Stability Screening

There are many types of races ranging from trans-oceanic races beyond the reach of any outside rescue facility to inshore races of short duration where rescue boats are available along the entire race course

Overview

In the Offshore Special Regulations (OSR) World Sailing has divided these into 7 categories (see OSR 2.01). Under OSR 3.04 World Sailing strongly recommends that for category 0 through 4 races the race organizer should require compliance with a minimum stability/buoyancy index. Sail Canada endorses that recommendation.

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. These may be used as a general guide to suitability for meeting the OSR categories (see table 1). When choosing the OSR category, race organizers should consider the maximum conditions for the ISO design categories.
Note that compliance with any of the screening systems mentioned herein does not guarantee total safety or total freedom of risk from capsizing or sinking.

Table 1
Summary of Maximum Design Category Conditions under ISO 12217-2

ISO Category A A B C
OSR Category 0 1 2 3 4
Significant was height* 7 7 4 2
Wind in Beaufort scale 10 10 8 6
Wind in knots 55 55 40 27

*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 races as special cases for which race organizers specify the criteria.
Since the cross-references in OSR 3.04.4 don’t mention ISO category D it won’t be found in this discussion.

If sustained conditions meeting or exceeding these are forecast or encountered, race organizers should consider postponing or abandoning the race.

Note that some one-design classes may specify other limits which should be followed.

Stability/Buoyancy Screening Systems

The preferred systems for stability/buoyancy screening are:

STIX

Any boat, no matter where she 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. The standard germain to this discussion is ISO 12217 Part 2 ( ISO 12217-2 ).

Since this is the most recent and arguably most sophisticated screening tool available to date it is the preferred method.

Boats independently verified as meeting the minimum sailing weight, STability IndeX (STIX) and Angle of Vanishing Stability (AVS) under ISO 12217-2 and meeting the other standards are assigned an ISO category as summarized in table 2.

Table 2
Corresponding ISO 12217-2 Minimum Values

OSR Category 0 1 2 3 4
ISO Category A A B C
STIX 32 32 23 14
AVS 130-0.002m
but always ≥100°
130-0.002m
but always ≥100°
130-0.005m
but always ≥95°
90°
Minimum Sailing Weight (m) 3,000 kg 3,000 kg 1,500 kg

OSR 3.04.4 recommends that race organizers accept the ISO category (A-C) appropriate for the race.

Race organizers may also choose to accept the ISO STIX or RORC STIX published where appropriate on an IRC certificate, in which case the minimum AVS and minimum sailing weight from table 2 should also be required.

Since under ISO a boat can’t achieve a category unless its minimum sailing weight exceeds the stated minimum, and since displacement is a readily available parameter, this is a useful minimum value that should be required regardless of the screening system(s) accepted.

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).

For races using ORR the minimum STBIX values published by ORR and duplicated in table 3 below may be accepted.

Table 3
ORC/ORR Minimum STBIX

OSR Category 0 1 2 3 4
STBIX 120 115 110

Table 4
Minimum SSN

OSR Category 0 1 2 3 4
SSSN 35 28 15 10

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 moveable ballast, wings or excessive flare.

Summary

Sail Canada recommends that stability screening should be required for category 0 through 4 races and that race organizers adopt the following in diminishing order of precedence:

  • minimum ISO (or IRC) category per table 2,
  • minimum sailing weight for the category per table 2,
  • minimum ISO or IRC STIX and minimum AVS per table 2,
  • minimum ORC or ORR STBIX per table 3,
  • minimum SSSN per table 4, except for boats with moveable ballast, wings or excessive flare.

  • Rolex
  • Helly Hansen
  • Skippers' Plan
  • Bank of Montreal
  • Government of Canada