BOAT HULL DESIGN 



2-3 



plies of resin impregnated reinforcement. This type of construction is used primarily for 

 small, open, low speed boats up to about eighteen feet in length. The term "unstiffened" 

 as used here means that no framing is added and the hull has an interior surface unbroken 

 by projecting ribs Fig. 2 -3a. Many times interior fittings such as thwarts and buoyancy 

 tanks are used to provide some support for the shell in addition to performing their 

 primary function. 



CENTERLINE 



PENT 



ERLINE 



CENTERLINE 



a. UNSTIFFENED 



b. FLAT AREAS ARE NOT RECOIkWENDED 

 FOR UNSTI FFENED SK INS 



C. MOLDEO-IN SPRAY STRIP 



Fig. 2-3. Unstiffened single skin hull construction 



The unstiffened single skin may be used with any of the molding methods discussed in 

 Chapter 4. In selecting this type of construction it must always be remembered that the 

 single skin derives considerable strength from the curved shape common to most small boats. 

 Large flat areas, Fig. 2 -3b, should be avoided. In vee -bottomed boats a molded in spray 

 strip, Fig. 2 -3c, may sometimes be used in lieu of stiff eners to provide the necessary 

 rigidity. For an example of a boat with an unstiffened single skin hull see Fig. 2-4. 



Single Skin with Framing 



As the size of the boat and the severity of 

 service increases, or when large flat sur- 

 faces are required, the unstiffened single skin 

 becomes too flexible or limber. Solving the 

 problem by means of large increases in thick- 

 ness, while theoretically possible, causes 

 molding difficulties and is also very uneco- 

 nomical. Therefore, for minimum weight and 

 cost, framing should be added to the hull. 

 Framing in this case may be a member which 

 serves only as a stiffener without any other 

 purpose or it may be a built-in bunk locker, 

 or other component already serving an addi- 

 tional function. 



The framing is usually oriented in two 

 basic directions; longitudinal, that is parallel 

 with the long axis of the boat, or transverse, 

 which is perpendicular to the long axis of the 

 boat, Fig. 2-5. In the case of longitudinal 

 framing, occasional large transverse frames 

 may be necessary to support the longitudinal 

 frames. The designation of the framing sys- 

 tem is determined by the direction of the 

 greater number of smaller frames which 



Fig. 2-4. Unstiffened 



Single skin hull (Courtesy Marscott 



Plastic Covlfiany) 



