Table 2
S
ISAF Offshore Special Regulations note that: ISO 12217-2 is expected
ISO Design Category A B C D
C
to be capable of being used as a guide to general suitability for R
competition in Special Regulations race categories. This comparison is
Minimum STIX or RORC STIX 32 23 14 5
Minimum AVS 130-0.002*m 130-0.005*m 90 75
E
also shown by Table 2, right.
SR Category 1-2 3 4 N/A E
It must be clearly noted that if categorisation of races is by definition of
N
the required ISO design category (A, B, C, or D), that boats must satisfy
Where ‘m’ is the boat’s Minimum Sailing Weight
both the STIX and AVS requirements for that design category.
I
N
Alternatively, Organising Authorities may instead specify the numerical
G
minimum qualifying requirements for STIX and AVS.

S
C
H
E
M
IRC Stability and Safety
E
Screening Indices –
Q: My boat has a ‘sistership’ which has a different keel. Can I use the
SSSN and STIX
same data or will I have to get my boat measured?
A: ‘Sisterships’ are boats that are to all intents and purposes identical. So
Q: What is STIX?
model variants require individual assessment.
A: It is the STability IndeX derived by the International Standard for the
Q: How do I know if my boat has been officially assessed or if the
stability and buoyancy of boats, ISO 12217-2. STIX is calculated from
required data is available
the physical characteristics of each boat and is a number generally in
A: The RORC Rating Office publishes a list of boats for which STIX and AVS
the range 5 to 50. In a similar manner to the SSSN system, a higher
data is available, either assessed by a Notified Body or as IRC STIX.
value suggests greater seaworthiness.
Please see this list on our website www.rorcrating.com.
Q: How does STIX work?
Q: What happens if some of the data is not available for my boat?
A: The International Standard defines four Design Categories for boats
A: If either of stability data or a hull file are not available, then neither STIX nor
broadly related to ocean, offshore, coastal and local sailing. These
Design Category can be calculated and your certificate will show SSSN
are analogous (but not directly related) to the ISAF Offshore Special
only.
Regulations Categories. Each design category has a minimum
qualifying STIX value and also a minimum qualifying AVS (see below)
Q: There is a hull file available for my boat, but stability has not been
which is linked to the minimum sailing weight (m in kg) of the boat.
assessed. What do I do?
A: Stability may be assessed either theoretically by a naval architect in
Design Category A B C D
accordance with the ISO Standard, or directly measured by an inclining
Wave height (metres) up to approx 7 4 2 0.5
test.
significant significant significant maximum
Q: What are the requirements for theoretical assessment?
Typical Beaufort wind force up to 10 up to 8 up to 6 up to 4
A: ISO 12217-2, Annex C, para C.2.2 Vertical Centre of Gravity, requires that
Calculation wind speed (m/s) 28 21 17 13
theoretical calculation of Vertical Centre of Gravity (VCG) shall be based
Minimum STIX Value 32 23 14 5
on the calculated mass and centres of gravity of individual components,
Minimum AVS (degrees) 130-0.002*m 130-0.005*m 90 75
raised by the addition of 5% of (Fm + Tc), where Fm is freeboard amidships
and Tc is draught of canoe body, in both cases in minimum operating
Q: What is ‘AVS’? condition.
A: Angle of Vanishing Stability, also known as LPS, Limit of Positive
Q: If theoretical assessment is not possible, how do I get my boat
Stability. It is the angle beyond which a boat becomes unstable and
inclined and what does this cost?
will capsize to an inverted position.
A: The RORC Rating Office and other National Authorities have measurers
Q: How is STIX calculated?
trained to carry out inclinings. A paper is available from our website (www.
A: In a similar manner to the calculation of SSSN, STIX is the product of a
rorcrating.com) detailing how the inclining is carried out and specifying
number of different safety/stability-related features. These are:
the required condition of the boat. The Rating Office charges £22/m
Length, Dynamic Stability, Inversion Recovery, Knockdown
LOA for the inclining itself. The owner will then need to consult the boat’s
Recovery, Displacement/Length, Beam/Displacement, Wind
designer or other qualified person to calculate the stability at the required
Moment, Downflooding, and Reserve Buoyancy.
angles of heel and supply the other required data.
Q: Will the calculation of STIX and AVS involve any additional
measurement?
Q: Will STIX and AVS be automatically calculated for my boat?
A: Since 1998, the European Union’s Recreational Craft Directive (RCD)
A: Yes, provided that the Rating Office holds all the necessary data and the
has required that all new boats (except pure racing boats) sold in the EU
owner has completed and returned the appropriate form available from
must have been assessed for STIX/AVS by a Notified Body – an officially

www.rorcrating.com.
recognised assessment agency. Many boats pre-dating this have also
Q: What happens if I modify my boat?
been assessed. These boats will not require anymore measurement.
A: Modifications to sails will not generally require any additional re-
Other boats may require some measurement. However, any which
measurement for STIX/AVS. Modifications to rig and/or ballasting may
have had their stability assessed (either theoretically or by an inclining
require that the boat’s stability is re-assessed. Modifications to the hull,
test) for any purpose or for which a sistership has been assessed
keel or rudder may require a new hull file. Please contact the Rating
and for which a hull file (a computerised lines plan) is available will be
Office with specific detail.
assessed by the IRC Rating Authority on receipt of a request by the
owner accompanied by the required data. In this case, certificates will
Q: How do I use the STIX number and the AVS value on my IRC
use the nomenclature IRC STIX to show that the calculation has been
certificate?
carried out by us rather than a Notified Body.
A: Exactly as with SSSN, the Race Committee will set minimum qualifying
values for each parameter depending on the nature of the race.
8
55
IRC 09 Screening P54-56_bs.indd 55 1/12/08 09:31:20
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