We derive a simple analytical model of the boundary layer induced by rotary tidal currents flowing over a horizontal bottom in the absence of strong vertical stratification, formulating explicit estimates of the shear velocity, dissipation, and friction factors relating inviscid velocity to bottom shear stress. Accordingly, there friction factors may be calibrated by an observed velocity profile through the rotary bottom boundary layer, thereby reducing the empiricism of existing depth averaged models of tidal hydraulics. We illustrate the procedure by analyzing measured M2 rotaty current profiles on the outer European shelf with good model accuracy; the local dissipation-based friction factor compares favorably numerical model of the northwest European shelf as well.