Motion Comfort Ratio (CR / MCR)
The Comfort Ratio (CR), also called the Motion Comfort Ratio (MCR), is naval architect Ted Brewer's metric for the "corkiness" of a sailing hull — how rapid and jerky its motion will be in confused seas. Crew fatigue and seasickness come from high-frequency snap-back accelerations, not from absolute heel angle. A slow, deep roll is far easier to endure on a 20-day passage than a fast, violent one.
Brewer first published the ratio in a Cruising World article in September 1990. He later wrote in Ted Brewer Explains Sailboat Design that he came up with it "tongue-in-cheek," but added: "the comfort ratio has been accepted by many as a measure of motion comfort, and indeed, it does provide a reasonable comparison."
You'll see CR cited in two places: cruising magazines and yacht broker write-ups (where it's offered as a proxy for "how seakindly is this boat?"), and in long-distance cruisers' own notes about boats they've owned. As a buyer, it's most useful when you're trying to predict how a boat will feel on multi-day passages — not after an afternoon of dock-walking. The intuition Brewer wanted to bake in: heavy boats with narrow beam and modest waterplane area roll slowly and comfortably; light, beamy boats roll fast and snappily. The formula puts displacement on top and a length-times-beam waterplane area on the bottom, with beam given extra weight because beam is what controls how fast the boat snaps back.
Formula
- D — Displacement in pounds
- LWL — Waterline length in feet
- LOA — Length overall in feet
- Beam — Maximum beam in feet
- 0.65 — A generic waterplane coefficient
The result has units of pounds per square foot (it's not dimensionless), so comparisons are only meaningful between boats measured in the same system.
What the equation is really saying
Look closely at the denominator: 0.65 × L × B is, structurally, the formula for a waterplane area, with a generic 65% block coefficient. The length term — 70% of LWL plus 30% of LOA — is an averaged "effective" waterline that's slightly longer than the static LWL, meant to simulate the dynamic wetted length as the boat moves through the water. Beam is raised to the 1.333 power (not just squared and not cubed) to give it stronger influence without overwhelming the equation.
The underlying physics: a boat's rolling acceleration is roughly proportional to metacentric height, and metacentric height is proportional to length × beam³. So motion roughness scales with L × B³. Brewer wanted "influence" rather than strict equality — exposing the equation to the cube of beam would have produced a tiny, awkward number — so he settled on the 1.333 power as a compromise that keeps CR in a reasonable, length-comparable range.
In Brewer's own words: "motion comfort depends on the rapidity of the motion; the faster the motion, the more upsetting it is to our human gyroscopes. Greater weight, or lesser area, means a lower motion, thus more comfort. Beam enters into it also as wider beam will generate a faster reaction, particularly in beam seas."
Interpretation
Brewer divided comfort zones into three rough bands:
| CR | Brewer's category | Feel |
|---|---|---|
| Under 20 | Lesser comfort | Quick, snappy, sometimes violent motion. Light racing boats and modern daysailers. |
| 20 – 30 | Average | Typical of coastal cruisers and modern production boats. Fine for weekends and short passages. |
| 30 – 40 | Greater | Moderate bluewater cruisers — comfortable on long ocean passages. |
| 40 – 50+ | Very high | Heavy oceangoing expedition vessels and classic full-keel yachts. |
Brewer himself noted that comfort ratios range from about 5.4 for a Lightning daysailer to the high 60s for a heavy Colin Archer pilot boat, and that "the moderate and successful ocean cruiser, such as the Whitby 42 or Bob Perry's Valiant 40, will be in the low to middle 30s."
In the Cruising World chart accompanying the original 1990 article, the boundaries between Greater / Average / Lesser were straight lines passing through the origin when plotted against LOA — meaning the boundary is essentially CR ≥ 0.835 × LOA for "Greater Comfort" and CR ≤ 0.626 × LOA for "Lesser Comfort." That's the cleanest length-aware version of the rule.
