Capsize screening formula calculator
CSF = beam(ft) ÷ ∛(displacement(lb) ÷ 64). Values of 2.0 or below are conventionally read as offshore-favorable.
The formula and the 2.0 threshold
CSF = Beam (ft) ÷ ∛( Displacement (lb) ÷ 64 )| CSF value | Reading |
|---|---|
| ≤ 2.0 | Offshore-favorable: relatively narrow and heavy for her size. |
| > 2.0 | Coastal profile: beamier and lighter; form stability over ballast. |
Intuition: wide, light boats resist heeling at first but are more stable upside down once capsized. Narrow, heavy boats are the reverse. The screen compresses that trade-off into one number from two published specs.
Use it as a flag, not a verdict
The CSF ignores ballast placement, keel shape, cockpit volume and the full stability curve. It was designed as a first-pass screen when no stability data is available, which is exactly the situation of someone reading a used-boat listing. Read it alongside the other ratios, then worry about condition.
The deeper story, including what the Fastnet committee actually studied, is in the capsize ratio guide. For all six numbers at once, use the design ratios calculator.
Knotworth computes the CSF and every other ratio straight from a listing URL, free, and flags what the ad doesn't say about the boat's condition.
Capsize screening questions
Which displacement number should I enter?+
Use the published light-ship displacement, the same figure listings and design databases show. Loading the boat with cruising gear makes her heavier and actually lowers the screening value slightly, so light-ship is the conservative, comparable choice.
Why does my result differ slightly from sailboatdata's?+
Usually rounding: metric-to-imperial conversion, or a different published displacement for the same model (builders revised specs across production years). Differences of a few hundredths are normal and never change the reading.
Can I use this calculator for a catamaran?+
No. The screening formula was built for ballasted monohulls; a catamaran's beam makes the number meaningless. Multihull stability is assessed with entirely different criteria.
My result is over 2.0. Does that rule the boat out?+
Not by itself. Above 2.0 conventionally reads as a coastal profile rather than an offshore one, and the screen ignores ballast placement and the full stability curve. What the threshold means, where it came from and its limitations are covered in the capsize ratio guide.