3-22 



DESIGN DETAILS 



Hard Spots 



Probably the major single trouble-maker in fiberglass construction is the so-called 

 "hard spot. " A hard spot is any isolated rigid point of support in a shell panel, and is 

 caused when the shell panel attempts to flex under load but is prevented from doing so 

 locally by a rigid support. This in turn causes an abrupt change in the deflected shape with 

 very high local stresses, Fig. 3-49. These stresses can cause damage to the laminate and, 

 in extreme cases, actual cracking of the shell will result. Examples of hard spots include 

 abrupt endings of stiffeners in the middle of a shell panel and abrupt endings of partial 

 bulkheads. The best cure for hard spots is to avoid them by continuing the offending member 

 to another point of support, as extending a longitudinal to the next transverse member, Figs. 

 3-50 and 3-51. An alternate, of considerably less merit, is to taper the end of the member 

 and to provide additional layers of reinforcement locally under the hard spot to reinforce 

 the shell. This should be done at the junction of the shell with bulkheads and interior decks 

 to prevent the formation of a hard line as indicated in Fig. 3-24. Location of bunks, seats, 

 etc., against the shell should also be avoided to prevent hard spots, unless each connection 

 is treated to minimize the abrupt change in panel deflection by local reinforcement. 



HMRD SPOT 



RIGID MEMBER 



LOCAL REINFORCEMENT 



/TRANSVERSE FLOOR 

 OR BULKHEAD 



-V- 



Fig. 3-50. Hard Spot 

 Longitudinal Endings 



NOT ACCEPTABLE 



FULL DEPTH LONGITUDINAL 



ENDING IN UNSUPPORTED 



PANEL 



HARD SPOT 



BETTER BUT TO BE_ A. VO I DED 

 LONGITUDINAL TAPERED TO 

 REDUCE RIGIDITY. LAMINATE 

 REINFORCED LOCALLY AT HARD 

 SPOT 



BEST 



LONGITUDINAL ENDS AT 



STRUCTURAL TRANSVERSE 



MEMBER 



s. NOT ACCEPTABLE 



PARTI AL FLOOR OR 

 TRANSVERSE BULKHEAO 



CABI N SOLE, DECK, 

 OR LONGl TUDI NAL 



FLOOR OR TRANSVERSE 

 BULKHEAD CARRIED TO 

 NEXT DECK. ACCESS 

 HOLE IF REQUIRED 



t). ACCEPTABLE 



Fig. 3-51. Hard Spot - Partial Floor or Transverse Bulkhead 



A final type of hard spot is one which is not at first obvious, but which could easily 

 cause failure in a high speed runabout. The condition, shown in Fig. 3-52a, occurs in 

 double bottom construction, which is relatively very stiff and intended to reduce bottom 

 deflection. The inner skin, usually a built-in flat, ends at the outer shell on the bottom 

 of the hull instead of being carried over to the side shell. This, in effect, means that the 



