Your statement was...the wood is the backbone and the strength of the stringer. This is simply not true. This should help everyone understand a bit better...
Stringers
What is a stringer and how does it work? The answer to the first part of the question is easy and widely understood. The answer to the second half of the question is much more likely to be misunderstood.
What is a stringer?
A stringer is a longitudinal support along the inside of the hull. Its role in boat construction is to stiffen the hull. Sounds simple right?
How does it work?
This is where things get a little trickier.
The most important characteristic of a fiberglass stringer is its profile, its shape in other words. Let me illustrate this point with an analogy. Imagine you have a flat piece of steel. The steel is 1/4? thick, only an inch wide, and it's 15' long. If you pick up the steel from one end and held it level to the ground, the other end would sag considerably.
Now imagine you have a cardboard tube. The tube is also 1/4? thick and 15' long. If you pick up the tube from one end and hold it level to the ground, you can imagine that the tube would hardly flex at all. This isn't because cardboard is stronger than steel. The cardboard is more rigid because the cardboard is a tube and the steel is flat. This is basically how fiberglass stringers work. It's their shape that provides the support, the core is simply there as a form.
In present day manufacturing of boats, builders are hyper conscious of Strength-to-Weight ratios. Stringers, deck beams and other supportive elements are formed in one continuous mold and glassed into a bare hull as a ?pan? with no core material whatsoever.
In Fiberglassic repair, we're dealing with the technology and knowledge that existed fifty or sixty years ago. In the early days of fiberglass construction (mid 50s and earlier), the stringer core material was given the structural duty, with a layer or two of fiberglass overlay simply to hold it in place. By the late 50s this mistake was largely rectified by reversing those roles. The structural element of the stringers became fiberglass while the core became more of a shape.
One thing to note about this role reversal though, is that boat builders, as recently as a few years ago, did not understand this principle and relied on the core for at least some of the structural duty of the stringer. For this reason, any repair to or removal of a stringer core should be followed by adding additional layers of mat and roving.
Side note: The previous sentence ended with the words ?mat and roving,? because additional thickness of the stringer is the goal. Adding layers of mat and cloth wouldn't build up thickness as quick.
Another side note: When building up stringer thicknesses, I use a crude formula. I have no idea if these numbers are ideal, too strong, or too weak, but I would be interested in hearing from a naval architect on the subject.
I build up the stringer wall thickness according to the length of the boat. Typically, I use ?? of thickness for every 10' of stringer length. There are variables to this formula, such as the number of stringers and their profile (some profiles are stronger than others). Box beam type stringers are stronger than half-circle profiles because it more closely resembles true I-beam construction.
The forms were made out of whatever was handy. In the Northwest, you'll find many forms made out of Spruce or Fir, In the Northeast, you'll find many made out of Pine. You'll find many boats all over that used plywood as a form. Usually the stringers will be in box form (a rectangular profile) because that was the simplest construction method. They are then glassed to the hull and ?Presto!? you have a stringer! On my Banshee, the stringers were preformed semicircles with no core, which were later glassed in.
When you understand that it's the shape of the stringer and not whats inside of it that provides the rigidity, it makes problems much easier to solve. The truth is, just about anything that will not melt from the styrene in fiberglass resins will work as a form.
Cardboard is actually a fantastic core material. It is light weight, readily available everywhere and so cheap it can often be found for free. The downside it that it is already rigid enough that you may have a hard time getting it to conform to the shape of the hull.
Urethane foam is probably the best substrate you can use. It can be easily shaped to any complex curve. It is perfectly compatible with resin and it is highly rot resistant. Unfortunately, it is cost prohibitive to most hobbyists and not always readily available.
Plywood will also work just fine as a core material. It is much more stable than solid wood. It's also available everywhere. It's cheap. And, though it's harder to shape than urethane, it's not beyond the ability of most DIYs. The problem with plywood is that it's susceptible to rot.
Although the profile of the stringer is by far the most important factor in building a fiberglass stringer, rot is still a concern that must be addressed. Rotted wood will add a considerable amount of weight via moisture absorption. Additionally, the rapid expansion of wood from moisture absorption can cause the stringer to delaminate from the hull. For this reason, whether using plywood as a core material or not, great care should be taken to ensure that moisture stays on the outside of the stringer.
The only medium for stringer construction I absolutely do not recommend is solid wood. Because plywood is made from alternating grain directions of lumber, it is orders of magnitude more stable than solid wood. Solid wood will expand and contract with humidity and temperature variations and can ultimately fracture the fiberglass lamination to the hull. Do Not Use Solid Wood As A Core Material!
