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Discussion of selected wood species bending strengths and stiffness

Ultimate bending stresses (psi) and Modulus of Elasticity (stiffness)

Results of tests on small, clear straight-grained specimens @ 12% moisture content, Source: Wood Handbook, Wood as an Engineering Material, US Dept. of Agriculture:

  bending strength modulus of elasticity
Basswood, American* 8700 1,460,000
Cedar:    
Alaska 11,100 1,420,000
Atlantic White 6,800 930,000
Eastern Red 8,800 880,000
Incense 8,000 1,040,000
Northern White 6,500 800,000
Port Orford 12,700 1,700,000
Western Red * 7,500 1,100,000
Spruce    
Black 10,300 1,530,000
Englemann* 9,300 1,300,000
Red 10,200 1,520,000
Sitka 10,200 1,570,000
White 9,800 1,340,000

* denotes used by US Blanks and others

 

Discussion of relative bending strengths increases at selected thickness

Bending strengths are a direct function of the thickness of a material. The strength increases in proportion to the square of the thickness of the material.

Therefore, the relative strength increases for the following thickness (2 ¾" as the base):
(A slightly thicker surfboard creates a much stronger surfboard!)

Thickness Thickness Increase Relative Strength Increase
  (from 2.75") (from 2.75")
2.75"    
3" 9% 19%
3.125" 9.75% 29%
3.25" 10.5% 39%
3.5" 12.25% 62%
3.75" 36.3% 85%

 

Discussion of elasticity (flex) of woods used in surfboard stringers

A surfboard is basically two surfaces of fiberglass separated by a foam core, otherwise known as "foam sandwich construction". The more a board will flex, the more likely the fiberglass will "buckle", and lose its strength. Once this occurs, the board begins to break. Thus, the stiffer the board (caused by stiffer woods and higher density core foam), the less likely it is to break! Basswood, as shown by the chart, is one of the least flexible woods, and also has a high bending strength. Western Red Cedar has the lowest combined strength of the woods used by US Blanks (and others).