The problem of optimal control has been applied in many fields with developing of computer. Authors studied here about the simulation of minimum time control and took the crossing of AKASHI STRAIT since strict natural condition is asked for the model. That is, ships run with 8^<k't>, 12^<k't> or 16^<k't> between AKASHI and IWAYA in most strong current. There are 12 cases, having respectively right and wrong current. Authors had to study the treatment of a boundary line in wrong current. It's that a μ's sign determined by the equation of the boundary line is related to the limit of extremal on the boundary line in theory and the boundary condition was considered with practical treatment.
