740 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 75.3 



CASE 1. WORKING LIMITS ON STRUCTURAL ROUGHNESS 

 FOR FLOW IN ANY DIRECTION PARALLEL TO THE 

 SURFACE, FOR CRAFT TRAVELING AT Tq<I.O, Fn<0.5 



JlO 



In d or less 



to 



.'^J 



or le»s 



n 



Lorqe Rivets/ 



I I I Mediom i 

 («-U*( Rivets |<-|dH 



InGenerol, Finished Rivet Points Are To Be Rounded, and 



Not Pointed 



If riveted butts normal to the flow are to be 

 lapped, the exposed plate edges should face 

 forward on slow-speed ships. Here the separation 

 drag abaft plate edges facing aft is greater than 

 the dynamic-pressure drag against edges facing 

 forward. When faced forward the exposed plate 

 edges are to be chamfered as indicated at 1 in 

 Fig. 75.C. 



CASE 2. WORKING LIMITS ON STRUCTURAL ROUGHNESS 



APPLYING TO EDGES GENERALLY NORMAL TO THE 

 DIRECTION OF FLOW, FOR SHIP SPEED RANGES OF Tq<I.O 



-* « Direction of Water Flow 



'i Inch Mox. for Plate ^ •» 



5 Thicknesst=|-lnch 4 Q ^"'^*' 



or More 



This Slope Preferred 

 in Reqion of 



Designed Waterline 



TrQilino-Edqe Lap 

 Direction of Water Flow ■* ' 



I Maximum Abrupt Joo When tj)>t2^, Minimum 

 ' '* To ^1 Transition Slope is I in 4 



.Up to -^ But Not More Than jg- Inch in Any Cose 

 ^ ^ ^- . —T'^// /V 



Wooden^ 



z:j:pian>i^Nll|vlMT[ 



■-(Outer LQ<^er)\ 



Diacjrams I and 



rZ Are to Apply 

 When Edges 

 Are Within, 



Fig. 75.B Working Limits on Structural Roughness, 

 Flow Parallel to the Surface, Case 1 



with respect to the prevailing flow, should not 

 project from the fair surface by more than the 

 limits indicated in diagrams 3, 4, and 5 of Fig. 

 75.B. 



When prefabricated sections of a ship are 

 welded together, they are found sometimes not 

 to fit properly. The alignment at the shell, con- 

 sidering smoothness only and not structural 

 continuity, should conform to the limits indicated 

 in diagram 7 of the figure. When the abutting 

 plates are of unequal thickness, and the excess 

 is on the outside, the length of the transition 

 taper is to be not less than 4 and preferably 6 

 times the difference in thickness; see diagram 6 of 

 Fig. 75. B. Corresponding smoothness and offset 

 hmits for the adjacent planks and the calking of 

 wooden boats are depicted in diagrams 8 and 9. 



Fig. 75. C Working Limits on Structural Roughness, 

 Case 2 



For ships of higher speed, with Hmits as yet 

 lurdetermined, the pressure drag on forward 

 edges exceeds the separation drag on after surfaces. 

 In this case, the exposed edges are best faced 

 aft. If a reasonable degree of smoothness is also 

 required, a cement filler or a rivet cement is 

 applied in the region abaft the exposed trailing 

 edge, diagrammed at 2 in Fig. 75. C. 



An exposed plate edge is considered "trans- 

 verse" if it lies within 30 deg of a line normal to 

 the adjacent water flow, indicated at 3 in the 

 figure. In some quarters, however, chamfering 



