An explanation of GSC Minimum Stability requirements

Global Solo Challenge Regulations are based on the best practices established by World Sailing and their renowned Offshore Special Regulations for Category 0 Events. Point 3.04 of the GSC regulations specify the minimum stability  requirements for the Event.

If a boat is able to demonstrate compliance with ISO 12217-2 design category A or higher, either by EC Recreational Craft Directive certification having obtained the CE mark or the designer’s declaration (3.04.1) this satisfy the minimum stability requirement.

If compliance with IS 12217-2 design category A or higher cannot be demonstrated than three different stability screening systems are available as an alternative and are discussed below.

The International Standards Organization (ISO) has created a standard for sailboats of hull length no less than 6m, ISO 12217-2 with design categories (A to D) for different types of sailing. Compliance with any of the screening systems does not guarantee the suitability of a boat to participate in the Event and only sets minimum standards against the risk of capsizing or sinking.

Self Righting capability of a boat and AVS

In addition to the same requirements found in OSR Cat 0, GSC Regulations specify at point 3.04.3 that a boat should be capable of self-righting in case of an inversion following a capsize. There is no set parameter for this requirement however the graph below gives some insight into this additional requirement. The curve is based on findings from the 1979 Fastnet which provided data to estimate the relationship between a boat’s Inverted Time before self-righting and its Angle of Vanishing Stability or AVS.

Boats with an AVS of 120 or more as likely to spend 2 minutes or less in an inverted position before a wave will help the boat self right. This is to be considered the minimum acceptable threshold. For this reason as a general rule participants should only consider boats with an AVS greater than 120 degrees. As boat displacement has an impact on the expected behaviour of an inverted boat however a detailed calculation of the minimum accepted is provided below in table 2 (130° -0.002m but always ≥100°).

It must be noted that only a full analysis of positive AND negative stability curves gives a full indication of the expected behaviour of a boat when inverted. Boats with larger beam, flatter decks, smaller coach roofs can have significant stability when inverted therefore each participant should consult with the boat designer or other suitably qualified professional to assess whether hull/deck shape affect negatively the likelihood of self-righting in under 2 minutes. If this is the case a full analysis must be carried out to evaluate mitigating solutions such as the addition of lateral or forward water ballast tanks to be used exclusively in the event of an inversion and if the boat does not self-right.

A further note must be made about masts, standing rigging or additional equipment such radars, mast steps, masthead sensors etc. Any weight above the waterline negatively affects AVS but even more the further any weight is placed from the waterline. The impact on AVS of weight at the mast head can be as much as fifteen times that of weight placed for example below decks.

Even when installing additional equipment of any kind, batteries, choosing the position of heavy equipment such anchors and chain etc all efforts must be made to keep weight central and as low as possible.

Summary of Maximum Design Category Conditions under ISO 12217-2

ISO CategoryAABCD
OSR Category (GSC)0123
Significant wave height*>777420.5
Wind in Beaufort scale>101010864
Wind in knots>555555402715

Table 1

*Height in meters. Some waves will be double this height.
*Ocean Navigator published an informative article discussing which waves pose a danger to a boat.
ISO considers category 0 Events as special cases for which no maximum Wave height or Wind strength is specified.

Stability Screening Systems

Under GSC Regulations 3.04 the precedence for monohull stability screening systems is:

  • ISO 12217-2
  • Where compliance with ISO 12217-2 cannot be demonstrated:
  1. use the following values taken from Table 2, below:
      • minimum STIX; and
      • minimum AVS; and
      • minimum m*AGZ; or
  2. the minimum ORC stability index taken from Table 3, below; or
  3. the minimum IRC SSS base value taken from Table 4, below.

ISO 12217-2

Any boat, no matter where it was built, sold in the European Economic Area (EEA) since June 15, 1998, is required to have undergone a certification process involving many ISO standards. This page refers to ISO 12217 Part 2 (ISO 12217-2).

STIX, AVS, m*AGZ, m

The Stability IndeX (STIX), Angle of Vanishing Stability (AVS) and minimum righting energy m*AGZ are the 3 measures which are required under ISO 12217-2. Boats independently verified as meeting those minimum values are assigned an ISO category as summarised in table 2. “m” is the boat empty mass in kgs as defined in ISO 12217-2.

Corresponding ISO 12217-2 Minimum Values

OSR Category (GSC)0123
ISO CategoryAABCD
STIX>32323223145
AVS>  130-0.002m
but always ≥100°
130-0.002m
but always ≥100°
130-0.002m
but always ≥100°
130-0.005m
but always ≥95°
90°75°
Minimum Righting Energy m*AGZ>172,000 kg metre degrees172,000 kg metre degrees172,000 kg metre degrees57,000 kg metre degrees

Table 2

IMS, ORC and ORR

In addition to using the hull shape to determine the rating, the International Measurement System (IMS) based rules such as Offshore Racing Congress (ORC) and Offshore Racing Rule (ORR) calculate a STaBility IndeX (STBIX).

In alternative to the he minimum Stability parameter in Table 2, STBIX values published by ORR and duplicated in table 3 below may be accepted.

ORC/ORR Minimum STBIX

OSR Category (GSC)01234
STBIX>120115110103

Table 3

Minimum SSN

OSR Category (GSC)01234
SSSN>3535281510

Table 4

Unlike the STIX and ORR systems, SSSN estimates stability from rather basic parameters as compared to the rigorous and detailed process used by the other systems. As such it should only be used where other screening numbers are unavailable.

SSSN is not intended, nor should be used, for unconventional boats including those with movable ballast or excessive flare.

Resources

ISO 12217-2: Boats sold in Europe typically have a capacity plate with the CE mark.

STIX, AVS etc.: IRC certificates will indicate these values if they’re available. The IRC website also posts a selection of values.

ORC, ORR STBIX: ORC and ORR certificates indicate the ORC/ORR stability index.

IRC SSS base value: IRC certificates will indicate this if the STIX values are unavailable

Article by Dave Gerr, Naval Architect.

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