B/D
Ballast / displacement ratio
B/D = (Ballast_weight / Displacement) * 100%
On brochure sheets, a 40% ballast ratio looks like an absolute guarantee of stiff sea-kindly performance. Out on the water in a 22-knot gust, reality is dictated by keel depth rather than brochure percentages. A yacht with a shallow cast-iron keel and 42% ballast can heel over to its toerail, forcing the helm to fight round-up tendencies and dump the mainsheet. Right beside it, a cruiser carrying only 33% ballast in a deep lead torpedo barely heels past 15 degrees, turning the puff directly into forward speed. The ballast to displacement ratio (B/D) calculates keel mass relative to total boat weight, but without keel draft and vertical centre of gravity, that percentage is dangerously deceptive.
Formula
B/D = (Ballast_weight / Displacement) * 100%
Dimensionless percentage. Both ballast and displacement must use identical mass units (either pounds or kilograms).
Try your numbers
B/D
Result
Enter positive numbers in all fields.
Rule of thumb
Cruising monohull benchmark bands: Under 30%: Modern wide-transom hulls with high initial form stability, shoal centerboarders, or motorsailers; they sail stiff up to 12 degrees then require early reefing once true wind hits 15 knots. 30% to 38%: Modern family production cruisers (Bavaria, Beneteau Oceanis, Dufour, Hanse); paired with standard draft of 1.8 to 2.1 m, this offers a predictable balance of comfort and hull loads. 38% to 45%: Heavy-weather ocean cruisers (Hallberg-Rassy, Najad, Sweden Yachts, Linjett); outstanding gust resistance, comfortable offshore motion, and high angle of vanishing stability (AVS). Above 45%: Traditional full-keel voyagers (Contessa 32, Rustler 36) requiring massive underwater mass to compensate for shallow draft.
Limits
Two critical specification traps: First, calculating ballast against brochure lightship displacement. A 7500 kg yacht with a 3000 kg keel shows 40.0% B/D on paper. Loading 300 litres of diesel, 400 litres of water, 70 metres of chain, dinghy, outboard, and stores increases live displacement to 9500 kg, dragging actual B/D down to 31.5%. Second, ballast ratio ignores keel material density. Lead (11.3 g/cm3) is 57% denser than cast iron (7.2 g/cm3), concentrating weight lower down and increasing the righting arm without adding hull weight.
Boat examples
- Hallberg-Rassy 36
Moderate ocean passage maker: 3450 kg lead keel in a 7500 kg hull provides 46% lightship B/D, holding high stiffness and soft motion in North Sea squalls.
- Beneteau First 31.7
Fast performance cruiser: 38% ballast ratio on a deep 1.9 m iron fin delivers quick helm response, but demands prompt first reefs in Baltic chop.
- Linjett 40
Modern Scandic cruiser: 37% ballast concentrated in a deep lead bulb provides the righting lever of a 45% shallow keel while saving total weight.
History
For over a century, yacht brokers have quoted ballast percentages as shorthand for whether a hull is stiff or tender. In the era of traditional long-keeled yachts like the Folkboat (50% ballast) and Contessa 26 (57%), ballast sat directly inside the hull belly. Without deep draft, huge ballast fractions were mandatory to create a righting lever. The arrival of modern high-aspect fin keels and bulb profiles proved that lowering the centre of gravity creates far more righting moment with significantly less deadweight.
FAQ
- Why can a 33% ballast boat be stiffer than a 42% boat?
- Righting moment equals ballast weight multiplied by righting arm. A deep lead bulb at 2.1 m draft creates a longer lever arm than shallow cast iron at 1.45 m draft.
- How does cruising gear affect real-world ballast ratio?
- Stores, full tanks, chain, and crew add 1.5 to 2.5 tonnes to a mid-sized cruiser. The keel weight stays fixed, causing real B/D to drop by 6 to 9 percentage points.
- Why does keel material matter if weight is identical?
- Lead is 57% denser than cast iron. A lead keel is more compact and lowers the vertical centre of gravity by 15-25 cm, increasing righting moment significantly.