SSSN, STIX and AVS

Since the late 1980s, IRC certificates have incorporated a 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).

SSSN
An SSS Numeral value, specific to that particular boat, is printed on each boat's IRC certificate issued by the RORC Rating Office. A high value indicates that the system evaluates a boat as being more seaworthy. SSSN is the sum of Base Value and Adjustment Value with the two values arrived at as follows.

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.

SSSN Adjustment Value (Table 1) allows for recognition of safety related features, principally compliance with a Category of ISAF Offshore Special Regulations (SR), not already evaluated by the rated parameters. Its application is optional and a zero value will be applied unless a request is made by an owner.

SSSN Table 1

SSSN Table 2

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 SSSN, 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 SSSN 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.

SSSN or STIX or IRC STIX?
For many races, SSSN 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:

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 Numerals
As examples only, the RORC typically uses the SSSN values shown in Table 3 as default values. There are however circumstances when the RORC will use different SSSN 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.