STIX, design categories and capsize screening
1The lesson in plain words
Design categories describe assessed conditions. STIX summarises stability factors for monohulls; neither alone guarantees seaworthiness.
Design categories
The RCD, or Recreational Craft Directive, categories describe wind and significant wave height, the mean height of the highest third of waves: A, above Force 8 and 4 m, excluding abnormal conditions; B, up to Force 8 and 4 m; C, up to Force 6 and 2 m; D, up to Force 4 and 0.3 m, with occasional waves up to 0.5 m. ISO 12217-2 assesses sailing boats 6–24 m. Follow the assessed loading limits in your owner’s manual.
STIX
The stability index, STIX, combines length, righting energy, AVS, knockdown and inversion recovery, displacement–length, beam–displacement, wind moment and downflooding. In this monohull assessment route, the minimums are A 32, B 23, C 14, D 5. A higher value indicates more favourable assessed stability factors; it does not establish overall seaworthiness or guarantee safety.
AVS minimums
In the monohull assessment route, each category also needs a minimum AVS. For A it is 130° − 0.002 × m (m in kg), but not below 100°; for B, 130° − 0.005 × m, not below 95°. C needs 90°, D 75°. A also needs at least 3,000 kg loaded mass, B 1,500 kg.
Capsize screening
The capsize screening formula, CSF, is a rough monohull screen: CSF = beam (ft) ÷ (displacement (lb) ÷ 64)^⅓. A value below 2 is traditionally favoured for offshore work. It uses only beam and displacement, ignoring ballast position and AVS. Use assessed stability information and the full GZ curve before judging suitability.
Multihulls
ISO 12217-2 treats habitable multihulls differently: it assesses how likely they are to invert, and asks for inverted flotation and a means of escape for boats that could. Their safety comes from staying upright and floating if not.
2Key data
- Monohull STIX minimums
- A 32 · B 23 · C 14 · D 5
- AVS, category A
- 130° − 0.002 m (kg), at least 100°
- AVS, category B
- 130° − 0.005 m (kg), at least 95°
- AVS, C and D
- C 90° · D 75°
- Capsize screening
- Monohull screen: B(ft) ÷ (Δ(lb) ÷ 64)^⅓; rough guide only
3Key points
- 1
ISO 12217-2 assigns A–D to sailing yachts of 6–24 m.
- 2
Monohull STIX minimums: A 32, B 23, C 14, D 5; no seaworthiness guarantee.
- 3
Monohull assessment also uses minimum AVS and, for A and B, minimum mass.
- 4
The capsize screening formula is only a rough monohull beam–weight screen.
- 5
Multihulls are judged on inversion risk, inverted flotation and escape.
4Frequent mistakes
Wrong: A category B yacht is unsuitable for any ocean passage.
Right: Categories describe design conditions, not routes. B covers up to Force 8 and 4 m significant wave height. Check expected conditions, assessed loading and present condition before planning a passage.
Wrong: A CSF under 2 proves a yacht is safe offshore.
Right: This rough monohull screen ignores ballast position, KG and AVS. Check assessed stability, the GZ curve, loading and condition.
5Practice case & exam keys
You compare two used 12 m cruisers for an Atlantic crossing. One is category A with STIX 38; the other is category B with STIX 27 and has since been fitted with in-mast furling, a heavy arch and a generator. What must you check before choosing?
Show the answer
Check both yachts’ condition, assessed loading and current equipment positions. The A-rated boat has the higher recorded index, but that alone does not decide suitability. Additions above G raise KG; a generator below G may lower it. Obtain updated stability advice after substantial modifications and confirm the information matches each yacht’s present configuration.
Exam keys
- ISO 12217-2: sailing boats 6–24 m. Monohull assessment uses STIX, AVS and mass.
- STIX minimums A 32, B 23, C 14, D 5.
- AVS: A ≥ 130 − 0.002m (min 100°), B ≥ 130 − 0.005m (min 95°), C 90°, D 75°.
- Monohull CSF = B / (Δ/64)^⅓ (ft, lb): < 2 traditionally favoured; rough only.
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