CSF
Capsize screening formula
The capsize screening formula (CSF) is a quick beam-versus-displacement screen from the USYRU work after the 1979 Fastnet storm. Lower CSF favours heavier, narrower form. FairHelm judgment: treat CSF under 2 as a coastal/offshore conversation starter, never as a certification.
Formula
CSF = Beam_ft / (Displacement_lb / 64)^(1/3)
Beam in feet. Displacement in pounds. Same imperial tradition as SA/D.
History
After Fastnet 1979, US researchers sought simple screens for boats that might be more prone to inversion. CSF entered popular yacht literature and still appears on formula sites and broker sheets. It was never a Class or ISO stability certificate.
Rule of thumb
CSF under 2.0 is often quoted as preferable for offshore-oriented monohulls. Values above 2.0 are common on beamy coastal designs. Do not reject a boat on 2.05 alone; investigate stability data and survey findings.
Limits
CSF ignores ballast location, freeboard, cabin trunk, and dynamic stability. Beamy modern cruisers can score poorly yet have strong form stability when upright. Wrong displacement or using metres without conversion ruins the number. FairHelm does not equate CSF with seaworthiness.
Boat examples
- Hallberg-Rassy 36
Classic Scandic cruiser proportions usually screen more kindly than wide charter hulls of similar LOA.
- Linjett 40
Compare published beam and displacement carefully; small input errors move CSF across the 2.0 line.
- Beneteau First 31.7
Sportier beam can push CSF higher; read that as a design choice, then check reefing and crew skill.
FAQ
- Is CSF under 2.0 a legal requirement?
- No. It is a screening heuristic from post-Fastnet literature, not a flag-state or Class rule. Insurance and charter companies may use other criteria.
- Why is Fastnet 1979 mentioned so often?
- The race losses drove research into stability and design screens. CSF is one popularised outcome, not the only lesson (weather, gear, and crew decisions mattered too).
- Can a catamaran use CSF?
- The classical CSF is for monohulls. Multihull stability uses different tools. Do not apply monohull CSF bands to cats.