Actually the beach cats are always in displacement mode, even at high speeds. The contradiction you underline is actually caused by the theoretical max hullspeed theory being wrong. For some reason this myth is impossible to kill. Many people, including maritime engineers, inteprete Froude's law in the wrong way and thus think that the max hull speed law has a scientific basis when it does not. These rest of the errors can be directly trashed back to this fictious law.
Drag consists of
1.Skin friction drag, which depends on the surface area in contact with water.
2. Form drag (or pressure drag), which depends on the shape of the moving object. Boats moving in the water surface and planes at transsonic and supersonic speeds create waves which create wave drag.
1. Skin friction drag:
Wouter, you are correct that reducing weight by reducing length reduces surface area more than reducing the same amount of weight by reducing hull width. Reducing weight of a rectangular box floating on the water surface by 10% in a 6 m boat by reducing length, reduces wet surface area by 9.37%. Reducing weight 10% by reducing width, reduces wet surface area by 3.7%.
2. Form drag (pressure drag)
Cross-section area of the hull and hull shape are of importance.
2.2. Wave-making drag is a component of form drag.
For displacement ships, the power needed increase speed above the (root of water-length * 1.34) increases abruptly. This rule is theoretically founded on the fact that max wave-speed depends on wavelength and gravity.
When increasing speed over theoretical max speed, you need to climb the bow wave, and this demands a lot of power.
One way around this limitation is the semi-planing mode. With a vertical flat stern the water detaches from the stern at a certain speed. Thereby, the stern wave is tricked into reattaching far behind the boat. Hence, the wave-making boat length is much longer than the water line.
The International 14 class guys use horizontal wings below the water surface to trick the water to behave as if the waterline length is much longer. (In addition, these wings prevent pitch-poling.)
Most of our beloved catamarans sail in the semi-planing mode.
(Some has maintained that cats like the M20 can sail in planing-mode, but there are different definitions of planing.)
Another way around the displacement speed, is to make the hulls so narrow that they only create very small waves.
By shortening the hull, you may as Wouter, has pointed out, reduce friction drag. However, increasing hull cross-sectional area to maintain volume, increases form drag. One part of form drag is wave drag.
Most sailboat classes have a maximum length.
HAVE YOU EVER SEEN SUCCESSFUL BOATS THAT ARE SIGNIFICANTLY SHORTER THAN THE ALLOWED CLASS LENGTH?
Stein
