The maritime industry is in a mind-set to save fuel and reduce emissions. How one achieves this end, however, can be a matter of some debate. Although substantial industry effort is being placed on things that can be optimized, to achieve real benefit and financial return one must first understand the details about how the vessel consumes propulsive energy during its mission. This article discusses a rational, simple, and effective systems engineering approach to identify power and fuel demands via computational propulsion analysis of the individual operating modes of a vessel’s duty cycle. It explains sensible consumption metrics that can be used to evaluate and compare different physical systems, strategic plans, or helm decisions. A duty cycle operating mode analysis calculation for a tugboat in multirole service (as a harbor tug and in long haul ocean barge towing) is demonstrated using commercial off-the-shelf software, including examples of design-side and shipboard decision options and consequences.