A keel smile is the crack along the joint where a bolted-on keel meets the hull. On most fin-keel boats you'll see one if you look, and that's the point: Don Casey, writing in SAIL, says a hairline crack is quite normal. What matters is how wide it is, whether water is getting to the keel bolts, and why the joint is moving.
Why it opens
Epoxyworks puts it down to a hull that isn't as stiff as when it was new: years of flexing move the joint more than the fairing putty can take. Casey adds that a pronounced smile might have less to do with the keel than with the hull, and that a grounding drives the keel up into the hull. Practical Sailor names two more causes: cast-in bolts pulling out of a lead keel, and the hull laminate compressing under the nuts in a cored hull.
How a surveyor checks it
The joint. With the boat in the slings or on blocks, look along the whole joint. Epoxyworks says hairline cracking typically begins at the front of the keel. Note the width and how far it runs.
Rust below it. Casey's line: rust stains running down from the joint mean water is probably reaching some, or all, of the keel bolts. On a lead keel, rust is a sure sign of corroding bolts. On an iron keel the smile tends to be broader and more continuous.
Inside. Look at the bolts and nuts in the sump for rust, weeping, loose nuts and laminate crushed under them. Practical Sailor's tell: thread showing above the nuts after repeated tightening means the bolts are almost certainly pulling out of the keel.
After a grounding. Spend time on the floors and tabbing round the keel, and ask about any grounding. Boats racing offshore under World Sailing's rules need a keel inspection after each unintentional grounding.
What decides the grade
Water and movement. An open joint with rust streaks, weeping bolts or a keel that moves is an A. A crack wider than a hairline on a dry joint is a B. A hairline crack in the fairing is a C to watch.