Air Cushion Vehicles, Hovercraft, and Surface Effect Ships 



Possible propulsion systems include turbojets and turbofans, which are 

 clearly suited to amphibious craft. Fully submerged or partially submerged 

 super cavitating propellers as well as waterjets with flush or ram inlets can be 

 used on nonamphibious types. The pure turbojet has an exhaust velocity which 

 makes it unsuited for even fast craft speeds. The engine efficiency is further 

 diluted with duct losses, gear losses, and drag induced by appendages. Putting 

 these factors together gives some measure of efficiency. The actual numbers 

 change with time as new developments appear, 



Mr. House presented an example showing that a specific 500-ton sidewall 

 craft does the greatest amount of work when operating in the 70 to 80 knot re- 

 gion. The example showed the interrelation among power plant efficiency, lift/ 

 drag ratio, and tons per mile per year with a constant 2000 hours of utilization. 

 The percentage of operating time spent at each speed as well as the assumed 

 sea conditions also affect the results. He also concluded that at these speeds 

 waterjet propulsion seemed to be the best choice. Mr. House answered Dr. St. 

 Denis that blade cooling and better materials would probably account for im- 

 proved turbine performance rather than the use of regenerative cycles. 



Mr. House assured Mr. Weller that his lift/drag calculations included the 

 lift power requirements. He noted that the seaway places an upper limit on 

 cushionborne performance. 



Mr. House informed Dr. Quant that the tradeoff between propulsion system 

 weight and propulsive efficiency depends heavily on how much time is spent at 

 the various operating speeds. His final point was that the conclusions are very 

 sensitive to the details of the specified mission. 



Mr. House suggested, in reply to Dr. Wang, that speeds of 150 to 200 knots 

 would give better propulsion efficiencies if you could build a craft to take it. 



Dr. Sheets then presented some comments on three new subjects: fan de- 

 sign, propulsion, and load and structures. 



The fan design characteristics govern the cushion characteristics, which in 

 turn affect speed, degradation in a seaway, and bubble leakage. He has studied 

 centrifugal and axial flow fans with variations in speed of rotation and pressure/ 

 quantity relationships plotted against horsepower. Craft of 100 tons, 500 tons, 

 and 4000 tons have been analyzed. In some cases, the propeller and fan are 

 driven by the same prime mover. 



On the subject of propulsion, Dr, Sheets agreed with the problems as stated 

 by Mr, House. Dr. Sheets compared his results with those of Fielding and 

 Stanton-Jones to show that these vehicles fall in an unoccupied area in the 

 Gabrielli/Von Karman line. 



On the subject of loading criteria and structural analysis. Dr. Sheets dem- 

 onstrated an approach using computer calculations and tank tests to assess the 

 validity of a structural model. A large number of loads and loading conditions 

 were included. He mentioned hogging, sagging, and torsional loads as well as 

 unusual loads caused by docking, towing, and hoisting. 



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