2-2 



BOAT HULL DESIGN 



standard scantlings for boats of a given size and type, such as the rules for the construction 

 of wooden yachts published by Lloyds (3), Herreschoff (4) and Nevins (5). These tables and 

 similar steel scantling tables are not however, applicable to fiberglass reinforced plastic 

 boats because of the difference between the basic materials and types of construction. A 

 frame and plank wooden boat must be made rigid to prevent the many joints from working and 

 starting leaks. This requirement usually results in scantlings larger than necessary to re- 

 sist the applied loads on a strength basis. Fig. 2-2 indicates the differences between frame 

 and plank and fiberglass construction. In steel boats corrosion allowances are necessary 

 which make up a high percentage of the thickness of the thin hull plates. 



MOLDED FIBERGLASS 

 STI FFENERS BONDED OR 

 MOLDED IN PLACE 



SCARF JOINT 

 THROUGH BOLTED 



y v 1U1 \ 



ONE PIECE 



FIBERGLASS 



SHELL 



a. FIBERGLASS 



PLANK TO 

 FRAME METAL 

 FASTENERS 



PLANKS WITH CAULKED 

 SEAMS 



b. WOOD FRAME AND PLANK 



Fig. 2-2. Fiberglass and wood frame and plank hull construction 



Since the fiberglass reinforced plastic hull eliminates the many seams and butts charac- 

 teristic of wooden construction and the corrosion allowance required in steel boats, determi- 

 nation of the hull scantlings should be based on the strength and other characteristics of 

 fiberglass laminates and not on simple equivalence to a comparable wood or steel hull. 



TYPES OF CONSTRUCTION 



Fiberglass boat hulls may be constructed of unstiffened single skin laminates, single 

 skin laminates stiffened with ribs or frames, sandwich construction of thin face laminates 

 with a low density core, or some combination of these types. Selection of the type of con- 

 struction is dependent upon the hull size, appearance, intended service, reinforcement, 

 fabricator experience, quantity, production facilities, and economic considerations. Some 

 doubt may exist as to the most suitable type of construction for any specific boat hull. Usually 

 more than one type of construction is suitable, but only one specific type will result in the 

 most economical design. In general, a fabricator developing a new small boat for quantity 

 production will produce a prototype to test the soundness of the hull structure, performance 

 of the hull form and suitability to mass production. Modifications to improve the original 

 prototype and reduce production costs, are usually necessary prior to and during quantity 

 production. This is usually impractical for larger boat hulls unless quantity production is 

 contemplated. In such a case, careful study, from both structural and economic viewpoints, 

 will usually indicate the best choice. 



Unstiffened Single Skin 



The simplest form of a fiberglass boat hull is the single skin without stiffening. This 

 construction consists of a laminate molded to the desired form, the laminate being several 



