BOAT HULL DESIGN 2-13 



In view of the complexity of the theoretical analysis required and the lack of needed ex- 

 perimental data, vibration analysis of fiberglass panels is not, at this time, a practical boat 

 design tool. 



At the present time the only practical cure for vibration problems is a trial and error 

 approach. This essentially consists of building the boat, testing it, and if objectionable 

 vibration occurs making suitable corrections. 



Examples of corrective action which might be taken include: additional plies of laminate; 

 additional stiffening to reduce panel size; and change the number of blades on the propeller if 

 the measured frequency of vibration equals the RPM times the number of propeller blades. 

 Of these three, the addition of extra stiffeners is recommended as the most practicable and 

 economical. 



Strength Criterion 



For certain hull components, notably the shell, several different criteria are proposed 

 depending on the type of boat being considered. Following the general discussion, design 

 examples are presented to illustrate suggested methods for the structural design of several 

 types of boats. 



Shell and Framing: The basic component of all boats is the shell. Structurally this is 

 a watertight envelope which must be maintained in its designed shape while resisting the ex- 

 terior water pressure. The framing, in addition to supporting the shell against this pressure 

 must also spread the local interior loads from the engine, mast etc., over a large area of 

 the shell. 



In small open boats the shell is usually the only structural member. Small, low 

 speed boats powered by an outboard motor of 10 horsepower or less, and day sailers are 

 examples of this type. 



The shell of a low speed open boat is generally subjected to low stresses from water 

 pressure on the bottom, torsion from the sails, and various other over-all loads. The 

 critical loads on these boats are generally local loads due to handling, grounding, minor 

 collision with docks, and so forth. Since these loads are similar for most of these low speed 

 boats, the size of the boat has surprisingly little effect on the laminate used. An exception 

 is the effect of torsion on open, as opposed to decked, sailboats. In order to prevent ex- 

 cessive twisting of the hull, the laminate for the larger sizes of open boats is generally in- 

 creased. The loads described above are not susceptible to detailed analysis, and the best 

 criteria available are the many successful boats in this category. 



Based on current practice, the following laminates are recommended: 



For round bottom boats under 12 feet in length - normal low speed pleasure service 



1 ply 10 ounce cloth on the outboard side 



1 ply 1-1/2 ounce mat 



1 ply 14-17 ounce woven roving 



For round bottom boats 12 to 18 feet in length - normal low speed pleasure service 



1 ply 10 ounce cloth on the outboard side 



1 ply 2 ounce mat 



1 ply 24-2 7 ounce woven roving 



