Base Value category
Rating Authority procedures or by an approved alternative method except

that boats of approved one-design classes may use class standard
stability data and true sisterships of designs already assessed may make
10 Short day race, usually with Typical Solent race/Cat 4
use of sistership data. In such cases, stability data may be reduced.
time limit
15 Passage race along or near RORC cross Channel
Subject to approval by the IRC Rating Authority, a lines plan from any
populated coast races/Cat 3
reasonable source may be used for the calculation of hydrostatic data.
28 Between 250 and 600 miles Fastnet Race/Cat 2
Owners are responsible for the supply of righting moment data from a
35 1,000 miles in all weather Plymouth to Gibraltar/Cat 1
source acceptable to the IRC Rating Authority in the form required by ISO
12217, Part 2, Paragraph 6. In addition to IRC STIX, certificates will show
the angle of vanishing stability as calculated by the boat’s designer or
other qualified person. STiX and irc STiX
ISO 12217 Part 2 defines four Design Categories broadly related to
Application of SSi by Organising Authorities ocean, offshore, coastal and local sailing. These four Design Categories
are analogous (but not directly related) to the ISAF Offshore Special
important notice Regulations Categories. The minimum permitted values of STIX and AVS
Organising Authorities may incorporate a minimum qualifying SSS for each design category is shown by Table 2, overleaf. IRC STIX is
Base Value and/or STiX/AVS and/or irc STiX/AVS for entry to a race. directly compatible with STIX.
8
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IRC2010 Screening STIX.indd 57 27/11/09 10:28:15
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