Sec. 71.9 



DESIGN OF MISCELLANEOUS PROPULSION DEVICES 



649 



be kept clear of protuberances or where some 

 similar characteristic is more important than the 

 downright efficiency of propulsion. In most 

 lifeboat installations (those operating from hfe- 

 saving stations ashore) the jet ducts can not 

 protrude beyond the fair hull hne, and a con- 

 siderable sacrifice in propulsive efficiency is 

 accepted. If, as may be expected for these craft, 

 good backing qualities are required, the pump 

 must be of the axial-flow or propeller type unless 

 flap valves in the ducts are employed. 



ManeuverabiUty and steering as well as pro- 

 pulsion are achieved with a pivoted jet, arranged 

 to discharge water in any relative direction 

 desired. As with a steering propeller, however, it 

 is mandatory to incorporate a low-friction thrust 

 bearing in the swiveUng head, because all the 

 propulsive thrust is exerted through this swiveling 

 connection between the jet elbow and the boat, 

 indicated in Fig. 71.C. If the friction is large 



sSwivelina 

 Stern Mechanism 



Bottom of Boat 



Water.from Pump 



Propelling 

 Jet 



Swivelinq-Nozz-le Assembly 

 Is Shown in 



Solid Black 



Fig. 71. C Schematic Arrangement of Rever- 

 sible Jet-Propulsion Device for a Small Boat 



in this large-diameter bearing, often incorporating 

 a watertight stuffing box as well, the craft can 

 not even be steered. 



When working up the design of one of these 

 systems the procedure divides itself naturally into 

 several steps: 



(1) Determination of the quantity-rate or amount 

 of water per unit time to be handled, the area(s) 

 of the jet(s), and the amount of increased velocity 

 to be imparted to this mass of water to produce 

 the thrust required 



(2) Working out the method whereby this 

 quantity-rate is to be taken in and led to the 

 pump or impeller which is to impart the augment 

 of pressure and velocity to it 



(3) The fashioning of the ducts and passages, for 



both the inlet and the outlet water, so that the 

 hydraulic losses will be a minimum 

 (4) Estimating or calculating the rating of the 

 pump or impeller and the power necessary to 

 drive it. 



There have been many recent successful devel- 

 opments of guide-vane assembUes by which the 

 flow of water can be changed in direction abruptly 

 yet efficiently. Relatively sharp turns can now be 

 worked into the ducts of the hydraulic propulsion 

 setup without excessive hydraulic losses. This 

 gives the designer considerable latitude in leading 

 a stream of water around inside a vessel. Prac- 

 tically all modern circulating-water channels and 

 variable-pressure water tunnels for the testing 

 of model propellers under cavitating conditions 

 embody corner guide vanes of this type. Up-to- 

 date design rules and criteria have been worked 

 out by the St. Anthony Falls Hydraulic Labora- 

 tory in Minneapolis, Minnesota. In fact, it is 

 possible by employing a large rotary valve with 

 vanes to "switch" the water from one duct to 

 another or to reverse its direction. 



Centrifugal pumps of good design show effi- 

 ciencies of well over 0.80 and reaching 0.90 at 

 the rated output. Propeller-type or axial-flow 

 impeller pumps with fixed blades should do at 

 least as well [Rouse, H., EH, 1950, Chap. XIII]. 

 Design and performance data on hydraulic 

 pumps of various kinds are given by J. W. Daily 

 in the reference cited and by G. F. WisUcenus 

 [FMTM, 1947]. 



Supercharging of the pump, to increase the 

 ambient pressure and to delay or prevent cavita- 

 tion of the blades, may be accomplished by 

 making use of the ram or dynamic pressure built 

 up at the duct inlet by the motion of the craft 

 through the water. This is also used to help force 

 the required quantity of water through the duct 

 toward the pump, against the friction resistance 

 of the walls. 



The hull of a craft designed for reasonably 

 efficient propulsion with fixed ducts requires 

 special forming in way of the jet intakes and 

 discharges, whether these openings are at the 

 bow or stern or along the hull. The design of any 

 installation of importance should be checked at 

 least by flow tests on a small model and preferably 

 by performance tests and pressure measurements 

 on a larger model. 



The curved-vane diffusers and the various 

 other means adopted by hydraulic engineers to 



