2-12 BOAT HULL DESIGN 



For small boats this same loading condition exists but the length to beam and depth re- 

 lationships are such that this is usually not a critical loading condition. 



In the following pages, the design loads for several general types of boats are discussed, 

 with the intention of covering the broad field of current fiberglass boat hulls. It is recognized 

 that in some instances, statements made regarding the loads which should be used may be 

 controversial. In all cases, the intention is that the loads described shall produce a safe, 

 trouble free boat for operation in the service as described. The loads have been chosen so 

 that the resulting scantlings are generally in accord with good modern practice. However, 

 it must be recognized that fiberglass laminates are a relatively new material for boat hulls 

 and that as more and more service experience is accumulated, it may become apparent that 

 some of the recommended loads are too severe and others perhaps not severe enough. Cer- 

 tainly the standards recommended are not intended as hard and fast rules, nor are they pre- 

 sented as the last word. In the final analysis the individual designer must use his own best 

 judgment in producing a balanced design suitable for the intended service. 



DESIGN LOADING 



In the design of the various components of a boat's structure, there are two basic criteria 

 which must be considered; vibration and strength. 



Vibration Criterion 



Critical vibration is familiar to everyone in one form or another. Any physical object 

 has a natural frequency of vibration; that is it tends to vibrate at a particular rate. This 

 rate depends on the geometry of the object, the loads imposed on it, and the material used. 

 If a varying load is imposed on a structure and the frequency of variation is nearly the same 

 as the natural frequency of that structure, the phenomenon of critical vibration or resonance 

 occurs. The amplitude of the critical vibration is high, and the resulting shaking or drum- 

 ming is unpleasant and may cause structural failure. This structural failure may be due to 

 fatigue at relatively low stress levels or, in the rare case of extreme vibration, to high 

 stresses induced by large deflections. 



To date, the vibration of fiberglass laminate panels has not received sufficient attention. 

 The theoretical analysis of orthotropic plate vibration has been investigated (7) but the analysis 

 is quite complex. A further complication is introduced because the modulus of elasticity 

 values generally measured are static values. For certain materials significantly different 

 moduli values are obtained under dynamic conditions such as vibration. Published data (8) 

 on Douglas fir indicates a 10 per cent increase in the dynamic modulus of elasticity compared 

 to the static modulus of elasticity. The only available published article on the vibration of 

 fiberglass laminate (9), which is very limited in scope and cannot be used without substanti- 

 ating tests, indicates a 50 per cent increase in the dynamic modulus compared to the static 

 modulus of elasticity. 



A characteristic of fiberglass laminates which may help alleviate vibration difficulties 

 is internal damping. This term refers to the ability of a material to absorb vibration energy 

 by converting it to heat produced by internal friction. Internal damping has two effects on the 

 vibratory characteristics of a material. First, it reduces, usually to a small degree, the 

 natural frequency. Second, and more important, it reduces the amplitude of vibration drasti- 

 cally at frequencies at or near resonance. The amount of internal damping actually present 

 in fiberglass laminates has not been measured experimentally, but it may be expected to be 

 substantial compared to metals. 



