Capsize Screening Ratio Calculator
The post-Fastnet capsize screen: beam against displacement's cube root, with the 2.0 offshore line — what it screens, and what it can't see.
Born from the 1979 Fastnet inquiry: wide-and-light boats resist initial heeling but, once inverted, stay there — beam stabilizes you upside down too. The screen flags that geometry; it cannot see ballast position, cabin buoyancy or the actual righting curve.
Formula
⚠️ For planning and education only — verify with your vessel's documentation, naval-architecture data and official sources. Not for navigation or stability decisions on real voyages without professional data.
The post-Fastnet capsize screen: beam against displacement's cube root, with the 2.0 offshore line — what it screens, and what it can't see.
About Capsize Screening Ratio Calculator
After the 1979 Fastnet storm killed fifteen sailors, the inquiry sought a number that flagged capsize-vulnerable designs from data anyone could find in a brochure: the result was the capsize screening ratio — beam over the cube root of displaced volume, with 2.0 as the offshore pass line. This calculator computes it and explains both the physics it captures (beamy-light boats are stable upside down) and the limits the word 'screening' was chosen to admit.
How to use Capsize Screening Ratio Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula CSR = beam ÷ (Δ_lb/64)^⅓ — beam against the cube root of displaced volume; ≤ 2.0 to pass substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Capsize Screening Ratio Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula CSR = beam ÷ (Δ_lb/64)^⅓ — beam against the cube root of displaced volume; ≤ 2.0 to pass with sources cited on the page
- ✓Born from the 1979 Fastnet inquiry: wide-and-light boats resist initial heeling but, once inverted, stay there — beam stabilizes you upside down too. The screen flags that geometry; it cannot see ballast position, cabin buoyancy or the actual righting curve.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why does beam appear in the numerator — isn't beam stabilizing?+
Right side up, yes — that's form stability. But the same geometry works inverted: a wide flat deck-and-hull is ALSO stable upside down, with a high re-righting threshold. The Fastnet boats that stayed inverted longest were beamy and light. Displacement (deep ballast implied) fights back in the denominator: weight low in the boat shrinks the inverted-stability range.
What does failing the screen actually mean?+
That the beam-to-weight geometry resembles designs with poor ultimate stability — nothing more precise. A failed screen invites the real analysis: the full righting-arm curve, angle of vanishing stability (offshore consensus: AVS ≥ 120°), and downflooding points. Some screen-failers have fine AVS thanks to deep bulbs; some screen-passers hide shallow ballast. The screen sorts the queue for the better tools.
Why was the criterion built from brochure numbers only?+
Deliberate accessibility: in 1985, righting curves existed for few production boats, and the committee wanted owners — not just designers — able to screen their own vessels from published specs. Beam and displacement appear in every listing. The trade for accessibility is bluntness, which the committee documented candidly; modern ISO STIX numbers do better with more data.
How do modern wide-stern cruisers fare on the screen?+
Many fashionable designs sit at 2.0–2.3 — the contemporary hull form (max beam carried aft, light displacement, twin rudders) is exactly what the formula penalizes. Their defenders point to deep bulb keels (good AVS despite beam) and the formula's blindness to ballast position; their critics point to inverted-stability physics that hasn't changed since 1979. For coastal sailing the debate is academic; for ocean crossing it's a real design conversation.
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