This ratio is abbreviated as V/√L. It is the speed in knots divided by the square root of the waterline length in feet. For example, a boat with 25 foot WL gives a square root of 5. Then, when she is moving at 5 knots her speed/length ratio is exactly one. At a speed of 10 knots her ratio would be two. The ratio is of significance for Displacement hulls (ie. those which don’t plane on the water) because it is rarely that such hulls achieve better than a speed/length ratio of 1.5. In fact most don’t do as well as that: a ballasted sailing boat of 25 foot waterline would do well to achieve a ratio of 1.3 and to sail at 6.5 knots, though she might easily do it with a 30 horsepower engine in calm water.