Understanding the Numbers

A quick screen developed after the 1979 Fastnet disaster to flag boats whose proportions make them more vulnerable to capsize in large breaking seas. Beam in feet, displacement in pounds.

How to read it: Lower is better. A result of 2.0 or below is the conventional cutoff for offshore suitability; below 1.8 is excellent. Wide, light boats score high; narrow, heavy boats score low.

Where it falls short: It uses only beam and displacement. It ignores ballast placement, hull shape, and cabin buoyancy, all of which matter enormously in a real capsize. A ballasted narrow boat and a narrow boat with concrete in the bilge can score identically. Treat it as a first filter, never a verdict.

Also called Limit of Positive Stability. The heel angle beyond which a boat no longer rights itself and will instead continue to a full inversion. Derived from the stability curve, which depends on hull shape, ballast, and deck/cabin geometry.

How to read it: Higher is better. Offshore racing rules typically require 120° or more; 130°+ is excellent for a bluewater cruiser. A boat with an AVS of 120° that is knocked flat (90°) still has a 30° margin before it turns turtle, and a high AVS also means the boat spends less time inverted if it does go over.

Where it falls short: Published AVS figures assume light-ship trim; a loaded cruiser with gear on deck, a dinghy on davits, and half tanks will have a lower real-world AVS. Figures are also scarce for older production boats; absence of data is not evidence of a problem, just of an era that didn’t publish stability curves.

A dimensionless measure of how heavy a boat is for its waterline length. Displacement in pounds (converted to long tons), LWL in feet.

How to read it: Under ~100 is ultralight, 100–200 light, 200–300 moderate, 300+ heavy. Most classic bluewater cruisers on this site fall in the 300–400 range. Heavier boats carry more stores and tend toward a slower, steadier motion; lighter boats are faster in light air and surf earlier downwind.

Where it falls short: Says nothing about where the weight is. A boat with weight concentrated low in ballast behaves very differently from one with the same D/L and a heavy deck. It also punishes long waterlines: two identical hulls with different LWL measurement conventions can score quite differently.

The sailing equivalent of a power-to-weight ratio. Sail area in square feet (usually 100% foretriangle plus mainsail), displacement in pounds converted to cubic feet of seawater.

How to read it: Under 15 is under-canvassed or heavy (motorsailer territory), 15–18 is typical for cruisers, 18–22 performance cruisers, above that racing boats. A low SA/D boat will need the engine in light air; a high SA/D boat needs earlier reefing.

Where it falls short: Based on nominal sail plan, not what a boat actually flies: a cruiser with a big genoa or an asymmetric can sail well above its published number. It also uses light-ship displacement, so a loaded cruiser’s real SA/D is lower than the brochure figure.

Ted Brewer’s tongue-in-cheek-but-useful estimate of how quickly a hull accelerates in a seaway, the jerkiness of motion, which is what actually fatigues a crew. Displacement in pounds, lengths and beam in feet.

How to read it: Higher is more comfortable. Under 20 is lively (daysailers, racers), 20–30 typical coastal cruisers, 30–40 moderate bluewater cruisers, 40+ the heavy traditional passage-makers. Narrow, heavy, longer boats score high.

Where it falls short: Brewer himself called it a bit of a joke that happened to work. It ignores hull sections, damping from the keel, and rig inertia. Use it to compare boats of broadly similar type, not as an absolute scale.

The percentage of a boat’s total displacement carried as fixed ballast.

How to read it: Most cruising sailboats fall between 30% and 45%. Higher generally means a stiffer boat that stands up to its sail plan and recovers from a knockdown more decisively.

Where it falls short: Ratio alone ignores ballast depth: 40% in a shallow full keel does less righting work than 35% in a deep bulb. Two boats with identical ratios can have very different stability curves. Read it together with draft and AVS, not alone. On a lifting-keel or centerboard boat (Ovni, Garcia, Boreal, and similar aluminum designs, among others in this catalog), the published ratio describes the board-down condition only; the same design intentionally trades a lower static ballast ratio for the ability to reduce leeway and skid away from a breaking sea with the board partly raised in survival conditions, a real, sourced offshore-safety strategy this single number cannot represent one way or the other.

The theoretical maximum speed of a displacement hull, reached when the boat’s bow and stern waves merge into a single wavelength. A 25 ft waterline gives about 6.7 knots.

