Enter the beam and displacement for the capsize screening formula. Add the waterline length, sail area and ballast to get the other three ratios brokers and buyers quote. Every number works in feet and pounds or metres and kilograms.
The formulas
Capsize screening formula. Beam ÷ ∛(displacement in lb ÷ 64). Dividing by 64 turns pounds into cubic feet of seawater. YachtDatabase gives the cutoff: 2 or less for ocean passages, over 2 for coastal cruising.
Displacement-length ratio. (Displacement in lb ÷ 2,240) ÷ (0.01 × waterline in ft)³. The first term is the displacement in long tons. Wikipedia's bands run from ultralight under 90 to ultraheavy at 360 and up.
Sail area-displacement ratio. Sail area in sq ft ÷ (displacement in lb ÷ 64)^⅔. SAIL counts under 15 as under-canvased, over 15 as reasonable performance and 18 to 20 or more as relatively high performance.
Ballast ratio. Ballast ÷ displacement × 100. SAIL says the higher it is, the more the boat resists heeling.
Which displacement to use
SAIL compares boats at half-load displacement: equipped for sailing, crew aboard, half the stores, water and fuel. Builders sometimes quote the lighter weight of the boat as shipped, which lowers the ratios and flatters the boat. Note which figure you used.
What the numbers don't tell you
The capsize screening formula ignores hull shape and ballast position, and Wikipedia gives the example of a light fin keeler with a deep bulb scoring the same as a heavy full keeler. For a CE-marked boat, the design category on the builder's plate comes from an ISO 12217 stability assessment: see ISO 12217 design categories.
Citing it in a report
"Capsize screening formula 2.07 from the builder's published beam of 12 ft and displacement of 12,500 lb; over the 2.0 often used as the cutoff for ocean passages. The boat is CE design category B." Give the figures and where they came from. Add the design category if the boat has one.