What the comfort ratio measures
The comfort ratio, devised by yacht designer Ted Brewer, estimates how quickly a boat responds to waves. Motion sickness and fatigue come less from how far a boat heels or pitches than from how sharply it starts and stops moving. A heavy boat with a narrow waterline plane reacts slowly to a passing wave; a light, beamy boat follows the wave surface and jerks. Brewer wanted a number that captures this, for comparing boats of a similar size.
The formula
In imperial units, with lengths in feet and displacement in pounds:
CR = Disp ÷ (0.65 × (0.7 × LWL + 0.3 × LOA) × Beam1.33).
In metric, with lengths in metres and displacement in kilograms, the same formula becomes
CR = 0.1384 × Disp ÷ (0.65 × (0.7 × LWL + 0.3 × LOA) × Beam1.33).
The constant converts kilograms to pounds and metres to feet, so both versions give the same number.
The weighted length (70% waterline, 30% overall length) stands in for the length that matters when the boat pitches in a seaway. The beam term is raised to the power 1.33 because beam sets the waterplane area, and with it how hard the water pushes the hull around.
How to read it
- Under 20: lightweight racing boats.
- 20 to 30: coastal cruisers.
- 30 to 40: moderate offshore cruisers.
- 40 to 50: heavy offshore boats.
- Over 50: extremely heavy bluewater boats.
Brewer himself warned that the ratio is mainly useful for boats of a similar type and size: larger boats always score higher. That is why the calculator compares your value with boats of the same length in our catalogue.
Limitations
The ratio rewards weight and penalises beam, so it favours older, narrow, heavy designs. It knows nothing about hull shape, the distribution of weight along the boat, or the height of the rig, all of which affect motion. A modern beamy boat with a low ratio can still be comfortable at anchor and on a reach, while it will slam more when beating into a short sea. Brewer described it, partly tongue in cheek, as a measure of how comfortable a boat is for the crew, not of seaworthiness or safety. For multihulls the number is meaningless, as their motion follows different rules.
Sources
The formula and bands are Ted Brewer's, as published on his website and in his writing for sailing magazines, and used by sailboatdata.com; this calculator reproduces sailboatdata's values to within 0.2%. See our sources and methodology, and compare with the capsize screening value.