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, these 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 rotary current profiles on the outer Europeann shelf with good model accuracy; the local dissipation-based friction factor compares favorably with a global value based upon a calibrated numerical model of the northwest European shelf as well.