Sec. 67.24 



UNDERWATER-HULL DESIGN 



539 



or .T-distance from the stem, of transverse hull 

 curvature, and of hull roughness 



(b) Boundary-layer velocity profile, which is a 

 function of hull roughness, transverse curvature, 

 and other factors. For example, a tip clearance 

 which may be greater than the boundary-layer 

 thickness 5 (delta) when the ship has a clean 

 bottom, freshly painted, may be much less than 

 5 when the bottom has been severely roughened 

 by barnacles and other marine growth. 



(c) The shape of hull endings, of skeg and bossing 

 terminations, and of objects ahead of the propeller 

 which may produce near-separation. 



Keeping the hull clear of the most intense part 

 of the blade-pressure field beyond the tip involves 

 much more knowledge of this field than exists at 

 present. For a given thrust loading it appears, 

 however, to be a direct function of the circulation 

 distribution at the tip. The assumption that this 

 distribution is roughly similar for all screw 

 propellers leads to one of the rules in present use, 

 which gives the hull tip clearance as a function 

 of the propeller diameter. Since the thrust loading 

 among different types of ships varies rather 

 widely, however, this latter factor can no longer 

 be neglected. Furthermore, the circulation is 

 increased and the pressure field is greatly inten- 

 sified when the tip swings through a high-wake 

 portion of the boundary layer. Both the factors 

 mentioned therefore require careful thought and 

 study when establishing hull tip clearances. 



The shape of the ship sections opposite a 

 screw-propeller position, whether concave and 

 generally concentric with the propeller axis or 

 convex to that axis, is an important feature, 

 although it is not yet known how this effect is 

 related to or combined with that of tip clearance. 



The volume of the adjacent hull or appendage 

 is at times a controlling factor. 



Blade tips often pass an appendage or a part 

 of the hull which occupies only a small area and 

 a small radial distance in the plane of the disc. 

 Examples are the shoe at the bottom of a stern- 

 post to carry a lower rudder bearing or a rope and 

 cable guard on a submarine. Only mechanical 

 clearance is then necessary, say 0.10 to 0.50 ft, 

 depending upon the size of the vessel. If the 

 appendage is liable to be bent toward the propeller 

 axis in service, as for the cable guard of the sub- 

 marine, this clearance may be increased by say 

 twice the dimension of the appendage, measured 

 radially from the propeller axis. Another example 



is the V-shaped portion of the hull, sometimes 

 rather narrow, lying above the arch of the propeller 

 aperture in a single-screw stern of the canoe or 

 whaleboat (cruiser) type. The volume is small 

 and the obstruction occupies only a small part 

 of the circumference around the tip circle. 



An adjacent structure of considerable area, 

 lying generally in a longitudinal plane passing 

 through or close to the propeller axis, calls for 

 greater tip clearance even though it is thin. 

 A larger area is exposed to the pressure fields 

 beyond the tips as the blades pass by. The tip 

 clearance for such a structure could possibly be 

 as small as O.IZ) or less for a lightly loaded pro- 

 peller, Ctl of the order of 1.0, yet as large as 

 0.2Z) or more for a heavily loaded one, with a 

 Ctl of the order of 3.0. 



An adjacent expanse of hull plating, generally 

 flat in shape and more or less normal to the plane 

 of the propeller disc, calls for an ample tip clear- 

 ance, following the reasoning of Sec. .33.3 and 

 of the preceding paragraphs. A logical and com- 

 prehensive rule has not yet been developed for 

 calculating a minimum or a desirable tip clearance 

 under these conditions. Incidentally, this clear- 

 ance is measured transversely, in the early stages 

 of a design, between the propeller disc or tip 

 circle and the hull section directly abreast it. 

 When it is known whether or not the propeller is 

 to be raked, and when the slope of the adjacent 

 hull surface can be determined, the minimum 

 clearance is measured from the tip circle of the 

 swept volume normal to the hull, as in diagrams 

 1 and 3 of Fig. 33.B. 



The recommended method for selecting screw- 

 propeller tip clearance abreast a generally flat 

 fore-and-aft structure is to determine first, from 

 Fig. 45.C or from Eq. (5.viii), the nominal 

 thickness of the turbulent boundary layer at the 

 a;-distance from the bow selected for the propeller 

 position. The sustained speed is the one used for 

 this estimate because it gives the greatest value 

 of 5. The boundary-layer thickness thus derived 

 is only a rough approximation for large values 

 of X, but it is at least an approximation. 



It is also estimated, from the turbulent-flow 

 velocity profiles of Fig. 5.K, that the friction-wake 

 velocities in the outer half-thickness of the bound- 

 ary layer are less than about 0.1 the ship velocity 

 V. At the same time it is known that the boundary- 

 layer thickness is increased by fouhng of the hull 

 surface, as in Fig. 22.H. It seems wise, therefore, 

 to fix the hull tip clearance at a value at least as 



