Motion Comfort Ratio (CR / MCR)

The Comfort Ratio (CR), also called the Motion Comfort Ratio (MCR), is naval architect Ted Brewer's way of estimating how "corky" a sailing hull will feel in confused seas. It is less about how far a boat heels and more about how quickly it snaps back. A slow, deep roll is much easier to live with on passage than a fast, jerky motion.

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 in cruising magazines, broker write-ups, and long-distance cruisers' notes about boats they've owned. As a buyer, it is most useful when you're trying to picture multi-day passages, not an afternoon at the dock. Brewer's intuition was straightforward: heavy, relatively narrow boats tend to roll slowly and comfortably; light, beamy boats tend to react quickly and feel snappier. The formula puts displacement on top, then divides by a length-and-beam estimate of waterplane area, with beam given extra influence.

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 is not dimensionless, so compare boats measured in the same unit system.

What the equation is really saying

Look closely at the denominator: 0.65 × L × B is basically a waterplane-area estimate, using a generic 65% coefficient. The length term — 70% of LWL plus 30% of LOA — acts like an "effective" waterline, slightly longer than the static LWL, to approximate the boat's moving waterline. Beam is raised to the 1.333 power so it matters more than simple width, without making the result unusably small.

The physics behind that choice is roll acceleration. Beam strongly affects how quickly a boat reacts after a wave or gust heels it. Brewer wanted beam to count, but not so much that the formula collapsed into awkward tiny numbers, so the 1.333 power is a practical compromise.

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:

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 useful, but it has a point of view:

• 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.

Treat 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. The larger boat has more length and mass, so it can bridge wave crests in ways the smaller boat simply cannot.

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 bigger picture

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 tell you both how fast the boat wants to go and how it may feel getting there. Either number alone only tells half the story.

Reading the number as a buyer

Forget the exponent for a moment. If a listing gives you CR — or you calculate it below — use the number to picture the ride: offshore, on a long weekend, or on the kind of choppy beat home that decides whether your crew wants to come back.

What the number feels like in real conditions:

• CR under 20. Light and snappy. In 3–4 ft chop, the bow lifts, drops, and lifts again. Crew weight and trim matter. Fun for an afternoon; tiring overnight. This is racing-boat and daysailer territory.
• 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.
• CR 40+. Heavy expedition territory. The motion is slow, sometimes ponderous, and largely indifferent to chop. The tradeoff is light-air performance — you'll motor more — but bad weather is where these boats earn their keep.

How to use it as a filter:

1. Match CR to your sailing. Weekend coastal? 20–30 is fine. Crossing the Pacific? Aim for 30+, ideally 35+.
2. 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."
3. 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.
4. 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: heavy, narrow, full-keel, and famous for a slow, damped roll offshore. The Beneteau Oceanis 38, much wider and lighter, scores in the high teens or low 20s. It can be quick and pleasant for an afternoon sail, but a crew beating into 6-ft seas for two days may arrive worn out.

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