3-24 



DESIGN DETAILS 



Re-entrant corners, generally called notches, should be avoided whenever possible, and 

 if they must exist should have a generous radius. Fig. 3-53 indicates a re-entrant corner. 



BEST CONSTRUCTION 

 GRADUAL CHANGE IN SHAPE 



-GOOD-RE-ENTRANT CORNER 

 IMPROVES WITH INCREASED RADIUS 



S I 5 NOT A RE-ENTRANT 

 CORNER AND CAUSES NO 

 STRESS CONCENTRATION 



BAD RE-ENTRANT CORNER 

 NO RADIUS 



Fig. 3-53. Re-entrant Corner due 

 to Change in Shape 



A common source of difficulty in steel 

 vessels is the high stress concentration at the 

 corner of a rectangular or square hole. These 

 corners are always given generous radii to 

 reduce this stress concentration. The 

 experimental work with glass cloth laminates 

 (4) indicates, surprisingly, that for 181 cloth 

 laminate a round cornered cut has slightly 

 less strength than a cut of the same over-all 

 size with square corners. No explanation is 

 given for this seeming discrepancy, but it is 

 presumed to be due to the orthotropic proper- 

 ties of the laminate. Since the reported 

 difference between strength for square and 



round corners is small for orthotropic materials and since most boat laminates contain 

 mat, which is essentially an isotropic material that should be much stronger with rounded 

 corners, it is recommended that square or rectangular cuts in boat structure be provided 

 with corner radii, as indicated in Fig. 3-54b. 



CUT I N DECK 

 FOR COCKPIT, 

 ACCESS HATCH, 

 ETC 



SQUARE CORNER 

 CAUSING HIGH 5TRESS 

 CONCENTRAT I ON 



CUT I N OECK 



ROUNDED CORNER 



RELIEVES STRESS CONCENTRATION 



a. NOT ACCEPTABLE 



b. ACCEPTABLE 



Fig. 3-54. Deck Cuts 



A common source of stress concentration due to structural discontinuity is the bonded 

 joint. Table 3-5 obtained from (5) indicates, in broad categories, the stress concentration 

 present in bonded joints between similar materials under three different types of loading. 

 The loads indicated are tension (T), compression (C), and bending (M). Joints whose 

 stress concentrations are listed as major for a particular type of loading should be avoided 

 for major structural parts loaded in a similar way. When stress concentrations are listed 

 as moderate, it is recommended that the joint be avoided unless the over-all stress level 

 is kept low. 



Notice the difference between the beveled joints and the unbeveled ones of the same 

 type. This is an excellent illustration of the benefits of gradual changes in shape as 

 opposed to abrupt changes. 



Knife Edge Crossing 



The knife edge crossing, illustrated in Fig. 3-55, is a problem which tends to occur 

 often, and can be more serious in larger boats. As the figure indicates, the stress level 



