580 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 69.14 



the neutral angle for each and this angle may 

 change with speed. An effect of this kind is more 

 pronounced when the rudders lie within the pro- 

 peller outflow jets. Furthermore, there is a 

 cumulative torque reaction from all the engines 

 which results in an appreciable listing or heeling 

 moment and an ever-present heel at moderate to 

 high speeds. 



This heeling moment due to torque should be 

 counteracted, not by a fixed moment, as of ballast 

 or rnachinery asymmetry, but by a torque which 

 varies generally as the engine torque, increasing 

 gradually with speed and power. This is best 

 accomplished by applying a hydrodynamic torque 

 which automatically increases with speed. For 

 right-hand engines and propellers, a positive 

 heeling moment is required, acting to produce a 

 starboard heel. On some V-bottom planing craft 

 [Bureau of Ships Bull, of Inform. 32, 1 Oct 1948] 

 the chine spray strip is modified to slope its lower 

 edge downward and outward, giving it a negative 

 dihedral angle with the bottom of the boat. The 

 water moving out transversely from under the 

 boat is deflected sharply downward, and an up- 

 ward force results from this change of direction. 



The hydrodynamic compensating torque is also 

 produced by a well-cambered hydrofoil placed in 

 an offset position in the propeller outflow jet, so 

 as to develop a lift force forming a torque opposite 

 to that of the propeller. Fig. 73. P illustrates and 

 Sec. 73.21 describes a pair of twisted hydrofoils in 

 the form of a cross, placed in the outflow jet of a 

 single screw propeller to accomplish this purpose. 



69.14 Propulsion-Device Design to Meet 

 Maneuvering Requirements. The design of ship 

 hulls to meet maneuvering requirements is dis- 

 cussed in Part 5 of Volume III. There are pre- 

 sented here a few features relative to the type, 

 position, and size of the propulsion devices when 

 maneuvering is a major consideration. For stop- 

 ping and running astern, some design comments 

 and pertinent references are given by J. E. Burk- 

 hardt [ME, 1942, Vol. I, pp. 35-38]. Fortunately 

 there are available sufficient data from tests of 

 model propellers running astern, listed in the 

 references of Part 5, to enable the designer to 

 check the ability of a propeller to produce a 

 specified thrust for astern operation. There are 

 also available in Sec. 60.18 the results of backing 

 tests on one self-propelled model. 



While it is true that ships can be and have been 

 steered by changing the rates of rotation of wing 

 propellers carried by them, the turning moments 



are usually too small to be of any considerable 

 benefit in rapid maneuvering. Indeed, if any 

 drastic change is to be made in a vessel's turning 

 path it is necessary to reverse the wing propeller(s) 

 on the inside of the turn. It is doubtful if any 

 vessel carrying screw propellers can have its 

 turning characteristics materially improved by 

 any practicable positioning of the wing screws or 

 wing shafts at a large distance from the center- 

 plane. 



Despite the overwhelming percentage of time 

 during which any ship is employed in ahead 

 operation, the requirements for developing astern 

 thrust and for maneuvering may and often do 

 influence the design of the propulsion device (s). 

 For example, on a paddle tug with independent 

 wheels, frequently employed in backing and 

 turning, the paddle blades are properly straight 

 rather than curved in section. They should, 

 furthermore, enter and leave the water vertically, 

 if this is feasible without too much complication. 

 An icebreaker which needs powerful astern 

 thrusts to fulfill its mission may advantageously 

 have screw propellers with blade sections nearly 

 or completely symmetrical, as for the screw 

 propellers of the double-ended ferryboat. 



By and large any special requirements for 

 starting, stopping, and turning are fulfilled by 

 propulsion devices with large thrust-producing 

 areas. The larger these areas the better, since 

 for a given speed of advance V a the thrust depends 

 upon the thrust-load factor and ultimately upon 

 the disc area. 



69.15 Relation of Propulsion-Device and Hull- 

 Vibration Frequencies. A discussion of machin- 

 ery and hull vibration as such is definitely outside 

 the scope of this book. Nevertheless, it is pointed 

 out here that the rate of rotation of the propulsion 

 devices, whatever their type, should not be fixed 

 until a study has been made of the probable 

 vibration characteristics of the hull and propelling 

 machinery under various loading conditions 

 [Lewis, F. M., ME, 1944, Vol. II, pp. 130-137; 

 Kane, J. R., and McGoldrick, R. T., SNAME, 

 1949, pp. 193-252]. This insures that the propul- 

 sion-device rpm or the blade frequencies n{Z) do 

 not coincide with a 2-noded or 3-noded hull 

 frequency in vertical or horizontal flexure, so that 

 a slight mechanical or hydrodynamic unbalance 

 is magnified over the whole ship. Strictly speaking, 

 the torsional hull frequencies should also be 

 estimated and compared with the proposed shaft 

 rpm. 



