Ballast to Displacement Ratio (B/D)
The B/D ratio — also called Ballast Ratio — is the simplest of all stability metrics: the fraction of a boat's total displacement that sits in the keel as ballast. It's offered everywhere as a shorthand for "stiffness" — how hard the boat fights to stay upright under wind pressure. Used correctly, it's a useful filter. Used carelessly, it's one of the most misleading numbers on a spec sheet.
Brokers, surveyors, and sailing magazines all quote it because it's the only stability number you can pull from a spec sheet without digging into a stability booklet — two values, one division. As a buyer scanning listings, you'll see it printed alongside displacement and sail area as a quick "what kind of boat is this" header. The underlying idea is intuitive: the more of the boat's total mass is concentrated in the keel, the harder the boat fights heeling forces. With that picture in mind, the formula is barely more than a definition.
Formula
Where ballast weight is the weight of dedicated ballast (typically lead or iron in the keel) and displacement is the boat's full sailing weight. The result is a percentage between roughly 25% and 50% for most monohulls.
Interpretation
| B/D | Stiffness |
|---|---|
| ≤ 25% | Tender. Heels easily. Usually unsuitable for severe offshore work unless the design leans heavily on form stability (wide beam) or carries its ballast very low. |
| 30 – 35% | Average. Standard coastal-cruiser range. Sensible balance of stiffness and total weight. |
| 35 – 40% | Stiff. Carries good sail area in moderate winds without crew on the rail. |
| 40 – 50% | Very stiff / powerful. Stands up to large rigs in heavy winds. Common on hardcore performance cruisers and racers from the IOR era. |
Practical Sailor's Measuring Performance and Ted Brewer's commentary in Good Old Boat both note that the historical 40% threshold meant something specific on traditional shallow-keel boats — and means something quite different on a modern deep-bulb design.
The big caveat: ballast placement is everything
B/D is blind to where the ballast actually sits. It calculates total weight as a percentage of total mass — nothing more. But the physics of righting moment are about leverage: weight × distance from the roll axis. Two boats with identical B/D can have wildly different righting moments depending on keel architecture.
Consider two 18,000-lb boats with identical 40% ballast ratios — both carry 7,200 lb of lead:
- Full keel: 7,200 lb of lead encapsulated in a 4-ft draft shallow bilge. Excellent grounding protection, beautiful tracking, easy to support on a yard cradle. But the short lever arm produces weak mechanical leverage. Most of the righting moment comes from form stability (beam).
- Deep fin with bulb: 7,200 lb concentrated in a torpedo bulb 8 ft below the waterline. Same B/D, exponentially more righting moment from leverage alone. The boat will feel stiffer in every condition that matters.
In practice, a naval architect can match the righting moment of a 45% shallow-keel boat with a 25% deep-bulb design. That sentence should be tattooed on every B/D number you see published.
Bottom line: B/D only means something once you've also looked at keel depth and geometry. A modern boat with B/D in the low 30s and a deep bulb is often stiffer than a heavy classic with B/D above 40 on a shallow encapsulated keel.
Form stability vs. ballast stability
There are two ways a sailboat resists heeling:
- Ballast stability — weight low in the hull creates a long lever that fights the heeling moment of the wind on the sails. This is what B/D measures, indirectly.
- Form stability — a wide hull, when heeled, immerses asymmetrically and generates a buoyancy moment that resists further heeling. This is what makes a beamy catamaran or a flat-bottomed skiff stiff initially.
Modern wide production cruisers lean heavily on form stability. They can run a relatively low B/D and still feel stiff in light to moderate winds. The tradeoff is what happens at extreme angles: form stability reverses past about 60–70° of heel (the wide hull becomes stable inverted), whereas ballast stability keeps trying to right the boat all the way to the limit of positive stability. This is why the Capsize Screening Formula penalizes beam — and why B/D alone is a poor measure of offshore stability.
At the opposite end, narrow classics with disproportionately heavy ballast deliver ratio numbers that look extreme today — but they were the design language of small offshore boats before form stability and wide-stern designs took over.
