Motion Comfort Ratio Calculator (Brewer)
Calculate Ted Brewer's motion comfort ratio from displacement, waterline and overall length and beam, interpret it from light racer to heavy offshore cruiser, and see how adding payload or beam changes the predicted motion in a seaway.
How it works
MCR = displacement (lb) ÷ [0.65 × (0.7 × LWL + 0.3 × LOA) × beam^1.33]
Yacht designer Ted Brewer proposed the comfort ratio as a rough index of how gently a boat moves in a seaway: heavier boats accelerate more slowly when a wave hits, longer boats bridge waves better, and beamy boats — with their high form stability — snap back upright more sharply, so beam appears with an exponent greater than one. Values under 20 describe racers and daysailers whose quick motion tires the crew; 30–40 is the comfortable bluewater band; over 50 is the slow roll of a heavy traditional cruiser. Like all such ratios it ignores hull sections, damping from a long keel, and how the boat is sailed, but among boats of similar type it ranks their motion surprisingly well.
Worked example
An 18,000 lb boat with 30 ft LWL, 36 ft LOA and 12 ft beam has MCR = 18,000 ÷ (0.65 × 31.8 × 27.3) = 32.0 — a moderate bluewater cruiser; loaded with 2,500 lb it rises to 36.4. One extra foot of beam would drop it to 28.7.
Frequently asked questions
Is a higher comfort ratio always better?
For motion, yes; for performance, no — the same weight and narrowness that soften motion make the boat slower and wetter to windward. Choose by intended use.
Does it apply to catamarans?
No. Multihull motion is governed by different mechanics (no roll, sharp pitch); the ratio would give absurdly low numbers because of the huge beam.
Related calculators
Design ratios are comparative indicators from published yacht-design practice (Brewer, Gerr, US Sailing), not seaworthiness certifications. Use the designer's data, stability curves (STIX/AVS) and the boat's actual loaded displacement; sea state, crew and preparation matter more than any ratio.