STIX and AVS FAQ

What is 'STIX'?

It is the STability IndeX derived by the International Standard for the stability and buoyancy of boats, ISO 12217-2. STIX is calculated from the physical characteristics of each boat and is a number generally in the range 5 to 50. In a similar manner to the SSSN system, a higher value suggests greater seaworthiness.

How does STIX work?

The International Standard defines 4 Design Categories for boats broadly related to ocean, offshore, coastal and local sailing. These are analogous (but not directly related) to the World Sailing Offshore Special Regulations Categories. Each design category has a minimum qualifying STIX value and also a minimum qualifying AVS which is linked to the minimum sailing weight (m in kg) of the boat.

What is ‘AVS’?

Angle of Vanishing Stability, also known as LPS, Limit of Positive Stability. It is the angle beyond which a boat becomes unstable and will capsize to an inverted position.

How is STIX calculated?

STIX is the product of a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy.

Will the calculation of STIX and AVS involve any additional measurement?

Since 1998, the European Union’s Recreational Craft Directive (RCD) has required that all new boats (except pure racing boats) sold in the EU must have been assessed for STIX/AVS by a Notified Body – an officially recognised assessment agency. Many boats pre-dating this have also been assessed. None of these boats will require any further measurement.

Other boats may require some measurement. However, any which have had their stability assessed (either theoretically or by an inclining test) for any purpose, or for which a sistership has been assessed and for which a hull file (a computerised lines plan) is available, may be assessed by the IRC Rating Authority on receipt of a request by the owner accompanied by the required data. In this case, certificates will use the identifier ‘IRC’ STIX to show that the calculation has been carried out by the IRC Rating Authority rather than a Notified Body.

My boat has a ‘sistership’ which has a different keel. Can I use the same data or will I have to get my boat measured?

‘Sisterships’ are boats that are to all intents and purposes identical. So, model variants such as different keel versions require individual assessment.

How do I know if my boat has been officially assessed or if the required data is available?

IRC publishes a list of boats for which STIX and AVS data are available, either assessed by a Notified Body or as IRC STIX. This list is updated periodically as more data is collected. You may find the most recent list on this page https://ircrating.org/irc-racing/race-management/ in the Safety and Stability section.

What happens if STIX and AVS data is not available for my boat?

If either of stability data or a hull file are not available, then neither STIX nor Design Category can be calculated and your IRC certificate will show SSS only. If you need STIX and AVS for racing and no data is listed on this website (see the previous question) either from a Notified Body or IRC calculated STIX, depending on the age of the boat you may be able to obtain this from the designer or builder; alternatively STIX can be calculated by the design office or the IRC Rating Authority.

See STIX information and application forms on this page https://ircrating.org/irc-racing/race-management/ in the Safety and Stability section.

Will STIX and AVS be automatically included on my IRC certificate?

Yes, provided that the Rating Authority either holds the data as a production boat, or has received the appropriate completed form: see STIX information and application forms on this page https://ircrating.org/irc-racing/race-management/ in the Safety and Stability section.

Data will remain on the IRC certificate unless there are modifications to the boat that could alter the calculated stability values, such as changes to the keel or ballast, among others. For further details, see the information provided below.

What happens if I modify my boat?

Modifications to sails will not generally require any additional re-measurement for STIX/AVS. Modifications to rig and/or ballasting may require that the boat’s stability is re-assessed. Modifications to the hull, keel or rudder may require a new hull file. Please contact your IRC Rule Authority in the first instance https://ircrating.org/irc-certificate/how-to-apply/

How do I use the STIX number and AVS value on my IRC certificate?

The World Sailing Offshore Special Regulations specify minimum STIX and AVS values for each race category (OSR 3.04). For some races it may possible to qualify using SSS instead of STIX and AVS.

https://www.sailing.org/inside-world-sailing/rules-regulations/offshore-special-regulations/

 

Are STIX and SSSN values directly related?

No. Although the two methods are aimed at answering the same general question, they use different data and functions to arrive at the answer. So while the two values will be broadly compatible, they are not in any way linked.

Where can I find out more about the International Standard and how it is applied?

A more detailed, historical paper is available on this website https://ircrating.org/irc-racing/race-management/ in the Safety and Stability section.

Alternatively, please contact the International Standards Organisation for a copy of the Standard, ISO 12217-2. https://www.iso.org/