Sail Area to Displacement Ratio (SA/D)

The marine equivalent of an automobile's horsepower-to-weight ratio. Wind on the sails is the driving force; displacement is the inertial mass that must be moved and the volume of water that has to be physically displaced for the boat to make way. SA/D captures both sides of that equation in a single dimensionless number that can be compared directly across boats of very different sizes.

You'll see SA/D printed in nearly every magazine review and on most broker spec sheets, because it's the closest thing sailing has to a published horsepower figure. The numerator is easy — square footage of canvas. The denominator is the part that takes a moment to absorb: you can't just divide by displacement, because pounds and square feet aren't the same kind of quantity. The trick is to convert displacement into the volume of seawater the boat pushes aside, then take the two-thirds power of that volume to get back to an equivalent area, so power and weight end up in comparable units. With that step in place, the same number compares meaningfully whether the boat is a 24-ft daysailer or an 80-ft cruiser.

Formula

In imperial units, with displacement in pounds:

• SA — Sail area in square feet (mainsail + 100% of the foretriangle — see the caveat below)
• D — Displacement in pounds
• 64 — Weight of one cubic foot of seawater in pounds
• V — Volume of displacement in cubic feet, i.e. $D/64$

The ratio is dimensionless — area in square feet divided by area in square feet — so it works directly in metric units as well (using sea-water density of 1.025 t/m³ to convert displacement to cubic meters). It is comparable across any size of boat in any unit system.

Why the 2/3 power?

Dividing displacement by 64 converts pounds to cubic feet of displaced water — a 3D volume. Raising that volume to the 2/3 power converts it into an equivalent 2D surface area, so it can be compared dimensionally against the square footage of the sail plan. Without the 2/3 power the ratio wouldn't be unit-consistent, and a 25-footer couldn't be compared with a 50-footer.

What it predicts

SA/D primarily indicates acceleration and light-to-moderate air performance. Wave-making resistance grows disproportionately as a boat approaches its theoretical hull speed, so SA/D is most useful as a measure of how easily and how quickly a boat will reach displacement speed — and how it will behave before the wind builds enough to push every hull near its limit.

It does not tell you about heavy-air performance, upwind ability, or stability under sail — those depend on hull form, ballast placement, and the GZ curve, not on the power-to-weight ratio alone.

Interpretation

Ted Brewer's classification, summarized in Ted Brewer Explains Sailboat Design and widely repeated, breaks the SA/D spectrum into recognizable boat types:

Open-class round-the-world racers (IMOCA 60s and prior generations of solo-offshore boats) live in an entirely different league — SA/D values in the high 30s to mid 40s are routine, and modern Volvo and Cup boats are higher still.

A useful rule of thumb from the IOR era (1970s–80s): SA/D above 17 was considered fast, below 16 was considered slow (Sail Magazine, Comparing Design Ratios). Modern carbon spars, synthetic rigging, and laminate sailcloth let production cruisers safely carry larger rigs than their predecessors — pushing the average modern cruiser closer to 20. A boat considered overpowered in 1985 is mainstream today.

For the owner: Are you looking for a gentle floating cottage (low SA/D) or a spirited sports car (high SA/D)? Your answer narrows the target range — and tells you how much rig management you're signing up for.

Caveat: sail-area inflation

The biggest source of distortion when comparing SA/D across boats is how the sail area is measured. The standard, comparable convention is:

Mainsail area + 100% of the foretriangle

The foretriangle is calculated from the rig dimensions I (mast height from the deck to the forestay attachment) and J (horizontal distance from the mast base to the forestay attachment at the bow):

Marketing brochures frequently inflate the published sail area by:

• Using a 130% or 150% overlapping genoa in place of the 100% foretriangle.
• Including the roach — the extra rounded area on a modern fully-battened mainsail's trailing edge.
• On cutter rigs, adding both the yankee jib and the inner staysail.

