For spacing of vane arrays, the information about strength of vortex downstream of a submerged vane is needed. A series of experiments were performed with rectangular submerged vane with and without a collar at Froude numbers 0.13 and 0.25. To define the strength of vortex, a new set of dimensionless parameters using moment of momentum are involved. The present study provides certain information on the decay of strength of vortex with downstream of vane. The decay of vane strength does not follow similar trend and appears sensitive to the location of center of vortex, Froude number, etc. The streamwise spacing of submerged vanes may be taken at least 40H at optimal angle of attack. Centre of vortex is also observed to change with the downstream of submerged vane. © 2006 Taylor & Francis Group, LLC.