Decide
In the monohull assessment route, what is the minimum STIX for category A?
The lesson in plain words
- 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.
Key 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
Key points
- ISO 12217-2 assigns A–D to sailing yachts of 6–24 m.
- Monohull STIX minimums: A 32, B 23, C 14, D 5; no seaworthiness guarantee.
- Monohull assessment also uses minimum AVS and, for A and B, minimum mass.
- The capsize screening formula is only a rough monohull beam–weight screen.
- Multihulls are judged on inversion risk, inverted flotation and escape.
Check what you have read
Which standard assigns stability-based design categories to sailing yachts of 6–24 m?
True or false?
Decide each statement before you see the answer.
Monohull assessment also uses minimum AVS and, for A and B, minimum mass.
The capsize screening formula is only a rough monohull beam–weight screen.
A CSF under 2 proves a yacht is safe offshore.
A category B yacht is unsuitable for any ocean passage.
Practice case & exam keys
Situation
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.