A challenge when designing water intake structures in rivers, channels and reservoirs is to reduce the entrainment of air through free air-core vortices. Entrained air can cause discharge reduction, vibration and cavitation in the water-conveying system. This study firstly investigates flow boundary effects at the critical submergence of an intake, which is the depth up to which air entrainment occurs. The friction effect of a boundary section close to the free surface dominates the boundary blockage effect and retards the occurrence of the air-entraining vortex. Potential flow solution methods overpredict the critical submergence of an intake under the influence of boundary friction, so a design criterion is presented to correct this. Furthermore, previous work shows that Froude similarity is not always reliable for predicting critical submergence at prototype intakes. In the second part of this study, kinematic similarity - the equality of the ratios of intake velocity to the velocity at the imaginary critical spherical sink surface between the model and prototype intakes - is proposed and validated against experimental results. In addition, a theoretical explanation of the equal intake velocity concept is given using kinematic similarity.