sail-past today of Windermere 17-footers for their 120th anniversary.
the seventeen feet is measured from bow to stern along the keel at the point at which is normally sits in the water (rigged/unrigged/ with/without crew, I don't know).
It is the shape of the keel's profile (you can tell I'm on dodgy ground re understanding here) that designers work on, to get the most efficiency, doing things like having it in different places in relation to the centre of the boat and changing angles.
Iconic racing yachts! Length is the prime measurement. As you say, design is usually at water line and all weights (rigging, average crew) considered in the calculations. These are two man racing yachts and reputedly quite challenging in handling. Keel design matters for speed and turn handling in the water. (Spot the wife of a Yachtmaster lol) OH could pontificate for hours on all things sailing far better than me! What a gorgeous sight to see them all out en masse. ⛵️
