A basic design problem for both sluices and spirals is to determine the required trough length. The present contribution examines sluice performance in relation to stratification length as a preliminary to the more complex issue of spiral length. Test results obtained from parallel sided sluices are analyzed and compared with computer based performance projections to determine how reliable the simulations are in forecasting actual results. The general trends appear correct and recent developments in computational fluid dynamics offer promise of quantitative simulations for both sluices and spirals. Test results obtained from both coal and mineral spirals having varying number of turns are compared and it is found that separation continues down the full length of the trough though at a diminishing rate. It is shown that mineral separations can be revitalized by employing repulpers, which offer the same advantages as constructing multiple shorter troughs on one column in a simple and more cost effective solution. The sedimentation rates in coal separations suggest that five or six turn troughs may be needed and in addition the reversed product locations compared with mineral spirals indicate that repulpers are likely to be ineffective. Test results are presented that support both premises and it is concluded that the longer troughs currently in use are the best choice for coal spirals.