In many SPH simulations of galaxy formation a lower limit is imposed on the kernel radius h equal to the gravitational softening length ∈. It has been found that such a constraint can in some circumstances compromise the spatial resolution to which hydrodynamical quantities are evaluated to the extent that the evolution of the gross features of a numerical galaxies are affected. Such effects can be avoided by allowing h to evolve freely to maintain a roughly constant number of neighbours in SPH summations. Here we focus on how imposing a constraint on h may affect the velocity field.