Did your ever heard the rule "length runs?"
This rule is based on the so-called hullspeed of a displacement vessel. It grows with the distance between bow and stern. This distance is called the length of the waterline. At hullspeed the vessel is "trapped" between its bow and stern wave. In the language of mathematics the hullspeed results from the length of the waterline (LWL) via the following equation:
hullspeed v in kilometer per hour [km/h]
hullspeed v in knots [kn]
The length of the water line (LWL) is measured in meter. That hullspeed is depending from the length of the waterline only, is the reason why larger vessels can reach larger speeds with the same propelling power. Therefore old shipbuilders say "length runs."
The following input form computes the hullspeed. LWL [m] is the length of the water line. v [m/s] is the hullspeed in metern per second. v [km/h] is hullspeed in kilometern per hour. v [kt] is the hullspeed in knots, i.e. nautical miles per hour.
You can make entries in all fields, the respective other fields will be computed. The present largest container vessel of the world, the Emma Mærsk, has a length of 397 meters and reaches a theoretical hullspeed of 48.22 knots! Try and you'll see, yes it's true - "length runs".