BOAT HULL DESIGN 



2-11 



where laminate cost is relatively a smaller portion of the total cost than it is for the small boats. 

 Fig. 2-11 also indicates the reason for the difference between Types B and C. For section 

 moduli below 0.007in3, Type B laminate with one ply of woven roving is less expensive than 

 Type C with two. For section moduli above this value, Type C becomes less expensive. 



Fig. 2-11 considers material cost based on unit prices for the date indicated and glass 

 percentages and thicknesses usually obtained by contact molding. The effect of changes in 

 unit prices or molding method should, of course, be considered. In addition, labor costs 

 for the different types of material, which vary somewhat with different fabricators, should 

 be considered. 



Fig. 2-12 indicates the relationship of weight per square foot of the three laminates 

 described above and the section moduli at the inboard face of the laminates. These data 

 are based on contact molding. The data given in Fig. 2-12 will enable the designer to con- 

 sider the effect of weight when selecting a laminate. 



BASIC DESIGN CONSIDERATIONS 



The most difficult technical problem in small boat hull design is the selection of the 

 loads to apply in determining the scantlings of the hull structure. The actual load on any 

 given part of a vessel in service is extremely complex; consisting not only of loads applied 

 by external forces such as water pressure which act directly on the part, but also of loads 

 transmitted from the parts of the boat adjacent to the section or member under considera- 

 tion. For example, a bulkhead may be loaded directly by equipment mounted on it, and in- 

 directly by water pressure loads which are transmitted from the shell to the frames sup- 

 porting the shell, and from the frames to the bulkhead which supports them. Although any 

 piece of a boat hull is directly or indirectly connected with all the other pieces, it is not 

 generally practicable to determine the distribution of the loads transmitted from one part 

 to another 



The small boat designer must, therefore, resort to more or less assumed design 

 loads. These loads, combined with a choice of the factor of safety which represents, in 

 part, the designer's best judgment regarding the accuracy of his assumed loading, are 

 applied to determine the required scantlings. 



The assumed design loads depend on the function of the part of the boat under considera- 

 tion, and on the size, type, and intended use of the boat. Large ships must be designed to 

 withstand me longitudinal bending stresses imposed on them in waves as shown in Fig. 2-13 

 in addition to the lateral loads due to water pressure and local loads due to equipment 

 and cargo. 



ess of 

 buoyancy over we 



LOCAL EXCESS OF 

 BUOYANCY OVER WE I 



WATER SURFACE 



TENS! ON 

 I N BOTTOM 



LOCAL EXCESS OF 

 WEIGHT OVEF 



Fig. 2-13. Forces on ship's hull causing 

 longitudinal bending stress 



