658 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 71. IS 



The rot.ating-l)lade propeller lends itself par- 

 ticularly well to craft which must hold position 

 against wind, tidal-current, and other forces 

 when stationary or nearly so. It is reported 

 [SBSR, 12 Mar 1953, p. 342], in an article on 

 "German Small Craft," that: 



"The Voith-Schneider propeller has proved of particular 

 value in applications where large athwartships thrusts 

 are required. It has therefore been fitted for buoy lifting 

 and laying vessels which have to be operated in particularly 

 difficult waters." 



The article is accompanied by the outboard 

 profile of a small lighthouse tender with a long, 

 sloping, cut-up stern and two Voith-Schneider 

 propellers, one well out on each side, set with 

 their axes pointing upward and inward. The 

 blade tips are well above the baseplane. However, 

 the tops of the blades come rather close, perhaps 

 too close, to the water surface. 



The large diameter of the rotating assembly of 

 a propeller of this type lends itself equally to a 

 high gear reduction from a high-speed engine or 

 motor or to a low-speed motor drive. The angular 

 speed of the propeller is usually not a major 

 design problem. 



On the other hand, the rotating-blade propeller, 

 of whatever type, has been plagued from the 

 beginning of its development by the unavoidable 

 complication and relative Aoilnerability of the 

 internal mechanical gear. With half a dozen or 

 more blades to position, not only for straight- 

 ahead steady running but for changing the pitch 

 and changing the direction of the resultant 

 thrust, the mechanical-design problem is difficult 

 at the best [Mueller, H. F., "Recent Develop- 

 ments in the Design and Application of the 

 Vertical Axis Propeller," SNAME, 1955, pp. 4-30]. 

 Simplifications have been effected to make the 

 parts more sturdy, but these changes have, more 

 often than not, involved some reduction of the 

 hydrodynamic efficiency. By dint of excellent 

 engineering the rotating-blade propeller continues 

 to run. It is possible that some of the developments 

 which have brought a greatly increased measure 

 of rehability to the controllable propeller may 

 do the same for the most useful rotating-blade 

 device. 



71.18 Airscrew Propulsion. An early type of 

 "skimming boat" with airscrew propulsion, appar- 

 ently intended for work in extremely shallow 

 water, was designed and built by Yarrow of 

 Glasgow about 1921 [Mar. Eng'g., Jul 1921, p. 



538]. An air-rescue task is clearly indicated, since 

 the wide, flat-bottomed craft was capable of a 

 speed of 50 miles per hour. A later type is shown 

 by D. Nicolson [NECI, 1937-1938, Vol. LIV, 

 Fig. 5, p. 117]. Sea sleds or inverted V-bottom 

 craft driven by airscrews are illustrated in Motor 

 Boating [New York, Jan 1946, p. 112]. Two 

 types of high-speed boats driven by airscrews are 

 shown by J. Baader, in a chapter in which he gives 

 design instructions and data for the powering of 

 this type of craft ["Cruceros y Lanchas Veloces 

 (Cruisers and Fast Launches)," Buenos Aires, 

 1951, pp. 222-224]. 

 Propulsion by airscrew (s) is selected when: 



(a) Weeds, grasses, and other marine growths 

 are so profuse and thick that hydrodynamic pro- 

 pulsion by weedproof screw propellers, or by 

 sculling propellers, paddlewheels, and similar 

 devices is out of the question 



(b) The hull and the engine are so high above the 

 water surface, as in a, hydrofoil-supported craft, 

 that it is undesirable or inconvenient to transmit 

 the power to a propeller under water 



(c) It is necessary to eliminate the noise and other 

 disturbance made under water by mechanical 

 propulsion devices. This may be mandatory for 

 certain fishing operations. 



(d) It is desired to measure the hull resistance of 

 a craft, free of all hydrodynamic propulsion 

 efTects. 



To compensate for the down-pitch moment of 

 the airscrew thrust, at a large relative height 

 above the line of action of the water-resistance 

 forces, any small boat carrjdng an airscrew must 

 have a large hydrodynamic moment resisting the 

 thrust moment. This is one reason for the use of 

 sled-type craft of rectangular cross section, re- 

 sembling the floats of early seaplanes. 



Airscrew propulsion is exceedingly inefficient — 

 indeed, it is almost ineffective — when the speed 

 of advance is low, as it would be for even a small 

 swamp boat plowing through a heavy growth of 

 weeds. Air flow through the propeller then re- 

 sembles that depicted in Figs. 16. K and 16. L, 

 when momentum is imparted to only a small mass 

 of "new" air in any given interval of time or given 

 distance of forward travel. The propulsive effi- 

 ciency mounts rapidly as the speed of advance 

 increases in proportion to the velocity of the 

 outflow jet of the propeller. For this type of 

 drive, therefore, the resistance of the craft to be 

 driven should be low and the resulting speed high. 



