572 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 69.6 



A subsequent commentary is equally applicable: 



"... of the ship in a big swell pitching, the bow and 

 stern moving through many feet and the amidships 

 movement only angular, and this space occupied by the 

 engine, and the engine was never seasick . . ." [Lord 

 Brabazon, SBSR, 23 Oct 1952, p. 555]. 



Comments on machinery-aft positions were made 

 by G. Gravier, concerning the performance of 

 the passenger steamer El Djezdir [SBSR, 23 Sep 

 1954, p. 397], and by A. C. Hardy [SBSR, 7 Oct 

 1954, p. 465]. 



More recently E. C. B. Corlett has made a 

 further study of the advantages and disadvan- 

 tages of installing machinery aft, based upon 

 modern conditions. He also discusses the question 

 of placing the navigating bridge and all the crew 

 accommodation aft [The Motor Ship, London, 

 Feb 1955, pp. 483-485; IME, Jun 1955, Vol. 

 LXVII, pp. 84-85]. 



69.6 Use of Systematic Wake Variations. It 

 has been said [author unknown] that "The more 

 uniform the wake, the simpler does it become to 

 design an efficient propeller." Fortunately, be- 

 cause of the practicability of changing the size, 

 form, and attitude of the blade sections along a 

 length or a radius, it is only needful that this 

 uniformity be maintained for the complete 

 travel path of any given blade section. For a 

 screw propeller this calls for circumferential 

 uniformity in the wake fraction at any radius. 



When examining wake diagrams such as those 

 in Chap. 60 the procedure is to look for systematic 

 variations which permit the propulsion device to 

 be adapted locally to them. The next step is to 

 determine the correct or proper average wake 

 characteristics in the regions where the variations 

 from this average are the smallest. 



When contemplating the design of a paddle- 

 wheel, for instance, it is known that, apart from a 

 consideration of ship-wave effects, the wake 

 velocities near the ship hull are positive because 

 of the viscous flow in the boundary layer along- 

 side. Farther from the ship, outside the boundary 

 layer, these velocities are negative because of the 

 accelerated regions of potential flow abreast the 

 ship, indicated in the velocity profiles of diagram 

 C of Fig. 6.B. Theoretically, the paddle blade 

 elements away from the ship should travel faster 

 than those next to the ship. However, this is not 

 feasible in a shipboard installation and it might 

 not be advantageous hydrodynamically for other 

 reasons. The blade load per unit area next to the 

 ship is therefore larger than at a distance from 



the ship. Were it necessary to equalize the blade 

 loads per unit length for any reason this could be 

 done by narrowing the blade at its inner end and 

 widening it at its outer end. 



For practical and mechanical reasons, it is 

 advantageous to have the greatest blade load 

 nearest to the point where the torque is delivered 

 to the wheel. The increased loading at the inner 

 end of a paddlewheel blade, or at the hull end of a 

 Kirsten- Boeing rotating-propeller blade, is accord- 

 ingly accepted. The rotating blades of a Voith- 

 Schneider propeller develop thrust on opposite 

 sides during any one revolution. It is possible to 

 take advantage of the velocity variation in and 

 beyond the boundary layer by local changes in the 

 width but not in the section of a blade. 



69.7 Rate and Direction of Rotation of Pro- 

 pulsion Devices. Rather extensive comment 

 concerning the rate of rotation of screw propellers 

 is given by J. E. Burkhardt [ME, 1942, Vol. I, 

 pp. 28-35]. These remarks, coupled with the 

 discussion of Sec. 70.10 on the rate of rotation of 

 screw propellers as an element in design, is 

 sufficiently general so that no further comment is 

 needed here about other propulsion devices. The 

 matter of selecting a rate of rotation that will 

 not cause vibration of the ship structure or of its 

 major parts in resonance with the shaft or blade 

 frequencies is discussed briefly in Sec. 69.15. 



The selection of the direction of rotation of 

 these devices for a new ship design usually depends 

 upon the relative importance of the propulsive 

 efficiency to be achieved and the maneuvering 

 and other qualities desired. For small craft it 

 may also depend upon the availability of pro- 

 pelling plants developing the necessary individual 

 shaft powers and which rotate in the directions 

 desired, left-hand or right-hand. For the smaller 

 vessels, where one may have to use available 

 stock machinery, rotation in a desired direction 

 may be too expensive because of necessary modifi- 

 cations or may involve the carrying of too many 

 spare parts on a craft equipped w^th engines 

 rotating to both hands. In large vessels practically 

 all propelhng plants, at least in the design stage, 

 can be made to operate in either direction. 



The interposition of reduction or multiplying 

 gears may or may not change an engine direction 

 to the desired propeller direction. Nevertheless, 

 it is generally possible, even in the construction 

 stage, to obtain a desired direction of rotation of 

 the propulsion-device shaft if there are sufficient 

 advantages to be gained thereby. 



