Why are Slow Boats slow?

Boats with hulls that are not designed to plane such as sailboats, canoes and kayaks, are often described as having a “hull speed”. The hull speed may be viewed as a maximum achievable speed, not because it is impossible to go faster, but because it would take an inordinate amount of power to reach higher speeds. The hull speed limitation is due to several factors. One is drag or friction between the moving hull and the water. At slow speeds in still air, friction of the water is a major factor. As the speed increases, additional energy is dissipated by the generation of waves (boat wake). The amount of energy used to generate wake increases rapidly with increasing speed. As long as wind and the presence other waves are ignored, the production of boat wake may be considered the dominant factor when hull speed is being approached.
Hull speed has been studied and modeled extensively. An empirical formula that is often used to calculate hull speed for small boats is:

Hull Speed = C*(Waterline Length)^0.5

If the length is expressed in feet, the hull speed will be in knots. C is a constant for any given boat that depends on the nature of the hull. Generally C is in the range from 1.2 to 4.0. The value most commonly used for sailboats is 1.34. Narrow, sleek hulls would have the highest value for C. . Assuming that C = 1.34, twenty feet of waterline will produce a hull speed of approximately 6 knots.

If one wishes to visualize how hull speed is established, it is helpful to consider the appearance of the boat wake. A wave crest is formed at the bow of the boat and another crest further back along the sides of the boat. As the boat speeds up, the second crest slips back near the stern. It may form right at the stern or slightly behind. This creates a wave trough in which the boat is essentially trapped. A planing boat has enough power to climb out of the trough, leaving the stern wave behind. The amount of power needed to reach the hull speed will depend on the weight of the boat and contents.

Energy expended attempting to exceed the hull speed, in a non-planing situation, is definitely energy wasted. Even at speeds less than the hull speed, the energy requirement will become greater as the speed approaches the hull speed. A rule of thumb often used at speeds less than the hull speed is, “the thrust needed to move the boat will increase with the square of the speed”. A “sweet speed” can usually be found that represents an acceptable balance between speed and fuel economy.