The capsize ratio, explained: what counts as a good value
Few numbers in a boat listing carry more emotional weight than the capsize ratio, and few are more misread. This guide covers what the formula actually measures, where the famous 2.0 threshold comes from, and the limits the committee that created it was honest about from the start.
If you just want the number, use the free capsize screening calculator. This page is the why behind it.
What is the capsize screening formula?
The CSF divides a boat's maximum beam, in feet, by the cube root of her displacement in cubic feet of seawater. Wide and light produces a high value, narrow and heavy a low one. The lower the number, the less prone the design is to capsize and to remaining upside down afterwards.
CSF = Beam (ft) ÷ ∛( Displacement (lb) ÷ 64 )The counterintuitive part is that beam works against you. Wide, flat boats feel stiff in normal sailing because of form stability, but the same geometry makes them stable inverted once a breaking wave puts them over. Heavy, narrow hulls give up initial stiffness and gain ultimate recovery.
Where the 2.0 threshold comes from
In the 1979 Fastnet Race, a Force-10 storm hit a fleet of 303 yachts; boats were rolled and abandoned and 15 racing sailors died, with more lives lost aboard boats shadowing the fleet. The Joint USYRU/SNAME Committee on Safety from Capsizing that followed studied why some designs capsized and stayed inverted while others recovered.
One output of the committee's 1985 final report was this screen: a deliberately simple test a buyer or race organizer could apply with two published numbers. The working threshold was set at 2.0, and race committees have used it, and more refined descendants of it, as a screening cutoff since. Its own authors called it exactly that, a screen, to be followed by real stability analysis where possible.
| CSF value | Conventional reading |
|---|---|
| 1.6 – 1.9 | Typical of narrow, heavy bluewater cruisers (a Westsail 32 screens about 1.6). |
| ≤ 2.0 | Offshore-favorable by the conventional threshold. |
| 2.0 – 2.2 | Common for moderate coastal cruisers and cruiser-racers. |
| > 2.2 | Beamy, light designs; form stability over ballast. |
What the capsize ratio does not measure
The screen ignores ballast amount and depth, hull shape, cockpit volume, deck and cabin structure, and the full righting-moment curve. Two boats with identical CSF can have very different real-world stability. It also says nothing about crew, preparation or condition, which decided outcomes at Fastnet as much as design did.
Ratio explainers from Good Old Boat and long-running forum debates on SailNet keep reaching the same conclusion: use the CSF to compare candidates and raise questions, never as a pass/fail safety certificate.
Read it alongside the comfort ratio and displacement-to-length: the three together sketch the boat's character in a seaway far better than any one alone.
How Knotworth uses the capsize screen
When you paste a listing URL, Knotworth matches the boat to her design data, computes the CSF with every other ratio, and reads them against your sailing plans. A coastal screen value on a boat you want for ocean passages becomes a mission-fit flag in the free pre-check, stated plainly with the reasoning.
Then it moves to the half the ratios cannot see: the age and condition of the systems that actually keep a boat upright and dry, which is where the full report earns its keep.
Paste the listing URL. The pre-check computes the capsize screen and every design ratio, checks mission fit, and flags what the ad leaves unsaid.
Capsize ratio questions
What is a good capsize ratio for a sailboat?+
A capsize screening value of 2.0 or below is conventionally read as offshore-favorable; above 2.0 suggests a coastal profile. Long-legged bluewater cruisers often screen between 1.6 and 1.9, while beamy inshore designs can exceed 2.2.
Is the capsize ratio the same as a stability curve?+
No. A stability curve (GZ curve) is a full engineering picture of righting force through every heel angle. The capsize screen is a two-input shortcut created for situations where no stability data exists, which is why it works from numbers found in any listing.
Can I trust the capsize ratio for a modern wide-stern design?+
Only loosely. The screen was calibrated on 1970s and 80s hull forms. Modern beamy designs with twin rudders and deep bulb keels read worse on the screen than their engineered stability suggests. Compare like with like, and prefer real stability data when it exists.
Does passing the capsize screen mean the boat is ready for offshore sailing?+
No. The screen describes the design as drawn. Whether this particular boat is offshore-ready depends on the condition of the rig, sails, seacocks, steering and safety gear, which is exactly what a listing tends to leave unsaid.
Where do I find the beam and displacement numbers?+
Both appear in most listings and on the designer specification pages of databases like sailboatdata.com. Use the boat's published light displacement consistently; loading a cruiser down changes the real number and slightly flatters the screen.