2-6 BOAT HULL DESIGN 



Method of analysis and graphs of section modulus and moment of inertia for half-round 

 and hat stiffeners are given in Chapter 6. 



These stiffener configurations give the designer the opportunity to vary the stiffness and 

 strength of the section simply by changing the cross-sectional dimensions instead of changing 

 the thickness of the fiberglass laminate. For maximum strength and rigidity the reinforce- 

 ment preferred for the stiffener laminate is woven roving, with the thickness approximately 

 equal to that of the shell laminate. In areas where the interior of the shell is covered with 

 cloth for appearance, this covering can be carried over the stiffeners. 



There are two basic ways to connect the frames to the skin. The preferable method is 

 to add the stiffeners during the molding of the shell before the shell laminate cures. This 

 may be done with any of the molding systems. Where matched metal molds are used, the 

 construction of the male portion of the molds is complicated, since it must be recessed to 

 allow for the stiffeners. The other method consists of bonding the stiffeners to the cured 

 shell. This eases the molding problem but makes it more difficult to obtain a good bond. 

 If this method, called secondary bonding is used, care must be taken to ensure that sufficient 

 faying, or contact, area is provided so that the horizontal shear stress in the bonded joint is 

 within allowable limits. A reasonable value for ultimate shear stress at the bond line, using 

 polyester resins, is 800 to 1000 PSI (6). Greater bond shear stress, 1200 to 1500 PSI, can 

 be obtained with epoxy resins (6). These values are all for the wet condition. 



Sandwich Construction 



The most complex type of construction, and the most difficult to fabricate, is the 

 sandwich. This consists of two fiberglass laminates separated by a core of lightweight 

 material. The purpose of this construction is to increase the rigidity of the flat panel by 

 increasing its thickness without the use of a solid laminate. A solid laminate of equivalent 

 thickness would be very heavy, extremely uneconomical, and would also present some mold- 

 ing difficulties. In sandwich construction it is usually assumed that the fiberglass skins re- 

 sist all the bending stresses and deflections, while the core resists the shear stresses and 

 deflections, withstands local crushing loads and prevents buckling of the laminate skins in 

 compression. This assumption is discussed in Chapter 6. 



Since the strength and rigidity of a sandwich depend on both skins working as a unit at 

 the required separation, the core material must bond firmly to each skin and be sufficiently 

 strong to withstand the loads previously mentioned. The core of a sandwich must therefore 

 be carefully chosen since failure of the core will lead to failure of the entire unit. 



As an example of the advantage to be gained from using sandwich construction to increase 

 the strength and rigidity of a given amount of fiberglass laminate, consider a strip of single 

 laminate 1 inch wide and 1/4 inch thick. This strip has a moment of inertia, I, of . 0013 in** 

 and a section modulus, Z, of . 0104 in^. The same amount of fiberglass divided into two 1/8 

 inch thick laminates, one on each side of a 1/2 inch thick core, has a moment of inertia, I, 

 of .02 6 in^, and a section modulus, Z, of .069 in3. Therefore with the same amount of fiber- 

 glass the strip is now 20 times as stiff and 6 times as strong. The total weight of the struc- 

 ture is increased by the weight of the core, which is much less than the additional weight of 

 fiberglass laminate required to provide equal strength and stiffness. In designing a sand- 

 wich, the skins must be made thick enough to withstand local impact, abrasions, and handling. 



It will be noticed that the effect of the core has not been considered in the preceding 

 example. The effect of the core on the over-all strength and rigidity of the sandwich varies 