Caveat: the math has biases
CR is a fast, useful screening number, but it's not neutral:
- Beam is penalized, so modern wide boats almost universally score poorly even when their hull form handles wave impacts efficiently.
- Long overhangs are rewarded (because LOA enters the denominator with smaller weight than LWL, but LOA itself is large), so old CCA-era designs with sweeping overhangs score higher than their dynamic seakeeping deserves.
- Heavy displacement is rewarded, so the metric implicitly assumes "heavier = more comfortable," which is true on average but ignores structural mass distribution and roll-damping appendages.
Read a low CR as a prompt to look harder at hull shape — not as a verdict.
Caveat: it doesn't scale with size
A 30-footer with CR 35 will not feel like a 45-footer with CR 35. Absolute mass and physical length let the larger boat bridge wave crests in ways a smaller hull simply can't — even at identical CR.
CR is most useful when comparing boats of similar length. Cross-length comparisons via CR alone are misleading. The length-aware version (CR ≥ 0.835 × LOA for "Greater Comfort") implicitly handles this — but most published CR numbers do not.
CR + S-Number: the comprehensive view
Sponberg's most useful contribution to CR analysis is plotting it against the S-Number. The result is a clean 2D map:
- Lower-left: low S#, high CR — slow, comfortable cruisers and lead sleds.
- Upper-right: high S#, low CR — racing machines, fast and snappy.
- Upper-left and lower-right: rare and interesting combinations.
Together, S# and CR capture both how fast the boat will go and how it will feel getting there. Either one alone tells only half the story.
Reading the number as a buyer
Forget the 4/3 power for a moment. If a listing tells you the Comfort Ratio — or you compute it below — here's how to translate the number into how the boat will feel offshore, on a long weekend, or in the kind of choppy beat home that defines whether your crew wants to come out again next weekend.
What the number feels like in real conditions:
- CR under 20. Light and snappy. In 3–4 ft chop, the boat hobby-horses — bow lifts, drops, lifts again. Crew weight and trim matter. Fun for an afternoon, exhausting overnight. This is racing-boat territory: a Melges 24, a J/24, a modern flat-bottomed daysailer.
- CR 20 – 30. The mainstream coastal range. The motion is lively but not violent. A weekend on the boat won't grind on you; multi-day passages are doable but tiring. Most modern production 30–40 footers live here.
- CR 30 – 40. Genuinely comfortable. Roll period is slower, accelerations are dampened. This is the band where a couple can sail across an ocean and not arrive worn down. Older, heavier cruisers (Valiant 40, Whitby 42) and modern serious bluewater designs (Hallberg-Rassy, Hylas) cluster here.
Tartan 37 - CR 40+. Heavy expedition territory. The motion is slow, sometimes ponderous, and almost indifferent to chop. The trade is light-air performance — you'll motor more — but in genuinely bad weather these boats are unbothered.
How to use it as a filter:
- Match CR to your sailing. Weekend coastal? 20–30 is fine. Crossing the Pacific? Aim for 30+, ideally 35+.
- Length matters. A CR of 30 on a 30-footer is a different beast than a CR of 30 on a 45-footer. The longer boat will physically bridge wave crests in ways the shorter boat can't. When in doubt, use the length-aware boundary: CR ≥ 0.835 × LOA approximates "Greater Comfort."
- Don't dismiss a low CR outright. Modern wide hulls always score poorly. A boat with CR 22 and a well-designed hull form can be more pleasant than a boat with CR 32 and an outdated rolly section.
- Cross-check with CSF. A comfortable boat that capsizes easily is not what you want offshore. Pair CR ≥ 30 with CSF ≤ 2.0 for a serious bluewater shortlist.
A quick example. The Westsail 32 scores in the high 40s — a heavy, narrow full-keel boat whose deliberate, dampened roll has put dozens of these across oceans short-handed. The Beneteau Oceanis 38, much wider and lighter, scores in the high teens or low 20s — fast and comfortable for an afternoon sail on the bay, but a crew that beats into 6-ft seas for two days will arrive exhausted.
