340 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 59.11 



"Water is drawn in through about two-thirds 

 of the length of the opening, measured from the 

 smaller end. The water flows into the cone in the 

 direction of rotation and the resultant spiral flow 

 causes the water to leave the cone with increased 

 velocity, thereby producing a reactive thrust 

 which takes effect upon the internal surfaces of 

 the cone." 



(5) SBSR, 10 Feb 1938, p. 176; 2 May 1940, pp. 444-445. 



The latter reference contains rather detailed draw- 

 ings of a 25-ft launch equipped with a double-cone 

 propeller designed by Donald V. Hotchkiss for 

 operation on the Irrawaddy River where floating 

 debris and weeds are encountered. A grille is pro- 

 vided to exclude "objects which might damage the 

 impellers if drawn into the intakes." 



(6) Baader, J., "Cruceros y Lanchas Veloces (Cruisers 



and Fast Launches)," Buenos Aires, 1951; Fig. 175 

 on p. 221 illustrates a Hotchkiss propeller fitted in 

 the side of a vessel 



(7) On pages 358 and 359 of the September 1952 issue of 



The Motor Boat and Yachting there is an article, 

 with drawings, about a small cone propeller installa- 

 tion suitable for dinghies. The following is quoted 

 from page 358: 



"Advantages of the system are that there is no 

 projection outside the hull so that the draft of a 

 craft using it is much reduced; the cone propeller 

 can pass through weed beds, or over ropes or 

 other obstructions without fouling, by reason of 

 the self-clearing grids provided; installation is a 

 simple matter and the cones can be installed in 

 the most suitable part of the boat. A further 

 important advantage is that the impeller can be 

 employed to pump out the bilges, by providing 

 piping connected to the small end of the cone." 



For the Gill propeller, the available information 

 is somewhat more scanty: 



(a) Gill, J. H. W., "Der Hydraulische Schiffsantrieb fiir 



besondere Fahrtverhaltnisse (The Hydraulic Ship 

 Propulsion for Special Ship Operating Conditions)," 

 Bull. Tech. du Bureau Veritas, 1921, p. 199 



(b) The Shipbuilder, 1921, p. 24 



(c) The Engineer, 1921, Vol. 1 of that year, pp. 140, 172 



(d) MENA, 1923, p. 345 



(e) SBSR, 19 Aug 1926, Vol. 28, pp. 202-204; 1939, Vol. 



54, pp. 111-115. 



59.11 Area Ratios, Blade Widths, and Blade- 

 Helix Angles of Screw Propellers. The various 

 blade-area ratios of a screw propeller are defined 

 rather precisely in Sec. 32.8 of Volume I and in 

 the "Explanatory Notes for Resistance and 

 Propulsion Data Sheets," SNAME Technical 

 and Research Bulletin 1-13, July 1953, page 16. 

 The expanded-area ratio Ae/Aq , also known 

 rather indefinitely as the blade-area ratio or the 

 disc-area ratio, is the one employed almost 

 exclusively in this book, particularly in the 



propeller-design discussion of Chap. 70 of the 

 present volume. 



It is often convenient, when laying out propeller 

 apertures and edge clearances by the rules laid 

 down in Sec. 67.24, to know the approximate 

 maximum blade width for a screw propeller 

 having Z blades and a specified (or approximate) 

 expanded-area ratio As/Ag , or for a propeller 

 having a given mean-Avidth ratio Cm/D. The 

 maximum width will depend to some extent upon 

 the blade outline and shape but approximate 

 values can be derived for blades of average shape, 

 to permit establishing aperture and edge clear- 

 ances in the preliminary-design stage. The broken- 

 line graph of Fig. 59. E enables a designer to 



Left-Hand Scole is Rotio of 



Expanded Chord Cf^ 



^eon Expanded Chord Length i 



Mean- Width Ratio -^ 



Fig. 59. E Gbaph for Estimating Maximum Expanded 

 Chord Length from Mean-Width Ratio 



estimate the maximum expanded chord length 

 Cm ax or the maximum blade width for a reasonable 

 range of mean- width ratio Cm/D. 



For example, the stock propeller (TMB 2294), 

 used for the self-propulsion tests of the ABC 

 transom-stern ship model, has a mean-width 

 ratio of 0.238. From Fig. 59.E, the value of 



CMax/CMean = 1 . 19, WheilCe Cwax = 1.19(0.238)/) = 



1.19(0.238)(20.0) = 5.66 ft. The chord-length 

 data on Fig. 78. Ma give a maximum length of 

 2.682 ft at the 0.772 for a 9.25-ft diameter. Step- 

 ping this up to the 20.0-ft diameter of the ultimate 

 propeller design for the ABC ship gives a c^^t, of 

 5.80 ft. This is only slightly greater than the value 

 estimated from Fig. 59. E. 



For the ABC propeller designed in Chap. 70, 

 the actual value of c^^k at the 0.7/2, from Table 

 70.i, is 5.098 ft. From Fig. 59.E, for a Cm/D ratio 

 of 0.229, derived in Sec. 70.31, the value of 



CMax/CMea- IS 1.182 aud Cwax = (1 . 182) (0.229) (D) 



= (1.182) (0.229) (20.0) = 5.42 ft. The estimated 

 value is thus somewhat large. The forward aper- 

 ture clearance of 4.7 ft, indicated in Fig. 06. Q, is 

 somewhat less than the Cmss of 5.098 ft for the 



