Non-traditional hydrodynamic propulsion systems such as pods, water jets, surface drives, and vertical axis propellers either inherently provide a steering capability or are sold with the steering element as an integral part. In addition, the appendage drag of these systems varies greatly. Thus the traditional measure of merit, the propulsive coefficient, which is mostly concerned with propeller efficiency and hull-propeller interaction, is not the best assessment of non-traditional propulsion systems.

A new proposed measure of merit called the Hydrodynamic system coefficient is presented It is the ratio of the "bare hull" effective power to the final delivered power. Bare hull in this case refers to the vessel configuration without it's steering and propulsion-related appendages. The Hydrodynamic system coefficient is presented for several cases of the following vessel configurations; twin-pod propulsion, traditional shaft and strut arrangements, and nonintegral type skegs. Additionally, the application is also explained for vertical axis propellers, water-jet propulsion, tunnel hull arrangements, and surface drives.

It is shown that the Hydrodynamic system coefficient is an excellent single parameter for comparing/assessing the performance among these various types of propulsion arrangements. The unadjusted model-test results are presented along with an adjusted calculation for cases where the rotating element of the model propulsor had poor performance.

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