What the capsize screening value measures
After the 1979 Fastnet race, in which a storm capsized and rolled many yachts and fifteen sailors died, the United States Yacht Racing Union and the Society of Naval Architects and Marine Engineers studied why some boats were knocked down and inverted while others survived. One outcome was a simple screening formula: wide, light boats were more likely to be capsized by a breaking wave, and once upside down they were more stable inverted than narrow, heavy ones.
The formula
The value divides the beam by the cube root of the displacement volume, so it has no units and is the same in metric and imperial.
- Metric:
CSV = Beam (m) ÷ (Disp (kg) ÷ 1025)1/3, with 1025 kg/m³ for seawater. - Imperial:
CSV = Beam (ft) ÷ (Disp (lb) ÷ 64)1/3, with 64 lb/ft³ for seawater.
How to read it
Lower is better. A value under 2.0 is the common guideline for boats intended to cross oceans; it is widely used by offshore racing organisers and cruising guides as a first check. Values above 2.0 do not mean a boat is unsafe, but that it deserves a closer look at its stability data before going offshore. Modern beamy production boats often score 2.0 to 2.2; narrow traditional cruisers often score 1.6 to 1.8.
Limitations
It is a screening value, not a stability assessment. It ignores where the weight is: a deep bulb keel and a light rig make a boat far more resistant to capsize than its beam and displacement suggest, and a heavy deck saloon does the opposite. It ignores the shape of the hull and the deck, the righting moment at large angles, and the angle of vanishing stability. For an informed judgement, look at the stability curve and the ISO 12217 stability index (STIX) on which the CE design categories are based; the ballast ratio adds a little more context.
The formula uses published light displacement. A cruising boat loaded for a passage is heavier, which lowers its value, although weight stowed high, such as solar arches and dinghies on davits, can reduce stability at the same time. The formula was never intended for multihulls, which are stable upside down by nature, and their values are not comparable.
Sources
The formula comes from the USYRU/SNAME Safety from Capsizing study that followed the 1979 Fastnet race. Our catalogue values come from sailboatdata.com, and this calculator reproduces them to within half a percent. See our sources and methodology.