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Stability Screening For Monohulls

 

Introduction 
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. In the Offshore Special Regulations (OSR) ISAF has divided these into 7 categories (see OSR 2.01). Under OSR 3.04.3 ISAF strongly recommends that for category 0 through 4 races the race organizer should require compliance with a minimum stability/buoyancy index. CYA 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 capsize 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 wave height*
-
7
7
4
2
Wind in Beaufort scale
-
10
10
8
6
Wind in knots
-
55
55
40
27

* Height in metres. 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 independantly 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 reqardless 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 Council 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
-
-



Stability, Safety and Screening Numeral 
The Stability, Safety and Screening Numeral ( 
SSSN ) is a system which can be calculated from the same input parameters as required for a PHRF certificate, thus for almost any sailboat. The minimum recommended SSSN values are listed in table 4.

Table 4 
Minimum SSSN 

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 rigourous 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 
CYA recommends that stability screening should be required for category 0 through 4 races and that race organers 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.