The compounding effect is large. A modern boat's headline SA/D can look 15–25% higher than a comparably-rigged classic simply because of how the manufacturer counted the sails. Always normalize both boats to the 100% foretriangle + nominal mainsail standard before comparing.

Caveat: payload sensitivity

SA/D assumes the displacement number on the spec sheet. Modern light-displacement boats (D/L < 150) are exceptionally sensitive to load. Adding a few thousand pounds of cruising gear — anchor chain, a watermaker, solar arrays, provisions for a long passage — submerges the wide flat stern, increases wetted-surface drag, and drops the effective SA/D meaningfully. A heavy-displacement boat barely settles an inch on the same load, and its SA/D stays close to its designed value. Liveaboard buyers should mentally adjust the published SA/D downward for any ultralight design they're considering.

Sister ratio: SA/WS (sail area to wetted surface)

For pure light-air performance, SA/D's sister ratio is SA/WS — sail area over total wetted surface (hull + keel + rudder):

When the wind is barely moving the air, wave-making drag is negligible and frictional drag (from the wetted surface dragging through the water) dominates. SA/WS captures the resulting power-to-drag ratio. Skene's Elements of Yacht Design gives typical keel-boat SA/WS values of about 1.9–2.4 at 25-ft LWL up to 2.9–3.3 at 80-ft LWL.

The catch: wetted surface is rarely published, so SA/WS is hard to look up. Offshore cruisers also carry enough hull, keel, and rudder area to support real loads, so wetted surface tends to be high — and SA/WS becomes a secondary concern compared with SA/D. Round-the-buoys racers in light-air venues care about it deeply; ocean voyagers, less so.

Reading the number as a buyer

Don't worry about the inputs. If a listing tells you the SA/D — or you've plugged numbers into the calculator below — here's how to translate the output into what you'll actually experience on the water.

What the number feels like at the helm:

• SA/D ≈ 14 – 16 — You'll motor more than you'd like to in summer. In a 10-knot breeze the boat moves, but slowly. The flip side: the rig is unstressed, you almost never reef, and the boat is forgiving when a squall hits before you can react.
• SA/D ≈ 17 – 19 — The mainstream cruising sweet spot. The boat sails meaningfully in 5–8 knots of breeze, accelerates respectably out of tacks, and still asks you to reef only as the wind builds past 18–20 knots. This is the band a couple can manage short-handed without anxiety.

  

• SA/D ≈ 20 – 22 — You'll feel the boat want to go in any wind. Expect to reef earlier than your dock-mates and to put more attention into trim. Light-air sailing is genuinely fun rather than a chore.
• SA/D > 22 — Treat it as a performance boat. The rig will overpower the hull before most cruisers would even think about reducing sail. Plan for active sail management as part of the experience, not a chore.

How to use it as a filter:

1. Decide what kind of sailing you want. Lazy summer evenings on a lake? An SA/D under 17 is fine and probably better. Coastal hops where you want to actually sail in 8-knot afternoons? Aim for 18+. Spirited weekend racing? 20+.
2. Always normalize. If one boat's brochure quotes a 150% genoa and another quotes the 100% foretriangle, the first looks 15–25% peppier than it really is. Use the calculator below with the 100% foretriangle + mainsail value for both.
3. Cross-check against displacement. A high SA/D on a heavy boat is genuine power. A high SA/D on an ultralight is a warning that the boat needs both wind and active sailing to behave.

A quick example: The Catalina 30 (~16) sails like an honest, manageable family cruiser — you might motor in a light morning, but the boat takes care of itself in 12 knots of breeze. The J/109 (~22) at the same dock will be ghosting along in 4 knots of wind and asking for a reef before the Catalina's owner has noticed conditions changed. The Melges 24 (~33) shouldn't be measured against either — it's a different category of boat entirely.

Calculator

Below are some example boats with their sail area and displacement values. Enter the 100% foretriangle + mainsail number for the result to be comparable to other boats here.