To date, probabilistic design methods of channel widths are still under development. This is partly because there are no suitable models available. Real-time simulators are too expensive and time consuming to be often applied for full probabalistic channel design. While autopilot fast-time simulation models exclude one very important factor – human behavior - so that the results need further interpretation. In order to fill the gap between the two existing simulation methods, a new simulation model, namely probabilistic fast-time simulation model is developed in the study. The new model takes four main variables related to human behavior into account, i.e. varied time intervals of command settings, rudder error, position threshold and rudder threshold. The properties of the four variables are derived from and verified with Ennore real-time simulations. The recommended input parameters of the new model are suitable for big vessels transiting straight one-way outer channel sections. The new simulation model is applied for probabilistic design of the access channel to the port of Coega. The resulting channel widths are reasonable. Therefore, the development and application of the new probabilistic fast-time simulation model can be regarded successful and promising.