Centerline plywood stringer - Replace with?

So, on my 44ft trimaran, there is a plywood stringer that goes in the center of the boat with same height as the bulkheads the sole is resting at. In the aft part of the boat there has been a fuel tank before where they removed two sections of stringers. I have now cut out to replace two of the bulkheads and was also planning to restore the missing stringers. But..can I do it differently? Can I get away with lower stringer, like a solid piece of wood instead?

 

I envision a ply stringer that was installed on edge. That is to say that the stringers long dimension is in the vertical plane. Solid wood of the same dimensions installed in the same way will likely be more rigid than the ply. The edge strength of the solid wood is more than likely stiffer than ply and could very well be more durable.

You could make the stringer, not so tall but I would not advise it. You can rely on the equation that describes the stiffness of an element of this sort when considering possible modification. The mass moment of inertia of a beam ( it is calculated as if it were a beam) of this kind can be calculated as width times depth raised to the third power the product divided by the constant 12.............thus (W x D^3)/12..................that only tells you about the potential strength of the element. The depth raised to the third power is the more influential of the numbers, so depth is far more important than width. The other factor that plays into the actual strength is the modulus of elasticity which varies with different kinds of material. I can not swear to it but I reckon that solid wood may have a better elastic modulus than ply but that depends somewhat on your selection of wood species. Elastic modulus is merely the relative strength in tension of whatever material is being assessed. Iron has a much larger modulus than candle wax etc. No doubt you will have some good wood choices there in Sweden. Choose one with favorable rot and fungi resistance.