Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec
The requirements in respect of AVS are given in Table 1 below. These requirements have since been carried over into the Code for Small Commercial Sailing Vessels, which includes charter yachts up to 24m length
Cat Area of Operation 0
unrestricted Min. AVS
1 up to 150nm from haven 17
2 up to 60nm from haven 3 up to 20nm from haven
90 60 90 60 90 60 90 60
24 LOA 24 LOA
20 24
LOA
25 24
5.2 LOA
17
Table 1: UK Minimum AVS Sail Training Vessels USA
Following the USYRU / SNAME research, the following suggested criterion for minimum AVS was put forward [15]:
minimum AVS = (160 – L')° but not less than 120° where:
L' = capsize length (feet) =
L BB I
IB
B
2
C C
B
2
L = measured length (feet) I = estimated roll moment inertia ≈ Δ1.744/35.5 (lb.ft2) Δ = displacement (pounds)
IB = base roll moment inertia = 0.135 L4.5 (lb.ft2) BB = base beam = (2 + L/4) (feet) B = measured beam (feet) CB = base centre of pressure above VCG = 2 (feet) C = estimated centre of pressure above VCG = (2- VCG) (feet)
For a boat having I, B and C equal to the base value, the required AVS is 140° at 20 feet (6.1m) length, reducing to 120° at 40 feet (12.2m) length.
5.3 ISO
ISO 12217 [16] assesses the stability of offshore monohull sailing yachts using two criteria. The Stability Index (STIX) is a development of the RORC SSS and RYA STOPS screening systems, and comprises the square root of the product of seven factors, multiplied by the Base Length. Three of the seven factors are based on properties of the righting lever curve, including the AVS and positive area up to downflooding. Minimum STIX values are given according to the Design Category.
In addition to STIX, the standard requires a minimum AVS. The derivation of both requirements is described in
B-100
In order to compare the different criteria for unrestricted operation directly, the following assumptions have been made:
(i) that for the US criterion the values of I, B and C are equal to the base values, and
(ii) that for the ISO criterion the mass in tonnes is equal to 0.005L3.
The resulting comparison is shown in Figure 6. Clearly, when these
assumptions are inaccurate, the AVS Figure 5: ISO12217-2 Minimum AVS (based on [16])
This requirement was the outcome of vigorous debate in order to secure international consensus. It can be seen that the margin to known casualties can be quite small, and that most known safe boats exceed this minimum by about 10° to 20°.
5.4 COMPARISON OF CRITERIA
[17], which explains that boat mass was found to give better discrimination than length between casualties and 'safe' designs. The final requirement in respect of AVS is not as given in this reference, but is as shown in Table 2. The final version of STIX is described in [18].
Design
Category A B C D
Minimum AVS
(130 – 0.002m)° but ≥ 100° (130 – 0.005m)° but ≥ 95° ≥ 90° ≥ 75°
m = mass of yacht in kg Table 2: ISO Minimum AVS (taken from [16])
These requirements are shown graphically in Figure 5, which also shows data relating to stability casualties and 'good' designs.
©2011: The Royal Institution of Naval Architect
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