Wing keels and shoal-draft variants
Many production boats are offered in both deep-keel and shoal-draft configurations. The shoal version typically has the same total ballast — same B/D — but with the lead spread horizontally (often as a wing keel with horizontal flares at the tip) to lower the center of gravity within the constraint of a shallower draft.
Wing keels lower the CG modestly compared to a chopped fin of the same draft, but they're a workaround, not a substitute for a deep bulb. They:
- Reduce upwind performance. Wings increase wetted surface and disturb flow off the keel tip.
- Compromise grounding recovery. A wing can dig into mud or sand and resist refloating in ways a clean fin won't.
- Don't fix the offshore stability gap. A boat's Limit of Positive Stability drops measurably between the deep and shoal versions of the same design — typically by 10° or more.
Use wing/shoal draft where they buy access to specific anchorages, mooring fields, or canals you can't otherwise reach. Don't read them as a free lunch.
What to look for instead
If you want a real picture of how stiff a boat is, ask for two numbers:
- Maximum righting moment at the angle of maximum righting arm (typically 50–60°). This combines weight, leverage, and hull form into a single physical force.
- Limit of Positive Stability (LPS) — the heel angle past which the boat won't right itself. Offshore monohulls should be 120° or higher. The deep-keel version of a given hull will measure noticeably higher than the shoal version.
Together, these tell you what B/D can only hint at: how the boat behaves under real sail loads, and how it behaves after a knockdown. See the Capsize Screening Formula page for more on the GZ curve (the curve of righting arms) — the underlying physics that B/D approximates.
Reading the number as a buyer
Don't worry about subtracting weights or chasing lead-vs-iron specifics. If a spec sheet hands you a B/D — or you compute one below — here's how to translate it into what you'll actually feel under sail, paired with the one caveat that matters: where the lead actually sits.
What the number feels like under sail:
- B/D ≤ 25%. The boat heels easily. In a 15-knot beam reach it will lay down to its rail with full sail up, and you'll be thinking about reefing earlier than your dock-mates. If the keel is shoal/wing, treat it as a fair-weather boat. If the keel is deep with a bulb, it can still be plenty stiff — and the ratio is misleading you.
- B/D 30 – 35%. The mainstream cruiser zone. Carries normal working canvas without crew on the rail; needs a reef when the wind comes up past 18–20 knots. Most modern production cruisers live here.
- B/D 35 – 40%. Stiff. Carries a full main and genoa comfortably into the low 20s of wind. Older offshore designs and stiffer cruiser-racers cluster here.
- B/D > 40%. Powerful and demanding. Common on IOR-era cruiser-racers and traditional bluewater boats. The boat will carry sail in conditions that would have a 30% boat over-canvased — if the rig is matched to it.
The caveat you can't skip: keel architecture.
Two boats with B/D 40% can feel completely different. A shoal full keel carrying its lead 4 feet down is less stiff than a deep fin with a bulb carrying 30% B/D eight feet down. Always pair B/D with draft and keel type before trusting it:
- Deep fin + bulb keel. B/D can be moderate (32–38%) and still feel stiffer than a 45% full-keel boat. Mechanical leverage wins.
- Modified fin with skeg. Stiffness scales roughly with B/D × draft. Good middle ground.
- Full keel. B/D needs to be higher to compensate for the short lever arm — but you trade light-air performance for tracking and protection.
- Wing/shoal-draft keel. Often the same B/D as the deep version, but with LPS down 10° or more. Acceptable for coastal work where you need the shallow draft; a real cost for serious offshore intent.
A quick example. The Catalina 30 and the J/109 both compute to roughly 40% B/D — but they sail completely differently. The Catalina carries its lead in a moderate-draft fin and feels stiff-enough for coastal sailing in moderate winds. The J/109's lead lives in a deep keel with a bulb, several feet farther below the waterline, and the boat will carry its full rig in winds that would have a Catalina owner well into a second reef.
Calculator
Below are some example boats with their ballast and displacement values. Remember to also note the keel type when comparing — see the caveat above.