688 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 73.13 



propeller. The reference embodies a stern view of 

 the vessel on the ways, showing the propellers, 

 nozzles, and rudders. There are si.x flanking rudders 

 in addition to the three steering rudders. The craft 

 has a length of 164 ft, a beam of 44 ft, a depth of 

 10 ft, and a draft of 6.5 ft. 



(14) Roscher, E. K., "Kort Nozzle Propulsion of Ships," 



Shipbuilding, 1955, Vol. I, p. 84 ff; abstracted in 

 IME, Apr 1956, Vol. LXVIII, pp. 105-106 



(15) Van Manen, J. D., "Recent Research on Propellers in 



Nozzles," SNAME, New York Sect., 30 Oct 1956. 



For design purposes the Horn and Amtsberg 

 references of 1950 are the most useful and valuable. 



Although it does not contain design rules as 

 such the marine architect who sets out to study 

 the advisability of using fixed shroudings with 

 screw propellers will require reference to the 

 paper "Open-Water Test Series with Propellers 

 in Nozzles," by J. D. van Manen [Inter. Shipbldg. 

 Prog., 1954, Vol. 1, No. 2, pp. 83-108]. 



D. S. Simpson has the following to say concern- 

 ing this type of installation: 



"The Kort nozzle, now almost universally used on the 

 river towboats, has (made) a definite contribution to all 

 vessels used principally for towing, although it shows 

 little change in free route performance. Experiments 

 indicate that the hull must be designed for it as nozzles 

 added to existing (hull) designs have not given the ex- 

 pected improvement in towing power" [SNAME, 1951, p. 

 560]. 



A very real problem associated with the pro- 

 vision of fixed shrouding is building the necessary 

 rigidity into it and attaching it firmly to the hull. 

 This appUes equally to the design of shrouding 

 intended for mechanical protection only and to 

 that installed for improving the efficiency of 

 propulsion. The shape of the shrouding is so 

 foreign to that of a normal ship that when added 

 as an appendage it never appears to belong to 

 the ship. To obtain an integrated design it may 

 eventually be necessary to design a whole new 

 type of afterbody. 



If the shrouding or nozzle is part of the initial 

 design, a stern with a shallow transverse arch 

 and gently sloping buttocks over the propeller 

 is indicated. The under side of the arch then 

 forms the top of the nozzle opening. The nozzle 

 proper benefits by two hull attachments, each as 

 long as the nozzle, and spread laterally by the 

 width of the arch. 



The application of a nozzle-shaped fixed 

 shrouding or enclosing duct to an existing ship 

 is best limited to propeller positions abaft large 

 skegs or to those underneath wide, rather flat 



sterns. Here a reasonable amount of rigid struc- 

 tural anchorage is available at or near the top 

 of the shrouding. Although a distance equal to 

 the fore-and-aft length of the shrouding is avail- 

 able for this attachment, the real need is for 

 width of anchorage, to hold the shrouding con- 

 centric with the propeller-tip circle. 



While a horizontal strut connection or tie to 

 the bottom of a large ship skeg carrying a nozzle- 

 enclosed propeller is not mandatory, it is greatly 

 to be preferred. It can not be very deep, otherwise 

 it would extend below the baseplane. Likewise, 

 it can not be very wide or it would interfere with 

 the contraction of the inflow jet to the lower part 

 of the propeller disc. Nevertheless, it is a tie to a 

 relatively rigid portion of the ship structure and 

 as such it should be utilized to the utmost. 



73.13 Shaping and Positioning of Contra- 

 Vanes Abaft Paddlewheels. The action of 

 contra-vanes forward of and abaft paddlewheels 

 and sternwheels is described in Sec. 32.4 and 

 illustrated in Fig. 32. C. These vanes are fixed 

 appendages applied solely to improve the efficiency 

 of propulsion. 



So far as known, the only model experiments on 

 and full-scale trials of either leading or trailing 

 contra-vanes were those made under the super- 

 vision of F. Siiberkriib ["Vergleichende Modell- 

 versuche mit Siiberkriib Leitflachen an einem 

 freifahrenden Schaufelrad, Teil II (Comparative 

 Model Experiments with Siiberkrub Guide Plates 

 On a Free-Running Paddlewheel, Part 2)," 

 HSVA Rep. 321, 3 Mar 1936 (in German), copy 

 in TMB library; "Neue Verbesserungen in der 

 Hydromechanik des Radantriebs (New Improve- 

 ments in the Hydromechanics of Paddlewheel 

 Propulsion)," WRH, 15 Sep 1941, pp. 269-271]. 

 The notes in this section are based partly on 

 these data and partly on the general hydrodynamic 

 knowledge set forth elsewhere in the book. 



Considering first the forward or leading vane, 

 it is pointed out in Sec. 71.6 that a side paddle- 

 wheel is best positioned so that a wave crest lies 

 about opposite the point where the blades enter 

 the water on the forward side of the wheel. If the 

 leading contra-vane is placed under this crest, 

 the water flows to it in a direction nearly hori- 

 zontal. As a hydrofoil, cambered to deflect the 

 water sUghtly downward, its lift is exerted in a 

 direction close to the vertical. There is very little 

 or no thrust component of this lift force; as the 

 lift is otherwise not useful it should be kept to a 

 minimum. The entrance of the contra-vane is 



