Pereira, esta é uma pergunta interessante. Voce nao precisa saber o motivo, e nem o Froude sabia, mas está relacionado à transição do fluxo laminar para turbulento. A proporção do comprimento do costado exposto aos dois regimes de fluxo tem alguma influência até o comprimento de 50 pés.
“Froude found 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 had acquired a forward motion and so had a lower relative velocity.” PG 07
In his own words, “it is at once seen that, at a length of 50 feet, the decrease, with increasing length, of the friction per square foot of every additional length is so small that it will make no very great difference in our estimate of the total resistance of a surface 300 ft long whether we assume such decrease to continue at the same rate throughout the last 250 feet of the surface, or to cease entirely after 50 feet; while it is perfectly certain that the truth must lie somewhere between these assumptions.” PG 08
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