CSF
Capsize screening formula
CSF = Beam_ft / (Displacement_lb / 64)^(1/3) = Beam_m / (Displacement_kg / 1025)^(1/3)
In a marina, a wide modern cruiser feels rock solid, barely leaning when stepping aboard. Yet during the 1979 Fastnet storm, 15 sailors died and over 75 yachts were knocked down or rolled 360 degrees. The inquiry found that beamy, light hulls were rolled most readily by breaking waves, and once inverted, their wide flat bottoms acted like stable rafts. The Capsize Screening Formula (CSF) was created to filter out roll-prone hull shapes before heading offshore.
Formula
CSF = Beam_ft / (Displacement_lb / 64)^(1/3) = Beam_m / (Displacement_kg / 1025)^(1/3)
Imperial: Beam in feet, displacement in pounds. 64 is the weight of one cubic foot of seawater in pounds. Metric: Beam in metres, displacement in kilograms, 1025 is seawater density in kg/m3. Both yield an identical dimensionless number.
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CSF
Result
Enter positive numbers in all fields.
Rule of thumb
Offshore benchmark: CSF of 2.0 or lower is traditionally recommended for ocean-going monohulls. Values above 2.0 are common among modern coastal cruisers with wide aft sections. A score of 2.05 does not make a yacht unseaworthy, but signals that heavy-weather stability requires deeper scrutiny.
Limits
CSF is completely blind to keel ballast. It contains no data on ballast ratio, keel bulb depth (VCG), coachroof buoyancy, or Angle of Vanishing Stability (AVS). A boat with a deep lead bulb at 2.5 m draft gets the same score as a shoal-draft unballasted hull of identical beam and displacement. Modern ISO 12217-2 STIX replaces CSF for official ocean certification.
Boat examples
- Hallberg-Rassy 36
Displacement cruiser: Beam 3.55 m, Disp 7500 kg calculates to CSF 1.83. Heavy displacement and moderate beam comfortably clear the ocean benchmark.
- Linjett 40
Fast Baltic cruiser: Beam 3.75 m, Disp 7600 kg gives CSF 1.92. Substantial cruising displacement keeps the modern hull under the 2.0 line.
- Beneteau First 31.7
Light performance cruiser: Beam 3.23 m, Disp 3750 kg yields CSF 2.10. Stepping over 2.0 calls for proactive reefing and active crew management in strong winds.
History
Born from the USYRU and SNAME investigations into the 1979 Fastnet disaster. Researchers noted that IOR racing designs with wide beam and light displacement suffered disproportionate inversions. Dividing displacement by seawater density gives hull volume; taking the cube root yields an equivalent hull dimension in feet, making CSF a pure dimensionless ratio.
FAQ
- Why divide displacement by 64?
- Seawater weighs approximately 64 pounds per cubic foot. Dividing displacement in pounds by 64 gives underwater volume in cubic feet; taking the cube root converts volume into feet, cancelling the units of beam.
- What is the inverted stability trap?
- Very wide, flat-bottom hulls have high form stability both right-side up and upside down. If flipped by a huge wave, a beamy hull can remain inverted like a raft, whereas a narrow deep-keel yacht rights itself immediately.
- Is CSF under 2.0 an official maritime rule?
- No. It is an empirical screening heuristic from post-Fastnet literature. Official European Category A ocean classification relies on the comprehensive ISO 12217-2 STIX standard.