Sec. 67.18 



UNDERWATER HULL DESIGN 



527 



detail design of the appendage itself is completed. 

 This is particularly true at the stern of a vessel, 

 in the vicinity of a screw propeller and a rudder. 

 A suitable procedure to be followed here is well 

 illustrated by the first design of the arch-type 

 stern for the ABC ship. It involved a closely spaced 

 pair of skegs and rudders, a large screw propeller 

 with exceptionally small tip clearance, an exposed 

 propeller shaft, a propeller-bearing housing abaft 

 the propeller, supported by multiple strut arms, 

 and a contra-propeller effect in the strut arms. 

 After this arch-type stern was completely designed 

 it was found that an installation very similar to 

 it had been embodied in the Dravo pushboat 

 Pioneer nearly two decades before, even including 

 the twist in the strut arms [SBSR, 14 Mar 1935, 

 pp. 291-293]. The general arrangement of these 

 appendages, as built into and tested on the self- 

 propelled model of the ABC vessel, is illustrated 

 in Figs. 73.F, 73.H, and 74.L in Chaps. 73 and 

 74 on fixed and movable appendages, respectively. 

 Supporting a propeller bearing within the 

 centerline tunnel requires either a V-strut of the 

 usual type ahead of the propeller or a strut of 

 special type abaft it. Since the tunnel is partly 

 obstructed by a short bossing where the shaft 

 comes out of the hull and a length of exposed 

 rotating shafting, sketched in Fig. 67.0, a V-strut 

 ahead of the propeller would only add to the 

 obstructing effect. Standard strut arms, because 

 of their isolated positions and short fore-and-aft 

 lengths, are not suitable as deflectors to impart 

 reversed rotation to the flow in the inflow jet; 

 see the discussion of the contra-propeller in 

 Sec. 36.9. The arch-stern arrangement permits 

 moving the propeller well aft, into a nearly 

 horizontal portion of the tunnel, but at the expense 

 of getting the rudders too near the propeller to 



Transom Immersion at t 



Transom Immersion O'^^r Ruddei 



take advantage of the augmented outflow velocity, 

 described in Sec. 33.21. However, there is no 

 obvious reason, except perhaps an instinctive 

 one, for maintaining this fore-and-aft distance, 

 expecially as the rudders do not lie within the 

 outflow jet. 



Moving the propeller bearing with its sup- 

 porting struts abaft the propeller clears the tunnel 

 of this obstruction and makes it possible to use 

 the strut arms as vanes for taking out the rotation 

 in the outflow jet and developing an additional 

 thrust, in the manner of a contra-rudder. Further- 

 more, by using three and possibly four arms, 

 curved to straighten the flow in the outflow jet, 

 these arms can have long, thin sections while 

 at the same time they provide adequate support 

 for the bearing housing. One or two arms, gen- 

 erally vertical, take the weight of the propeller 

 and the after half of the exposed shaft while the 

 two side arms steady the bearing laterally. The 

 arms are so spaced as to encounter the blade- 

 pressure fields at random or in succession rather 

 than simultaneously, indicated in Fig. 67. P. 

 The arms are thin and of relatively large chord, 

 to give good hydrodynamic performance. The 

 multiple-strut and bearing-housing combination 

 can possibly be assembled in place and welded 

 into the ship, if considered advisable, as an 

 integral part of the structure. 



To remove the propeller in such a setup it is 

 necessary either to split the propeller-bearing 

 housing and drop the lower half or to provide a 

 separate section of exposed shafting. Removal of 

 such a section enables the propeller to be pulled 

 forward a short distance, far enough to get the 

 shaft extension out of the after bearing. Normally 

 the first scheme is objectionable because with 

 the standard method of propeller attachment of 



ELEVATION, LOOKING TO, PORT, 



WITH NEAR SKEG REMOVED 



~f i9 Stations ' 18 ' TT 



Fig. 67.0 Arch Stern Underwater Profile of ABC Ship, with Near-Side Skeg Removed 



