Resistance

William Froude was the man who pioneered the study of resistance in water. He said that the "resistance of a floating body in motion was the sum of two parts - frictional resistance and residual resistance. Frictional resistance comes from the hull's contact with the water. Residual resistance is all of the other resisting factors of which wavemaking, form, and yaw are the most important.

The combined effects of wetted surface, surface condition, surface length and speed make the resistance due to friction and can be figured out with the formula:

Rf = 0.97 x CF x WS x V2
where:

_{Rf = Resistance in pounds
Cf = Coefficient of friction
SW = Wetted surface
V = Velocity in ft/sec
0.97 = Constant for fresh water}

The friction in question is not actually created between the boat's surface and the water. The initial friction is but as this friction gets stronger the heat slows down the water molecules. This creates a layer of water that moves with the hull. the calculated friction is actually the friction between this layer and the other water.

Two types of waves comprise a boat's wake. First there are the divergent waves. These fan out backward and offer litte resistance. Then there are transverse waves whose crests are at rt. angles to the boat's direction. These waves' lengths are equal to the natural wavelength of a wave traveling the same speed as the boat. William Froude figured that a waves speed could be figured by this equation. speed(knots) = 1.34 x (1/2)Length(ft) So, as the craft speeds up the fewer transverse waves there will be. When these waves are reduced to one at the bow and one at stern the craft has reached hull speed.