526 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.1S 



gain in power from the expected high average 

 wake drops the propeller or shaft power below 

 the value to be expected with an orthodox type 

 of stern. It is recalled in this connection that, 

 practically without exception, the model tests of 

 normal-form sterns and twin-skeg sterns, on a 

 comparative basis, showed higher effective powers 

 but lower propeller or shaft powers for the twin- 

 skeg form. 



By retaining the high tunnel slopes, at values 

 which would just avoid separation or — Ap's 

 along the roof of the arch, it was hoped to create 

 artificially a forward wake current over most of 

 the tunnel area. This is exactly what is accom- 

 plished by a bulb at the bottom of a skeg ending, 

 sketched in Fig. 25. L. It appeared, furthermore, 

 that the wake velocities within the arch should 

 be higher than they were in the original twin-skeg 

 tunnels of the late 1930's and the early 1940's 

 [SNAME, 1947, pp. 97-169]. These velocities 

 should also be considerably more uniform across 

 the tunnel area because of its circular section, 

 without the inside corners of the earlier tunnels on 

 twin-skeg ships. Furthermore, by having the 

 propeller disc fill nearly all of the tunnel area it 

 should be possible to take advantage of the 



Fig. 67.N View from Aft of Arch-Stern 

 Assembly on ABC Ship Model 



forward wake velocities in all the water passing 

 through it. However, as the tunnel does not cover 

 all the propeller disc in the ABC design, it was 

 expected that below the shaft axis the wake 

 velocities would be appreciably lower than those 

 above the axis. 



In relatively slow-speed tunnel-stern pushboats 

 and towboats built to operate in shallow waters 

 there appears to be no great difficulty in getting 

 water through a narrow bed clearance under the 

 vessel, into the tunnel (s), and thence aft to the 

 propeller(s). However, this does not necessarily 

 assure the designer that this flow is adequate in a 

 deep-water vessel with the same type of stern, 

 which has to traverse shallow waters occasionally, 

 like the ABC ship. What saves the situation here 

 is the necessity for the deep-water vessel to 

 slow down if the bed clearance is small, else it 

 squats and its stern drags on the channel bed. 



Finally, it is realized that the increased disc 

 area and improved flow pattern expected with 

 this arch type of single-screw stern, pictured in 

 Fig. 67. N, are offset to some extent by the adverse 

 effect of the items listed hereunder, based on a 

 comparison with a single-screw stern of normal 

 form: 



(a) Increased wetted surface of the side skegs. 

 This has been reduced somewhat by cutting up 

 the skegs 2.33 ft, the height of two docking blocks. 



(b) Necessity for a shaft strut within the arch 

 to support the propeller bearing 



(c) Drag of the exposed propeller shaft and of a 

 short fairing or bossing at its forward end 



(d) Necessity for two rudders, neither of which 

 lies in the propeller outflow jet 



(e) Probable necessity for placing, farther for- 

 ward than in a normal single-skeg stern, that 

 part of the propelling machinery directly attached 

 to the shaft, because of the reduced rate of 

 rotation associated with the larger propeller and 

 the larger main gear. 



Despite these initial disadvantages, the ABC 

 type of arch stern was considered by several 

 experienced naval architects who examined it to 

 have sufficient promise to justify the building and 

 testing of a model. In the present state of the art, 

 this is the best that can be done with any new 

 design. 



67.18 Design of Hull and Appendage Com- 

 binations. Very frequentlj' the final design of 

 that portion of a ship hull adjacent to a fixed or 

 movable appendage is only possible after the 



