744 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 75.S 



Elevation from 

 — jtl — --. Outside 



Vertical Section 

 Normol U> 5liell 



Fig. 75. G Recessed Mooring Bitt 



craft-carrier hulls in canal locks have led to the 

 development of a number of recessed fittings for 

 lashing, lifting, and mooring. Two types of simple, 

 welded, recessed lifting fittings are diagrammed in 

 Fig. 75. F, adaptable to any position or slope of 

 shell plating. A single recessed bitt or Dutch 

 dolphin is depicted in Fig. 75. G. A strap is easily 

 passed through the lifting fittings and an eye is 

 quickly thrown over or lifted off the bitt. While 

 designed as an abovewater installation this 

 dolphin may be used in positions which are under 

 water at some load conditions. 



75.8 Fairing the Enlargements Around Ex- 

 posed Propeller Shafts. The hubs of shaft struts, 

 the external or exposed couplings of the propeller 

 shafts passing through them, and the hubs of the 

 propellers carried by these shafts require fairings 

 as units. In other words, the leading fairing, the 

 enlarged strut-hub body, the propeller hub, and 

 the trailing fairing are treated as parts of a single 

 body. They should, theoretically, form a contin- 

 uous streamlined surface for a considerable length 

 along the shaft axis. One solution for the fairing 

 of such an assembly is represented by the design 

 of these parts on the wing shafts of the World 

 War II German cruiser Prinz Eugcn, illustrated 

 in Fig. 75. H. The fact that the maximum diameter 

 of the integrated fairing combination is consider- 

 ably larger than that of the strut hub is not to be 

 taken as an indication of good or recommended 

 design. This shape was adopted for many German 

 men-of-war of the 1930's and 1940's. Its use can 

 be justified, at least partly, by the following line 

 of reasoning. 



The best flow to the root sections of the blades 

 of a screw propeller, where the interference 

 effects are large and uniform flow is a useful 

 factor, is obtained when the structure surrounding 

 the propeller shaft bearing and the strut hub is 

 absolutely fair for a rather long distance ahead 



of the propeller hub. This requirement is met in 

 the assembly drawn at 1 in Fig. 75. H. Not the 

 least important part of this continuous fair form 

 is the easy curve at the forward end, Avhere the 

 diameter of the assembly has to increase from the 

 diameter of the shaft to one nearly three times 

 as large. The shght decrease in diameter from 

 the forward end of the assembly back through 

 the propeller hub conforms to the pattern of the 

 flow lines in the inflow jet and contributes a 

 slight inward component of velocity which partly 

 compensates for the centrifugal force due to 

 induced rotation in the jet at the propeller. When 

 a propeller is located abaft a large skeg or long 

 bossing this reduction of radius through the 

 propeller hub is a natural consequence of the 

 tapering form of the skeg or bossing. It is natural 

 in cases of this kind to give the propeller blades 

 a reasonable amount of rake but the designer of 

 the Prinz Eugen propellers did not see fit to do so. 

 Good design for minimum resistance and easy 

 water flow calls for the use of curved profiles from 

 leading to traihng edge of this or an equivalent 

 assembly. There should be no sharp disconti- 

 nuities such as are sometimes encountered when 

 the leading fairing in front of a cyhndrical strut 

 hub is made of straight conical form. If such an 

 assembly can be expected to remain completely 

 submerged under all except the most severe 

 pitching conditions in waves, the maximum 

 diameter may be at the propeller hub. If a large 

 diameter is not required there for other reasons 

 it may be in the strut hub, as in the German 

 design illustrated. Manifestly, the farther aft 



Direction of Flow 



Moximum Diameter of Foirinij i5 

 I 2.77^mes Shoft 

 \J)iometer 



■ Tonqent 

 'Lie5 0t32de(j 



1 Foinnq on Outboard Shafts, Germon Cruiser PRINZ. EUGEN *'"^ ■S'^""' 

 ' Center 



Fig. 75.H Fairings for Strut and Propeller Hubs 



