Motion Comfort Ratio Calculator (Brewer)
Ted Brewer's tongue-in-cheek-turned-canonical comfort ratio: how corky or steady a boat feels in a seaway, from displacement, length and beam.
Brewer devised it half in jest; the fleet adopted it in earnest because it works: heavy boats with moderate beam accelerate gently in waves (comfort = low acceleration), while wide light hulls snap. Beam's 1.33 exponent is the snap.
Formula
โ ๏ธ For planning and education only โ verify with your vessel's documentation, naval-architecture data and official sources. Not for navigation or stability decisions on real voyages without professional data.
Ted Brewer's tongue-in-cheek-turned-canonical comfort ratio: how corky or steady a boat feels in a seaway, from displacement, length and beam.
About Motion Comfort Ratio Calculator (Brewer)
Seasickness is an acceleration problem, and Ted Brewer's comfort ratio โ invented, he admitted, half as a joke โ captures it well enough that brokers quote it fifty years later: displacement divided by a beam-and-length term approximating how abruptly waves can toss the hull. Under 20 is dinghy-lively; over 40 is sip-your-coffee-on-passage. This calculator computes it with the standard bands and the honest footnote about what one number can't know.
How to use Motion Comfort Ratio Calculator (Brewer)
- 1Enter โ sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula CR = ฮ รท [0.65 ร (0.7ยทLWL + 0.3ยทLOA) ร B^1.33] โ weight per unit of motion-driving geometry substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Motion Comfort Ratio Calculator (Brewer)?
- โInstant, free and private โ every calculation runs in your browser, nothing is uploaded
- โBuilt on the published formula CR = ฮ รท [0.65 ร (0.7ยทLWL + 0.3ยทLOA) ร B^1.33] โ weight per unit of motion-driving geometry with sources cited on the page
- โBrewer devised it half in jest; the fleet adopted it in earnest because it works: heavy boats with moderate beam accelerate gently in waves (comfort = low acceleration), while wide light hulls snap. Beam's 1.33 exponent is the snap.
- โSwitch units, tweak any input and watch every result update live
Frequently asked questions
What physically makes a high-comfort-ratio boat comfortable?+
Inertia versus wave force: a heavy hull presenting moderate beam to the sea accelerates slowly when a wave shoulders it โ and human inner ears object to acceleration, not motion per se. Light, beamy hulls (great for form stability and interior volume) get snapped quickly by every wave face: less roll ANGLE sometimes, but far more roll ACCELERATION, which is what stomachs score.
Why does beam appear with an exponent of 1.33?+
Brewer's empirical capture of beam's double mischief: wider hulls both generate larger righting moments per degree (stiffer, snappier roll) and present more waterplane for waves to lever. The super-linear exponent encodes that beam hurts motion comfort faster than it grows โ the design tension behind every modern wide-stern cruiser, which buys interior and downwind power at the comfort ratio's expense.
How should I actually use the number when boat shopping?+
As a comparison within type, not an absolute verdict: between two 35-footers under consideration, CR 22 vs 34 predicts a real difference on a lumpy channel crossing. It can't see daggers like a slamming flat forefoot, a corky stern, or rig-induced snap-roll โ sea trials in chop remain the truth. Pair it with D/L (heaviness) and capsize ratio (beam vs displacement) for the standard broker-spec triad.
Do catamarans and trimarans break the formula?+
Completely โ it was derived for ballasted monohulls. Multihulls ride on form stability with no ballast and entirely different motion physics: quick, small-amplitude motions instead of rolling. Their comfort metrics (bridge-deck clearance, weight-to-length, beam-to-length) are a different toolbox; running a cat through this formula produces a number with no meaning.
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