Unconventional Propulsion — Silverleaf 



propulsive efficiency must be uniform, comprehensive, and unambiguous if it is 

 to have value in comparisons between different systems. 



The hydrodynamic performance of a marine propulsion device operating in 

 isolation can be defined by its thrust efficiency, which is the ratio of the power 

 output based on the effective thrust from the device, to the power input to the 

 device. If the inflow velocity is taken as the mean velocity in the nonuniform 

 flow conditions in which the device operates when propelling the ship, then this 

 thrust efficiency is equal to the "behind" efficiency used in conventional ship- 

 powering analyses, as defined in Ref, (4) and elsewhere. 



When the device is part of the propulsion system of a ship, it has to be 

 physically linked to the hull; this generally requires some external appendages, 

 such as shaft supports or water inlets, and their net drag may increase the total 

 resistance of the ship above that of the bare or naked hull. The flow induced by 

 the propulsion device generally further increases the resistance of the hull, and 

 the propulsive thrust must overcome this augmented resistance; there is a fur- 

 ther interaction effect because in these conditions the mean inflow velocity to the 

 device is less than the speed of the ship. The flow interaction effects between 

 hull and propulsion device can be expressed as a single factor linking the thrust 

 efficiency of the device alone to its propulsive efficiency when part of the pro- 

 pulsion system; this hull interaction factor is identical to the hull efficiency cus- 

 tomarily used in ship powering analyses and cannot be ignored in assessing the 

 relative merits of different types of propulsion device in real operating conditions. 



The overall efficiency of the complete propulsion system, including prime 

 mover, is the ratio of the useful power to the power output of the engine. In 

 conventional ship powering analyses it is customary to consider that this useful 

 power is the effective or tow-rope horsepower of the hull including any external 

 propulsion appendages. However, the ship designer is primarily interested in 

 the power required to propel the bare hull, and the power absorbed in overcom- 

 ing the drag or resistance of external appendages directly associated with the 

 propulsion device should not be regarded as useful output; consequently in com- 

 paring the efficiencies of different propulsion devices, the useful power should 

 be related to the resistance of the naked hull alone. This gives a useful propul- 

 sive efficiency defined by the ratio of the effective horsepower for the naked hull 

 to the power output of the prime mover. 



Thrust Efficiency and Its Components 



Although the thrust efficiency rjj alone is not a sound index for comparing 

 the performance of different propulsion devices, it is a useful part of such an 

 index, and it can also be resolved into components which have some value. Al- 

 most all practicable marine propulsion devices are of the reaction-screw-type, 

 in which thrust is developed by a rotating pump or rotor, which imparts energy 

 to accelerate a jet of water. The ideal or maximum efficiency r/j of such an a 

 accelerated jet system can be readily derived by simple axial momentum or 

 actuator disk theory which ignores viscous effects and other losses such as 

 those due to flow rotation. The realizable thrust efficiency 77^ is then obtained 

 by applying a pump or hydraulic efficiency factor rjp to take account of these 

 losses in the rotor. Some propulsion devices, such as water-jet systems, 



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