Capsize Screening Formula (CSF)
The Capsize Screening Formula is a fast, two-input check on a specific failure mode: a boat that gets rolled by a breaking wave and stays inverted. Unlike most of the other ratios on this site, it wasn't invented to predict speed or comfort — it was a triage tool, designed to separate "probably safe offshore" from "probably not" using only beam and displacement.
Origin: the 1979 Fastnet Race
On 14 August 1979, the Fastnet Race fleet ran into a Force 10 storm in the Western Approaches with wave conditions that exceeded anything anyone had reasonably anticipated. Of 303 starting boats, 24 were abandoned, 5 sank, and 19 sailors died. Many boats were rolled — and crucially, some stayed inverted long enough for crews to be lost.
The Cruising Club of America's technical committee (USYRU/SNAME joint investigation) responded with a screen that any sailor could compute from a brochure spec sheet, designed to flag designs at elevated risk of staying inverted after a knockdown. The formula they settled on is what's known today as the CSF (Wikipedia, Capsize screening formula).
The core intuition the committee wanted to capture: a wide, light boat is easy to roll and stays inverted once it's there; a narrow, heavy boat is harder to roll and recovers quickly when rolled. Beam and displacement, the only two inputs, are on every spec sheet — so the screen could be applied to any boat in the market without specialized data. Here's the formula they settled on.
Formula
- Beam — Maximum beam in feet
- D — Displacement in pounds
- 64 — Weight of one cubic foot of seawater in pounds
The cube root of yields a characteristic length — the side length of a cube of seawater equal in weight to the boat. Dividing the maximum beam by that length produces a dimensionless ratio that compares hull width against hull mass. A wide-but-light boat scores high; a narrow-but-heavy boat scores low.
Interpretation
| CSF | Reading |
|---|---|
| ≤ 2.0 | Passes the screen. Considered appropriate for offshore, bluewater passages. Bluewater-focused designs typically aim for ~1.7–1.8. |
| > 2.0 | Higher inverted stability — more likely to remain upside-down after a wave strike. Many modern beamy production cruisers land here. Not "unsafe" for coastal sailing, but a flag for serious offshore intent. |
The 2.0 threshold isn't a hard physical limit — it's a triage line. Many boats above 2.0 have crossed oceans without trouble, and many below 2.0 have been damaged in storms. CSF screens for one specific failure mode, not seaworthiness in general.
What the math actually says
CSF penalizes beam (linearly) and rewards displacement (under a cube root):
- Wide boats are stable inverted. A flat raft is harder to flip back upright than a narrow log. Once a wide hull is rolled past its limit of positive stability, the same form stability that made it feel stiff right-side-up now keeps it stable upside-down.
- Heavy boats sink deeper and ride lower. This puts more mass below the waterline, lowers the center of gravity relative to the rolling moment of the wave, and gives the boat the inertia to be picked up and rolled back upright by the next wave train.
This is exactly why narrow heavy classics (full-keel ketches, English ocean racers from the 1960s) score so low on CSF, while wide modern production cruisers — even structurally sound ones — score above 2.
The deeper picture: the GZ curve
CSF is a single-number approximation of what naval architects measure precisely with the curve of righting arms (GZ curve), which plots restoring lever against heel angle from 0° all the way to 180°. Two values on that curve matter most for offshore work:
- Limit of Positive Stability (LPS), also called Angle of Vanishing Stability (AVS). The heel angle past which the boat will not right itself. Offshore monohulls should have an LPS of 120° or higher. A deep-draft version of a given hull will measure higher than the shoal-draft version of the same design — often by 10–20°.
- Ratio of positive area to negative area under the curve. Integrate the area to the right of the LPS (where the boat is fighting to come back up) versus the area to the left of the LPS (where it's stable inverted). A deep-bulb keel pushes positive area up and negative area down, meaning the next wave is much more likely to roll the boat back upright. A shoal-draft hull with a high beam has a meaningfully larger inverted-stable region — once flipped, it can stay there.
If you're seriously evaluating a boat for offshore work, ask the builder or the design office for the stability curve (sometimes called a "stability booklet" if the boat is large enough to be category-certified). CSF and B/D are proxies; the GZ curve is the truth.
A useful related metric to know is STIX (Stability Index) — the ISO 12217 standardized score that combines GZ-curve shape, downflooding, and several other factors into a single number. Boats with a STIX of 32+ are certified for Category A (ocean) work. Where it's published, STIX is a far more comprehensive indicator than CSF.
Caveat: multihulls don't play
The CSF is calibrated for monohulls. Catamarans and trimarans have extremely high CSFs (often 3–4) because the formula sees beam without understanding that the beam is distributed across two or three slender hulls, not one wide one. A 40-ft catamaran does not capsize in the same way a 40-ft beamy monohull does, and CSF doesn't tell you anything useful about it.
Reading the number as a buyer
Don't worry about cube roots. If a listing tells you the CSF — or you compute it below — here's what the output actually says about the boat's offshore character.
What different CSF values mean:
- CSF below 1.8. Pedigreed offshore design. Narrow-for-its-weight, sits deep, hard to invert and almost certain to recover quickly if it does. Westsail 32 and other traditional bluewater designs sit here.
- CSF 1.8 – 2.0. Comfortably passes the screen. Most older offshore cruisers (Valiant, Pacific Seacraft, Hallberg-Rassy, etc.) land in this band. A reasonable margin for serious passage-making.
- CSF 2.0 – 2.2. The borderline that catches most modern production cruisers. Not unsafe — but a flag if you intend to be offshore in genuine weather. Pair with B/D, hull form, and ideally the stability curve.
- CSF above 2.2. Distinctly modern wide-and-light territory. Charter cats and beamy production cruisers. Fine for coastal sailing and protected waters; serious storm exposure is a different question.
How to use it as a filter:
- Compute it from the spec sheet using only beam and displacement — two numbers every listing has. It's the cheapest filter you can run.
- If you intend serious offshore work and CSF is above 2.0, dig deeper into hull form, ballast placement, and — ideally — the published stability curve.
- Pair CSF with B/D. A low B/D and a high CSF is the worst case for inversion resistance.
- Don't reject a boat purely on CSF. A coastal sailor who never expects breaking-wave conditions has little reason to fear values above 2.0. A bluewater voyager has every reason to care.
A quick example. Compare a 32-ft Westsail (11 ft beam, ~20,000 lb) to a modern 46-ft production cruiser like the Beneteau Oceanis 46.1 (14.8 ft beam, ~23,000 lb). The Westsail scores around 1.6; the Oceanis lands just above 2.0. Both will sail well on a sunny coastal afternoon. In a Force 10 storm with breaking waves, the math says the Westsail is markedly more likely to roll back upright.
