Which sails quicker the rowing boat "trainera" or "trainerilla" ?

The great mathematics magazine + Plus Magazine, in its May 2012 issue published an article entitled "Outer space: Canoeing and kayaking", authored by John D. Barrow, which raises the question of whether these boats run more or less when they double the crew, which we do not know in advance because if the propulsive power is doubled so does the displacement and therefore the hydrodynamic resistance.

+ Plus presents simple and reasonable equations to estimate the variation of this resistance and determine from it the necessary power increase, which compares with the increased power provided by the additional rowers. He gets a theoretical conclusion that  a double crew boat increases its speed by 8 to 10%, what matches well with the actual data recorded in kayacs and canoes races. This conclusion appears consistent with the intuitive impression we have that larger similar boats are faster than smaller ones.