This paper is focused on the thrust allocation algorithm, which is a part of a Dynamic Positioning (DP) system in marine vessels with diesel-electric power system. In this paper the focus is on using the thrust allocation to make the diesel generators on board the vessel work more fuel efficiently, by reducing the total fuel consumption of all online diesel generators. A static model for the fuel consumption of a diesel generator as a function of its produced power is derived from data, and this model is used to create a convex Quadratic Programming (QP)-problem which finds the most fuel efficient thrust allocation solutions. The simulation scenarios shown in this paper typically give a fuel reduction of a rather common Platform Supply Vessel (PSV) of up to 2% of its maximum possible fuel consumption. The fuel optimization can be implemented as a standard QP-problem by recalculation of its cost function weights based on linear and quadratic model approximations at the current operation point.