Composite Material Stiffness

Here is a material question for you boat and paddle builders. Which material will be stiffer and lighter on its own and when reinforced with glass or carbon on both sides you say? Marine plywood ¼” thick by 4” wide by 8 feet long or a plank of pine wood of comparable dimensions?

I’m posting this question on some of the boat building forums too, so if you see it there too feel free to reply wherever.

How about balsa wood or other lightweight “core”? I know material like that will be lighter and more fragile when on its own compared to wood or plywood. But when reinforced to the same strength as the plywood or wood will it be lighter and stiffer?

In a summary, what will be the stiffest, lightest, and strongest DIY layup that will not break the bank? I already have some carbon cloth and fiberglass tape that I could use to reinforce the plank with. I am also considering adding some Kevlar so that I can avoid catastrophic failure if the thing breaks. I can vacuum bag that whole thing too.

As a background, I will use this as a stringer for internal reinforcement of a lightweight Greenland style paddle. I plan to have reinforcements placed at 90 degree angle to it so that stringer will not be the only thing inside but that does not matter for the question above. More FYI - in addition to this “skeleton” the paddle will have external composite shell (made from carbon or fiberglass or carbon/Kevlar sleeve) and foam to fill the void and provide shape for the shell while curing. I know others who have done similar paddles and were happy with the results, just curious if I could improve on the plywood for the stringer they used without going into much more expense.

What would be a good thickness for the board? Would ¼” sandwiched b/w 2 layers of carbon on each side be weaker and more flexible than a sandwich of two 1/8” planks with three layers of carbon?

I am pretty sure a core b/w layers of fabric will be stiffer than three layers of fabric without a core but if someone has a different opinion, please share -:wink: