1. A computer system comprising memory and a processor, the computer system being programmed to perform steps comprising: providing a mathematical model containing equations that model transportation scheduling and inventory management of a bulk product from multiple supply locations to multiple demand locations to minimize cost per unit volume or mass of the bulk product, the model includes a parameter for demurrage rate and constraints for minimum flow rates at the demand locations, maximum flow rates at the demand locations, and vessel draft limits, and the model includes decision variables for: (i) the amount of bulk product to be loaded from a supply location onto a vessel, (ii) the amount of bulk product to be discharged from a vessel to a demand location, and (iii) demurrage time for a vessel; receiving data relating to the characteristics of a fleet of vessels, supply locations, and demand locations; applying the data to the mathematical model; and minimizing cost per unit volume or mass of the bulk product using a mathematical optimization solver to solve the mathematical model based upon the constraints and decision variables.