686 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 73.10 



tion and eddying at this point are avoided at the 

 expense of an insignificant increase in volume 

 displacement, weight, and wetted surface. 



73.10 Design Rules for Deflection-Type or 

 Contra-Guide Bossings. Ship designers have 

 been reluctant to use deflection-type or contra- 

 guide bossings on vessels with wing propellers 

 because of the uncertainty as to just how much 

 twist can be used and just how it can be worked 

 into them. They fear that this twist may cause 

 eddying, introduce vibration, and do more ulti- 

 mate harm than good to the propulsion charac- 

 teristics and behavior of the vessel as a whole. 

 True, a deflection-type bossing, even though 

 designed only to reduce the unfavorable compo- 

 nent of flow as it enters the propeller disc, requires 

 much greater care in shaping than the fairing type 

 of long bossing. Further, it calls for more thorough 

 checking by model tests. Separation of flow may 

 well occur with careless or improper design, 

 leading to ensuing vibration and other troubles. 



As wdth a contra-guide skeg ending the twist 

 is imparted in a direction contrary to the rotation 

 of the propeller blades when running ahead. If 

 the wing propellers are definitely to turn outward, 

 and the stern is of normal form, the bossing 

 termination lies at a small slope fi with the 

 horizontal. With inward-turning propellers the 

 slope is larger than for a bossing of the fairing 

 type. Indeed, the termination may approach a 

 vertical position, with a slope of 70 or 80 deg or 

 more. 



The amount of twist which can be imparted is 

 a function of the fore-and-aft length of the bossing 

 and of the smallness of the reentrant angle against 

 the shell, on the reduced-pressure side of the 

 bossing, convex to the flow. Fig. 36. H illustrates 

 these features as applied successfully to the U. S. 

 destroyer Warden (DD 352), where the reentrant 

 angle on top of the bossing was much smaller 

 than normal. It is true that separation generally 

 occurs alongside a surface when the slope of that 

 surface with the direction of motion exceeds a 

 limiting angle. However, a twist imparted to the 

 water by a long bossing with easy curves may 

 possibly act to diminish this critical slope by 

 changing the general direction of flow at its after 

 end. 



When designing a contra-guide bossing it is 

 kept constantly in mind that the water is being 

 forcibly deflected on the straight or concave side 

 and must follow the bossing-flowplane curvature 

 there. It is only constrained to follow the convex 



side by the conversion of some kinetic energy into 

 sufficient potential energy and pressure, with an 

 adequate pressure gradient, to accelerate it 

 inward toward the bossing surface, at right 

 angles to the latter. 



In the present state of knowledge, it is perhaps 

 well to hmit the angle between the convex side 

 of a deflection-type bossing and the corresponding 

 side of a fairing bossing for the same ship design 

 to a maximum of about 8 or 10 deg. It is also 

 well to limit the reentrant angle at the after end 

 of a deflection-type bossing to a minimum of some 

 60 or 65 deg, reckoned from the adjacent hull 

 surface. 



When the designer has done his best on paper 

 he may try his hand in modeling clay. The bossing 

 so developed is added to the model and run for 

 flow directions. This may be in a model basin but, 

 if at all possible, the flow should be observed in a 

 circulating-water channel. A flow test and a wake 

 survey, including measurements of velocity mag- 

 nitude and direction in the propeller disc, are 

 much more necessary for a deflection-type bossing 

 than for one of the fairing type. 



The designer who is looking — and hoping — for 

 a real reduction in shaft power, of the order of 5 

 or 10 per cent, such as that achieved on the 

 U.S.S. Warden, should not be discouraged when 

 a first attempt at laying out a deflection-type 

 bossing produces erratic flow around the propeller 

 position. It may produce no reduction in shaft 

 power at all. Indeed, the design knowledge 

 relating to this type of bossing is still so limited 

 that only by accident could a designer expect 

 to arrive at the proper shape and proportions on 

 the first trial. Modified bossings are rather easily 

 applied to a model and more easily checked for 

 flow in a tuft test in a circulating-water channel. 

 In fact, it can be stated as an inflexible rule that 

 no contra-guide bossing should be incorporated 

 in a ship design and in the construction drawings 

 without the most complete flow investigation on 

 a model, both with and without the propeller 

 working. This involves measuring on the model, 

 if it is practicable, the transient variations in 

 torque and thrust as each propeller blade passes 

 through a complete revolution. At some time in 

 the future it should involve measurements of the 

 periodic pressure fluctuations and force variations 

 on the bossing and adjacent hull. 



73.11 Vertical Bossings as Docking Keels. 

 For vessels which are wide aft in proportion to 

 their immersed depth, with rather flat stern 



