Principios navais de arquiteura

3.1 – General:
The frictional resistance accounts for 80 to 85 percent of the total resistance in slow-speed ships and as much as 50 percent in high-speed ships.
3.2 – Froud’s experiments on friction:
Froud conclude that the model-ship extrapolation could only be solved dy dividing the resistance into two components.
Froude fount that at any given speed the specific resistance per unit of surface area was less for a long plank than for a shorter one, which he attributed to the fact that towards the after end of the long plank the water hat acquired a forward motion ant so hat a lower relative velocity. He gave an empirical formula for the resistance in the form
R = f SVn
R = resistance, kN or Ib
S = total area of surface, m2 or ft2
V = speed, m/sec or ft/sec f = depende do comprimento e da natureza da placa n = normalmente considerar 2 (entre 1,87 e 2)
Froud uses the idea of “equivalent plank”: the immersed skin was calculated and it is considered that the resistance due to it is equivalent of that of a rectangular surface of equal area and length.
But the actual ship resistance was everywhere higher than that predicted from model. Froud assumed that it was because of the surface assuming that the copper-sheathed was equivalent of smooth varnish over 23 and to calico of the rest.
The values of frictional coefficients were considerably above those now generally accepted for smooth surfaces.
3.3 – Two-dimensional frictional resistance formulations:
Reynolds made water flow through a glass tube, introducing a thin stream of dye on the centerline at the entrance to the tube. When the velocity was small, the dye remained as a straight filament parallel to the axis of the tube with diameter D. At a certain velocity, which Reynolds called the critical velocity Vc, the filament began to waver, became sinuous and finally lost all definiteness of outline, the dye filling the whole tube. The resistance experienced by the fluid over a given length of pipe was