: ... but I don't think an
: arbitrary 10 inches in from the point of the bow or stern is the place for
: measuring the rocker height. Rather, just measure it at the points where
: the waterline crosses the stems. Just my humble suggestion.

The reason you don't measure at the stems is because two boats with similar measurements at the stem may have completely different rocker.

"Rocker" is some measurement of the curvature from the intersection of waterline (DWL) with the stems and the lowest point of the keel (the baseline). If you just measure from the DWL to the baseline all you end up with is the draft. This doesn't tell you anything about the curvature.

To get a handle on the curvature you must measure some intermediate point between the stems and the point of greatest draft. By measuring in from the ends you get this intermediate point. The thing is at this time, there is no agreement on where to make this measurement and what dimensions to measure.

10" is arbitrary, but any measurement will be fairly arbitrary. You could also look for the point where the depth is 50% of the distance between the DWL and the baseline, then measure the relative distance of that point from the ends vs the distance from the ends of the lowest point. This would be a dimensionless ratio (distance to 50% point/distance to 100% point) where smaller number indicate less rocker. An advantage of a dimensionless measurement is when you scale a boat up or down, the measurement does not change.

But again the choice of 50% is arbitrary. 25% or 10% might also work.

The problem is not so much coming up with a system, it is getting agreement on which arbitrary measurement is best.