I have read Open Water Kayak’s “Standard Engineering Model” and have some questions. I have attempted to cross reference the concepts with E.P. Popov’s Mechanics of Materials..

(Smax I)/
((Tw+Tg)/2)

Popov calls this the “Flexure Formula”;

((Tw+Tg)3 - Tw3) + Tw3(Ew/Eg) /12

Popov describes this formula for moment of inertia as “transformation of a section”.

(M2 L )/
(2 I Eg)

This formula is the maximum amount of strain energy a cantilevered beam can store without failing.

98/12

The density of glass is needed to determine the thickness of the glass portion of the composite.

22/12

The density of wood is needed to determine the weight of the wood portion of the composite.

(WPw W)/Dw

The thickness of the wood is needed to calculate moment of inertia.

((1-WPw) W)/Dg

The thickness of the glass is needed to calculate moment of inertia.

600*Tg

Where is this used?

.05 Eg = Ew

This is used in the transformation method of computing the composite section moment of inertia.

(((Tw+Tg)3 - Tw3) + Tw3(Ew/Eg)) Eg /12

This is the moment of inetia times the modulus of glass, as described in Popov’s transformation of sections.

18W3Eg (1331+13794WPw+47652WPw2-(62777 +117649Ew/Eg)WPw3 ) /156590819

I tried to expand the equations above and did not come up with this result. Can you show some intermediate steps?

In the above equation, the static strength does appear to vary according to the square of the weight, when wood as a % of weight is constant.

In the above equation, the static strength does appear to vary according to the tensile strength, when wood as a % of weight is constant.

Then, fixing all variables except the wood as a % of weight you found a curve with a peak at .63%. I used the calculation for the moment of inetia times the modulus of glass, as above before the substitutions to calculate the curve as a check. Keeping W constant and keeping the wood as a percent of weight constant, the curve did not have a peak. Similarly, the static and dynamic curves did not peak. Should I have seen a peak using equations for before the substitutions were made?

Your dimensional analysis approach is very intriguing, and would be very useful provided it is understood. I am ready to start fiberglassing, and this discussion has already given me a lot to consider.

Thanks in advance for any help you can give. I like many others am very grateful for the level of commitment and effort that the builders on this site have for helping us novices.

Tom