How to read it: Longer waterline = faster boat, which is why LWL matters more than LOA for passage times. Real-world sustained passage speeds are typically 60–75% of hull speed once weather, sea state, and sail changes are averaged in.

Where it falls short: It’s a soft wall, not a hard one: light boats can exceed it by surfing or planing, and heavy boats may lack the power to ever reach it. The 1.34 constant is a rule of thumb for traditional hulls; modern flat-sectioned hulls routinely beat it downwind.

The general, underlying relationship that the hull-speed entry above is one named point on. S/L expresses how fast a boat is moving relative to its own waterline length, independent of absolute size, and serves the same purpose as the naval architect’s Froude number in different units.

How to read it: An S/L of 1.34 is the classic hull-speed threshold: the bow and stern wave systems interfere and a traditional displacement hull needs disproportionately more power to go faster. Below that, a boat is comfortably within its displacement mode. Well above it, a hull is planing, semi-planing, or otherwise escaping displacement-mode wave drag, the territory of racing multihulls and some modern flat-sectioned monohulls surfing downwind. Where a boat’s real speed sits on this ratio describes how fast it feels for its size better than raw knots do.

Where it falls short: The ratio alone says nothing about hull shape, only length and speed, so two boats at an identical S/L number can behave very differently. Naval architects and builders discussing chine versus round-bilge hulls note that the same hull-shape choice matters more or less depending on where a boat sits on the S/L curve, and that chines on true planing-speed vessels are a different subject from chines on the displacement and semi-displacement hulls this catalog mostly covers. Whether a given hull shape helps or hurts at a specific S/L number is still something naval architects determine empirically, tank testing or CFD, treated as indicative rather than accurately predictive, not something a formula settles on its own. Like hull speed, a published S/L figure describes a light-ship boat in ideal conditions; load, trim, and sea state change how much of it a real passage actually delivers.

A full keel runs most of the hull’s length, often with the rudder hung on its trailing edge; many designs soften it with a cutaway forefoot. A fin keel is a shorter, deeper appendage, usually paired with a separate rudder. A centerboard or lifting keel raises and lowers, most often on aluminum or steel expedition-style designs, trading a fixed draft for the ability to explore shoal anchorages or dry out intentionally.

How to read it: Full keels track steadily, heave-to well, protect the propeller and (via a keel-hung rudder) the rudder itself in a grounding or debris strike, and take the ground more gracefully, at the cost of speed, pointing ability, and maneuverability in marinas. Fin keels are faster, closer-winded, and far handier under power. Keel type and rudder type are separate design choices, not a package deal: a fin keel or a centerboard/lifting keel is just as free to carry a skeg ahead of the rudder for directional stability and impact protection as it is to carry a fully exposed balanced spade, and plenty of respected bluewater designs pair a fin or lifting keel with a skeg-hung rudder for exactly that reason. See Skeg-Hung vs. Spade Rudder below for that trade-off on its own terms; don't assume a boat's rudder protection from its keel type alone. A lifting keel adds real offshore range beyond convenience: sailors and builders of this type (Ovni, Garcia, Boreal, and similar designs) report that reducing draft in survival conditions lets the boat skid sideways away from a breaking sea rather than tripping over a fixed deep keel, a genuine dynamic-stability strategy distinct from a fixed keel's static righting moment.

Where it falls short: Keel type is a poor proxy for seaworthiness by itself; execution matters more than category, and rudder protection specifically is a separate spec to check rather than one keel type carries by default. A well-engineered fin-keeler has circumnavigated many more times than dock wisdom suggests, and a badly built full-keeler is still a badly built boat. A lifting keel also adds real mechanical complexity (the lifting mechanism itself, and the keel box/trunk it retracts into) that a fixed keel simply doesn't have, worth a specific survey line item rather than assuming it away.

A skeg-hung rudder sits behind a fixed fin (the skeg) that supports it top and bottom. A spade rudder hangs on its stock alone, fully balanced and unprotected.

How to read it: The skeg protects the rudder from impact and takes bending loads off the stock, reassuring for remote-waters cruising. A spade gives lighter, more precise steering and less drag, and because part of its area sits ahead of the stock it needs less helm effort. Keel-hung rudders (on full-keel boats) are the most protected and least efficient of all.

Where it falls short: A skeg is only as good as its construction: a weakly built skeg can itself be the failure point, tearing hull laminate when struck. Conversely, modern spade stocks are heavily engineered. As with keels, build quality beats configuration.