CM 



HYDRODYNAMICS IN SHIP DESIGN 



See. 71.S 



of Sec. 59.6, E is 0.24 meter, D i.s 3.56 meters and 

 L(blade-crank length) is 0.36 meter. Tlie eccen- 

 tricity ratio is 0.5/0.8 = 0.625 in the first case 

 and 6.24/0.36 = 0.667 in the second. The eccen- 

 tricity ratios of the more recent vessels of Bragg's 

 table vary from 0.584 to 0.69. That of the ABC 

 paddlewheel in Fig. 71.B is 2.625/4.0 = 0.656. 



It is possible, by raising the eccentric center 

 B above the level of the wheel axis A, to: 



(1) Help bring the leaving blade more nearly 

 tangent to the resultant-velocity vector on the 

 after side 



(2) Avoid or reduce the mechanical interference 

 between a blade and the feathering link which 

 operates it, on the forward side of the wheel. 

 In diagram 1 of Fig. 32.B, the nearly horizontal 

 feathering link on the forward side just clears 

 the upper edge of its blade. The same is true in 

 Fig. 71. A but, for the ne.xt blade above, the 

 feathering link actually fouls the inner edge of 

 the blade. A similar situation in practice is met 

 either by making the feathering links of rec- 

 tangular section and bending them so as to clear 

 the blades [Teubert, 0., "Binnenschiffahrt," 1912, 

 p. 444], or by notching the inner edges of the 

 blades to clear the hnks [Hart, M., ATM A, 

 1906, Vol. 17, Pis. V and VI]. 



With the usual feathering mechanism, having 

 an eccentric center forward of the shaft center, 

 the feathering links are in compression, especially 

 when the blades are entering or leaving the water. 

 The links can not, therefore, be too slender for 

 their length without risk of buckling. J. Scott 

 Russell proposed a variation of the usual geo- 

 metric arrangement whereby the eccentric center 

 (B in Fig. 71.A) Hes abaft the wheel axis. The 

 blade cranks are thus on the after or +Ap sides 

 of the blades, and the feathering Unks are, when 

 loaded heavily, always in tension. 



71.8 Variations from Normal Paddlewheel 

 Design. Where damaging debris is frequently 

 encountered and repairs to blades must be made 

 on the spot or locally, often by the ship's force, 

 radial blades are used, bolted to the wheel arms. 

 The blades can be shifted in or out, radially, to 

 suit a more-or-less permanent change in draft or 

 trim. They can be varied in width, or shifted 

 radially, to permit the wheel torque and speed 

 to be varied so as to obtain the maximum engine 

 power with the greatest ship speed or towline 

 tension. The radial blades are simple and cheap 

 and they produce thrust but they are not par- 



ticularly efficient. They impart useless upward 

 and downward components of motion to the 

 water [Teubert, 0., "Binnenschiffahrt," 1912, 

 Figs. 305, 306, p. 441; S and P, 1943, Figs. 168, 

 169, p. 149] and they leave a great deal of energy 

 in a series of short, steep waves trailing astern. 



Wheels with fiat, smooth blades geared to a 

 shaft so as always to stand with their heights 

 truly vertical, and with annular support rings 

 always above the waterline, as devised by Georg 

 Fricke of Lembruch, Germany, have proved 

 excellent for propulsion in calm waters, grown 

 thick with grasses and weeds. The blades press 

 the weeds down vertically and do not become 

 foul [Deetjen, R., Schiff und Hafen, Mar 1952, 

 pp. 80-81; this German article is abstracted in 

 SBSR, 8 May 1952, p. 579]. 



A partial list of references in the technical 

 literature on paddlewheels, relating to both 

 model and full-scale devices, is to be found in 

 Sec. 59.6. 



It should be clear from the foregoing that, 

 whatever its place in the scheme of things pro- 

 pulsive, the analysis and design of a paddlewheel 

 represents a marvelous exercise in practical 

 hydrodynamics and practical machine design. 

 Not only does the paddlewheel need an extension 

 of the motion analysis published by M. Hart 

 [ATMA, 1906, Vol. 17, Pis. II and III] but the 

 experience gained in such an analysis would be 

 invaluable when analyzing the action of other 

 propulsion devices and in preparing systematic 

 rules for their design. 



71.9 Design Notes for Hydraulic-Jet and 

 Pump- Jet Propulsion. Methods of achieving 

 what is often termed hydraulic propulsion are 

 described in Sec. 15.8 and elaborated upon in 

 Sec. 32.5. The efficiencies of the principal systems 

 are discussed in Sec. 34.13. In Sec. 59.8 there is 

 given a list of the principal references on this 

 subject, both historical and technical. Some of 

 them are valuable in design as indicating the 

 pitfalls to be avoided. 



It is pointed out in Sec. 15.8, and it is again 

 emphasized here that the air, gas, or water jet 

 employed for propulsion may, hke that from the 

 airscrew, be discharged into the atmosphere 

 above the water. An example of this is the Russian 

 shallow-draft river launch propelled by twin 

 water jets and pictured in The Illustrated London 

 News [11 Dec 1954, p. 1070]. 



Hydraulic-jet propulsion lends itself to craft for 

 special duty, where the outside of the hull must 



