Multi-rate simulation techniques offer advantages to the computer simulation of large scale dynamic systems. Each part of the system is solved using the most appropriate time step and numerical integration method. The approach can be particularly advantageous in real-time applications, where it is essential to complete each set of calculations in the allotted real-time interval. The ESL Simulation language has parallel segment features which makes it particularly suited to the realization of multi-rate simulations. Experiences of using ESL and the Virtual Test Bed (VTB) to realize a multi-rate simulation of an underwater unmanned vehicle (UUV) are described. A non-real-time version of the simulation with 5 different frame rates has achieved speed increases of the order of 500 times with no significant loss of accuracy, making a real-time implementation feasible.This research has included analysis of the stability of multi-rate methods and these are summarized in the paper.