Tidal turbines in strong flows have the potential to produce significant power. However, not all of this potential can be realized when gaps between turbines are required to allow navigation along a channel. A review of recent works is used to estimate the scale of farm required to realize a significant fraction of a channel's potential. These works provide the first physically coherent approach to estimating the maximum power output from a given number of turbines in a channel. The fraction of the potential realizable from a number of turbines, a farm's fluid dynamic efficiency, is constrained by how much of the channel's cross-section the turbines are permitted to occupy and an environmentally acceptable flow speed reduction. Farm efficiency increases as optimally tuned turbines are added to its cross-section, while output per turbine increases in tidal straits and decreases in shallow channels. Adding rows of optimally tuned turbines also increases farm efficiency, but with a diminishing return on additional rows. The diminishing return and flow reduction are strongly influenced by how much of the cross-section can be occupied and the dynamical balance of the undisturbed channel. Estimates for two example channels show that realizing much of the MW potential of shallow channels may well be possible with existing turbines. However unless high blockage ratios are possible, it will be more difficult to realize the proportionately larger potential of tidal straits until larger turbines with a lower optimum operating velocity are developed.