Safety and Stability Indices (SSI)

Since the late 1980s, IRC certificates have incorporated an SSS Numeral for use by race committees in determining suitability of a particular boat for a race. When the information is available, they now in addition carry a STIX (STability IndeX) value and AVS (Angle of Vanishing Stability). For further information on the latter, see the FAQs at:

www.ircrating.org/en/technical-a-certification/sssn-a-stix.html.

With effect from 1st January 2009, the adjustment element of SSS for compliance with a particular category of ISAF Offshore Special Regulations (OSR) has been discontinued.

Prior to 1st January 2009, an Organising Authority stated in its Notice of Race the minimum qualifying SSS Numeral. This SSSN was made up of the SSS Base Value plus any adjustment for compliance with a category of OSR. With effect from 1st January 2009, Organising Authorities will specify the minimum qualifying SSS Base Value. In overall terms, the effects will be zero except that the requirement for a boat to comply with the conditions of any adjusted SSS Numeral at all times irrespective of the OSR category of the race she is competing in will disappear.

SSS

An SSS Base Value, specific to that particular boat, is printed on each boat's IRC certificate issued by the IRC Rating Authority. A high value indicates that the system evaluates a boat as being more seaworthy.

The Base Value for the hull, rig and appendages is computed from the boat's dimensions and rated parameters. It is the product of several factors, each representing a different safety related feature. A Base Size factor, calculated from the boat's principal dimensions, is modified by each of the other specific factors to take account of variations from the norm. The modifying factors are: displacement/length, beam/displacement, sail area/displacement, beam/length, self righting, rig, keel, engine, and dayboat.

STIX and IRC STIX

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 of data by a boat’s owner, STIX and AVS will be printed on each boat’s IRC certificate. In a similar manner to SSS, STIX or IRC STIX is a number representing the perceived 'seaworthiness' of the design, with again a higher value reflecting a more 'seaworthy' boat.

Each boat's STIX or IRC STIX is calculated in accordance with ISO 12217 Part 2 by the combination of factors related to dynamic stability, inversion recovery, knockdown recovery, displacement-length, beam-displacement, wind moment and downflooding, using the following data and methods.

IRC STIX Numbers are calculated using declared or measured IRC data. Each boat shall have its stability assessed in accordance with IRC 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 use of sistership data. In such cases, stability data may be reduced. Subject to approval by the IRC Rating Authority, a lines plan from any reasonable source may be used for the calculation of hydrostatic data. Owners are responsible for the supply of righting moment data from a 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.

Application of SSI by Organising Authorities
Important Notice
Organising Authorities may incorporate a minimum qualifying SSS Base Value and/or STIX/AVS and/or IRC STIX/AVS for entry to a race. Responsibility for selection of which is to be used, for the minimum qualifying value(s), lies with the Organising Authority for a race. It is not possible for the IRC Rating Authority, nor any other body remote from the organisation of a race, to lay down firm recommendations or guidance. Only the organisers of a race can be fully aware of the circumstances of a particular race.

SSS or STIX or IRC STIX?

For many races, SSS will continue to be an entirely valid method by which Organising Authorities may screen the suitability of particular boats for a race.

Qualification for RORC organised races will depend on the ISAF Offshore Special Regulations Category of the particular race and also the age of the boat as follows:

Cat 1 and 2 races:
Boats with series date of 1995 and later will be categorised under STIX only.

Boats with series date before 1995 may be categorised under either STIX or SSS.

Cat 3 races:
Boats with series date of 2000 and later will be categorised under STIX only.

Boats with series date before 2000 may be categorised under either STIX or SSS.

Cat 4 races:
Boats may be categorised under either STIX or SSS.

Organising Authorities are therefore recommended whenever possible to incorporate qualification for races using STIX and AVS into their race rules, particularly for races of a more onerous nature, or if there is 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 boat's stability.

SSS Base Values

As examples only, the RORC typically uses the SSS Base Values shown in Table 3 as default values. With effect from 1st January 2009, the SSS Base Values below replace the previous SSS Numeral values. There are however circumstances when the RORC will use different minima for a variety of reasons. These qualifying values for RORC organised races were reviewed for 2006 onwards and in the cases of category 1 and 2 races increased slightly. Additionally, no RORC recommendation is now made for category 0 races, these being considered to be so specific that no general guidance can be offered.

Table 3

STIX and IRC STIX

ISO 12217 Part 2 defines 4 Design Categories broadly related to ocean, offshore, coastal and local sailing. These 4 Design Categories are analogous (but not directly related) to the ISAF Offshore Special Regulations Categories. The minimum permitted values of STIX and AVS for each design category is shown by Table 4, below. IRC STIX is directly compatible with STIX. ISAF Offshore Special Regulations note that: ISO 12217-2 is expected to be capable of being used as a guide to general suitability for competition in Special Regulations race categories. This comparison is also shown by Table 4.

Table 4

Where m is the boat’s Minimum Sailing Weight.

It must be clearly noted that if categorisation of races is by definition of the required ISO design category (A, B, C, or D), that boats must satisfy both the STIX and AVS requirements for that design category. Alternatively, Organising Authorities may instead specify the numerical minimum qualifying requirements for STIX and AVS.