B/D
Ballast / displacement ratio
Ballast ratio is ballast weight divided by displacement, usually as a percent. It is a crude stiffness hint. FairHelm judgment: a 40% ratio with a shallow wing keel is not the same boat as 35% with a deep lead fin. Read keel geometry before trusting the percentage.
Formula
Ballast ratio = (Ballast_weight / Displacement) * 100
Same mass units for ballast and displacement (kg or lb). Result is a percentage.
History
Yards and brokers have quoted ballast percentages for decades as a proxy for 'stiff' or 'tender'. The number travelled with lead keels and iron keels alike, often without stating density or vertical centre of gravity.
Rule of thumb
Many cruising monohulls sit roughly 30-45%. Below about 30% invites questions about form stability and keel design. Above about 45% is less common on production cruisers and may signal race-oriented ballast packages.
Limits
Ratio ignores VCG and draft. Water ballast and movable ballast systems break the simple static reading. Iron versus lead changes volume for the same mass. Brochure displacement errors flow straight into the percent.
Boat examples
- Hallberg-Rassy 36
Deep encapsulated or bolted keel packages: ask surveyors about keel bolts and matrix, not only the percent.
- Beneteau First 31.7
Fin-keel performance cruiser: ballast ratio alone will not tell you how it reefs in a Baltic squall.
- Sweden Yachts 390
Compare published ballast to sisterships; marketing rounded percentages are common.
FAQ
- Is a higher ballast ratio always safer?
- No. Safety depends on stability curve, downflooding, and condition. A high ratio with a high VCG can still disappoint. Prefer a stability booklet or ORC/IMS data when available.
- Does water ballast count?
- Only when tanks are full, and even then the system dynamics differ from fixed lead. State tank state if you quote a number.
- How does this relate to CSF?
- CSF uses beam and displacement, not ballast. Use both: ballast ratio for weight distribution questions, CSF for beamy vs heavy form screening.