The comfort ratio, explained: what the number really predicts

Ask cruising couples why they sold the boat and you will hear about motion sickness at least as often as about storms. The comfort ratio is the one listing number that speaks to it. Below: what the formula measures, how to read the bands, and why its own creator admitted he invented it half as a joke.

For the number itself on a specific boat, the free design ratio calculator computes it alongside every other ratio.

What is the comfort ratio?

The comfort ratio divides a boat's displacement by an approximation of her waterplane, the footprint of hull sitting in the water. A heavy boat on a small footprint accelerates slowly when a wave lifts her, so she scores high. A light boat on a wide footprint gets tossed, and scores low.

The logic is simple physics. When a wave passes under the hull, it pushes up on the waterplane, and the boat's mass resists that push. More displacement or less waterplane means slower vertical acceleration, and it is acceleration, not motion itself, that turns stomachs.

Beam is raised to the 1.33 power because width punishes comfort disproportionately. A wide, flat hull snaps back upright quickly, which feels reassuring at the dock and exhausting on day three of a passage.

How do you read the number?

Under 20 is racer or daysailer territory: quick, jerky, wet and fun in small doses. The 20 to 30 band covers most production coastal cruisers. From 30 to 40 you are in moderate bluewater country, and above 40 sit the heavy offshore cruisers and motorsailers with the gentlest ride of all.

Some worked examples make the bands concrete, using published figures from sailboatdata.com. A J/24 scores around 12, a Catalina 30 around 25, and a Westsail 32, the archetypal heavy double-ender, around 43.

Anyone who has sailed all three will recognize the ranking instantly. That is the ratio's real value: it puts a number on a character difference you would otherwise only discover offshore.

A ratio invented tongue in cheek

Ted Brewer, the naval architect behind hundreds of cruising designs, has been unusually honest about this formula's origins. He wrote that it is "a ratio I dreamed up, tongue-in-cheek, as a measure of motion comfort," only to watch the sailing world adopt it as gospel.

He also stood by its core idea: it "does provide a reasonable comparison between yachts of similar type," because faster motion genuinely is more upsetting to the average person. You can read his own account in his Good Old Boat piece on design numbers.

That origin story is the right frame for using it. A back-of-the-envelope index that survived decades of scrutiny is useful, but it was never engineering, and Brewer never pretended otherwise.

What does the comfort ratio miss?

The formula sees only displacement, length and beam. It ignores hull sections, bilge shape, where the ballast sits, and the damping a deep keel and rudder provide. It says nothing about speed, safety or seaworthiness, and modern wide-stern designs read harsher on paper than they behave at sea.

Two boats with identical ratios can move very differently. Slack bilges and low, central ballast produce a softer roll than the formula can see, while a fin keel and skeg rudder add damping it cannot credit. Naval architect Eric Sponberg's survey of the design ratios treats it as a rough comparator for exactly these reasons.

Ratio explainers from Good Old Boat and Charles Doane's Wave Train analysis land in the same place: compare boats of similar type, and never read the number as a verdict on a boat you want.

Read it alongside the capsize ratio and displacement-to-length: the three together sketch the boat's character in a seaway far better than any one alone. The full set lives in the design ratios guide.

How Knotworth uses the comfort ratio

Tell Knotworth you plan multi-day passages with a crew that has never done a night at sea, and the comfort ratio starts to matter. The same Westsail 32 that scores 43 and a performance cruiser scoring 16 might both fit your budget; only one of them will let that crew arrive wanting more.

So the free pre-check reads the number against your stated plans, not against an abstract ideal, and says when the mismatch is worth a second thought. Motion is also one of the few things you cannot fix later: rigging can be replaced, a hull's motion cannot, which is why this flag comes before any talk of cost.