650 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 71.10 



raise the operating efficiencies of propeller-type 

 pumps can all be applied to the propulsion of 

 a ship, if the hull has a shape which lends itself 

 to the building in of the necessary ducts or jet 

 boundaries. It is hardly to be expected that a 

 form of underwater hull which, over many 

 decades, has evolved into one suited to the screw 

 propeller will be found readily adaptable for 

 the efficient use of fixed propeller shrouding, pro- 

 peller-type pump-jets, or hydraulic jets. Experi- 

 ence with the Kort nozzle, described in Sec. 36.19, 

 indicates that it is not easily fitted to a vessel of 

 normal form. 



The same basic hydrodynamic laws and rela- 

 tionships are used for the design of axial-flow 

 impeller pumps with casings as for open-stream 

 ship propellers but the attack on the problem is 

 quite different. The available engineering data 

 are in such form as to apply only to the design 

 of each class of devices by itself. For this dis- 

 cussion they are known as impellers and propellers, 

 respectively. 



The quantity rate of flow Q = ¥t = AUt, 

 inside the solid casing boundaries of cross-section 

 area A , must remain the same from inlet to outlet, 

 hence it is used as a basic quantity in the impeller- 

 pump design. If Af7 is the increase in velocity 

 imparted by the impeller from the casing inlet 

 to the casing outlet, the impeller thrust T is, 

 from Newton's second law of motion, 



T = [ p{AU) dQ = pQ(AU) (71 .v) 



Jq 



This is the same as Eq. (71.ii) of Sec. 71.4 and 

 of Eq. (34.xxix) of Sec. 34.13. 



Assuming that the inlet velocity is the differ- 

 ence between the ship speed V and the wake 

 speed V w at the inlet 'position, reckoned here the 

 same as the speed of advance V a at a propeller posi- 

 tion, then Finiet = Fx = V— Vw ■ The increase 

 in velocity AC/ through the casing and impeller 

 is taken as a fraction of the inlet velocity Va ■ 

 Also Q = T/{pAU). The pressure or pumping 

 head hp then required of the impeller is equal to 

 the increase in kinetic energy of the water passing 

 through the pump, or 



hp = 



KVa + Avy - VI] 



From here on the design problem becomes 

 lengthy and complicated, to be solved by methods 



developed by hydraulic engineers for the design 

 of ducts and pump impellers rather than by 

 those worked up by marine engineers for pro- 

 pulsion devices working in the open. Unfor- 

 tunately, the design data on hydraulic-jet 

 propulsion apparatus are relatively meager, par- 

 ticularly because there has been little in the way 

 of logical, progressive, modern development 

 except on classified projects for combatant 

 vessels and new weapons. It should be kept in 

 mind always, however, that efficient hydraulic- jet 

 propulsion requires the largest practicable diam- 

 eter of jet and the smallest relative velocity of 

 jet water, reckoned with respect to the surround- 

 ing undisturbed water. 



It is proposed in Sec. 34.13, and repeated here, 

 that the whole jet-propulsion system be designed 

 on an energy or work basis, rather than on a 

 pressure and force basis, as is customary for 

 screw propellers. 



71.10 The Design of Surface Propellers. 

 There are no systematic data, so far as known, 

 for the design of surface screw propellers. These 

 are deliberately intended to run with only 

 partial immersion, either because of draft limita- 

 tions or because of the lift force that is to be 

 obtained from them, described in Sec. 33.11. 

 A few references pertaining to this particular 

 design phase are: 



(a) Smith-Keary, E. M., "The Effect of Immersion on 



Propellers," NECI, 1931-1932, Vol. XLVIII, pp. 

 26-44 and D1-D17 



(b) Kempf, G., "Immersion of Propellers," NECI, 1933- 



1934, Vol. L, pp. 225-248 and D123-D138 



(c) Kempf, G., "The Influence of Viscosity on Thrust 



and Torque of a Propeller Working Near the 

 Surface," INA, 1934, pp. 321-326 and PL XXXIV 



(d) De Santis, R., "The Effect of Inclination, Immersion, 



and Scale on Propellers in Open Water," INA, 

 1934, pp. 380-385 and Pis. XXXVI-XXXVIII 



(e) Baker, G. S., "The Qualities of a Propeller Alone and 



Behind a Ship," NECI, 1937-1938, Vol. LIV, pp. 

 239-250 and D135-D146. 



General comments and design data on "Par- 

 tially Immersed Propellers" are given by W. P. A. 

 van Lammeren, L. Troost, and J. G. Koning 

 [RPSS, 1948, pp. 262-263]. 



Some practical pointers for the design of surface 

 propellers on high-speed racing motorboats are 

 published by E. C. B. Corlett in "Trends in 

 Very High Speed Craft" [The Motor Boat and 

 Yachting, Sep 1954, pp. 386-388]. 



The thrust-load factor for a surface propeller 

 is of course based upon the fractional disc area 



