Background

The US Navy operates a fleet of 6 T-AGS Class Oceanographic Survey vessels, powered by two azimuthing Z-drives that provide both propulsion and directional control of the vessel.  Recent ship modifications have included the addition of a larger skeg.  A skeg is essentially a fin located along the bottom-rear centerline of the ship, and its purpose is to provide greater directional stability.  An increase in the size of the skeg makes the ship more resistant to turning.  This results in less steering corrections that need to be made using the Z-drives and greater overall propulsive efficiency, which in turn, should lower fuel consumption.  The disadvantages of a larger skeg are a larger wetted surface area thus greater drag and a slight increase in flow irregularity into the propellers that can cause some propeller inefficiency. 

Prior to acceptance of the modifications, towing tank and computational fluid dynamics analyses were performed.  The analyses suggested a level of fuel savings would occur in response to the modifications.  However, since the modifications have been made to the vessel, no comprehensive analysis of the performance improvements has been performed.  The purpose of our analysis will be to validate that the ship modifications have indeed increased efficiency by lowering fuel consumption.

System

Based on the background information and in preparation of scoping the projectís problem, the Team identified the system of interest.  A system diagram identifies the attributes acting on the system, and the systems diagram was developed (see figure below).  The system diagram was used to gain a better understanding of the system and identifying all the variables that act on the vessel.  This diagram allowed the Team to develop and scope the projectís problem.

Figure 1.  System Diagram

The project was focused on a vessel and its fuel consumption (the system of interest, denoted by the yellow circle in the figure about) and it operates in the ocean.  Through discussion with the customer and research, the Team determined that there are multiple attributes that act on the system affecting the fuel consumption.  Some of the attributes are: the oceanís current, wind speed and direction, temperature of the ocean, sea state, and many more.  The Team identified a large number of attributes acting on the system and due to the restricted timeframe, not all attributes could be analyzed.  Instead, the Team identified two attributes to be analyzed: vessel speed, and sea state.  These attributes were selected because they had an additive effect on the vesselís fuel consumption.  Both the sea state and vessel speed are assumed to be influenced by the wind speed and direction, ocean current, and sea temperature.  Therefore, to develop a mathematical model that would incorporate as many variables as possible, variables with additive effect would provide the most value.