Sail Area to Displacement Calculator
Written to exclude boats, not to certify them.
Work out Sail Area to Displacement. Written to exclude boats, not to certify them. Every rate is an input, not an assertion.
Sail area to displacement
17.38
D/L 191 · capsize screening 1.95 · comfort 23.4
SA/D under 14 is a motorsailer, 17 to 20 a well-powered cruiser, above 24 a racing rig. D/L under 100 is ultralight, 200 to 300 moderate and above 300 heavy — and D/L falls with waterline length cubed, which is why long boats look lighter on this scale than they feel. The capsize screening formula was written after the 1979 Fastnet to screen out obviously unsuitable boats, with 2.0 as the threshold. It was designed to exclude, not to certify, and passing it says considerably less than failing it does. Motion comfort ratio is Ted Brewer's, and he described it himself as tongue-in-cheek. It correlates with how a hull feels in a seaway and knows nothing about rig, keel or how the boat is loaded.
How the Sail Area to Displacement Calculator works
SA/D, D/L, ballast ratio, capsize screening and motion comfort computed together from one set of dimensions, with what each one can and cannot tell you.
Also known as: sa d ratio for my boat · capsize screening formula · displacement length ratio · is this boat suitable offshore
Frequently asked questions
What is a good sail area to displacement ratio?
Under 14 is a motorsailer, 17 to 20 a well-powered cruiser, above 24 a racing rig. It measures power relative to weight, not speed.
What does displacement to length mean?
How heavy a boat is for its waterline. Under 100 is ultralight, 200 to 300 moderate, above 300 heavy — and it falls with waterline cubed, so long boats look lighter than they feel.
What is the capsize screening formula?
Beam divided by the cube root of displacement volume, with 2.0 as the threshold. It was written after the 1979 Fastnet to screen out obviously unsuitable boats — designed to exclude, not to certify.
Is the motion comfort ratio meaningful?
Somewhat. Ted Brewer devised it partly tongue-in-cheek, and it correlates with how a hull feels in a seaway while knowing nothing about rig, keel or loading.
Can I compare two similar boats with these?
Not reliably. They are crude numbers from dimensions alone and separate broad categories usefully — which is exactly not what they get used for.
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